Lecture 10, 11 & 12

Telephone Network

Telephone Network

TELEPHONE NETWORKTELEPHONE NETWORK

The telephone network had its beginnings in the late 1800s. It was originally designed for voice communication only. The entire network was originally an analog system using analog signals to transmit voice. With the advent of the computer era, the network, in the 1980s, began to carry data in addition to voice. During the last decade, the telephone network has undergone many technical changes. The network is now digital as well as analog.

14.4

Major ComponentsMajor Components

The telephone network, as shown in next Figure, is made of three major components: 1.local loops, 2.trunks, and 3.switching offices.

The telephone network has several levels of switching offices such as end offices, tandem offices, and regional offices.

Figure : A telephone system

Local loops• Local loop is a twisted pair cable that is used for connection for

connecting a subscribers telephone to the nearest end office.• The local loop has a bandwidth of 4 kHz if it is used for voice

communication.• In an 8- digit local telephone number the first four digits define

the office and the next four digits define the local loop number.

Trunks• Trunks are the transmission media that handle the

communication between offices.• The signals originating from many end offices are multiplexed

to form a common signal which is then transmitted over the trunks.

• The trunks thus handle hundreds or thousands of connections simultaneously. The transmission media used as trunks is usually optical fiber cables or satellites links.

Switching offices • Connecting every subscriber permanently with all the others

subscriber using wires will be impossible things.• Hence the telephone companies use switching located at the

switching offices to establish connection as and when required, between the calling and the called subscriber.

• Earlier the electromechanical rotary switches but being used but now in electronic exchanges, the electronic switches are used.

14.8

Local Access Transport Areas(LATAs)LATAs)

The LATA is an American concept. The United States was divided into more than 200 local-access transport areas (LATAs). The number of LATAs has increased since then. A LATA can be a small or large metropolitan area. A small state may have a single LATA; a large state may have several LATAs. A LATA boundary may overlap the boundary of a state; part of a LATA can be in one state, part in another state.

14.9

Figure : Switching offices in a LATA

• Types of LATA1. Intra LATA2. Inter LATA

Intra-LATA services

These services are provided by local exchange carriers. Since 1996, there are two types of LECs:

1.incumbent local exchange carriers (ILEC)

2.competitive local exchange carriers (CLEC)

Inter LATA Services The services between LATAs are Handled by interchange carriers (IXCs). These carriers are also called as long distances companies that provide communication services between two customers in deferent LATAs.

communications in circuit switching networks•Communication via circuit switching takes place over three phases of operation as follows:

• Circuit establishment • Data transfer• Circuit disconnect

1. Circuit establishmentIn a circuit switching network, before any signal is transmitted, it is necessary to establish as end-to-end link.The node to node links are usually multiplexed. they either use FDM or TDM

2. Data transferThe data can be analog or digital depending on the nature of the networkGenerally all the internal connection are duplex.

3. Circuit DisconnectAfter some time the connection between two user transmitted usually by the action of one or two stations.Circuit switching is insufficient in most of the application.

•The entire channel capacity id dedicated for the duration of connection, even if the data is not being transferred.

•Once the circuit is established, the network is effectively transparent to the user with no delays involved.

PSTNPSTN (public switched telephone network) is the world's collection of interconnected voice-oriented public telephone networks, both commercial and government-owned. It's also referred to as the Plain Old Telephone Service (POTS). A public telecommunication network can be described using four generic architectural components.1.Subscribers2.Subscribers line3.Exchanges4.Trunks

Refer fig to get the idea about these components.

FIG OF PSTN

Subscribers • These are the services that attach to the

network. Example is telephone, PCs etcSubscriber line• The line between the subscriber and the network

is called as subscriber line.• It is also called as subscriber loop or local loop.• All the local loop connection are made are using

the twisted wires.• The length of local loop cab be in the range of

few kilometers or few hundred kilometers.

Exchanges • The switching centers in the networks are called

as exchanges.• End office is a switching center which directly

supports the subscriber.• Typically as end office can support a few

thousand subscribers.Trunks• Trunks are the branches between the exchanges.• The signal travelling on them are either FDM or

TDM.

ROUTING IN CIRCUIT SWITCHING• The modern telephone network is developed in order to provide the basic telephone services.• The basic telephone service provides the two way real time transmission of voice signals.• The bandwidth of each voice signal is 4 kHz.• The telephone network operates on the principle of circuit switching. The block diagram of the

basic telephone network .• Following are the three phase of connection oriented communication.• When the user pickup his telephone, it activate a switch in the local telephone office of call

request.• A dial tone is given to the calling party and the dialed number of the called subscriber is

recognized.• The source switch uses the telephone signaling network to find the rout and allocate resource

to the network of destination office.• The destination office then alerts the called party about the incoming call by ringing its phone.• This ring is sent back to the calling party . the conversation can begin when the called party

lifts the telephone.

Making A Connection

The subscriber telephones are connected through the local loops, to end office or central office.•The switching station at the end office is accessed through dialing.•Two types of dialing are practically used.

