ee119 telecommunication principles and networks
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HNDE Colombo 15TRANSCRIPT
SRI LANKA INSTITUTE of ADVANCED TECHNOLOGICAL EDUCATION
Training Unit
Telecommunication
Principles and Networks Theory
No: EE 119
Electrical and Electronic Engineering
Instructor Manual
INDUSTRIETECHNIKINDUSTRIETECHNIK
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Training Unit
Telecommunication Principles and Networks
Theoretical Part
No.: EE 119
Edition: 2009 All Rights Reserved Editor : MCE Industrietechnik Linz GmbH & Co Education and Training Systems, DM-1 Lunzerstrasse 64 P.O.Box 36, A 4031 Linz / Austria Tel. (+ 43 / 732) 6987 – 3475 Fax (+ 43 / 732) 6980 – 4271 Website: www.mcelinz.com
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LIST OF CONTENT Learning Objectives..................................................................................................................6
1 Introduction to Topic and Course Program .......................................................................1
2 Basics to techniques .........................................................................................................2
2.1 Analogue and digital .................................................................................................2
2.2 Bauds, bits, bytes and codes....................................................................................4
2.3 Bandwidth.................................................................................................................5
2.4 Multiplexing and compression ..................................................................................6
2.5 Computer networks and types ..................................................................................6
3 Telephone systems .........................................................................................................10
3.1 PBX-system............................................................................................................11
3.2 Centrex-system ......................................................................................................12
3.3 Key-system.............................................................................................................13
3.4 Add-on peripherals .................................................................................................14
3.5 ACD to handle large volumes of calls.....................................................................14
3.6 Transport media, wireless, Twisted Pair Copper, fiber-optics ................................16
4 Development of network service providers .....................................................................19
4.1 History of bell system and regulatory......................................................................19
4.2 Telecommunication Act of 1996 .............................................................................20
4.3 Developments after Telecommunication Act of 1996 .............................................22
5 Local network service providing ......................................................................................23
5.1 Local strategies in competition ...............................................................................23
5.2 Carriers IEX ............................................................................................................24
5.3 Trading of bandwidth and the market .....................................................................24
5.4 Providers of local services......................................................................................25
6 Public networks ...............................................................................................................27
6.1 Local and long distance calls..................................................................................28
6.2 Topology of networks .............................................................................................29
6.3 Virtual private networks ..........................................................................................31
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6.4 Access networks.....................................................................................................33
6.5 Fiber optical networks.............................................................................................35
6.6 Signaling in networks..............................................................................................38
6.7 Hardware development ..........................................................................................41
7 Special network services and types ................................................................................42
7.1 ISDN .......................................................................................................................42
7.2 T-1 24 Channels .....................................................................................................44
7.3 T-3 the Capacity of 28 T-1 Lines, 672 Channels ....................................................46
7.4 DSL Digital Subscriber Line Technology ................................................................46
7.5 Gigabit Ethernet......................................................................................................49
7.6 Frame Relay (Shared WAN)...................................................................................50
7.7 ATM-Asynchronous Transfer Mode........................................................................51
7.8 SONET-Synchronous Optical Network...................................................................53
8 Modems and access devices ..........................................................................................55
8.1 Transferring computer data over telephone lines ...................................................55
8.2 DCE-connections to telephone lines.......................................................................56
8.3 Modems for analogue telephone lines....................................................................57
8.4 Connecting devices to ISDN...................................................................................58
8.5 Digital Service Unit/Channel Service Unit...............................................................59
8.6 Cable Modems .......................................................................................................60
8.7 Cable TV Set-Top Boxes........................................................................................61
8.8 Electric cables as data carriers...............................................................................63
8.9 Modem standards...................................................................................................63
9 The Internet.....................................................................................................................65
9.1 Brief history of the internet......................................................................................65
9.2 HTML......................................................................................................................67
9.3 Emailing..................................................................................................................68
9.4 Addressing in internet .............................................................................................70
9.5 Intranet ...................................................................................................................71
9.6 Extranet ..................................................................................................................72
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9.7 Security issues .......................................................................................................73
9.8 Reliability and capacity ...........................................................................................74
10 Converged networks...................................................................................................75
11 Wireless services........................................................................................................76
11.1 Brief history of mobile and cellular services............................................................76
11.2 Cellular telephone service technologies .................................................................77
11.2.1 AMPS..............................................................................................................77
11.2.2 D-AMPS..........................................................................................................77
11.2.3 GSM................................................................................................................78
11.2.4 UMTS-3G........................................................................................................79
11.2.5 Nextel..............................................................................................................80
11.2.6 Paging services ..............................................................................................81
11.3 3rd Generation Networks........................................................................................83
11.3.1 GPRS data carried as packets .......................................................................83
11.3.2 EDGE..............................................................................................................84
11.3.3 cdma2000 .......................................................................................................85
11.3.4 2.5 and 3G Services .......................................................................................86
11.3.5 4G-the future...................................................................................................87
11.4 Messaging services (SMS) .....................................................................................88
11.5 Mobile internet access............................................................................................89
11.6 Blue Tooth ..............................................................................................................90
11.7 Low Earth Orbiting Satellite Networks (LEOs) and Middle Earth Orbiting Satellites
(MEOs) ...............................................................................................................................91
11.8 Time division multiple access and code division multiple access...........................91
12 Global issues ..............................................................................................................93
12.1 Deregulation ...........................................................................................................93
12.2 Asia.........................................................................................................................93
12.3 The rest of the world ...............................................................................................94
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Learning Objectives
The Student should…
… be able to gain a knowledge of all the underlying technologies of telecommunication
networking.
… be able to describe the basic components in telecommunications.
… be able to explain how is voice transmitted over network.
… be able to describe analog and digital transmissions.
… be able to explain telecommunication protocols.
… be able to describe different types of telecommunication media.
… be able to describe different types of telecommunication systems.
… be able to identify different types of telecommunication networks.
… be able to identify different types of computer networks.
… be able to gain basic knowledge about modems and access devices.
… be able to identify basic components of public network.
… be able to identify special network services and types.
… be able to understand how mobile and cellular networks operate.
… be able to identify different types of cellular services.
… be able to gain basic knowledge about internet.
… be able to understand what is HTML and World Wide Web.
… be able to understand Internet Protocol (IP).
… be able to describe addressing in internet.
… be able to understand what happens to data within the Internet and within networks.
… be able to understand international standards and read documentary of equipment
manuals.
1 Introduction to Topic and Course Program
Telecommunications is one of the fastest growing business sectors of modern
information technologies. A couple of decades ago, to have a basic understanding of
telecommunications, it was enough to know how the telephone network operated.
Today, the field of telecommunications encompasses a vast variety of modern
technologies and services. Some services, such as the fixed telephone service in
developed countries, have become mature, and some have been exploding (e.g.,
cellular mobile communications and the Internet). The deregulation of the
telecommunications industry has increased business growth, even though, maybe
because tariffs have decreased.
The present telecommunications environment, in which each of us has to make
choices, has become complicated. In the past, there was only one local telephone
network operator that we chose to use or not use. Currently, many operators offer us
ADSL or cable modem for Internet access and we have many options for telephone
service as well.
Telecommunications is a strategically important resource for most modern
corporations and its importance continues to increase. Special attention has to be
paid to the security aspects and costs of services. The everchanging
telecommunications environment provides new options for users, and we should be
more aware of telecommunications as a whole to be able to capitalize on the
possibilities available today.
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2 Basics to techniques
2.1 Analogue and digital
Along one dimension, communications fall into two categories, analog and digital. In
the analog form of electronic communications, information is represented as a
continuous electromagnetic waveform. Digital communications involves modulating
(i.e., changing) the analog waveform in order to represent information in binary form
(1 s and 0 s) through a series of blips or pulses of discrete values, as measured at
precise points in time or intervals of time.
Fig. 01 Analog and Digital transmission Analog is best explained by examining the transmission of a natural form of
information, such as sound or human speech, over an electrified copper wire. In its
native form, human speech is an oscillatory disturbance in the air that varies in terms
of its volume or power (amplitude) and its pitch or tone (frequency). In this native
acoustical mode, the variations in amplitude cause the physical matter in the air to
vibrate with greater or lesser intensity and the variations in frequency cause the
physical matter in the air to vibrate with greater or lesser frequency. So, the physical
matter in the space between the speaker’s mouth (transmitter) and the listener’s ear
(receiver) serves to conduct the signal.
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That same physical matter, however, also serves to attenuate (weaken) the signal.
The longer the distance is between mouth and ear, the more profound the effect. As
a result, it is difficult, if not impossible, to communicate acoustically over distances of
any significance, especially between rooms separated by doors and walls or between
floors separated by floors and ceilings. In order to overcome these obvious
limitations, native voice acoustical signals are converted into electromagnetic signals
and sent over networks, with the compression waves falling onto a microphone in a
transmitter embedded in a handset or speakerphone. The microphone converts the
acoustical signals into analogous (approximate) variations in the continuous electrical
waveforms over an electrical circuit, hence the term analog. Those waveforms
maintain their various shapes across the wire until they fall on the speaker embedded
in the receiver. The speaker converts them back into their original acoustical form of
variations in air pressure, which can be received by the human ear and understood
by the human brain.
While the natural world is analog in nature, the decidedly unnatural world of
contemporary computers is digital in nature. Computers process, store, and
communicate information in binary form. That is to say that a unique combination of 1
s and 0 s has a specific meaning in a computer coding scheme, which is much like
an alphabet. A bit (binary digit) is an individual 1 or 0. The output of a computer is in
the form of a digital bit stream. Digital communication originates in telegraphy, in
which the varying length (in time) of making and breaking an electrical circuit results
in a series of dots (short pulses) and dashes (long pulses) that, in a particular
combination, communicate a character or series of characters. Early mechanical
computers used a similar concept for input and output. Contemporary computer
systems communicate in binary mode through variations in electrical voltage. Digital
signaling, in an electrical network, involves a signal that varies in voltage to represent
one of two discrete and well-defined states. Two of the simplest approaches are
unipolar signaling, which makes use of a positive (+) voltage and a null, or zero (0),
voltage, and bipolar signaling, which makes use of a positive (+) or a negative (−)
voltage. The transmitter creates the signal at a specific carrier frequency and for a
specific duration (bit time), and the receiver monitors the signal to determine its state
(+ or −).
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Various data transmission protocols employ different physical signal states, such as
voltage level, voltage transition, or the direction of the transition. Because of the
discrete nature of each bit transmitted, the bit form is often referred to as a square
wave. Digital devices benefit greatly from communications over digital transmission
facilities, which are not only faster but also relatively free from noise impairments.
Digital signaling in an optical network can involve either the pulsing on and off of a
light source or a discrete variation in the intensity of the light signal. Digital
transmission over radio systems (e.g., microwave, cellular, or satellite) can be
accomplished by discretely varying the amplitude, frequency, or phase of the signal.
Bandwidth, in the digital world, is measured in bits per second. The amount of
bandwidth required depends on the amount of raw data to be sent, the desired speed
of transmission of that set of data, and issues of transmission cost. Compression of
data files prior to transmission is fairly routined as it improves the efficiency of
transmission, reduces the transmission time, and thereby reduces transmission
costs.
2.2 Bauds, bits, bytes and codes
Baud is an old term that refers to the number of signal events (i.e., signal changes or
signal transitions) occurring per second over an analog circuit. The baud rate can
never be higher than the raw bandwidth of the channel, as measured in Hz. Baud
rate and bit rate often and incorrectly are used interchangeably. The relationship
between baud rate and bit rate depends on the sophistication of the modulation
scheme used to manipulate the carrier. The bit rate and baud rate can be the same if
each bit is represented by a signal transition. The bit rate typically is higher that the
baud rate as a single signal transition can represent multiple bits.
Quite simply, bps (lowercase b) is the bit rate, or the number of bits transmitted over
a circuit per second. It is the measurement of bandwidth over digital circuits and
should not be confused with the speed of the electromagnetic signal, that is, the
velocity of propagation. In other words, bps refers to the number of bits that pass a
given point in a circuit, not the speed at which they travel over a distance. Over an
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analog circuit, you can manipulate the electromagnetic waveforms to support the
transmission of multiple bits per baud. As a result, the bit rate (bps) can be a multiple
of the baud rate, even without the application of special compression techniques. A
thousand (1000) bps is a kilobit per second, or kbps; a million (1,000,000) bps is a
Megabit per second, or Mbps; a billion (1,000,000,000) bps is a Gigabit per second,
or Gbps; and a trillion (1,000,000,000,000) bps is a terabit per second or Tbps. Bps
(uppercase B) refers to the number of bytes transmitted over a circuit per second.
Bps is used exclusively in the context of storage networking, as storage is byte
oriented. Storage technologies such as Fibre Channel and ESCON (Enterprise
Systems CONnection) measure the speed of information in bytes per second.
2.3 Bandwidth
Bandwidth is a measure of the capacity of a circuit or channel. More specifically, it
refers to the total frequency on the available carrier for the transmission of data.
There is a direct relationship between the bandwidth of a circuit or channel and both
its frequency and the difference between the minimum and maximum frequencies
supported. The bandwidth of an analog service is the difference between the highest
and lowest frequency within which the medium carries traffic. Greater the difference
between the highest and lowest frequency result in greater capacity or bandwidth.
While the information signal (bandwidth usable for data transmission) does not
occupy the total capacity of a circuit, it generally and ideally occupies most of it. The
balance of the capacity of the circuit may be used for various signaling and control
(overhead) purposes. In other words, the total signaling rate of the circuit typically is
greater than the effective transmission rate.
For digital services such as ISDN, T-1 and ATM, speed is stated in bits per second.
Simply put, it is the number of bits that can be transmitted in one second. T-1, for
example, has a bandwidth of 1.54 million bits per second.
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2.4 Multiplexing and compression
The term multiplex has its roots in the Latin words multi (many) and plex (fold).
Multiplexers (muxes) act as both concentrators and contention devices that enable
multiple relatively low speed terminal devices to share a single high-capacity circuit
(physical path) between two points in a network. The benefit of multiplexers is simply
that they enable carriers and end users to take advantage of the economies of scale.
Just as a multilane highway can carry large volumes of traffic in multiple lanes at high
speeds and at relatively low incremental cost per lane, a high-capacity circuit can
carry multiple conversations in multiple channels at relatively low incremental cost
per channel.
Contemporary multiplexers rely on four-wire circuits, which enable multiple logical
channels to derive from a single physical circuit and permit high-speed transmission
simultaneously in both directions. In this manner, multiple communications (either
unidirectional or bidirectional) can be supported. Multiplexing is used commonly
across all transmission media, including twisted pair, coaxial and fiber-optic cables,
and microwave, satellite, and other radio systems. Traditional multiplexing comes in
several varieties, presented in the following sections in chronological order of
development and evolution. Included are Frequency Division Multiplexing (FDM),
Time Division Multiplexing (TDM) and Statistical Time Division Multiplexing (STDM).
Wavelength Division Multiplexing (WDM), a relatively recent development, is used in
fiber-optic cable systems.
2.5 Computer networks and types
A computer network is a collection of computers and devices connected to each
other. The network allows computers to communicate with each other and share
resources and information. One way to categorize the different types of computer
network designs is by their scope or scale. For historical reasons, the networking
industry refers to nearly every type of design as some kind of area network. Common
examples of area network types are:
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LAN-Local Area Network
MAN-Metropolitan Area Network
WAN-Wide Area Network
A LAN connects network devices over a relatively short distance. A networked office
building, school, or home usually contains a single LAN, though sometimes one
building will contain a few small LANs (perhaps one per room), and occasionally a
LAN will span a group of nearby buildings. In addition to operating in a limited space,
LANs are also typically owned, controlled, and managed by a single person or
organization. They also tend to use certain connectivity technologies, primarily
Ethernet and Token Ring.
LAN are privately owned networks within a single building or campus, up to the size
of a few km. Usually it’s used to connect PCs, workstations to share resources such
as storage devices on servers, printers or other output devices and in some
circumstances also to share access computing power.
Fig. 02 Local Area Network (LAN)
Each device connected to the local area network can communicate with every other
device. The connections between devices may be any of the following: twisted pair,
coaxial cable, fiber optics or wireless media. Cables or wires that connect devices are
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also called Backbone. For the most part, devices are connected to a LAN by twisted
pair cabling. In computer networking, layout of connected devices is called network
topology.
All networks are made up of basic hardware building blocks to interconnect network
nodes, such as Network Interface Cards (NICs), Bridges, Hubs, Switches, and
Routers.
Network Card, Network Adapter or NIC: A network card, network adapter, network
interface controller (NIC), network interface card, or LAN adapter is a computer
hardware component allowing computers to communicate over a computer network.
It provides physical access to a networking medium. A low-level addressing system is
provided by the use of MAC addresses. It allows interconnectivity either by cable or
by the use of cables.
Hub: A network hub or repeater hub is a device for connecting multiple twisted pair or
fiber optic Ethernet devices together and making them act as a single network
segment. Hubs work at the physical layer (layer 1) of the OSI model (which will be
referred to later in this course).
The availability of low-priced network switches has made hubs obsolete but they are
still seen in older installations and more specialized applications.
Bridge: A network bridge connects multiple network segments. It is also often
referred to as layer 2 switch, since it connects the network segments at the data link
layer (layer 2) of the OSI model. Bridges function similar to repeaters or network
hubs. However, with bridging, the network traffic is rather managed than simply
rebroadcast to adjacent network segments. Bridges tend to be more complex than
hubs or repeaters because bridges are capable of analyzing incoming data packets
on a network to determine if the bridge is able to send the given packet to another
segment of that same network.
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Switches: A network switch is a hardware device that joins multiple PCs or hosts
together within one local area network (LAN). Network switches operate at the data
link layer (layer 2) of the OSI model, like the bridge. Network switches are almost
identical to network hubs. A switch however contains more intelligence than a hub.
Network switches are capable of inspecting data packets as they are received,
determining the source and destination device of each packet, and forwarding them
appropriately. Because of this operation a switch saves network resources, like
bandwidth and has better performance than a hub.
Router: A router is a networking device which is tailored to route and forward
information. For instance in internet the information is directed to various paths by
routers. Routers connect two or more logical subnets, which do not necessarily map
one-to-one to the physical interfaces of the router. A router is also sometimes
referred to as layer 3 switch, and also often simply switch.
MAN-Metropolitan Area Network
A metropolitan area network (MAN) is a network that connects two or more local area
networks or campus area networks together but does not extend beyond the
boundaries of the immediate town/city. Routers, switches and hubs are connected to
create a metropolitan area network.
WAN-Wide Area Network
As the term implies, a WAN spans a large physical distance. The Internet is the
largest WAN, spanning the Earth. A WAN is a geographically-dispersed collection of
LANs. A network device called a router connects LANs to a WAN.
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3 Telephone systems
When the telephone was invented in 1877, there were only telephone sets and wires
between them. The first commercial sets consisted of a single piece of wood with a
single piece of equipment serving as both transmitter and receiver. At first telephone
sets come in pairs and were directly wired. One year later, first practical exchange
switch, was invented and placed into service. In the field of telecommunications, a
telephone exchange or telephone switch is a system of electronic components that
connects telephone calls and central office is the physical building used to house
inside plant equipment including telephone switches, which make telephone calls
"work" in the sense of making connections and relaying the speech information. This
manual exchange, housed in Central Offices (COs), allowed circuits to be connected
manually, on demand and as available. Through these central points of
interconnection, each subscriber required only one terminal device and one wired
connection to the central switch. Central offices or Central Office Exchanges (COEs)
handled all switching of calls whether they involved parties from different parts of the
city, across the hall, or across the office. Soon it became clear that extending a
physical partition of the COE to the customer premises presented a better approach.
This approach would considerably reduce the cost of cabling as well as the number
of ports required to connect those local loops to the switch. The shift to the user
organization of the functional responsibility for switching both incoming and outgoing
calls would also relieve the telco of that burden, allowing the exchange switch to
serve more end users. Finally, internal (station-to-station) calls could be connected
entirely through the premises equipment, without requiring a connection through the
telco's CO. As station-to-station calls are a very large percentage of the total calling
traffic in most large organizations, shifting this functional responsibility to the user
organization could have considerable impact on the telephone company’s capital
investment and labor costs. The first devices to accomplish this feat were Private
Branch eXchanges (PBXs) in 1879. Key Telephone Systems (KTSs) did not arrive on
the scene until 1938. Central exchange (Centrex), a CO-based solution, followed in
the 1960s. Automatic Call Distributors (ACDs) did not make an appearance until
1973.
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3.1 PBX-system
A PBX is an on-site telephone system that connects organizations to the public
switched telephone network. The central office switch is the precursor to on-site
private branch exchange (PBX) telephone systems. A central office switch is centrally
located and routes calls between users in the public network. PBXs are private and
located within an enterprise.
Just as a central office switch eliminates the need to wire each telephone to every
other telephone, with a PBX, each telephone is wired to the PBX-not to each
telephone in the company. Because the PBX is wired to the central office, each
telephone does not need its own line wire to the central office. In essence, with a
PBX, each employee does not have to pay for his or her own telephone line to the
local telephone company. Nor are there charges for calls between people in the
same office.
Fig. 03 PBX connected to CO with trunks
PBXs are connected to telephone company central offices by trunks that carry calls
between the PBX and the telephone company. Depending on the volume of calls
generated by the staff, eight to ten users can share each trunk. A PBX with 100 users
might share 12 trunks. Most companies use T-1 for their trunking. Instead of having
24 separate pairs of wires, the T-1 can carry 24 incoming and/or outgoing calls on
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two pairs of wire or on fiber optic cable. Fiber optic cables have the ability to carry
multiple T-1s.
3.2 Centrex-system
Centrex is a PBX-like service providing switching at the central office instead of at the
customer's premises. Typically, the telephone company owns and manages all the
communications equipment and software necessary to implement the Centrex
service and then sells various services to the customer. No switching equipment
resides on the customer premise as the service is supplied and managed directly
from the phone company's exchange site, with lines being delivered to the premises
either as individual lines over traditional copper pairs or by multiplexing a number of
lines over a single fiber optic or copper link. In effect, Centrex provides an emulation
of a hardware PBX, by using special software programming at the central office,
which can be customized to meet a particular customer's needs. As with a PBX,
stations inside the group can call each other with 3, 4 or 5 digits, depending on how
large the group, instead of an entire telephone number.
Centrex was invented in the mid 1960s by the Engineering Department of New York
Telephone to replace the PBX switchboards of large customers. It was a feature
package of the 5XB crossbar system. Much equipment had to be redesigned,
including incoming trunks and markers. The redesigned equipment was so expensive
that usually a separate 5XB switch was used just for Centrex customers, while POTS
(Plain Old Telephone Service) customers were wired to an unmodified exchange.
The Centrex customer is not restricted to using the features available to POTS
customers, but can choose from a wide variety of special services and features. In
fact, telecommunications companies generally offer numerous types of Centrex
service, including "Packaged Centrex", "Centrex Data", and "Customized Centrex".
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3.3 Key-system
Key telephone systems are business communications systems intended for small
businesses, typically defined in this context as involving no more than 50 stations.
The term key telephone dates to the beginnings of telegraphy and telephony when
mechanical keys were employed to open and close a circuit. The buttons on a key
telephone set, also referred to as keys, mechanically opened and closed the line
circuit. While contemporary KTSs provide much the same feature content as small
PBXs, and while they also act as contention devices for network access, KTSs are
not switches. That is, they do not possess the intelligence to accept a call request
from a user station, determine the most appropriate circuit from a shared pool of
circuits, and set up the connection through common switching equipment. Rather, the
end user must make the determination and select the appropriate facility (local line,
tie line) from a group of pooled facilities. KTS control relies on gray ware (i.e., gray
matter, or human brain power), rather than software (i.e., computer programs).
Therefore, the local loops associated with KTSs are lines, rather than trunks. A line is
a single-channel facility that is associated with a single telephone number and that
connects an endpoint to the Public Switched Telephone Network (PSTN). A trunk
typically is a multi channel, rather than a single channel, facility that interconnects
switches and is not necessarily associated with a telephone number. Rather, a trunk
serves a group of users and a group of telephone numbers through an intelligent
switching device that is designed to manage contention between users and channels.
Telco rate and tariff logic assumes that a trunk will be used more intensely than a
line, so the cost is greater.
Most small KTS’s are squared, meaning that every key set is configured alike, with
every outside line appearing on every set. Thereby, every station user can access
every outside line for both incoming and outgoing calls, and all feature presentations
are consistent. In larger systems, the physical size of the telephone sets required to
maintain the squaring convention would be impractical, but departmental subgroups
often are squared.
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3.4 Add-on peripherals
Peripheral devices such as voice mail, automatic call distributors and call accounting
systems can be added to PBX, Centrex or Kay systems to extend its functionality.
Call accounting systems track each telephone call made by individual users. They
provide accountability for call usage. They also indicate the amount of traffic on each
telephone line or trunk so that organizations can determine when there are too many
or too few outside telephone lines. Call accounting systems, also called station
message detail recording (SMDR) and call detail recording (CDR) generally are
installed on PCs. The PC is connected through a serial port to the telephone system.
Unified messaging is an optional feature of most new voice mail systems. It provides
the capability to retrieve fax, email and voice mail messages from a single device
such as a PC. Retrieving messages from PCs gives users the ability to prioritize
messages and listen to the most important ones first. It eliminates the need to hear
all messages before getting to the critical ones. Systems with only fax and voice mail
integration also are considered unified messaging systems. These systems store
incoming facsimile messages on the voice mail system's hard drive. When users call
in to pick up their messages, the system tells them how many faxes they have. The
user can have them printed at their default fax number programmed into the voice
mail system or provide the telephone number of a different fax machine. For
example, people that travel can receive fax messages at their hotel. In these
systems, fax and voice mail notification also can be obtained at the user's computer.
3.5 ACD to handle large volumes of calls
ACDs essentially are highly sophisticated PBXs designed specifically to switch
incoming calls in call center applications. As call centers are highly active, with
relatively large numbers of callers queued for a much smaller number of agents,
ACDs generally are non blocking systems. Call centers may be specific to the user
organization but often are set up on a service bureau basis. A service bureau might
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answer calls for a large number of clients, on either a primary or overflow basis, with
software and scripting specific to the individual client’s requirements. The system
identifies the target client by the telephone number called, with a separate telephone
number or set of telephone numbers associated with a specific client, a special
promotion, a specific service offering, or a special subset of each client’s customers
through Dialed Number Identification Service (DNIS), a service offered by the carriers
and generally associated with toll-free numbers (800, 888, 877, and 866 in the United
States and 0800 and 0500 in much of the rest of the world). The DNIS information is
passed to the ACD in advance of the call.
The process, typically involves a front-end voice processing system that prompts the
caller through a menu for call routing purposes. Based on factors such as the
originating telephone number, the specific number dialed, the menu option selected,
and other information input by the caller (e.g., account number and password), the
system then can route a call to an appropriate agent group, queue it if an agent is not
available, and deliver it to the first available qualified agent.
Fig. 04 Automatic call distributors (ACD)
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Multiple call centers can be networked, with calls routed to the most appropriate call
center closest to the caller, in consideration of network costs. In the event that the
closest call center’s queue length exceeds user-definable parameters, the call is then
served to the next nearest call center with a queue of acceptable length. Recent
developments allow the first call center to examine the queue lengths of all call
centers in the network, determine each call center’s ability to handle a call in
consideration of service-level parameters, and forward the call to the call center most
likely to satisfy those objectives based on look-ahead routing logic. This approach
reduces unnecessary levels of congestion and unacceptable numbers of unhappy
customers.
3.6 Transport media, wireless, Twisted Pair Copper, fiber-optics
Wireless
Radio is defined as the transmission and reception of electrical impulses or signals
by means of electromagnetic waves without the use of wires. Basically, radio waves
are electromagnetic radiation transmitted through the air to a receiver.
Radio waves are classified by their frequency, which describes the number of times a
signal cycles per second, commonly referred to as Hertz (Hz), in honor of Heinrich
Hertz. The wavelength is the distance between repeating units of a wave pattern. In a
sine wave, the wavelength is the distance between any point on a wave and the
corresponding point on the next wave in the wave train. There is an inverse
relationship between frequency and wavelength: As the frequency increases, the
wavelength decreases.
There are performance differences between radio frequencies. Low frequencies can
travel much further without losing power (i.e., attenuating), but they carry much less
information because the bandwidth (i.e., the difference between the highest and
lowest frequency carried in the band) is much lower. High frequencies (those in the
HF band, from 3MHz to 30MHz) offer much greater bandwidth than lower
frequencies, but they are greatly affected by interference from a variety of sources.
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Very high frequencies (those in the SHF band, from 3GHz to 30GHz) suffer greatly
from adverse weather conditions, particularly precipitation. This problem is even
greater in the extremely high frequencies (EHF band, from 30GHz to 300GHz).
Twisted pair cable
A twisted pair cable consists of two copper strings twisted like a DNS-string. It’s one
of the oldest, but it is still the most common media. For some physical reasons the
twisting causes the cable to produce less radiation and causing interferences.
The land line telephone system is based on twisted pair cables. These cables reach
several km without amplifier. Due to low cost and adequate performance twisted pair
will also be a medium with a future. There are several varieties of twisted pairs cables
and associated plugs:
Category 3 cable (Cat 3) consist of a bundle of four twisted pairs of insulated
copper wires come in a PVC-sheath. Up to for regular phones or two ISDN
phones could be connected.
Category 5 cable (Cat 5) is similar to CAT 3 but have more twists per cm.
This way the signal quality over longer distances is increased.
Category 6 cable (Cat 6) is a cable standard for Gigabit Ethernet and other
network protocols that is backward compatible with the Category 5/5e and
Category 3 cable standards. Cat-6 features more stringent specifications for
crosstalk and system noise. The cable standard provides performance of up to
250 MHz and is suitable for 10BASE-T / 100BASE-TX and 1000BASE-T
(Gigabit Ethernet).
Category 7 cable (Cat 7) is a cable standard for Ethernet and other network
technologies. It has backwards compatibility with Cat 5 and Cat 6 Ethernet
cables. Cat 7 features even more strict specifications for crosstalk and system
noise than Cat 6. To achieve this, shielding has been added for individual wire
pairs and the cable as a whole.
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Fiber-optics
An optical fiber (or fibre) is a glass or plastic fiber that carries light along its length.
Optical fibers permits transmission over longer distances and at higher bandwidths
(data rates) than other forms of communications. Fibers are used instead of metal
wires because signals travel along them with less loss, and they are also immune to
electromagnetic interference. Optical fiber is used as a medium for
telecommunication and networking because it is flexible and can be bundled as
cables. It is especially advantageous for long-distance communications, because
light propagates through the fiber with little attenuation compared to electrical cables.
This allows long distances to be spanned with few repeaters. Additionally, the per-
channel light signals propagating in the fiber can be modulated at rates as high as
111 gigabits per second, although 10 or 40 Gb/s is typical in deployed systems. Each
fiber can carry many independent channels, each using a different wavelength of
light (wavelength-division multiplexing (WDM)).
Over short distances, such as networking within a building, fiber saves space in cable
ducts because a single fiber can carry much more data than a single electrical cable.
Fiber is also immune to electrical interference; there is no cross-talk between signals
in different cables and no pickup of environmental noise. Non-armored fiber cables
do not conduct electricity, which makes fiber a good solution for protecting
communications equipment located in high voltage environments such as power
generation facilities, or metal communication structures prone to lightning strikes.
They can also be used in environments where explosive fumes are present, without
danger of ignition. Wiretapping is more difficult compared to electrical connections,
and there are concentric dual core fibers that are said to be tap-proof.
Although fibers can be made out of transparent plastic, glass, or a combination of the
two, the fibers used in long-distance telecommunications applications are always
glass, because of the lower optical attenuation.
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4 Development of network service providers
The breakup of AT&T was initiated by the filing in 1974 by the U.S. Department of
Justice of an antitrust lawsuit against AT&T, which was at the time the only phone
company in the United States. The case, United States v. AT&T, led to a settlement
finalized on January 8, 1982, under which "Ma Bell" agreed to divest its local
exchange service operating companies, in return for a chance to go into the
computer business, AT&T Computer Systems. Effective January 1, 1984, AT&T's
local operations were split into seven independent Regional Holding Companies, also
known as Regional Bell Operating Companies (RBOCs), or "Baby Bells". Afterwards,
AT&T, reduced in value by approximately 70%, continued to operate all of its long-
distance services, although in the ensuing years it lost portions of its market share to
competitors such as MCI and Sprint.
4.1 History of bell system and regulatory
The telecommunications landscape has changed radically since 1984 when AT&T
had a near monopoly on telephone service. In the United States competition for sales
of telephone systems started in the 1960s. However, the most dramatic impetus for
competition for long distance service occurred in 1984.
By 1974, so many complaints had been filed by long distance competitors with the
Justice Department about AT&T's lack of cooperation in supplying connections to
local phone companies that the Justice Department filed an antitrust suit against
AT&T. In 1984 the suit was resolved. The Justice Department divested AT&T of its
22 local phone companies. The resolution of the Justice Department case against
AT&T is known as the Modified Final Judgment, or divestiture.
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Ownership of the 22 local phone companies was transferred from AT&T to seven
Regional Bell Operating Companies (RBOCs). The seven RBOCs at that time were:
Ameritech Corporation
Bell Atlantic Corporation
BellSouth Corporation
NYNEX Corporation
Pacific Telesis Group
Southwestern Bell Corporation
U S WEST, Inc.
The breakup led to a surge of competition in the long distance telecommunications
market by companies such as Sprint and MCI. AT&T's gambit in exchange for its
divestiture, AT&T Computer Systems, failed, and after spinning off its manufacturing
operations and other misguided acquisitions, it was left with only its core business
with roots as AT&T Long Lines and its successor AT&T Communications. It was at
this point that AT&T was purchased by one of its own spin-offs, SBC
Communications, which started as Southwestern Bell Communications.
4.2 Telecommunication Act of 1996
The Telecommunications Act of 1996 was the first major overhaul of United States
telecommunications law in nearly 62 years, amending the Communications Act of
1934. It was approved by the 104th Congress on January 3, 1996 and signed into
law on February 8, 1996.
The 1996 Telecommunications Act is divided into seven Titles: Telecommunications
Service, Broadcast Services, Cable Services, Regulatory Reform, Obscenity and
Violence, Effect on Other Laws and Miscellaneous Provisions.
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The Act makes a significant distinction between providers of telecommunications
services and information services. The term “telecommunications service” means the
offering of telecommunications for a fee directly to the public or to such classes of
users as to be effectively available directly to the public, regardless of the facilities
used. On the other hand, the term “information service” means the offering of a
capability for generating, acquiring, storing, transforming, processing, retrieving,
utilizing, or making available information via telecommunications, and includes
electronic publishing, but does not include any use of any such capability for the
management, control, or operation of a telecommunications system or the
management of a telecommunications service. The distinction comes into play when
a carrier provides information services. A carrier providing information services is not
a “telecommunications carrier” under the act. For example, a carrier is not a
“telecommunications carrier” when it is selling broadband Internet access. This
distinction becomes particularly important because the act enforces specific
regulations against “telecommunications carriers” but not against carriers providing
information services. With the convergence of telephone, cable, and internet
providers, this distinction has created much controversy.
The Act both deregulated and created new regulations. Congress forced local
telephone companies to share their lines with competitors at regulated rates if the
failure to provide access to such network elements would impair the ability of the
telecommunications carrier seeking access to provide the services that it seeks to
offer. This led to the creation of a new group of telephone companies, Competitive
Local Exchange Carriers (CLECs), that compete with ILECs or incumbent local
exchange carriers.
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4.3 Developments after Telecommunication Act of 1996
Implementation of the Telecommunications Act of 1996 was initially hampered by
RBOC legal challenges. Disagreements over pricing, implementation and order
placing snafus have further hindered competitors that wished to sell service using
incumbents' facilities.
The Telecommunications Act of 1996 mandated that the very organizations that
compete with new entrants, the RBOCs, must also supply connections and services
for competitors. The local Bell companies are offered the carrot of entrance into new
businesses, out-of-region long distance and manufacturing. Nonetheless, conflicts of
interest are inherent in pricing for and arranging for resale and access to Bell
resources. It is no surprise that issues of pricing for resale and interconnection were
contested in court.
Enforcement of provisions and details of implementation of the Act were left, for the
most part, to the FCC. Its rulings on wholesale rates and its rights to set rates were
challenged by the state public utilities, local telephone companies and independent
telephone companies. They contended that the 1934 Communications Act granted
state utilities the prerogative of setting resale and wholesale discounts in their states.
The Supreme Court ruled in January of 1999 that the FCC has jurisdiction on pricing.
It also ruled that the Act is constitutional in setting conditions for only the RBOCs but
not the independent telephone companies for entry into interregion long distance.
The following are factors that have slowed local competition:
Legal challenges to the Act
Interconnection disagreements between incumbents and new local carriers
Service interruptions when customers change from RBOCs to competitors
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5 Local network service providing
5.1 Local strategies in competition
There are technical, legal and financial considerations in providing local telephone
service. Understanding how calls are passed between competing local carriers and
between local carriers, resellers and interexchange carriers is important in
comprehending the structure of the industry.
The following should be considered within the context of local calling:
Transport-The line from a home or business to the central office.
Switching-The central office switch directs calls to their destination. It also has
links to billing and enhanced feature systems such as voice mail and caller ID.