1. Rotary or pulse dialing. 2. Touch tone dialing (DTMF dialing)

Pulsed Dialing:• As shown in fig (A) the telephone instrument has a routing dial.

Corresponding to each digit dialed a group of pulses is generated and set to the exchange. E.g. if digit “3” is dialed then a group of 3 pulses is produced.

• Two pulses are produced for digit 2 and so on. Ten pulse are produced when a “0” is dialed .

• The total of each pulse i.e. P remains always constant. The make to break ratio M:B is 33% 66% as shown in fig.

• An inter digit pulse is automatically generated in order to give sufficient time to the exchange switching circuits.

• The time taken to send a digit will not be constant. “1” will take shortest time (only 1 pulse) whereas a “0” will take longest time (10 pulse).

DTMF dialing [Dual Tone Multi-Frequency Dialing] •A rotary dial is very slow in dialing e.g. it takes about 12 seconds to dial a 7 digit numbers.•To increase the speed of dialing the so called “touch tone” push button key pad is used in the modern telephone instruments.•In this scheme a key pad of 12 push buttons arranged in four rows with three keys per row is used.•As we touch a push button on the key pad a voice frequency “tone” is produced.•This tone is unique for each push button. It is generated by the combination of two frequencies. One from the lower band and the other from the upper band.•Therefore the dialing is also known as the dual tone multi frequency dialing. (DTMF). This is as shown in fig.

• If the digit “8” is pressed the tone that is produced and sent to the exchange is a combination of 852 Hz from the lower band and 1336 Hz from the upper band.

• Special frequency selective filters are used in the telephone exchange to pass only the frequencies for DTMF.

• DTMF is much faster than the pulsed dialing. The time required to recognize any digit tone is only 50 m sec.

• The inter digit interval is of 50 m sec. which makes the total tome required to send any digit to be 100 m sec. this time does not depend on which digit is dialed. It is same for all digits.

Comparison of Pulse and DTMF Dialing

S. no

Pulse Dialing DTMF Dialing

1 A train of pulse is produced to represent the dialled digit. The number of pulses represent the dialed digit

A unique tone is produced by combination of two frequencies. The tones for the various digits are different each other.

2 Low speed dialing It is a high speed dialing

3 Mechanically rotating dial is used. Key pad is used

4 Time taken to send different digits is different from each other

Same tome is required for sending any digit.

Types of services Given By Telephone Networks

• Analog services– Analog switched services– Analog leased services

• Digital Services

Analog switched services

This is the normal telephone service. The signal on a local loop is analog and it has the bandwidth between 0 and 4 kHz.•The call between any two subscribers is established via a series of switches or exchange.•Some of the commonly provided services are as follows:1.Local call services:•It is the service normally provided for a flat monthly rate

Analog Switched Service

Analog Leased Service

2. Toll call services:• A toll call can be intra-local office or inter local office.• Depending on the geographical area, the call may have

to be routed via a toll office.• The subscriber is supposed to pay the fee for call.3. 800 services :• This service is used for providing free calls. The call is

free for the caller but it is paid by the called party.• But the rates are less expensive than the normal long

distance call

4. WATs: • WAT is a wide area telephone service. it is opposite to the 800

services.• For this service the outgoing calls are paid by the subscriber. The

service is less expensive and the charges. Are based on number of calls. • This service is applicable to calls inside a state or even outside the

state.

5. 900 services:• The 900 services are like 800 services. Here the call is paid by the caller

and it is more expensive than the normal long distance call.

14.34

Dial-Up ServiceDial-Up Service

Traditional telephone lines can carry frequencies between 300 and 3300 Hz, giving them a bandwidth of 3000 Hz.

A dial-up service uses a modem to send data through telephone lines.

The term modem is a composite word that refers to the two functional entities that make up the device: a signal modulator and a signal demodulator.

Figure : Telephone line bandwidth

Figure : Modulation/demodulation

Figure : Dial-up network to provide Internet access

Analog Hierarchy

Digital services: • Digital services are being offered in the recent

times.• These are less sensitive to noise than the

analog services. So less interference is observed.

• Two commonly used digital services are:1. Switched/56 services 2. Digital data services (DDS).3. DS

Switched/56 services• It allows data rates to go upto 56kbps.• A subscriber using normal telephone line cannot connect to a telephone or

computer using switched/56 even if he is using a modem.• The line in switched/56 services is already digital so that user doesn’t have

to use modems.• But they need to use another device called Digital Services Unit (DSU)• The DSU changes the rate of the data created by the subscriber to 56 kbps.• This service supports bandwidth on demand.• This service can support the video conferencing, multimedia, fast facsimile

and fast data transfer.

Switched/56 Service

Digital Data Service (DDS)• DDS is the digital counterpart of an analog leased

line.• It is a digital leased line with data rates of 64

kbps.