Terminating transport-The transmission of the call to its end site, or
destination.
Signaling-Signals in the network include telephone number dialed, busy
signals, ringing and the diagnostic signals generated by carriers for repair and
maintenance of the network.
Companies that compete with incumbent local telephone companies (ILECs) either
build their own infrastructure or resell incumbents' facilities. Many use a combination
of both strategies. Competitors buy elements such as transport, switching and
terminating services at discounts approved by the local utility commissions under
guidelines set by the FCC. Their profits are realized in their markup to end users.
Resellers bill end users and handle repair and customer service calls. If a repair
problem is on Bell lines, the reseller reports the problem to the Bell Company, which
either fixes it remotely or dispatches a technician.
For the most part, competitors for local service sell to the business, government,
education and health care industries. New cable TV companies are the largest
telecommunications segment that target residential customers. Competitors to
incumbent cable TV companies are known as overbuilders. Overbuilders lay fiber
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and hybrid coaxial cable fiber systems to which they connect central office switches,
television antennas and routers and switches for high-speed Internet access.
5.2 Carriers IEX
Prior to the Telecommunications Act of 1996, interexchange carriers (IEXs) sold long
distance services primarily between states and to international locations. These
carriers now sell local, international, data services and high-speed Internet access as
well as voice long distance. The largest IEXs own most of the switching and
transmission equipment over which their interstate traffic is routed. For example, they
own fiber optic cabling, microwave towers, multiplexing equipment (to send multiple
voice and data conversations over the same fiber cable) and switches that route
calls. The distinction between local and long distance service carriers is
disappearing. Now that Qwest owns the former RBOC US West, it is an
interexchange carrier only in the territory outside of that region. The four largest
interexchange carriers in the United States are:
AT&T
WorldCom
Sprint
Qwest
5.3 Trading of bandwidth and the market
Just as commodities such as natural gas, oil and electricity are traded, bandwidth
trading in telecommunications is just emerging as a way to buy and sell capacity and
hedge the risks of building new routes and buying capacity. For example, a
competitive local exchange carrier (CLEC) that sells both long distance and local
service can decrease its risk of price spikes by locking in a future price for bandwidth
leases. The seller, a carrier, can cut its risk of adding expensive electronics such as
wavelength division multiplexing (WDM) to dark fiber by selling future capacity. A
future sale locks in a price floor and future revenues. It guarantees the price won't
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drop below a certain price for the seller and fixes a price ceiling for the buyer.
Bandwidth trading is the listing, buying and selling of various telecommunications
services through participants such as brokers, traders and exchanges.
Bandwidth trading is still in its early stages. It is evolving as a result of growing
capacities in long distance networks and commoditization of networks. Technological
innovations such as wavelength division multiplexing have added tremendous
capacity to telecommunications networks in Europe and the United States. Not only
is there more capacity, but services are reliable and there is very little difference to
end users and other carriers among the networks. Finally, capital is drying up for
expansion of more diverse routes. Thus, when a carrier is faced with the decision of
building new routes or leasing them, it often chooses to lease capacity from other
carriers to conserve capital. For carriers of existing networks, selling spare capacity
lets them fill their "pipes" more fully.
5.4 Providers of local services
In addition to incumbent telephone companies, a variety of providers sell local
services. These include interexchange carriers, resellers, utilities and agents. A new
type of provider building local exchange carriers, sell services to tenants in office
parks and multi-tenant buildings.
Local Exchange Carrier (LEC) is a regulatory term in telecommunications for the local
telephone company. In the United States, telephone companies are divided into two
large categories: long distance (interexchange carrier or IXCs) and local (local
exchange carrier or LEC). This structure is a result of 1984 divestiture of then
regulated monopoly carrier American Telephone & Telegraph. Local telephone
companies at the time of the divestiture are also known as Incumbent Local
Exchange Carriers (ILEC).
Local phone calls are defined as calls originating and terminating within a local
access and transport area (LATA) which is defined by the Federal Communications
Commission. LECs typically operate businesses in more than one LATA yet their
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services of local telephone calls are still defined by LATA boundaries, not their
business areas.
A Competitive Local Exchange Carrier (CLEC) is a telecommunications provider
company (sometimes called a "carrier") that competes with other, already established
carriers (generally the incumbent local exchange carrier (ILEC)). CLEC sell Internet
access, data communications services, white and yellow page listings, toll-free (800
and 888) service, long distance and 911 services. CLECs refer to themselves as
ICPs when they sell local and data services over their own fiber optic or wireless
infrastructure.
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6 Public networks
The public switched telephone network (PSTN) is made up of switches, cabling and
equipment that simultaneously transmit multiple telephone calls over single pairs of
fiber cabling. The extraordinary characteristic of public networks is that carriers from
all parts of the world have agreed on ways to transmit calls to each other. Enormous
efforts have been made to ensure that systems are reliable and dependable and
function as much as possible during power blackouts, hurricanes and national
emergencies.
Switching is the primary vehicle for carrying voice, facsimile and dialup modem traffic
worldwide. Public network switched services are dialup; users dial a telephone
number to create a temporary connection to anyone on the public network. Examples
of switched services are home telephone, cellular, dialup Internet access and main
business lines. Switched services are used for data as well as voice. Switches
connect segments of local area networks (LANs) to each other. Asynchronous
transfer mode (ATM) and optical switches in public data and voice networks switch
vast quantities of traffic between cities.
Dedicated services, also called private lines, are more specialized than switched
lines. Organizations use them to save money when they need to transmit large
amounts of data or place hours of voice telephone calls to particular sites. Imagine
two tin cans and a string between two locations. This is something like a private line.
Dedicated lines are expensive and complex to manage. If a firm only has dedicated
lines between a few locations, maintenance might not be a problem. However, once
private lines connect many locations, they become cumbersome to manage.
Moreover, private lines are costly because carriers can't share the dedicated private
lines among many customers. For these reasons, many companies are choosing
carrier-managed, value-added virtual private networks (VPNs). Virtual private
networks are "virtually" private. They have many of the features of private networks;
however, network capacity is shared by many customers.
Signaling is the glue that holds the public switched network together. Routing, billing
and transferring calls between carrier networks depend on signaling. Network
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maintenance information also is carried on signaling systems. The way signals are
transported impacts network efficiency, costs, reliability and introduction of new
services.
Optical technologies have had a major impact on efficiencies in Internet, long
distance and local networks. They have lowered the costs significantly of building
high-capacity data and voice networks. Major efforts in development of new optical
technology are bringing the benefits of fiber optics closer to homes and small and
medium-sized businesses. Applications such as document sharing, email, Web
browsing and remote access to corporate files are affordable to individuals and small
and medium-sized businesses.
6.1 Local and long distance calls
The public switched telephone network is analogous to a network of major highways
originally built by a single organization but added to and expanded by multiple
organizations. Traffic enters and exits these highways (backbone networks) from
multiple "ramps" built by still more carriers e.g., the incumbent local telephone
companies, cable TV providers and competitive local exchange carriers (CLECs).
AT&T constructed the "highway" system that is the basis of the public switched
network in the United States. Prior to the 1984 divestiture, AT&T set standards via its
research arm, Bell Laboratories (now part of Lucent Technologies), such that all
central office switches and all lines that carried calls met prescribed standards. As a
result of these standards, anyone with a telephone can talk to anyone else. Dialing,
ringing, routing and telephone numbering are uniform.
The International Telecommunications Union (ITU) defines switching as "the
establishment on demand, of an individual connection from a desired inlet to a
desired outlet within a set of inlets and outlets for as long as is required for the
transfer of information." The inlets are the lines from customers to telephone
company equipment. The outlet is the party called, the connection from the central
office switch to the customer. A circuit or connection is established for as long as
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desired within the network until one party hangs up. Switched calls carry voice, data,
video and graphics. They operate on landline and cellular networks.
Telephone calls are routed to destinations based on the number dialed. This is the
addressing function. Telephones on landline connections send dual tone multi-
frequency (DTMF) tones over the network. At the central office, these tones or
frequencies are decoded to address signals. In cellular networks, users first dial the
number they wish to reach, and then press Send. The telephone number is sent as
digital bits within packets to the mobile switching office.
In the North American Numbering Plan (NANP), which covers the United States,
Canada and the Caribbean, three-digit area codes are assigned to metropolitan
areas. Exchanges, the next three digits of a phone number, are assigned to a rate
center, and the last four digits, the line number, are assigned to a specific business or
residential customer. In the rest of the world, each country has country codes, city
codes and user numbers. The digits of each vary in length: Country codes and city
codes are one to three digits long and numbers assigned to users generally are five
to ten digits long. There is no uniform worldwide numbering pattern.
6.2 Topology of networks
The term topology refers to the geometric shape of the physical connection of the
lines in a network, or the "view from the top." The shape of the network, the
configuration in which lines are connected to each other, impacts cost, reliability and
accessibility. Network topologies are categorized into the three basic types bus, ring
and star topology.
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Fig. 05 Bus, Ring and Star topologies
Bus Topology: Bus networks use a common backbone to connect all devices. A
single cable, the backbone, functions as a shared communication medium that
devices attach or tap into with an interface connector. A device wanting to
communicate with another device on the network sends a broadcast message onto
the wire that all other devices see, but only the intended recipient actually accepts
and processes the message.
Ring Topology: A ring network is a network topology in which each node connects
to exactly two other nodes, forming a single continuous pathway for signals through
each node - a ring. Data travels from node to node, with each node along the way
handling every packet.
Star Topology: Many home networks use the star topology. A star network features
a central connection point called a "hub" that may be a hub, switch or router.
Compared to the bus topology, a star network generally requires more cable, but a
failure in any star network cable will only take down one computer's network access
and not the entire LAN.
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6.3 Virtual private networks
A virtual private network (VPN) has all of the features of a private network where
dedicated lines tie sites together. However with VPNs, the network connections
between sites are shared by multiple organizations. A carrier manages the network.
The customer connects dedicated or dialup lines to the carrier's network. The carrier
is responsible for connections between customer sites. Many VPNs are based on the
Internet Protocol. The three most common applications for virtual private networks
are:
Remote access for telecommuters and employees that travel
Intranet connectivity for branch offices
Extranet links for business partners
Fig. 06 Virtual private network (VPN)
In recent years, many organizations have increased the mobility of their workers by
allowing more employees to telecommute. Employees also continue to travel and
face a growing need to stay connected to their company networks.
Commercial organizations use either remote access servers (RAS) or VPN switches.
If they use VPN switches they may also use network-based VPN services from
various network providers for additional security. Until recently, most corporations
used remote access server (RAS) devices to support dial-in access for employees. A
RAS device is a "box" with multiple modem and ISDN ports for dial-in connectivity.
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However, RAS devices do not support cable and DSL modems. With VPN switch and
network solutions, employees' calls are routed into the organization on the same links
used for Internet access. VPNs support dial-in and high-speed cable and DSL
modem access. Client software is installed on employees' computers to support the
VPN service.
As indicated in the VPN provider carries remote user’s calls over its public network
and routes them to an existing T-1 or dedicated line connected to the customer's site.
For an additional monthly fee, carriers will manage a customer's on-site router or
switch, and security such as a firewall. A firewall is software that screens incoming
traffic to prevent hacker’s access to files.
Besides using virtual private networks for remote access, a VPN can also bridge two
networks together. In this mode of operation, an entire remote network (rather than
just a single remote client) can join to a different company network to form an
extended intranet. This solution uses a VPN server to VPN server connection.
Internal networks may also utilize VPN technology to implement controlled access to
individual subnets within a private network. In this mode of operation, VPN clients
connect to a VPN server that acts as the network gateway.
This type of VPN use does not involve an Internet Service Provider (ISP) or public
network cabling. However, it allows the security benefits of VPN to be deployed
inside an organization.
Tunneling is a way to provide security on VPNs. Because virtual networks are shared
services, security is an important issue. Traffic from multiple organizations is carried
on the same "pipes" or telephone lines in the public carrier networks. Tunnels
surround customer packets with an extra header on each packet to provide security.
Encryption, or scrambling of bits, is an important element of tunneling. The encryption
makes hacking into a company's data more difficult. An alternative to tunneling is
network-based IP address filtering. With address filtering, the software looks at a
user's IP address and accepts or rejects it based on the IP address. Address filtering
is also commonly used on security software located in organizations' premises.
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Whether they use tunneling or address filtering, VPNs still use authentication and
authorization.
6.4 Access networks
The portion of the public network from the central office to the end user's location is
the access network, or "last mile," as illustrated in Figure 6.2. A major bottleneck of
analog services exists in the cabling to residential and small businesses from the
telephone company central office.
Fig. 07 Access network or last mile
Cable modem service is provided over the last mile. Cable modems operate on
hybrid fiber coaxial (HFC) cable infrastructure. Fiber runs from the cable company's
facility to the neighborhood. Coaxial cable is usually used to connect each residential
customer to the network. Cable modems are a nonswitched, always-on data
communications and Internet access service. Cable TV companies are investing
huge amounts of money to convert their cabling from one-way only cable TV service
to two-way systems for cable modems and telephone service.
In contrast to the last mile in residential areas in which twisted pair copper is used,
telephone companies and competitive access providers often lay fiber cables capable
34
of transmitting digital services directly to office buildings. The expense of supplying
fiber cable to office and apartment buildings with multiple tenants can be spread
across many customers. On the other hand, a fiber cable that terminates at a single
household must be paid for from the revenue generated by that one household.
Central offices switch calls between end users. There are two types of central offices:
end and tandem offices in incumbent telephone companies' networks. Tandem
offices do not have connections to end users. They have trunks to other carriers,
other tandem offices and end offices. They provide the connections for central office
traffic to other central offices, and central office to interexchange carriers' (IEXs')
switches. These switches carry high volumes of calls on paths called trunks. The
tandem office–to–tandem office portion of Bell networks is their backbone. It also is
referred to as the metropolitan network. It carries large amounts of traffic between
tandem offices. Without tandem offices, every end office would have to be connected
to every other end –office, creating a more complex network with many more trunks
to manage.
End central offices connect directly to business, commercial and residential
customers as well as to tandem offices. Long distance carriers connect to tandem
offices using toll offices. Toll offices are similar in structure to tandem switches. They
also are referred to as Class 4 switches. End offices are sometimes referred to as
Class 5 switches. The volume of calls between end offices and customers, and
between end offices and tandem offices, is lower than on trunks between tandem
offices.
Digital loop carriers (DLCs) are used to economically bring fiber closer to customers.
When telephone companies first started using fiber, they used it rather than copper if
the distance was longer than 1.5 miles. This is because signals on copper deteriorate
and need to be boosted at these distances. Rather than put amplifiers on the line,
carriers ran fiber to the neighborhood and terminated it in digital loop carriers. No
amplifiers are needed because signals on fiber can travel 64 miles without
deteriorating. Fiber also is more reliable and requires less maintenance than copper.
With the decreasing cost of electronics for DLC equipment, incumbent telephone
companies now use fiber on shorter runs also.
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Carrier hotels are locations where network service providers locate their switches and
routers and connect to each others' networks.
Rather than construct their own buildings to house their switches, carriers lease
space in carrier hotels. They place their equipment in cages in the carrier hotel.
Locked wire cages surround the equipment and access to the equipment is available
only to the carrier that owns the equipment. Leasing space in carrier hotels saves
network providers the expense of providing their own: Physical security against
break-ins, Access to large amounts of power, Access to backup power, Backup
generators, Dual air conditioning systems, Uninterrupted power supplies, Fire
detection and fire suppression equipment, Alarming to fire departments and police
departments, Staff to plan and, maintain the facilities, Construction of earthquake-
resistant facilities
6.5 Fiber optical networks
New passive optical network (PON) technologies lower the cost of deploying fiber
optic cabling in cable TV and landline local access networks. They essentially enable
one fiber pair from the network provider's facility to the neighborhood to be shared by
many customers. They have the added benefits of lowering the maintenance and
operating costs of these networks. Changes can be made by computer commands
rather than by dispatching a technician. Passive optical networks also have lower
space and power requirements than alternative technologies.
Passive optical networks (PONs) are devices located in the access network that
enable carriers to dynamically allocate capacity on a single strand or pair of fibers to
multiple small and medium-sized customers. Access networks comprise the cabling
and infrastructure between the customer and the telephone company. The allocation
of bandwidth is done through computer control rather than by dispatching a
technician when more bandwidth is requested. PONs increases the capacity,
flexibility and efficiency of fiber deployed in the last mile.
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The equipment consists of a switch that sits at the central office or the cable TV head
end called an Optical Line Terminal (OLT). The Optical Line Terminal controls
splitters, Optical Network Unit (ONU) and Optical Network Termination (ONT)
devices. The Optical Network Termination sits at the customer's premise. The
following are an overview of PON devices as defined by FSAN:
The Optical Line Terminal is located at the central office and has multiple
cards, each of which supports up to 32 end users. It has ports for the fiber and
backup fiber connected to the splitter. It also interfaces to the network service
providers' high-speed backbone network.
The splitter is like a garden hose with a T splitter that splits the capacity of the
fiber among up to 32 end users.
The Optical Network Unit can be used in cable TV networks or traditional
telephone company networks. It converts optical signals to those compatible
with coaxial cable and twisted pair. It has interfaces for DSL, cable TV and
plain old telephone service (POTS). It brings fiber to the curb or fiber to a
neighborhood cabinet.
The Optical Network Termination, which brings fiber to the building, has cards
that connect the fiber to customer premise equipment (CPE). These interfaces
include T-1 (24 voice and/or data channels) and E-1 (the European version of
T-1 with 30 channels) and LAN ports. Twisted pair copper or fiber connections
are supported.
Optical add and drop multiplexers (OADMs) reroute traffic that comes into a carriers'
point of presence (POP) from the backbone network. For example, a pair of fibers
might carry traffic on 40 different wavelengths. Each wavelength also is called a
lambda. Each wavelength (lambda) is essentially a high-speed path of, for example,
10 gigabits per second (Gbps) of data. A filter in an add and drop multiplexer reroutes
a single wavelength in the strand of fiber at, for example, the Detroit POP and "drops"
it off, or reroutes it to, Cleveland. Add and drop multiplexer obviates the need for
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more expensive amplifiers in Detroit to convert the signal back to an electronic format
before sending the traffic to Cleveland.
Optical cross connect (OXC) systems, (optical switches) have more capability than
optical add and drop multiplexers (OADMs). They are used in POPs with higher
traffic than those where OADMs are used. They switch light waves in core, high-
speed carrier networks. In the future, they may also be used in the backbone, high-
traffic portion of metropolitan networks between central office switches. Optical cross
connect (OXC) systems let carriers redirect wavelengths through computer
commands without physically unplugging fiber optic cables. A technician at a
computer can redirect an individual wavelength from one destination to another on
the network using software. Demultiplexers in the optical cross connect separate out
the wavelengths to their different routes.
Optical electrical optical (OEO) switches have direct optical interfaces. Fiber optic
cables plug directly into them. However, they process traffic electronically. The chips
that perform the switching do so electrically. The bits in the electrical signals tell the
switch where the traffic should be routed. OEO switches use the same protocols
used in the Internet Protocol (IP) such as multi-protocol label switching (MPLS).
MPLS looks at the address in the first packet in a stream of data, puts that in
hardware on the switch and routes the rest of the stream using the abbreviated
address in the hardware. It doesn't have to keep looking up the address. The
Resource Reservation Protocol (RSVP) is part of MPLS. It routes packets and
circuits based on a user's requested quality of service (QoS).
All optical switches (OOO) have direct fiber connections and they switch wavelengths
(lambdas) as light pulses without converting them to electrical signals. The OOO
refers to optical incoming, optical switching (internal) and optical outgoing. All optical
switches use out-of-band signals to direct light pulses to routes. The traffic itself
remains optical and the out-of-band electrical signals generated by routers tell the
switch where to send the traffic. The advantage of all optical switches is speed. They
are potentially faster than optical electrical optical (OEO) switches because they
aren't slowed down by the optical-to-electrical and electrical-back-to-optical
conversions. All optical switches use MPLS (previously described), as well as
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automated switched transport network (ASTN). ASTN is an emerging standard for
controlling wavelengths. It provides dynamic class of service assignment and flexible
restoration of service. Routers at carriers' points of presence (POPs) send out of
band signals to each other for managing the traffic handled by optical switches. The
signals are sent separately from the traffic.
6.6 Signaling in networks
Signaling is the process of sending information between two parts of a network to
control, route and maintain a telephone call. For example, lifting the handset of a
telephone from the receiver sends a signal to the central office, "I want to make a
phone call". The central office sends a signal back to the user in the form of a dial
tone indicating the network is ready to carry the call.
The three types of signals are:
Supervisory signals-Supervisory signals monitor the busy or idle condition of a
telephone. They also are used to request service. They tell the central office
when the telephone handset is lifted (off-hook requesting service) or hung-up
(on-hook in an idle condition).
Alerting signals-these are bell signals, tones or strobe lights that alert end
users that a call has arrived.
Addressing signals-these are touch tones or data pulses that tell the network
where to send the call. A computer or person dialing a call sends addressing
signals over the network.
Signals can be sent over the same channel as voice or data conversation, or over a
separate channel. Prior to 1976, all signals were sent over the same path as voice
and data traffic. This is called in-band signaling. In-band signaling resulted in
inefficient use of telephone lines. When a call was dialed, the network checked for an
available path and tied up an entire path through the network before it sent the call
through to the distant end. For example, a call from Miami to Los Angeles tied up a
path throughout the network after the digits were dialed, but before the call started.
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Common channel interoffice signaling, also known as out-of-band signaling, is in
reality a data communications network laid over carrier’s switching networks. It has
opened markets for new products and enhancements to carrier’s features. Common
channel signaling was developed as a way to increase network efficiency by setting
up separate channels for signals. It evolved into the basis for intelligent networks.
Routing instructions, database information and specialized programs are stored in
computers in the carriers' networks and are accessible over out-of-band signaling
links.
The Signaling System 7 (SS7) protocol, which is based on common channel
signaling, is a factor in lowering barriers to entry into the common carrier market.
Routing intelligence is located in lower cost computer-based peripherals rather than
in central office switches. For example, powerful parallel processing computers hold
massive databases with information such as routing instructions for toll-free and 900
calls. One processor with its database supports multiple central office switches. In
this case, each central office switch is not required to maintain sophisticated routing
information. The expense of the upgrade is shared among many central offices.
SS7 is a separate data network that carries all of the signaling in each carrier's
network. The efficiency of common channel signaling is achieved by having one
signaling link support multiple voice and data transmissions. The fact that one
signaling link supports many trunks (high-speed links between telephone switches)
highlights the requirement for reliability. If one signaling link crashes, many trunks are
out of service. Redundancy is an important consideration in the design of carriers'
signaling networks.
SS7 components include:
Packet switches, or signal transfer points that route signals between
databases and central offices
Service switching points, software and ports in central offices that enable
switches to query databases
Service control points, specialized databases with billing and customer feature
information
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Not all carriers own and operate their SS7 networks. They may, for example, use
SS7 networks from companies such as Illuminet. Carriers that use Illuminet have
links from their central office to the Illuminet signal transfer points.
Signal Transfer Points (STPs) are packet switches that route signals between
central offices and specialized databases. Messages are sent between points on the
SS7 network in variable-length packets with addresses attached. (Think of the
packets as envelopes of data containing user information such as the called and
calling telephone number, error correction information and sequencing numbers so
that the correct packets or envelopes are grouped together in the correct order at the
receiving end). Signal transfer switches read only the address portion of the packets
and forward the messages accordingly.
The fact that the signals are sent in a packet format is a significant factor in SS7's
efficiency. Packets associated with multiple calls share the same pipe. Packets from
transmissions a, b, c, and so forth are broken into small chunks (packets) and sent
down over the same 64 or 1540 kilobit SS7 links and reassembled at the destination.
Service Switching Points (SSPs) enable central offices to initiate queries to
databases and specialized computers. Service switching points consist of software
capable of sending specialized messages to databases and ports connected to the
S77 network. For example, when a 900 call is dialed, SSPs set up a special query to
a 900 database (the service control point) for information on routing the call.
Service switching points convert the central office query from the central office
"machine language" to SS7 language. When signals are received from the signaling
network, the service switching points convert the SS7 language to language readable
by the central office switch.
Service Control Points (SCPs) hold specialized databases with routing instructions
for each call based on the calling party and/or the called party. For example, service
control points tell the network which carrier to route an 800 call to. Services such as
network-based voice mail, fax applications and voice-activated dialing are located on
service control points or intelligent peripherals.
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6.7 Hardware development
Technical advances are improving the quality of voice and video carried on packet
networks. They also are lowering the cost of building high-capacity networks capable
of carrying voice along with data over backbone networks.
The technical advances include:
Improvements in routers - Faster routers enhance the quality of voice and
video carried on Internet Protocol networks.
Faster digital signal processors (DSPs) - DSPs are special purpose
microprocessors on pieces of silicon that execute instructions. They are good
at performing a small number of repetitive tasks such as compressing voice,
packetizing voice and converting analog voice into digital.
Dense wavelength division multiplexing (DWDM) - New DWDM equipment is
capable of carrying 160 channels of data over a single pair of fibers. Each
channel has a speed of OC-192 (10 gigabits). The single pair thus carries 1.6
terabits (a trillion bits) per second.
High-capacity optical switches - increase the capacity and routing flexibility of
backbone networks by switching thousands of light waves simultaneously.
Lower cost, programmable switches, called softswitches - Softswitches are
central office switches built on standard computer platforms for sending voice
over packet networks. Softswitches are made using standard protocols so that
they can easily interface with network-based applications such as unified
messaging and billing systems. They also interface with SS7 services for
sending traffic to proprietary central office switches.
Protocols that improve the quality of voice and video over packet networks.
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7 Special network services and types
7.1 ISDN
Integrated Services Digital Network is a telephone system network. Prior to the
ISDN, the phone system was viewed as a way to transport voice, with some special
services available for data. The key feature of the ISDN is that it integrates speech
and data on the same lines, adding features that were not available in the classic
telephone system. There are several kinds of access interfaces to the ISDN defined:
Basic Rate Interface (BRI), Primary Rate Interface (PRI) and Broadband-ISDN (B-
ISDN).
ISDN is a circuit-switched telephone network system that also provides access to
packet switched networks, designed to allow digital transmission of voice and data
over ordinary telephone copper wires, resulting in better voice quality than an analog
phone. It offers circuit-switched connections (for either voice or data), and packet-
switched connections (for data), in increments of 64 kbit/s. Another major market
application is Internet access, where ISDN typically provides a maximum of 128 kbit/s
in both, upstream and downstream directions (which can be considered to be
broadband speed, since it exceeds the narrowband speeds of standard analog 56k
telephone lines). ISDN B-channels can be bonded to achieve a greater data rate
typically 3 or 4 BRIs (6 to 8 64 kbit/s channels) are bonded.
Integrated Services refers to ISDN's ability to deliver at minimum two simultaneous
connections, in any combination of data, voice, video, and fax, over a single line.
Multiple devices can be attached to the line, and used as needed. That means an
ISDN line can take care of most people's complete communications needs at a much
higher transmission rate, without forcing the purchase of multiple analog phone lines.
Basic Rate Interface (BRI), also known as Basic Rate Access (BRA) and 2B + D,
provides two Bearer (B), or information bearing channels, each operating at the clear
channel rate of 64 kbps. Each B channel can carry digital data, digitized voice (PCM
encoded at 64 kbps or a lower rate), or a mixture of low-speed (sub rate) data as long
as it all is intended for the same destination. BRI also provides a Data (D) channel at
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16 kbps, which is intended primarily for purposes of signaling and control,
messaging, and network management. The D channel also generally is made
available for X.25 packet data transmission and low-speed telemetry when not in use
for signaling purposes; cost effective applications include credit card authorization,
which involves very small bursts of data. BRI is used primarily for residential, small
business, Centrex, and telecommuting applications that are not particularly
bandwidth intensive. The B channels can be aggregated, or bonded, to provide up to
128 kbps to a given conversation, such as a videoconference or Internet experience.
Additionally, multiple BRIs can be bonded for even greater capacity. Whether bonded
or not, ISDN BRI provides multiple channels over a single physical loop, which is a
great advantage.
A single BRI line can support up to 16 devices that contend for access to the BRI
channels through a Terminal Adapter (TA). The devices can be in a variety of forms,
including telephones, facsimile machines, computers, and video cameras.
Primary Rate Interface (PRI) also is known as 23B+D in the United States and
Japan. The European or international version is known as Primary Rate Access
(PRA) or 30B+D. PRI offers 23 B (Bearer) channels plus 1 D (Data) channel and is
backward compatible with T1 and J1 transmission systems, respectively. PRA offers
30 B channels plus 1 D channel and is backward compatible with E1 transmission.
PRI and PRA both provide a full-duplex (FDX) point-to-point connection through an
NT2-type intelligent CPE switching device, such as a PBX or router, for interfacing
with the carrier CO switch. The DS-0 is the basic building block of both PRI and PRA,
as both the B and D channels operate on clear channels at 64 kbps. As is the case
with BRI, the B channels can be used individually or can be bonded for voice, data,
video, facsimile, any other data and any multimedia combination, but the D channel is
reserved exclusively for signaling.
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Fig. 08 ISDN PRI connecting PBX to Central Office (CO)
While designed for transmission over a standard DS-1 trunk, PRI is a significant
improvement over T1 or E1, because the channels can be allocated dynamically. In
other words, each channel can act as an incoming, outgoing, combination, or DID
trunk, as the need arises. The nature of the channel can be determined as required,
based on user definable parameters. Additionally, multiple B channels can be
aggregated to serve bandwidth intensive applications, such as videoconferencing.
On the negative side, PRI does not compare favorably with T1 in terms of the raw
number of B channels, and PRI can be considerably more expensive, depending on
tariff specifics.
7.2 T-1 24 Channels
The T-1 multiplexing scheme was developed by AT&T in the 1960s as a way to save
money on cabling between telephone company switches by enabling one circuit to
carry 24 voice or data conversations. The speed of a T-1 circuit is 1.54 million bits
per second. The letters "DS" stand for digital signal level. DS-0 refers to the 56-Kbps
or 64-Kbps speed of each of the 24 individual channels of the T-1 or E-1 circuit. DS-1
refers to the entire 1.54-megabit T-1 line. The terms DS-1 and T-1 are used
interchangeably. All DS-0s run at 64,000 bps. However, depending on the signaling
available in the telephone company's network, 8000 of the bits might be required for
signaling and maintenance functions, leaving only 56 Kbps for user data. Clear
channel signaling must be available with the chosen carrier to be able to use the full
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64 Kbps for user data. With clear channel signaling, the 8000 bits don't have to be
"robbed" for network maintenance. Thus, the full 64,000 bits are available for user
data.
The total bandwidth of a T-1 circuit is higher than the sum of all of the channels - 24 x
64,000, which equals 1,536,000. The extra 8000 bits (1,544,000 – 1,536,000 = 8000)
are used for synchronization, keeping the timing set between frames. A frame is a
grouping of bits with samples of data from each of the 24 channels. Data from
devices connected to the T-1 are sampled, put into frames and sent sequentially on
the T-1 line.
T-1 can be installed on a variety of media, including the following:
Fiber optics
Twisted pair
Coaxial cabling
Microwave
Infrared light
The only digital signal speed that is standard throughout the world is the DS-0 speed
of 64 kilobits. There are two standard DS-1 speeds: The U.S., Canada and Japan
use 1.544 for T-1 with 24 channels, while the rest of the world uses 2.048 with 32
channels - 30 channels for user data, one channel for signaling and a channel for
framing and remote maintenance.
Digital Signal Levels
Level North America Japan Europe
User Channels
Speed User Channels
Speed User Channels
Speed
DS-0 1 64 Kb 1 64 Kb 1 64 Kb
T-1 (DS-1) 24 1.544 Mb 24 1.544 Mb 30 2.048 Mb
T-2 (DS-2) 96 6.312 Mb 96 6.312 Mb 120 8.448 Mb
T-3 (DS-3) 672 44.7 Mb 480 32.06 Mb 480 34.368 Mb
T-4 (DS-4) 4032 274.17 Mb
5760 400.4 Mb 1920 139.3 Mb
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7.3 T-3 the Capacity of 28 T-1 Lines, 672 Channels
A T-3 circuit is equivalent to 28 T-1s, or 672 channels (28 x 24 = 672). The total
speed of a T-3 line is 44.736 megabits per second. This speed is higher than 28 x
1.544 because some bits are needed for overhead (i.e., signaling and maintenance).
Large call centers often require several T-1 circuits at a single location. Rather than
install multiple T-1s, companies install T-3 circuits. T-3 services start to cost less than
multiple T-1s when customers have between eight to ten T-1s at the same site.
Catalog sales, financial institutions, insurance companies and service bureaus that
provide call center functions for smaller companies are examples of call centers that
might require T-3 capacity.
Fortune 100 companies use T-3 service to carry voice, video and data traffic on
private lines connecting their largest sites.
Internet service providers and telephone companies utilize the 672-channel capacity
of T-3. Large ISPs use T-3 to connect their switches to the Internet backbone. They
also provide T-3 service as a point where customers access their network. For
example, customers call into the T-3 network entrance points from individual 56 Kbps
to 1.54 T-1 lines. Many telephone carriers and Internet service providers are
upgrading from T-3 to optical carrier-level service in their backbone networks.
7.4 DSL Digital Subscriber Line Technology
DSL is a technology that provides high-speed data transmissions over the so-called
“last-mile” of “local-loop” of the telephone network, i.e., the twisted copper wire that
connects home and small office users to the telephone company central offices
(COs). Demand for high-speed access methods is increasing with growing Internet
access, electronic commerce, IP telephony, and videoconferencing. A number of
methods for providing this bandwidth are available, including DSL technologies,
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cable (CATV) networks, and wireless and satellite technologies. All of these fit into
the category of “residential broadband services.”
DSL technologies can enhance copper wire infrastructure to be the most effective
way of delivering broadband services to the greatest number of people. In some
cases, data rates up to 52 Mbits/sec can be achieved. DSL makes the local loop a
multiservice access network that can support not only Internet access, but video and
telephony services. No wiring upgrade is necessary for DSL. Only the equipment at
the user end and at the telephone company end of the cable must be upgraded to
new equipment.
While DSL transmission can share the same wire used to transmit traditional analog
voice calls, it can also support multiple lines of digital telephony within the frequency
range that it operates. DSL connections are point-to-point dedicated circuits, meaning
that they are always connected. There is no dial-up. There is also no switching, which
means that the line is a direct connection into the carrier’s system. DSL modems are
required at the customer site and the carrier site. Because there are different
modulation techniques, users must ensure compatibility between their equipment and
the carrier’s equipment. The carrier will usually recommend suitable equipment.
There are actually seven types of DSL service, ranging in speeds from 16 Kbits/sec
to 52 Mbits/sec. The services are either symmetric (traffic flows at the same speed in
both directions) or asymmetric (the downstream capacity is higher than the upstream
capacity). Asymmetric services are good for Internet users because more information
is usually downloaded than uploaded.
Following is a description of the different versions of DSL. Note that these versions
are often collectively referred to as xDSL:
HDSL (High-Speed Digital Subscriber Line) HDSL is the most common and mature
of the DSL services. It delivers data symmetrically at T1 data rates of 1.544 Mbits/sec
over lines that are up to 3.6 kilometers (12,000 feet) in length. Generally, HDSL is a
T1 service that requires no repeaters but does use two lines. Voice telephone
services cannot operate on the same lines. It is not intended for home users, but
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instead is intended for the telephone company’s own feeder lines, interexchange
connections, Internet servers, and private data networks.
SDSL (Symmetric Digital Subscriber Line) SDSL is a symmetric bidirectional DSL
service that is basically the same as HDSL, but operates on one twisted-pair wire. It
can provide data rates up to the T1 rate of 1.544 Mbits/sec.
ADSL (Asymmetric Digital Subscriber Line) ADSL is an asymmetric technology,
meaning that the downstream data rate is much higher than the upstream data rate.
As mentioned, this works well for a typical Internet session in which more information
is downloaded from Web servers than is uploaded. ADSL operates in a frequency
range that is above the frequency range of voice services, so the same wire can
carry both analog voice and digital data transmissions. The upstream rates range
from 16 Kbits/ sec to as high as 768 Kbits/sec. The downstream rates and distances
are listed here.
VDSL (Very High–Data-Rate Digital Subscriber Line) VDSL is basically ADSL at
much higher data rates. It is asymmetric and, thus, has a higher downstream rate
than upstream rate. The upstream rates are from 1.5 Mbits/sec to 2.3 Mbits/sec. The
downstream rates and distances are listed in the following table. VDSL is seen as a
way to provide very high-speed access for streaming video, combined data and
video, video-conferencing, data distribution in campus environments, and the support
of multiple connections within apartment buildings.
RADSL (Rate-Adaptive Digital Subscriber Line) This service is also similar to ADSL,
but it has a rate-adaptive feature that will adjust the transmission speed to match the
quality of the line and the length of the line. A line-polling technique is used to
establish a connection speed when the line is first established.
DSL Lite (or G.Lite) DSL Lite is considered a “jump-start” technology that is meant to
deliver DSL to the greatest number of users, as fast as possible. While it has a lower
data rate than other DSLs, it does not require that the telephone company do
anything to the lines. In addition, equipment to handle DSL Lite is becoming readily
available at a low price.
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7.5 Gigabit Ethernet
Gigabit Ethernet is a high-speed method for Internet access and data
communications. It currently operates at one-megabit per second to one gigabit per
second, but manufacturers are developing 10-gigabit products. They use the
Ethernet LAN standard, the most prevalent LAN protocol.
Gigabit Ethernet providers also are called optical local exchange carriers (OLECs).
They sell high-speed data communications service over fiber optic cabling to
business customers. In addition to the network connection, OLECs provide and
manage equipment inside the customer's building that is used to connect customer
LANs to outside fiber networks and to the Internet.
Ethernet is the most common protocol used in LANs. Gigabit Ethernet is based on
the 802.3 standard for transmitting 1000 million bits per second (1 billion). Although a
standard is not defined for 10-Gigabit Ethernet, new equipment is emerging that
operates at 10 gigabits over Ethernet. Because of its speed, Gigabit Ethernet
supports transmissions between organizations and access to the Internet.
Because it works on a standard protocol, manufacturing costs for Ethernet products
are low compared to, for example, ATM gear. Because these Gigabit Ethernet
manufacturers use Ethernet, their costs are lower than carriers that use SONET or
ATM in their infrastructure. Manufacturing standardized products in quantity costs
less than producing specialized products in lower number. Ethernet-based network
interface cards (NICs) are an example of a product based on a standard that has
decreased in cost to under $100. Network interface cards are installed in slots of PCs
and connect to Ethernet LANs.
Most LANs use Ethernet and are thus already compatible with Gigabit Ethernet.
Thus, installation of Ethernet switches at customer locations is not complex and
customers aren't faced with expensive upgrades when they implement the service.
No CSU/DSU (digital modems) or T-1 multiplexer is required. An on-site switch
transmits and receives data directly to and from the outside fiber to a port on the
metropolitan area–based Ethernet switch. Using Gigabit Ethernet is essentially like
linking customer premise local area networks to local area networks located at carrier
facilities in the metropolitan area network (MAN).
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7.6 Frame Relay (Shared WAN)
Frame Relay was originally intended as a data-only service. First implemented in
1992, Frame Relay is a public network offering that enables customers to transmit
data between LANs in multiple locations. It is also used to access the Internet. By
using Frame Relay, organizations do not have to plan, build and maintain their own
duplicate paths to each of their sites. Multiple users share the Frame Relay networks.
The line that connects customer to the Frame Relay network is called an access line.
It provides access from the user equipment to the Frame Relay network. Access lines
to Frame Relay networks run at various speeds depending on the amount of traffic
generated at each site. Sites at different locations in the same organization can be
configured with access lines at different speeds.
Frame relay is a packet-switched network which means that data is transmitted
through the network as a series of variable-length frames that can transport any kind
of data. Variable-length packets are used for more efficient and flexible data
transfers. Frame Relay provides a connection-oriented service, just like X.25. Frames
are transmitted across virtual circuits, either permanent (PVC) or switched (SVC).
Permanent virtual circuits (PVC): The Frame Relay operator provides the PVC
between two end points. This is done by programming the Frame Relay switches in
the network to assign a series of links between the switches to form the PVC
between the two end points. The PVC is permanently available to the two parties at
both ends.
Switched virtual circuits (SVC): Unlike permanent virtual circuits, SVC is based on
usage. Temporary connections are set up between points on a Frame Relay network.
SVCs can be used to carry voice traffic if volumes are low. Thus, users only pay for
what they use instead of incurring fixed monthly fees associated with permanent
virtual circuits.
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Frame Relay virtual circuits are identified by data-link connection identifiers (DLCIs).
DLCI values typically are assigned by the Frame Relay service provider (for example,
the telephone company). Each link can support multiple connections.
7.7 ATM-Asynchronous Transfer Mode
Asynchronous Transfer Mode (ATM) is an electronic digital data transmission
technology. ATM is implemented as a network protocol and was first developed in the
mid 1980s. The goal was to design a single networking strategy that could transport
real-time video conference and audio as well as image files, text and email. Two
groups, the International Telecommunications Union and the ATM Forum were
involved in the creation of the standards.
ATM is a packet switching protocol that encodes data into small fixed-sized cells (cell
relay) and provides data link layer services that run over OSI Layer 1 physical links.
This differs from other technologies based on packet-switched networks (such as the
Internet Protocol or Ethernet), in which variable sized packets (known as frames
when referencing Layer 2) are used. All ATM cells are the exact same length, 53
bytes. Of those 53 bytes, 5 are used for network processing overhead and the
remaining 48 are used to carry user payloads (user data such as voice or video).
ATM exposes properties from both circuit switched and small packet switched
networking, making it suitable for wide area data networking as well as real-time
media transport. ATM uses a connection-oriented model and establishes a virtual
circuit between two endpoints before the actual data exchange begins.
A significant reason why ATM is fast is that the cells are switched in the hardware.
This means that an ATM switch does not have to look up each cell's address in
software. Rather, an ATM switch sets up a route through the network when it sees
the first cell of a transmission. It puts this information into its hardware and sends
each cell with the same header routing information down the virtual path previously
established. ATM traffic consists of three basic types:
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Constant Bit Rate (CBR) traffic provides the highest priority and lowest delay
through a network. Typical applications include videoconferencing, voice,
television and video-on-demand.
Variable Bit Rate (VBR) traffic, where more delay and variation on speeds can
be tolerated. Compressed voice and video and bursty data traffic fit into this
category.
Available Bit Rate (ABR) traffic, also known as best effort ATM, supports
bursty LAN traffic and other traffic that can adjust their requirements according
to the speed of the available network resources.
Access to an ATM backbone network is provided through a User Network Interface
(UNI). The UNI, or user network interface, is the dedicated digital telephone line
connection between the customer and the ATM equipment. The dedicated
connection to ATM can be implemented at various speeds including: T-1, T-3,
Fractional T-3, OC-1 (52 megabits per second), OC3 (155 megabits per second),
OC12 (622 megabits per second) and above.
ATM supports multiple, parallel communications. For example, a videoconference
can be transmitted on the same line carrying large file transfers. Thus, even though
there is only one physical connection, multiple communications are taking place in
parallel. This is a major strength of ATM. Predefined paths between network
locations are called PVCs or Permanent Virtual Connections. The two types of
permanent virtual connections are:
Virtual path connection (VPC): Has many virtual channels running within it.
This is analogous to conduit carrying many cables.
Virtual Channel Connection (VCC): A single channel within the ATM circuit
that is defined when the service is put in place.
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7.8 SONET-Synchronous Optical Network
SONET is a Layer 1 transport service used on fiber optic cabling. Layer 1 functions
define interfaces to physical media such as copper and fiber optic cabling. SONET
takes data and transports it at high speeds called OC (optical carrier) speeds. In
contrast, ATM is a Layer 2 service; it performs switching, addressing and error
checking. SONET links carry data from ATM switches, IP networks, T-1 and T-3
multiplexers from close to where they enter carrier networks in fiber optic long-haul
and metropolitan area networks.
SDH, or synchronous digital hierarchy, essentially is the European version of
synchronous optical speeds. SDH signals are carried at Synchronous Transfer Mode
(STM) speeds. Europe's time division hierarchy is based on E1 (2-megabit) and E3
(34-megabit) signals. E1 circuits carry 30 channels at 64 kilobits per channel. E3
circuits carry 512 channels at 64 kilobits per channel. Traffic that is carried between
cities in Europe or in undersea cables is often referred to as being carried at STM-1
or STM-16 rates.
SONET has four functions or layers:
The photonic layer converts electrical signals to optical signals and vice versa.
If electrical signals from media such as copper are connected to SONET
multiplexers, the SONET equipment converts the electrical signals to light
signals suitable for fiber optic cabling. At the receiving end of the transmission,
it converts optical signals back to electrical signals.
The section layer monitors the condition of the transmission between the
SONET equipment and optical amplifiers. (Amplifiers are used to strengthen
optical signals that fade over distance.)
The line layer synchronizes and multiplexes multiple streams into one stream
or "pipe" of traffic. It also provides monitoring and administration of SONET
multiplexers.
The path layer assembles and disassembles voice and data carried on
SONET into frames. Frames are arrangements of bits that carry user
information as well as bits for monitoring and maintaining the line.
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To ensure reliability, telephone companies' SONET deployments often use
bidirectional ring topology. One set of fiber strands is used for sending and receiving;
the other is a spare set, also called the protect ring. If one set of fiber strands is
broken, the spare (protect) ring reroutes traffic in the other direction.
Wave division multiplexers increase the capacity of fiber connected to SONET
equipment. SONET equipment receives voice and data communications from
multiple sources. It converts these streams into optical light and sends them
uniformly at high speeds on one pair of fiber. If wave division multiplexing (WDM) is
performed, multiple SONET streams can be carried on one fiber. A wave division
multiplexer takes SONET streams and sends them out on many different colors of
light so that one fiber strand can handle up to 96 times the capacity of a SONET
multiplexer.
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8 Modems and access devices
Anyone sending data, video or images over a telephone line needs a device between
the telephone line and the equipment communicating. The vast majority of Internet
subscribers worldwide use analog dialup service. Modems convert their computer's
digital signals to analog in such a way that the signals are compatible with analog
telephone lines. All devices connected to public and private networks need devices
for functions such as error correction and timing between computers, multiplexers or
local area networks (LANs) and service providers' facilities.
8.1 Transferring computer data over telephone lines
The equipment that makes computer signals compatible with networks for
communications is called data circuit-terminating equipment (DCE). Analog and
digital telephone lines require different types of DCE devices. Data circuit-terminating
equipment (DCE) gear with remote diagnostics can reduce the finger pointing that is
often present when maintenance staff try to determine whether repair problems are
located in the network, computer, cables or modem. Problem determination, which
often involves computer suppliers, network vendors and modem suppliers, is a major
dilemma with companies responsible for telecommunications networks. One way
technicians pinpoint repair problems is by sending test data bits to the DCE device. If
the DCE device receives the data, the assumption is made that the problem is not in
the telephone line or DCE.
Data circuit-terminating equipment serves multiple purposes. The function it provides
depends on the network services with which it is used.
Functions of data circuit-terminating equipment (DCE) include:
ensuring that data flows in an even,
making sure the proper voltages are present,
performing error detection and
correction and compressing data.
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For analog lines, DCE has functions of converting digital computer signals to analog
and vice-versa.
8.2 DCE-connections to telephone lines
Data circuit-terminating equipment (DCE) is typically a modem or other type of
communication device. The DCE sits between the DTE (data terminal equipment)
and a transmission circuit such as a phone line. Originally, the DTE was a dumb
terminal or printer, but today it is a computer, or a bridge or router that interconnects
local area networks.
A DCE provides a connection for the DTE into a communication network and back
again. In addition, it terminates and provides clocking for a circuit. When analog
telephone lines are the communication media, the DCE is a modem. When the lines
are digital, the DCE is a CSU/DSU (channel service unit/data service unit).
DTE and DCE interfaces are defined by the physical layer in the OSI (Open Systems
Interconnection) model. The most common standards for DTE/DCE devices are EIA
(Electronic Industries Association) RS-232-C and RS-232-D. Outside the United
States, these standards are the same as the V.24 standard of the CCITT
(Consultative Committee for International Telegraphy and Telephony). Other
DTE/DCE standards include the EIA RS-366-A, as well as the CCITT X.20, X.21, and
V.35 standards. The later standards are used for high-speed communication over
telephone lines.
DTE and DCE devices send and receive data on separate wires that terminate at a
25-pin connector. It is useful to know that DTE devices transmit on pin connector 2
and receive on pin 3. DCE devices are just the opposite - pin 3 transmits and pin 2
receives.
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8.3 Modems for analogue telephone lines
Modems convert digital signals received from computers into analog signals and
transmit them over analog telephone lines. The process of converting digital signals
to analog and modifying them for transmission is called modulation. This makes the
signals compatible with analog telephone lines. At the receiving end, the modem
demodulates the signal or converts it from analog to digital and transmits it to the
data terminal equipment (DTE) (e.g., computer or T-1 multiplexer).
56-Kbps (V.90) modem speeds are asymmetric. "Upstream" from the subscriber to
the service provider is slower, (33.6 kilobits) than "downstream" (up to 53 kilobits)
from the Internet service provider (ISP) to the subscriber. This is because the
subscriber has an analog line to the central office and the Internet service provider
has a digital line. The assumption with 56-kilobit modems is that the ISP has digital
PRI ISDN service in its remote access server (RAS).
Fig. 09 PC connected to CO with modem
Two problems exist with 56-Kbps modems: power requirements at the local
telephone companies and conditions in the analog portion of the public network. To
achieve the 56-Kbps speed downstream, certain levels of power must be provided by
local telephone companies. If these levels are not present, the modems can only
achieve 53-Kbps speeds.
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PCMCIA stands for Personal Computer Memory Card International Association.
PCMCIA cards are 3.37 inches long by 2.13 inches wide and plug into slots on
portable computers such as laptops. They were initially designed as cards with extra
memory for laptops. For example, if the hard drive on the laptop was too small, a
PCMCIA card was installed to store extra documents or programs. These PCMCIA
slots are now commonly used for modems and fax/modems. PCMCIA cards can be
used with analog POTS lines Wireless LAN service and
cellular services.
PCMCIA modems are manufactured in a variety of speeds, including 56 Kbps. When
plugged into a standard analog telephone line, they work the same way as standard,
full-sized modems. An RJ11 jack for a telephone cord is attached to the end of the
card. Some PCMCIA cards have connections for Ethernet LAN, cellular and ISDN
connections as well as landline service. Thus, people can use the same PCMCIA
card at their work and home locations and with their cellular phone when they're
traveling. People with newer laptop computers have internal modems and Ethernet
connections so they don't need the PCMCIA card. They plug the telephone line or
Ethernet cable directly into a port of their computer. The small size of PCMCIA
modems is made possible by advances in silicon technology such that all of the
modem's functionality can be put onto one chip.
8.4 Connecting devices to ISDN
Devices such as video teleconference units, PCs, PBXs, key systems and
multiplexers that are connected to ISDN lines need an NT1 interface to the ISDN line.
ISDN enables voice, data and video to share one telephone circuit. The network
termination type 1 (NT1) corrects the voltage on the signals. It provides the electrical
and physical terminations to the carrier's network. In addition, the NT1 provides a
point from which line monitoring and maintenance functions can take place. On BRI
ISDN services, NT1 devices change the ISDN circuit from two wires that come into
the building from the central office to the four wires that are needed by ISDN
equipment. (BRI ISDN circuits carry two channels of voice, video or data). In the
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U.S., the FCC requires that the customer be responsible for supplying the NT1. In the
rest of the world, telephone carriers supply the NT1.
A terminal adapter (TA) that performs the multiplexing and signaling function on ISDN
services is also required. Multiplexing enables one line to be used simultaneously for
two voice or data calls. ISDN telephones have built-in terminal adapters. Non-ISDN
telephones, fax machines, video systems and computers can use ISDN if they are
connected to an external terminal adapter or a terminal adapter included in their NT1
device.
8.5 Digital Service Unit/Channel Service Unit
Channel Service Units (CSUs) and Digital Service Units (DSUs) are devices that, in
combination, serve to interface the user environment to an electrically based, digital
local loop. In contemporary systems, CSUs and DSUs generally combine into a
single device known variously as a CSU/DSU, CDSU, or ISU (Integrated Service
Unit), which typically appears in the form of a chipset on a printed circuit board found
under the skin of another device such as a channel bank, multiplexer (mux), switch,
or router. They are used in a wide variety of digital voice and data networks, including
DDS, T-carrier, and E – carrier.
Channel Service Unit Channel service units are circuit – terminating equipment that
provide the customer interface to the circuit. They also permit the isolation of the
DTE/CPE (Data terminal equipment/Customer premises equipment) from the network
for purposes of network testing. CSU functions include electrical isolation from the
circuit for purposes of protection from aberrant voltages, serving the same function as
a protector in the voice world. Additionally, the CSU can respond to a command from
the carrier to close a contact, temporarily isolating the DTE domain from the carrier
domain. This enables the carrier to conduct a loopback test in order to test the
performance characteristics of the local loop from the serving CO to the CSU and
back to the CO. Many contemporary CSUs also have the ability to perform various
line analyses, including monitoring the signal level. Such intelligent CSUs also often
have the ability to initiate loopback tests, although arrangements must be made with
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the carrier in advance. The CSU also serves to interface the DTE domain to the
carrier domain in an electrical environment. Within the DTE, for example, 1 bits
commonly are represented as positive (+) voltages and 0 bits as null (zero) voltages.
The network requires that 1 bits be alternating positive and negative voltages and
that the 0 bits be zero voltages. Further, the network requires assurance that 1s
density is achieved. Depending on the carrier network, 15-80 zeros can be
transmitted in a row as long as the density of 1s is at least 12.5 percent (1 in 8) over
a specified interval of time. CSUs insert, or stuff, 1 bits on a periodic basis in order to
ensure that the various network elements maintain synchronization. The CSU also
serves to provide signal regeneration and generates keep – alive signals to maintain
the circuit in the event of a DTE transmission failure. Finally, the CSU stores various
performance data in temporary memory for consideration by an upstream network
management system. Smart CSUs increasingly are positioned as Integrated Access
Devices (IADs). These multiport devices support interfaces to voice, data, and video
devices such as PBXs, routers, and videoconferencing units. The programmable IAD
supports bandwidth allocation for the various devices, enabling them to share a
single T1 or other digital facility.
Data Service Unit Data service units convert the DTE unipolar signal into a bipolar
signal demanded by the network. DSU functions variously include regeneration of
digital signals, insertion of control signals, signal timing, and reformatting. Some of
these functions can be ceded to either the CSU or the terminal equipment. In any
case, the functions must be performed, even though the CSUs and DSUs lose their
identity.
8.6 Cable Modems
A cable modem is a type of modem that provides bi-directional data communication
via radio frequency channels on a cable television (CATV) infrastructure. Cable
modems are primarily used to deliver broadband Internet access in the form of cable
Internet, taking advantage of the high bandwidth of a cable television network. They
are commonly deployed in Australia, Europe, and North and South America. In the
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USA alone there were 22.5 million cable modem users during the first quarter of
2005, up from 17.4 million in the first quarter of 2004.
Head end and end-user cable modems provide the following functionality:
Equalization to compensate for signal distortion
Address filtering so that the modem only accepts messages intended for the
correct recipient
Transmitting and receiving functions
Automatic power adjustments to compensate for power fluctuations
Adjustments in amplitude (signal strength or wave height) due to temperature
changes
Modulation of the signal (i.e., analog-to-digital conversions, and vice versa)
Compensation for delays caused by variable distances from the headend.
In network topology, a cable modem is a network bridge that conforms to IEEE
802.1D for Ethernet networking (with some modifications). The cable modem bridges
Ethernet frames between a customer LAN and the coax cable network.
With respect to the OSI model of network design, a cable modem is both Physical
Layer (Layer 1) device and a Data Link Layer (Layer 2) forwarder. As an IP
addressable network node, cable modems support functionalities at other layers.
8.7 Cable TV Set-Top Boxes
Cable TV set-top boxes are interfaces between televisions, satellite TV and cable TV
networks for access to television and other services. At the most basic level, they are
tuners. Cable and satellite TV operators remotely administer filters and traps in set-
top boxes to allow subscribers access to basic cable TV or premium channels. The
set-top box also has a security function. It scrambles and unscrambles TV signals
and also has links to billing systems for information on which channels to allow the
subscriber to receive. Credit information is also stored in some set-top boxes.
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Digital set-top boxes are available to take advantage of the two-way capability of
digital cable TV and satellite TV. These capabilities include:
Advanced digital security so that the security is placed on a card in the set-top
box that can be installed separately. If a consumer buys a set-top box from a
retailer, the cable TV provider can install the security feature on the card.
(Because security is proprietary to each provider, it is not available in retail
outlets.)
Advanced programming with 30 days worth of programming information.
Embedded modems that will enable televisions to be used as computers for
Internet access. For example, someone watching a football game will be able
to view statistics from the Internet in a window of the television. The set-top
box will also include infrared links to keyboards and computer mice.
Compression so that 6 to 12 compressed digital TV signals can be carried in
the same amount of frequency as one analog TV signal. The set-top box
converts digital cable TV or satellite TV into analog signals compatible with
analog television. It also can be built directly into digital televisions when the
industry agrees on standards compatible with digital cable TV. Some of these
extra channels can be used for interactive games for which subscribers will be
charged extra on their monthly cable bills.
Computer operating systems, software and possibly a hard disc for
programming guides and potential new services such as picture-in-picture for
viewing statistics while watching sports programs.
An Ethernet plug on the back of the set-top box so that computers or home
routers can be connected to the set-top box. A set-top box can be used to
send caller ID to the television screen. For this to work, subscribers must get
their telephone service from their cable TV provider.
Video on demand so callers do not have to place a separate telephone call to
order a premium movie. The movie can be ordered from the set-top box.
Open platform standards so consumers can purchase set-top boxes from a
variety of retailers and know they will work with all cable systems.
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8.8 Electric cables as data carriers
The electrical properties of copper cabling create resistance and interference. Signals
weaken the farther they are transmitted on copper wires. The electrical property of
copper cabling is the key factor that limits its transmission speeds.
Signals sent over copper wire are, for the most part, direct-current electrical signals.
Signals near these wires can introduce interference and noise into the transmission.
In particular, copiers, magnetic sources, manufacturing devices and radio stations all
can introduce noise. It is not uncommon for office and residential users to complain
that they can hear a nearby radio station's programming on their telephone calls. This
is the result of interference.
Within homes and businesses, crosstalk is another example of "leaking" electrical
transmissions. In homes with two lines, a person speaking on one line often can hear
the faint conversation on the other line. Current from one pair of wires has "leaked"
into the other wire. One way in which copper cabling is protected from crosstalk and
noise introduced from nearby wires is by twisting each copper wire of a two-wire pair.
Noise induced into each wire of the twisted pair is canceled at the twist in the wire.
8.9 Modem standards
The CCITT, an international committee that specifies the way modems and fax
machines transmit information to ensure compatibility among modems, has classified
dial-up modems according to the following modulation standards:
Bell 103M & 212A: Older standards, Bell 103 transmits at 300 bit/s at 300
baud and 212A transmits at 1200 bit/s at 600 baud.
V.21: Capable of only 300 bit/s, it is an international standard used mainly
outside of the U.S.
V.22: Capable of 1200 bit/s at 600 baud. Used mainly outside the U.S.
V.22bis: Used in the U.S. and out, it is capable of 2400 bit/s at 600 baud.
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V.23: Used mainly in Europe, it allows the modem to send and receive data at
the same time at 75 bit/s.
V.29: A one-way (half-duplex) standard that is used mostly for fax machines.
Capable of 9600 bit/s.
V.32: A full-duplex standard capable of 9600 bit/s at 2400 baud. V.32 modems
automatically adjust their transmission speeds based on the quality of the
lines.
V.32bis: A second version of V.32, it is capable of 14,400 bit/s. It will also
fallback onto V.32 if the phone line is impaired.
V.32ter: The third version of V.32, capable of 19,200 bit/s.
V.34: Capable of 28,000 bit/s or fallback to 24,000 and 19,200 bit/s. This
standard is backwards compatible with V.32 and V.32bis.
V.34bis: Capable of 33,600 bit/s or fallback to 31,200 bit/s.
V.42: Same transfer rate as V.34 but is more reliable because of error
correction.
V.42bis: A data compression protocol that can enable modems to achieve a
data transfer rate of 34,000 bit/s.
V.44: Allows for compression of Web pages at the ISP end and
decompression by the V.44-compliant modem, so transmitting the same
information requires fewer data packets.
V.90: The fastest transmissions standard available for analog transmission, it
is capable of 56,000 bit/s.
V.92: Transmits at the same speed as V.90 but offers a reduced handshake
time and an on-hold feature.
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9 The Internet
The Internet is a global system of interconnected computer networks. A computer
that connects to the Internet can access information from a vast number of servers
and other computers. An Internet connection also allows the computer to send
information onto the network; that information may be saved and ultimately accessed
by a variety of servers and other computers. Much of the widely accessible
information on the Internet consists of the interlinked hypertext documents and other
resources of the World Wide Web (WWW). Web users typically send and receive
information using a web browser; other software for interacting with computer
networks includes specialized programs for electronic mail, online chat, file transfer
and file sharing.
Information is moved around the Internet by packet switching using the standardized
Internet Protocol Suite (TCP/IP). It is a "network of networks" that consists of millions
of private and public, academic, business, and government networks of local to
global scope that are linked by copper wires, fiber-optic cables, wireless connections,
and other technologies.
9.1 Brief history of the internet
The Department of Defense's Advanced Research Projects Agency (DARPA) started
the Internet in 1969, in a computer room at the University of California, Los Angeles.
It wanted to enable scientists at multiple universities to share research information.
Advanced Research Projects Agency NETwork (ARPANET), the predecessor to the
Internet, was created 12 years after Sputnik, during the Cold War. DARPA's original
goal was to develop a network secure enough to withstand a nuclear attack.
The first communications switch that routed messages on the ARPANET was
developed at Bolt Beranek and Newman (BBN) in Cambridge, Massachusetts. (BBN
was bought by GTE. Bell Atlantic acquired GTE, changed its name to Verizon and
spun off BBN as Genuity). ARPANET's network used packet switching developed by
Rand Corporation in 1962. Data was broken up into "envelopes" of information that
contain addressing, error checking and user data. One advantage of packet
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switching is that packets from multiple computers can share the same circuit. A
separate connection is not needed for each transmission. Moreover, in the case of an
attack, if one computer goes down, data can be rerouted to other computers in the
packet network. TCP/IP, the protocol still used on the Internet, was developed in
1974 by Vint Cerf and Robert Kahn. It supports a suite of services such as email, file
transfer and logging onto remote computers.
In 1984, as more sites were added to ARPANET, the term Internet started to be
used. The ARPANET was shut down in 1984, but the Internet was left intact. In 1987,
oversight of the Internet was transferred from the Department of Defense to the
National Science Foundation.
While still used largely by universities and technical organizations, applications on the
Internet expanded from its original defense work. In particular, newsgroups used by
computer hobbyists, college faculty and students were formed around special
interests such as cooking, specialized technology and lifestyles. The lifestyles
newsgroups included sexual orientation (gay and lesbian), religion and gender
issues. Computer-literate people were also using the Internet to log onto computers
at distant universities for research and to send electronic mail.
The Internet was completely text prior to 1990. There were no graphics, pictures or
color. All tasks were done without the point-and-click assistance of browsers, such as
Netscape and Internet Explorer. Rather, people had to learn, for example, UNIX
commands. UNIX is a computer operating system developed in 1972 by Bell Labs.
UNIX commands include: m for Get Mail, j for Go to the Next Mail Message, d for
Delete Mail and u for Undelete Mail. The Internet was not for the timid or for
computer neophytes.
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9.2 HTML
The World Wide Web was conceived as a way to make using and navigating the
Internet easier. It is not a separate part of the Internet, but a graphical way to use the
Internet. The World Wide Web enables users to hear sound and see color, video and
graphical representations of information. Moreover, it provides links to information
using text and graphic images embedded in documents to "navigate" to other Web
sites. These links are in the form of highlighted text and graphics. Users click on them
with a mouse to move from one document to another or from one site to another.
These two capabilities, linking and graphics are the strengths of the World Wide
Web.
The World Wide Web was created in 1989 by Tim Berners-Lee at CERN, the
European Laboratory for Particle Physics. The goal of creating the Web was to
merge the techniques of client-server networking and hypertext to make it easy to
find information worldwide. The basic concept is that any type of client, the PC,
should be able to find information without needing to know a particular computer
language or without needing a particular type of terminal.
The name of the protocol used to link sites is Hypertext Transfer Protocol (HTTP).
The letters, http, start Web addresses. When a browser sees http, it knows that this is
an address for linking to another site.
HTML, an acronym for Hyper Text Markup Language, is the predominant markup
language for web pages. It provides a means to describe the structure of text-based
information in a document - by denoting certain text as links, headings, paragraphs,
lists, etc. - and to supplement that text with interactive forms, embedded images, and
other objects. HTML is written in the form of "tags" that are surrounded by angle
brackets. HTML can also describe, to some degree, the appearance and semantics
of a document, and can include embedded scripting language code (such as
JavaScript) that can affect the behavior of Web browsers and other HTML
processors.
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9.3 Emailing
Electronic mail, often abbreviated as e-mail or email, is a method of exchanging
digital messages, designed primarily for human use. A message at least consists of
its content, an author address and one or more recipient addresses. The foundation
for today's global Internet email service was created in the early Arpanet and was
codified as a standard for encoding of messages. An email sent in the early 1970s
looked very similar to one sent on the Internet today. Conversion from Arpanet to
Internet in the early 1980s produced the modern details of the current, core service,
with transport provided by the Simple Mail Transfer Protocol (SMTP), first published
as Internet Standard 10 in 1982. Email systems that operate over a network (rather
than being limited to a single, shared machine) are based on a store-and-forward
model in which email computer server systems accept, forward, deliver or store
messages on behalf of users, who only need to connect to the email infrastructure
with their personal computer or other network-enabled device for the duration of
message submission to, or retrieval from, their designated server. Rarely is email
transmitted directly from one user's device to another's.
Internet e-mail messages consist of two major sections:
Header-Structured into fields such as summary, sender, receiver, and other
information about the e-mail.
Body-The message itself as unstructured text.
Both plain text and HTML are used to convey e-mail. While text is certain to be read
by all users without problems, there is a perception that HTML-based e-mail has a
higher aesthetic value. Advantages of HTML include the ability to include inline links
and images, set apart previous messages in block quotes, wrap naturally on any
display, use emphasis such as underlines and italics, and change font styles.
Multipurpose Internet Mail Extensions (MIME) is an Internet standard that extends
the format of e-mail. MIME enables users to send video, foreign language and audio
file attachments. The MIME standard includes a way to attach bits at the beginning
and end of the attachment. These bits tell the receiving computer what type of file is
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attached and when the attachment ends. For example, the bits may tell the computer,
"This is a Microsoft Word for Windows file." The receiving computer then opens the
document as a Word file. MIME does not entirely solve the attachment problem. The
sending and receiving computers need compatible software platforms and programs
for reading attachments. Some early releases of spreadsheet and word processing
programs cannot open newer versions of these programs.
Instant messaging (IM) is a collection of technologies that create the possibility of
real-time text-based communication between two or more participants over the
internet or some form of internal network/intranet. Instant messaging happens in real-
time and require two or more persons to be logged on. Some systems allow the
sending of messages to people not currently logged on (offline messages).
Instant messaging based on Internet relay chat (IRC) protocol has been available
since the 1980s. Jarkko Oikarinen of Finland designed Internet relay chat in 1988.
Internet relay chat (IRC) protocol is based on a client-server model with "channels"
defined in the IRC protocol. A channel is the path defined to carry messages to chat
rooms or "buddy" lists where everyone receives the same message. The IRC
protocol defines how a group of clients (end-user computers) all receive the same
message from the server to which they're all connected. Chat programs relay a
message from single users to a predefined group. This is feasible because IRC is
based on TCP/IP, which can deliver packets containing the same message to many
computers. Each client that is part of the IRC group of networks downloads special
client Internet relay chat software
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9.4 Addressing in internet
An Internet Protocol (IP) address is a numerical identification and logical address
that is assigned to devices participating in a computer network utilizing the Internet
Protocol for communication between its nodes. Although IP addresses are stored as
binary numbers, they are usually displayed in human-readable notations, such as
208.77.188.166.
The current generation of IP is called IP version 4 (IPv4). IPv4 uses 32-bit (4-byte)
addresses, which limits the address space to 4,294,967,296 possible unique
addresses. However, IPv4 reserves some addresses for special purposes such as
private networks (~18 million addresses) or multicast addresses (~270 million
addresses). This reduces the number of addresses that can be allocated as public
Internet addresses, and as the number of addresses available is consumed, an IPv4
address shortage appears to be inevitable in the long run. This limitation has helped
stimulate the push towards IPv6, which is currently in the early stages of deployment
and is currently the only offering to replace IPv4.
IPv4 addresses are usually represented in dot-decimal notation (four numbers, each
ranging from 0 to 255, separated by dots, e.g. 208.77.188.166). Each part represents
8 bits of the address, and is therefore called an octet. In less common cases of
technical writing, IPv4 addresses may be presented in hexadecimal, octal, or binary
representations. When converting, each octet is usually treated as a separate
number.
The Internet is divided into logical domains, which are identified as a 32 bit portion of
the total address, under the terms of IPv4. Addresses in the Domain Name System
(DNS), the administration of which is the responsibility of ICANN, follow a standard
convention: [email protected]. The vast majority of the 147 million or so
registered Top Level Domains (TLDs) are commercial in nature. TLDs, which are
identified as the domain address suffix, are of two types: generic Top Level
Domains (gTLDs) and country codes.
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The following are six generic top-level domain names:
.com - Commercial businesses worldwide
.org - Nonprofit organizations worldwide
.net - Network providers worldwide
.edu - Educational institutions in the United States
.gov - United States governmental bodies
.mil - Branches of the United States military
Countries outside of the U.S. use country-specific, geographic, top-level domain
names referred to as country code top-level domain names (ccTLDs). For
instance, .jp is for Japan, .cn is for China and .uk is for the United Kingdom. Sites
may use generic domain names preceding the country code top-level domain
(ccTLD) name. Country-specific top-level domain names are approved by ICANN in
Latin character sets only.
9.5 Intranet
An intranet is a private computer network that uses Internet technologies to securely
share any part of an organization's information or operational systems with its
employees. Sometimes the term refers only to the organization's internal website, but
often it is a more extensive part of the organization's computer infrastructure and
private websites are an important component and focal point of internal
communication and collaboration.
An intranet is built from the same concepts and technologies used for the Internet,
such as client-server computing and the Internet Protocol Suite (TCP/IP). Any of the
well known Internet protocols may be found in an intranet, such as HTTP (web
services), SMTP (e-mail), and FTP (file transfer). Internet technologies are often
deployed to provide modern interfaces to legacy information systems hosting
corporate data.
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An intranet can be understood as a private version of the Internet, or as a private
extension of the Internet confined to an organization. The first intranet websites and
home pages began to appear in organizations in 1990-1991. Although not officially
noted, the term intranet first became common-place inside early adopters, such as
universities and technology corporations, in 1992.
Intranets differ from extranets in that the former are generally restricted to employees
of the organization while extranets may also be accessed by customers, suppliers, or
other approved parties. Extranets extend a private network onto the Internet with
special provisions for access, authorization and authentication.
An organization's intranet does not necessarily have to provide access to the
Internet. When such access is provided it is usually through a network gateway with a
firewall, shielding the intranet from unauthorized external access. The gateway often
also implements user authentication, encryption of messages, and often virtual
private network (VPN) connectivity for off-site employees to access company
information, computing resources and internal communications.
9.6 Extranet
An extranet is a private network that uses Internet protocols, network connectivity,
and possibly the public telecommunication system to securely share part of an
organization's information or operations with suppliers, vendors, partners, customers
or other businesses. An extranet can be viewed as part of a company's intranet that
is extended to users outside the company (e.g.: normally over the Internet). It has
also been described as a "state of mind" in which the Internet is perceived as a way
to do business with a pre approved set of other companies business-to-business, in
isolation from all other Internet users. In contrast, business-to-consumer involves
known server(s) of one or more companies, communicating with previously unknown
consumer users.
Briefly, an extranet can be understood as an intranet mapped onto the public Internet
or some other transmission system not accessible to the general public, but managed
by more than one company's administrator(s). For example, military networks of
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different security levels may map onto a common military radio transmission system
that never connects to the Internet. Any private network mapped onto a public one is
a virtual private network (VPN). In contrast, an intranet is a VPN under the control of
a single company's administrator(s).
An extranet requires security. These can include firewalls, server management, the
issuance and use of digital certificates or similar means of user authentication,
encryption of messages, and the use of virtual private networks (VPNs) that tunnel
through the public network. Many technical specifications describe methods of
implementing extranets, but often never explicitly define an extranet.
9.7 Security issues
The Internet is rife with risks. Hackers, crackers, saboteurs, and other unsavory
characters abound, eagerly attacking the Net and its users at every opportunity. The
risks include system intrusion, unauthorized data access, system sabotage, planting
of viruses, theft of data, theft of credit card numbers, and theft of passwords.
Securing data on the internet can be done using Encryption and Firewalls.
Encryption involves scrambling and compressing the data prior to transmission; the
receiving device is provided with the necessary logic in the form of a key to decrypt
the transmitted information. Encryption logic generally resides in firmware included in
stand-alone devices, although it can be built into virtually any device. Such logic now,
for example, is incorporated into routers, which can encrypt/decrypt data on a packet-
by-packet basis. Encryption comes in two basic flavors:
Private-key encryption, also known as single-key or secret-key encryption,
uses the same key for both encryption (encoding) and decryption (decoding).
This approach requires that the key be kept secret through some form of
secure key transmission prior to the ensuing data transfer.
Public-key encryption involves the RSA encryption key that can be used by
all authorized network users. The key for decryption is kept secret. Public –
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key encryption is much slower than private key, but the dissemination of the
key is accomplished much more quickly.
Firewalls comprise application software that can reside in a communication router,
server, or some other device. That device physically and/or logically is a first point of
access into a networked system. On an active basis, the device can block access to
unauthorized entities, effectively acting as a security firewall. Firewalls provide
logging, auditing, and sucker traps to identify access attempts and to separate
legitimate users from intruders. Firewalls can be in the form of a programmable router
or a full set of software, hardware, and consulting services
9.8 Reliability and capacity
Despite all its advances over the past couple decades, the Internet is challenged
today. It is still limited in bandwidth (capacity) at various points. One reason the
Internet needs more bandwidth is that traffic keeps increasing at an astonishing rate.
People are drawn to Web sites that provide pictures of products in order to engage in
demonstrations and to conduct multimedia communications. Multimedia, visual, and
interactive applications demand greater bandwidth and more control over latencies
and losses. This means that we frequently have bottlenecks at the ISP level, at the
backbone level (i.e., the NSP level), and at the Internet exchange points (IXPs)
where backbones interconnect to exchange traffic between providers. These
bottlenecks greatly affect our ability to roll out new time-sensitive, loss-sensitive
applications, such as Internet telephony, VoIP, VPNs, streaming media, and IPTV.
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10 Converged networks
The definition of convergence varies throughout the telecommunications industry.
Convergence can be considered as capability of one public network to carry all types
of traffic - voice, data and video - as packets. Converged networks either use Internet
protocol (IP)-based routers or asynchronous transfer mode (ATM) switches, which
send information in fixed-sized packets called cells. Internet backbone networks are
generally based on IP, a protocol used for routing packets in the Internet and in
private networks. Wholesale carrier’s networks that carry a mix of voice, data and
video tend to be based on ATM- and IP-based routers.
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11 Wireless services
11.1 Brief history of mobile and cellular services
Prior to the first deployment of analog cellular car telephones in 1984, users who
wanted to place telephone calls from their cars used mobile non-cellular telephone
networks that had connections to the public switched network. The first mobile
telephone system was started in 1946 in St. Louis, Missouri. Costs for car telephones
were high, between $2000 and $2500, and capacity was limited. The local telephone
company in each city operated one transmitter and receiver for the entire area. Thus,
the entire area covered by the one transmitter shared the same channels. This meant
that only a limited number (25 to 35) of simultaneous calls could be placed on each
city's mobile system. In addition to limited capacity, the quality of service was spotty
with considerable static and breaking up of calls.
Mobile radio service was more widespread than mobile telephone service prior to the
mid-1980s. Mobile radio is a "closed" service without connections to the public
switched network. Mobile radio operators can only reach people on their closed
network. For example, users on one taxicab service's system cannot call users on
another cab's system. Police departments were early pioneers of car radios. The
Detroit police department used mobile radio service in 1921. In the 1930s, mobile
radio use spread to other public safety agencies such as fire departments. Mobile
radio systems are now used for aviation, trucking, taxis and marine applications.
Mobile radio is half-duplex: Calls are two-way but only one user at a time can
transmit. For example, when one person is done speaking, he uses a convention
such as "over and out" to let the other person know he is finished talking. People
using mobile radio push a button to talk.
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11.2 Cellular telephone service technologies
11.2.1 AMPS
Advanced Mobile Phone System (AMPS) was the first cellular technology deployed in
the United States. Developed by Motorola and AT&T, AMPS is an analog technology
operating on 50 MHz in the 800-MHz band and supporting 666 (in some areas 832)
channels. In the United States, 25 MHz and 333 (in some areas 416) channels, each
are provided to the A-carrier, or nonwireline carrier, and the B-carrier, or wireline
carrier (incumbent telco or telco consortium).
Of the total number of channels awarded to each carrier, 21 channels are non
conversational channels dedicated to call setup, call hand-off, and call teardown. The
remaining communications channels are split into 30-kHz voice channels, with
separation of 45 MHz between the forward and reverse channels. Based on FDMA
and FDD transmission, AMPS does not handle data well, with modem transmission
generally limited to 6.8 kbps. Although once widely deployed in the United States,
Australia, the Philippines, and other countries, AMPS has almost entirely been
replaced by digital technology. Australian regulators mandated a cutover from
analogue AMPS to digital GSM and CDMA beginning December 31, 1999, in
Melbourne, gradually extending throughout the country during 2000. As noted above,
the FCC in the United States has authorized carriers to cease support for analog
systems as of March 1, 2008.
11.2.2 D-AMPS
Digital-AMPS (D-AMPS), also known as IS-54, IS-136 and TDMA, is a North
American digital cellular standard that operates in the same 800-MHz band as the
earlier analog AMPS. In fact, the two can coexist in the same network. D-AMPS use
the same 30-kHz bands as AMPS and supports up to 416 frequency channels per
carrier. Through Time Division Multiplexing (TDM), each frequency channel is
subdivided into six time slots, each of which operates at 8 kbps. Each call initially
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uses two time slots (e.g., 1 and 4, 2 and 5, and 3 and 6) in each direction, for a total
of 16 kbps, which supports the data transfer plus overhead for call processing. While
the standard recommends speech compression at 8 kbps (actually 7.95 kbps) using
Vector-Sum Excited Linear Predictive Coding (VSELP), that is an average rate
because each call can burst up to 48 kbps. D-AMPS yield a 3:1 advantage over
AMPS in terms of bandwidth utilization. IS-136 is known as a dual-mode standard
because both D-AMPS and AMPS can coexist on the same network, with both using
the same 21 control channels for call setup, call hand-off, and call teardown.
Thereby, IS-136 offers carriers the advantage of a graceful transition from analog to
digital. IS-136 also includes a nonintrusive Digital Control Channel (DCCH), which is
used for Short Message Service (SMS) and caller ID. SMS supports information
transfer for applications such as weather reports, sports scores, traffic reports, and
stock quotes, as well as short e-mail-like messages, which may be entered through
the service provider’s website. Some service providers also allow the cellular user to
respond to e-mails via two-way SMS. Data communications is supported at up to 9.6
kbps per channel (paired time slots), and as many as three channels can be
aggregated for speeds up to 28.8 kbps. Group 3 facsimile also can be supported.
The Radio Frequency (RF) modulation technique is Differential Quaternary Phase
Shift Keying (DQPSK).
11.2.3 GSM
Global System for Mobile Communications (GSM) was adopted by the CEPT in 1987
as the standard for pan-European cellular systems and was first introduced in 1991.
GSM operates in the 800-MHz and 900-MHz frequency bands and is ISDN
compatible. GSM carves each 200-kHz band into eight TDMA channels of 33.8 kbps,
each of which supports a voice call at 13 kbps using Linear Predictive Coding (LPC).
Data throughput generally is limited to 9.6 kbps, due to FEC and encryption
overhead. GSM commonly employs a four-cell reuse plan, rather than the seven-cell
plan used in AMPS, and divides each cell into 12 sectors. GSM commonly uses
frequency hopping and time-slot hopping, which also is used in CDMA systems. GSM
offers additional security in the form of a Subscriber Identification Module (SIM),
which plugs into a card slot in the handset, much as a PCMCIA card fits into a laptop
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computer. The SIM contains user profile data, a description of access privileges and
features, and identification of the cellular carrier that hosts the home registry. The
SIM can be used with any GSM set, thereby providing complete mobility across
nations and carriers supporting GSM, assuming that cross-billing relationships are in
place. GSM clearly developed to be the international standard of choice. Like D-
AMPS, GSM supports SMS text messaging, which generally is two-way. GSM is in
place in over 475 networks in more than 190 countries and predominates throughout
Europe and much of Asia, supporting full roaming privileges from country to country.
With minor modifications, GSM is the basis for DCS 1800 , also known as PCN
(Personal Communications Network), in Europe. DCS 1800, in large part, is an up
banded version of GSM, operating in the 1800-MHz (1.8-GHz) range. Also with minor
modifications, it is the basis for PCS 1900 in the United States, where it also is known
as GSM. PCS 1900 is the ANSI standard (J-STD-007, 1995) for PCS at 1900 MHz
(1.9 GHz). Unfortunately, PCS 1900 is not compatible with the original European
GSM, due to the difference in frequency bands. T-Mobile (owned by Deutsche
Telekom, which explains a lot) has deployed PCS 1900, and Cingular built a GSM
network as an overlay to its D-AMPS network.
11.2.4 UMTS-3G
Universal Mobile Telecommunications System (UMTS) also known as Wideband
CDMA (W-CDMA) is a 3G technology that is seen as a logical upgrade to GSM,
although the two are not compatible. UMTS runs over a carrier 5 MHz wide,
compared to the 200 – kHz carrier used for narrowband CDMA. UMTS specifications
provide for both TDD mode and FDD mode, with TDD largely used in Europe and
FDD in the United States. The FDD specifications call for the downlink to run in the
2100-MHz range (2110 – 2200 MHz) and the uplink in the 1900-MHz range (1885 –
2025 MHz). As is the case with all true 3G systems, UMTS specifications include 128
kbps for high-mobility applications, 384 kbps for pedestrian speed applications, and 2
Mbps (1.920 Mbps) for fixed in-building applications. In reality, UMTS currently caps
the transmission rate at a theoretical 384 kbps. UMTS was first deployed in Japan
(2000), where it is known as Freedom of Mobile Multimedia Access (FOMA).
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UMTS networks currently are being upgraded with High-Speed Downlink Packet
Access (HSDPA), which sometimes is characterized as a 3.5G technology. HSDPA
promises to increase theoretical downlink data rates to 14.4 Mbps, although current
implementations support speeds more typically in the range of 400 – 700 kbps,
bursting up to 3.6 Mbps for short periods of time using an adaptive modulation
technique to throttle bit rates up and down as the link permits. HSDPA has been
introduced on a limited basis in Austria, Finland, Japan, South Africa, and the United
States. Work has begun in the standards bodies on High-Speed Uplink Packet
Access (HSUPA). Once increased speeds are in place on both the downlink and
uplink, simultaneous voice, data, and even video calls will be quite possible.
11.2.5 Nextel
Nextel was founded in 1987 and initially offered data communications over analog
radio facilities. Nextel's newer wireless telephone service is carried over digital
facilities in its 800 to 900 MHz spectrum. The service is used with Motorola
telephones and is geared toward small and medium-sized businesses. In early 1999,
Nextel upgraded its network to support browser-equipped Motorola telephones. It
targets commercial, not residential, customers. Motorola Corporation and the McCaw
family each own 20% of Nextel (The McCaw cellular company was purchased by
AT&T in 1993).
Nextel phones have a liquid crystal display that can be used for text and numeric
paging. Nextel coverage is in 185 of the top 200 markets and within reach of 77% of
the U.S. These areas are largely metropolitan locations. Vast areas of sparsely
populated sections of the country will not be covered. Nextel is accessible from major
interstate highways. However, it will not have towers or service in remote locations
with few businesses such as North Dakota and Montana.
In addition, Nextel offers Nextel Direct Connect. This service enables employees in
the same company to have direct connections to each other by pushing a button on
their telephone.
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Because Nextel service does not work on the 1800 to 1900 megahertz frequencies, it
did not have to participate in costly bidding for new frequencies. However, it has a
limited amount of spectrum. Nextel offers the same features as PCS service such as
short messaging text paging, email and voice mail. These phones use a technology,
iDEN (integrated Digital Enhanced Network) developed by Motorola that breaks each
25 kilohertz channel into up to six time slots able to carry voice, paging traffic, data
and dispatch messages. It compresses the voice small enough so that it can fit into
one of the six time slots.
High-end Nextel phones have email capability embedded in them that works with
Microsoft Outlook and Lotus as well as email provided by Internet service providers.
The Nextel email server converts these email formats to that compatible with Nextel.
Subscribers access their email by pressing the email button on their telephone and
entering their password. A cookie in the phone sends the user name and email
account information. The phone can be set up to receive all or some email
messages. This functionality doesn't work when roaming. A cable is available to
connect the phone to a laptop computer so that email can be stored on a computer.
Nextel offers a dual band, GSM and 900-megahertz TDMA Motorola telephone that
operates in over 60 countries with which it has roaming agreements. Nextel sells
service directly and through Nextel Partners, an affiliate who sells in small to
medium-sized markets in 30 states within the United States. Its single mode phone
operates in parts of Canada, Latin America and the Philippines where Nextel
International operates networks in the same frequency and access methods as those
in the United States. Nextel's lower frequency network requires fewer towers
because lower frequency, longer wavelength signals travel farther without
deteriorating than PCS signals at higher (1900 megahertz) frequencies.
11.2.6 Paging services
Paging is a subscription service offered in a variety of plans and options to meet the
needs of a subscriber and the type of device used. In general, all pagers are given
unique phone numbers while alphanumeric pagers are given an email address,
usually consisting of the phone number. Upon calling a phone number assigned to a
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pager, the calling party reaches a recorded greeting asking the caller to enter a
numeric message, and sometimes giving the caller an option to leave a voice mail
message. Generally, the paged person will receive an alert from the pager with the
phone number to return the call and/or a pager code within a few minutes. In the
case of email paging, the text is displayed.
Numeric pagers are the simplest of the type of devices offering only a numeric
display of the phone number to be called and pager codes
Alphanumeric pagers are essentially modified versions of numeric pagers with
sophisticated display to accommodate text. These devices are usually given an email
address to receive text messages.
Two-way Alphanumeric pagers are alphanumeric pagers capable of both sending
and receiving text messages and email. To do this, the units either have a small built
in keypad that allows the user to input messages, or the message can be typed from
a wireless keyboard and is received by the pager.
Most modern paging systems use simulcast delivery by satellite controlled networks.
This type of distributed system makes them inherently more reliable than terrestrial
based cellular networks for message delivery. Many paging transmitters may overlap
a coverage area, while cellular systems are built to fill holes in existing networks.
When terrestrial networks go down in an emergency, satellite systems continue to
perform. Because of superior building penetration and availability of service in
disaster situations, pagers are often used by first responders in emergencies.
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11.3 3rd Generation Networks
11.3.1 GPRS data carried as packets
General Packet Radio Service (GPRS) is the 2.5G data service enhancement for
GSM host networks. GPRS specifications were developed in 1997 by ETSI, which
has since passed that responsibility on to the 3rd Generation Partnership Project
(3GPP). GPRS is a packet-switched service that takes advantage of available GSM
time slots for data communications and supports both X.25 and TCP/IP packet
protocols with QoS. GPRS, an important component in the GSM evolution, enables
high-speed mobile data com usage and is considered most useful for bursty data
applications such as mobile Internet browsing, e-mail, and various push technologies.
Through linking together as many as eight GSM channels, GPRS has a theoretical
transmission rate as high as 171.2 kbps, although it realistically is limited to 115.2
kbps and more typically 30 – 60 kbps. In practice, however, GSM system operators
are unlikely to allow a single user to access eight channels. GPRS maintains the
same GMSK modulation scheme used by GSM and provides always- on access,
although charges apply only for actual data traffic. Notably, GPRS will support
simultaneous voice and data communications over the same wireless link, with voice
taking precedence as always. GPRS defines three classes of terminal equipment:
Class A terminals support simultaneous circuit-switched GSM voice and SMS
service as well as GPRS packet-switched data traffic.
Class B terminals will support non simultaneous circuit-switched voice and
packet-switched data, automatically switching between the two. A Class B
terminal, for example, will suspend an active data session in the event of an
incoming voice call or SMS message. Most GPRS terminals are Class B.
Class C terminals support either circuit-switched voice and SMS service or
packet-switched services but must be manually switched from one to the
other.
The bit rate is sensitive to the encoding scheme in use, of which there are four. CS-4
supports a bit rate of 20.0 kbps per time slot but can be used only when the Mobile
Station (MS) and Base Station (BS) are in proximity. As the distance increases
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between the MS and BS, the encoding scheme must be more robust to compensate
for attenuation, and the bit rate accordingly must adjust downward. At the edge of the
cell, for example, CS-1 supports a bit rate of only 8.0 kbps. The four Coding
Schemes (CSs) and associated bit rates are as follows:
CS-4: 20.0 kbps
CS-3: 12.0 kbps
CS-2: 14.4 kbps
CS-1: 8.0 kbps
GPRS can run in either the symmetric or asymmetric mode, with the speed in either
direction sensitive to the multislot service class selected, of which there are 12. The
multislot service class determines the number of time slots in each direction, with
each time slot supporting a theoretical nominal data rate of 20 kbps (actually 21.4
kbps). The simplest is service class 1, which supports one time slot in each direction.
The most capable is service class 12, which supports four time slots in each
direction. Generally speaking, the most common service classes are asymmetric in
nature, which suits data-oriented Web applications in much the same way as do the
asymmetric local loop technologies of ADSL, PON, and WiMAX. The U.S. carriers
deploying GPRS include Cingular (800 and 1900 MHz) and T-Mobile (1900 MHz).
Upgrades from GPRS to 3G will be in the direction of UMTS, often through EDGE as
an intermediate step.
11.3.2 EDGE
Enhanced Data Rates for GSM Evolution (EDGE) is a 2.5G standard developed by
ETSI in 1999 and touted as the final stage in the evolution of data communications
within the existing GSM standards. The only IMT-2000 specification based on TDMA,
EDGE supports data transmission rates up to 473.6 kbps over GSM FDD channels
200 kHz wide through an improved modulation technique. 8-Phase Shift Keying (8-
PSK) involves eight levels of phase shift and, therefore, supports three bits per
symbol. EDGE supports 124 FDD channels, each of which supports eight time
slots/users. EDGE supports two modes of operation:
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Enhanced GPRS (EGPRS) is a packet-switched transmission mode that will support
data rates as high as 473.6 kbps. EGPRS estimates link quality in order to adapt the
Modulation and Coding Scheme (MCS), of which there are nine levels. If the system
estimates that the quality of the link is good, it will select the more efficient 8-PSK
modulation technique and, therefore, realize higher signaling rates per time slot and
higher data throughput. If the link quality is estimated to be poor, the system will
ratchet down to the less capable GMSK. Incremental Redundancy (IR) is an
enhanced Automatic Repeat Request (ARQ) technique. As transmission begins, IR
initially transmits packets with little FEC overhead in an attempt to maximize
efficiency, that is, maximize payload by minimizing overhead. If the initial
transmission cannot be successfully decoded by the receiver, IR ratchets up the FEC
overhead until it finds a level at which the receiver can successfully decode the
transmission.
Enhanced Circuit-Switched Data (ECSD) is an enhancement of the native GSM
circuit-switched protocol. ECSD adds 8-PSK as a modulation option, thereby
increasing the efficiency of data transmission. A GMSK connection requires four time
slots to support a 57.6-kbps data rate, but ECSD requires only two. EDGE also runs
over IS-136 TDMA networks in the United States. In either case, EDGE is an
intermediate step between 2G TDMA and 3G WCDMA, although some TDMA-based
carriers may stop at EDGE. At the time of this writing (June 2007), the Cingular
Wireless network (800 and 1900 MHz) largely supports EDGE and the T-Mobile
network (1900 MHz) is fully upgraded. Maximum transmission rates typically are in
the range of 75-150 kbps.
11.3.3 cdma2000
Developed by Qualcomm, the company that commercialized CDMA, Code Division
Multiple Access 2000 (CDMA2000) is a 3G system based on earlier CDMA versions
(also known as TIA/EIA IS-95a and IS-95b). CDMA2000 (also known as IS-856) has
been approved by the ITU-R as part of the IMT-2000 family. The initial version,
known as CDMA2000 1×RTT (one times Radio Transmission Technology, with one
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times referring to standard channel width), offers 2.5G capabilities within a single
standard 1.25-MHz channel, effectively doubling the voice capacity of the
predecessor 2G cdmaOne systems and offering theoretical data speeds up to 153
kbps (throughput in the range of 70–90 kbps) through the use of QPSK modulation.
An enhanced 3G version known as 1×EV-DO (one carrier EVolution-Data Optimized)
is a High-Data-Rate (HDR) version that employs 16-QPSK modulation in support of a
peak data rate of 2.4 Mbps on the downlink and 153 kbps on the uplink. 1×EV-DO
supports average aggregate throughput in a fully loaded three-sectored cell of 4.1
Mbps on the downlinks and 660 kbps on the uplinks, with dynamically assigned data
rates providing each user with optimum throughput at any given moment. 1×EV–DO
can run in any band (e.g., 450, 800, 1800, and 1900 MHz) and can coexist in any
type of network (e.g., CDMA2000, cdmaOne, GSM, TDMA, and AMPS). CDMA2000
runs in the 800-MHz and 1.8-2.0-GHz spectrum. GSM1x is a version designed as a
transition specification for GSM operators, involving dual-mode phones. Also known
as IS-2000-A, 3 x is an enhancement that uses three cdmaOne carriers for total
bandwidth of 3.75 MHz. This supports data rates up to 2 Mbps by spreading a
multicast signal over the three carriers. In North America, Bell Canada, Sprint Nextel,
and Verizon Wireless (800 MHz) have committed to CDMA2000.
11.3.4 2.5 and 3G Services
The first major step in the evolution to 3G occurred with the introduction of General
Packet Radio Service (GPRS). So the cellular services combined with GPRS
became' 2.5G.'
GPRS could provide data rates from 56 kbit/s up to 114 kbit/s. It can be used for
services such as Wireless Application Protocol (WAP) access, Short Message
Service (SMS), Multimedia Messaging Service (MMS), and for Internet
communication services such as email and World Wide Web access.
GPRS is a best-effort packet switched service, as opposed to circuit switching, where
a certain Quality of Service (QoS) is guaranteed during the connection for non-mobile
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users. It provides moderate speed data transfer, by using unused Time division
multiple access (TDMA) channels. Originally there was some thought to extend
GPRS to cover other standards, but instead those networks are being converted to
use the GSM standard, so that GSM is and newer releases.
GPRS networks evolved to EDGE networks. Enhanced Data rates for GSM Evolution
(EDGE), Enhanced GPRS (EGPRS), or IMT Single Carrier (IMT-SC) is a backward-
compatible digital mobile phone technology that allows improved data transmission
rates, as an extension on top of standard GSM. EDGE can be considered a 3G radio
technology and is part of ITU's 3G definition, but is most frequently referred to as
2.75G. EDGE can be used for any packet switched application, such as an Internet,
video and other multimedia.
3G networks enable network operators to offer users a wider range of more
advanced services while achieving greater network capacity through improved
spectral efficiency. Services include wide-area wireless voice telephone, video calls,
and broadband wireless data, all in a mobile environment. Additional features also
include HSPA data transmission capabilities, which provide users with data rates up
to 14.4 Mbit/s on the downlink and 5.8 Mbit/s on the uplink.
Unlike IEEE 802.11 networks, which are commonly called Wi-Fi or WLAN networks,
3G networks are wide-area cellular telephone networks which provide High-speed
Internet access and video telephony to 3G Network subscribers. IEEE 802.11
networks are short range, high-bandwidth networks primarily developed for data.
11.3.5 4G-the future
The introduction of 3G technology provides a huge expansion in mobile capacity and
bandwidth. 4G will do the same for the spectrum of broadband communications. By
supporting mobility, and by being faster and cheaper to deploy, it is expected that 4G
will disrupt today's wired broadband access alternatives, including DSL and cable
modems. 4G is also expected to serve the next billion Web users in developing
countries. Generally speaking, 4G is an evolution not only to move beyond the
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limitations and problems associated with 3G but also to increase the bandwidth,
enhance the quality of services, and reduce the cost of the resource. The main
distinguishing characteristics between 3G and 4G will be increased data rates,
enhanced multimedia services, new transmission techniques, new Internet access
technology, greater compatibility in interfacing with wired backbone networks, and the
addition of QoS and security mechanisms. 4G should also be able to provide smooth
global roaming ubiquitously, at lower cost. At the very least, this means a new air
interface supporting higher data rates and also a change in the way data transport is
handled end to end. Unlike 3G networks, which are a mix of circuit-switched and
packet-switched networks, 4G will be based solely on packet switching.
11.4 Messaging services (SMS)
Short Message Service (SMS) is a text-messaging service available on most digital
cellular telephone networks. SMS was originally designed to support one-way
information transfer for applications such as weather reports, sports scores, traffic
reports, and stock quotes as well as short e-mail-like messages, which may be
entered through the service provider’s website. Most service providers also allow the
cellular user to respond to e-mails via two-way SMS using the cell phone keypad.
Personal digital assistants and other wireless-enabled devices often have much more
functional keyboards, of course. Contemporary SMS supports two-way
communication between cell phones, other wireless devices, and computers
connected to the Web.
Initially SMS was defined in the Global System for Mobile Communications (GSM)
standards, but it also has been available on a number of other digital cell networks
since the late 1980s or so. SMS is a store-and-forward messaging technology that
generally includes a chat option that operates in near-real-time synchronous mode,
much like IM. In fact, many IM systems support mobile communications via interfaces
to SMS systems. As SMS messages use the same SMTP as is specified in the
TCP/IP suite for e-mail, messaging interconnection issues are relatively modest.
Interconnection of cellular networks for SMS applications presents something of a
challenge in the United States as there are so many transport protocol variations.
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Second-generation (2G) network protocols include Time Division Multiple Access
(TDMA), GSM and Code Division Multiple Access (CDMA). In every case, and
whether the SMS message originates or terminates in a mobile phone, the message
travels through a centralized message center, also known as a Short Message
Service Center (SMSC). Also in every case, the SMS data are packetized, but in
TDMA and GSM networks the data packet transport is over the signaling channel via
Signaling System 7 (SS7), while data packet transport in CDMA networks is over the
normal data channels. Where other coding schemes are employed, the number of
characters per packet varies. For example, a double-byte scheme such as that used
in support of complex alphabets (e.g., Chinese and Japanese) limits the packet size
to 70 characters. Content exceeding this limitation must be fragmented prior to
transmission and reconstituted upon reception. In most countries, GSM is the sole
2G network standard, so gateway issues are much less significant. All cellular
networks are limited in terms of bandwidth, of course, although the newer 2.5 G and
3 G networks are much less so. The user interface is always an issue with mobile
devices as display size and keyboard size are naturally limited.
11.5 Mobile internet access
WAP is a proposed standard designed for wireless Internet access. People with
WAP-enabled cellular phones access Internet sites, which are written in a special
programming language. The object is to make information downloaded from the site
fit into cellular devices' small screens. The Wireless Application Protocol is a menu-
driven method for downloading information such as flight schedules and bank
balances to cellular phones from the Internet. WAP service was introduced in Europe
in 2000. However, its slow speed, incompatibilities with some phones and technical
glitches resulted in user dissatisfaction. In addition, there were not a great many sites
available where operators had taken the trouble to re-write them for WAP access.
In addition to the preceding, WAP is a "dialup" service, and connection and download
speeds are slow. It is possible that as networks are upgraded for packet data service
and WAP technology is improved that WAP services may become more popular.
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11.6 Blue Tooth
Bluetooth technology allows for the replacement of proprietary cables that connect
one digital device to another with a universal short-haul radio link. Mobile computers,
cellular handsets, printers, keyboards, and many other devices can be embedded
with Bluetooth radios. Bluetooth was developed by the Bluetooth Special Interest
Group (SIG), founded by Ericsson, IBM, Intel, Nokia, and Toshiba.
A small wireless Bluetooth network connecting, for example, a user’s computer to its
peripherals is called a personal area network (PAN). PAN contains one or more
piconets. One Bluetooth piconet contains a single master and up to seven active
slaves. The master polls slaves and orders each of them to transmit in turn. For voice
applications Bluetooth specifies a synchronous channel that transmits at a
bidirectional 64-Kbps constant bit rate between a master and a slave. This can be
used to implement cordless telephones or hands-free sets for a cellular telephone.
Bluetooth systems use the same 2.4-GHz license free frequency band as WLANs
and they can coexist in the same area. The wideband WLAN signals and narrowband
Bluetooth signals do not interfere much.
Bluetooth uses frequency hopping spread-spectrum (FHSS) technology, in which
data are transmitted in bursts and the carrier frequency is changed after each burst.
There are 79 carrier frequencies with 1-MHz spacing over which the transmission
frequency hops. Each piconet uses a different pseudorandom hopping sequence
over the 79 carriers. Several piconets can operate in the same area simultaneously
because their signals interfere only at a time when they happen to occupy the same
frequency channel. The modulation rate of Bluetooth is 1 Mbps, which all devices and
both transmission directions in the piconet share. If we compare WLAN and
Bluetooth technologies we see that WLAN is a system for a work group (LAN) and
Bluetooth is for only a single user (PAN). The number of devices in the Bluetooth
network is very limited and data rate available for each device is quite low.
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11.7 Low Earth Orbiting Satellite Networks (LEOs) and Middle Earth Orbiting
Satellites (MEOs)
Low-Earth Orbiting and Middle-Earth Orbiting satellites (LEO and MEO) satellites
operate at low altitudes of several hundred miles or so in a variety of nonequatorial
orbital planes. This compares with GEO satellites, which always are placed in
equatorial orbital slots at an altitude of approximately 22,300 miles. LEO satellites
operate at altitudes of 644 – 2415 km. Although the term is not tightly defined, little
LEO systems involve a relatively small number of satellites and operate at
frequencies below 1 GHz in support of low-bit-rate data traffic (e.g., telemetry, vehicle
messaging, and personal messaging). Big LEO systems are bigger networks that
operate at higher frequencies in support of voice and higher speed data
communications.
MEO satellites operate at altitudes of 10,062-20,940 km. LEO and MEO systems are
configured as constellations of small, low-power satellites. In combination, the
satellites in such a constellation generally provide full coverage of major land
masses, and some have been designed to provide full coverage of virtually every
square inch of the earth’s surface. The various proposals have included as many as
840 satellites and are intended to provide various combinations of voice and data
services. These systems also are known as Mobile Satellite Systems (MSSs), as
opposed to the Fixed Satellite Systems (FSSs) in geostatic orbit. Whizzing around
the earth like electrons whizzing around the nucleus of an atom and LEO and MEO
networks are designed so that a satellite is always within reach of a terrestrial
terminal.
11.8 Time division multiple access and code division multiple access
In Time division multiple access (TDMA), the transmission channel is broken into a
number of time slots - for example, six. Three of the time slots carry traffic from three
devices, and three are not used. The three unused time slots ensure that there is no
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interference between traffic on the time slots carrying traffic. Time division multiple
access has three to five times the capacity of analog cellular service.
While time division multiple access is used both in the U.S. and the rest of the world,
the methods do not interoperate with each other. The standard in Europe and most of
the rest of the world is called GSM (Groupe Speciale Mobile). GSM works by dividing
channels of 200-kilohertz spectrum into eight time slots. Seven of the time slots carry
traffic and the eighth carries control signals. The U.S. has settled on a standard
called IS-136. It is also used in Latin America, Russia, parts of Asia and the Ukraine.
Code division multiple access (CDMA) is a "spread spectrum" technology. Each
conversation transmitted is spread over multiple frequencies as it is sent. This is
accomplished through the use of unique 40-bit codes assigned to each telephone
transmission. These codes are called Walesh codes. Having a unique code assigned
to each data or voice transmission allows multiple users to share spectrum or air
space. CDMA has more capacity than TDMA.
In addition to capacity, CDMA handsets use low amounts of power. This can be
significant in light of consumer concerns about cellular handsets causing cancer.
Lower power emissions translate to less threats of wireless service causing cancer.
Finally, calls are transferred (handed off), from cell to cell by a soft handoff method
superior to TDMA and analog cellular handoff. With a soft handoff, a call is rarely
dropped during the handoff. For a short period of time during the handoff or transfer,
the call is held as it is received and as the cell hands it off. Unfortunately, the decision
to use what they perceived as a superior multiplexing method cost many carriers in
the U.S. a high price in lost compatibility.
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12 Global issues
12.1 Deregulation
Parts of Europe were deregulated in 1997 and Asia, including Japan and Singapore,
are just opening their markets as is Australia. Latin America began opening its
markets in 1997. Two factors are pushing deregulation: the desire to be part of the
World Trade Organization (WTO) and the prospect of upgraded network
infrastructures. A requirement for joining the WTO is that countries liberalize their
market for imports and foreign companies. WTO members derive benefits from
having more companies to buy and sell services with on a quid-pro-quo basis.
Many countries created telecommunications monopolies to ensure the presence of
secure telecommunications. Given the availability of new technologies, this is no
longer required. However, over the years, incumbent telephone companies had no
incentive to upgrade networks, improve customer service, lower prices or improve
technology. Many countries looked to events in the United States where free market
conditions in the 1980s and 90s resulted in innovation, lower prices for long distance
and more choices for end users.
12.2 Asia
Because of their vast populations, there is a great deal of interest in
telecommunications in Asia. In Japan, large sums of money are being invested in
fiber directly to businesses and residences for high-speed Internet access.
The government controls China's telecommunications carriers. The main focus since
1999 has been in building up the infrastructure for cellular, broadband data networks,
Internet services and fixed-line voice telephony. While there is no official competition
to government-controlled monopolies, unofficial unauthorized competition exists in
the form of IP for voice phone calls. A gray market has existed in China where people
make calls through IP gateways linked to the Internet. Fees are lower than those
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charged by traditional carriers and are seen by the government as losses in revenue
worth millions of dollars.
In 1999, the Ministry of Information Industries broke the largest fixed-wireline
company, China Telecom, into four companies. The goal was to introduce
competition in the telecommunications sector and encourage construction of an
advanced infrastructure. There are six licensed carriers in China. All are government
backed: China Telecom, China Mobile, China Unicom,
12.3 The rest of the world
The UK, France and Germany are the three largest countries in Europe. Between
them, they control 60% of the telecommunications market. There are many carriers
that provide broadband fiber networks and long distance services for consumers and
businesses across Europe. Competition for these as well as cellular services is
strong.
The European Union (EU) issues telecommunications regulations, reviews mergers
and sets technical standards for its 15-member European nations. Unlike the U.S.,
where the Federal Communications Commission (FCC) enforces as well as issues
regulations, in Europe enforcement of regulations is up to national regulatory
agencies in member countries. This has led to uneven implementation of European
Commission deregulation directives. The European Union (EU) deregulated long
distance telecommunications in most of Europe in 1998. In November 1999, the EU
requested local loop unbundling by January 1, 2001. On December 5, 2000 the
European Parliament, the body consisting of voting members, approved the
unbundling of the local loop, the portion of the network from an end user to the
telephone company's equipment. However, implementation has stalled, primarily due
to political clout and noncooperation by incumbents who don't want to lose further
market share. Incumbent telephone companies still have the lion's share of local
landline service.
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EE119
Computer Networks
TEST
1. Describe in few words analog and digital transmissions.
2. What is bandwidth and how it is expressed in analog and digital
transmissions?
3. Network topologies are categorized into the three basic types bus, ring and
star. Describe ring topology in few words.
4. What is Data circuit-terminating equipment (DCE)?
5. What is signalling in networks? What is difference between in-bound and out-
bound signalling?
6. What is PBX?
7. What is the main difference between a CAT 3 and CAT 7 twisted pair cables.
Describe in few words why cables are twisted in pairs.
8. What is Asynchronous Transfer Mode (ATM)?
9. What is a Code division multiple access (CDMA)?
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EE119
Computer Networks
TEST
1. In the analog form of electronic communications, information is represented as
a continuous electromagnetic waveform. Digital communications involves
modulating (changing) the analog waveform in order to represent information
in binary form (1 s and 0 s) through a series of blips or pulses
2. The transmission capacity of a telecommunications pathway, electronic or
optical. Bandwidth refers to the range of frequencies, expressed in Hertz (Hz),
that comprise a given transmission channel; in other words, it is the difference
between the lowest and highest frequencies carried on the channel. The
bandwidth determines the rate at which information can be transmitted
through a circuit. The greater the bandwidth, the more data that can be
carried, and in digital facilities, bandwidth is expressed in bits per second.
3. A ring network is a network topology in which each node connects to exactly
two other nodes, forming a single continuous pathway for signals through
each node - a ring. Data travels from node to node, with each node along the
way handling every packet.
4. Data circuit-terminating equipment (DCE) is typically a modem or other type of
communication device. The DCE sits between the DTE (data terminal
equipment) and a transmission circuit such as a phone line. Originally, the
DTE was a dumb terminal or printer, but today it is a computer, or a bridge or
router that interconnects local area networks.
5. Signalling is the process of sending information between two parts of a
network to control, route and maintain a telephone call. When signals were
sent over the same path as voice and data traffic it is called In-band signalling.
Out-of-band signalling send signals on separate channels.
6. A PBX is an on-site telephone system that connects organizations to the
public switched telephone network. The central office switch is the precursor
to on-site private branch exchange (PBX) telephone systems. A central office
switch is centrally located and routes calls between users in the public
network. PBXs are private and located within an enterprise.
7. Cables are twisted because this way they cause less radiation and less
interferences. They are less vulnerable to interferences from the outside. CAT
7 Cables are tighter twisted than CAT 3, therefore allow higher speed and
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each wiring pair is shielded individually against the others. CAT 3 is
unshielded.
8. An international packet-switching standard that uses a cell-based approach in
which each packet of information features a uniform cell size of 53 bytes. ATM
is a high-bandwidth, fast packet-switching and multiplexing technique that
allows the seamless end-to-end transmission of voice, data, image, and video
traffic.
9. Code division multiple access (CDMA) is a "spread spectrum" technology.
Each conversation transmitted is spread over multiple frequencies as it is
sent. This is accomplished through the use of unique 40-bit codes assigned to
each telephone transmission.
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KEY TO EVALUATION
PER CENT
MARK
88 – 100
1
75 – 87
2
62 – 74
3
50 – 61
4
0 – 49
5