DDS

3. DS Hierarchy

T-1 Line

SignalingSignaling

The telephone network, at its beginning, used a circuit-switched network with dedicated links to transfer voice communication. The operator connected the two parties by using a wire with two plugs inserted into the corresponding two jacks. Later, the signaling system became automatic. Rotary telephones were invented that sent a digital signal defining each digit in a multi-digit telephone number. As telephone networks evolved into a complex network, the functionality of the signaling system increased. The signaling system was required to perform other tasks.

Control Signaling• A circuit switching network uses the control signals

to manage the following function.1. To establish calls.2. To maintain calls.3. To terminate calls.

• For large public telecommunication network a relatively complex control signaling scheme is required to be used.

signaling function:Some of most important signaling function are as follows : • To provide dial tone, ring tone and busy tone and busy tone to the

subscribers as and when required.• To transmit the number dialed by the subscriber to the switching

offices which attempt to complete the connection.• To transmit information between switches to indicate that a call

cannot be completed.• To transmit information between switches to indicate that the call

has ended.• To send a signal to make a telephone ring (at the called subscriber)

Control Signal•The network management control signals are used for controlling the overall routing selection process such as changing the previously planned routes, or for modifying the operating characteristics of the network in the event of overloading or failure condition.•Signaling can also be classified functionally as follows.

Supervisory function• These functions generally have a binary character (true /

false or on/ off).• These example of supervisory functions are request for

service, answer alerting and return to idle.• These signals are used to check if the desired resources is

available or not and also to communicate the status of the requested resources.

Address Signals• These are used for identifying a subscriber.• An address signal is generated by a calling party when he

dials a telephone number.• This address propagates through the network to help the

routing function and to locate the called subscriber.

Call information• it includes those signals which provide

information about the status of the call to the subscriber.

• These are audible tones.

Network management signals• These are used for maintenance, trouble

shooting and overall operation of the network.

• These signals can be in the form of message.

Common channel signaling• Traditionally the control signaling in a circuit

switching network is the in channel signaling.• The same channel which carries the voice

information (call) is used to carry the control signals as well.

• The advantage of in channel signaling is that no additional transmission facilities are needed for signaling.

Types of inchannel signaling• There are two types of inchannel signaling as

follows.1. Inband signaling2. Out of band signaling

Inband signaling• Inband signaling not only use the same physical

path as the voice but it also uses the same frequency band as the voice signal.

• This type of signaling has certain advantages. One of them is that due to some electromagnetic properties as those of the voice, the control signals can go everywhere the voice signal can go.

• Another advantage is that if the path is faulty then it is not possible to establish a call because the control signals which set up the call themselves cannot travel on the faulty path.

Out of band Signaling• The voice signal occupies the frequency band from 300Hz to 3400 Hz as shown

in fig. but a standard band of 0 to 4000 Hz has been allotted to the voice signal.• Hence the band from 3400 to 4000 Hz remains un-utilized.• The out of band signaling uses this unutilized narrow band of frequencies for

accommodating the control signals as shown in fig.• The advantage of out of band signaling is that control signals can be sent with

or without the presence of voice signals on the line.• This allows a continuous supervision and control of a call.• But the disadvantage of out of band signaling is that additional circuitry is

required to handle the signaling band plus the signaling rate are slower because of the narrow bandwidth used for the signaling purpose.

Figure : Data transfer and signaling network

• To transmit information used for the billing purpose.• To transmit information giving the status of equipment

or trunks in the network. This information can then used for the purpose routing and maintenance.

• To transmit information which is useful for the diagnosis of faults and for the isolation in case of system failure?

• To control special equipments such as the satellite channel equipment.

14.59

Digital Subscriber Line (DSL)Digital Subscriber Line (DSL)

After traditional modems reached their peak data rate, telephone companies developed another technology, DSL, to provide higher-speed access to the Internet. Digital subscriber line (DSL) technology is one of the most promising for supporting high-speed digital communication over the existing telephone. DSL technology is a set of technologies, each differing in the first letter (ADSL, VDSL, HDSL, and SDSL).

Digital Subscriber Lines (DSL)• Earlier we have seen the use of MODEM to produce digital

signals, so as to facilitate the internal access through telephone.

• But the traditional MODEMS cannot provide very high data rates.

• This disadvantage can be overcome with the help of following technologies.

1. The digital subscriber line (DSL)technology2. Cable modem3. SONET

DSL Technology• DSL stands for digital subscriber line and it has

been developed for providing a high speed access to the internet.

• It support the high speed digital communication over the existing local loops.

• The DSL technology is classified as follow:• Thus DSL technology is actually a set of

technologies which is referred to as x DSL where x can be A,V,H or S.

ADSL• Short for Asymmetric Digital Subscriber Line.• ADSL is a type of DSL Broadband communications technology used for connecting to the

Internet. • ADSL allows more data to be sent over existing copper telephone lines (POTS), when

compared to traditional modem lines. • A special filter, called a microfilter, is installed on a subscriber's telephone line to allow

both ADSL and regular voice (telephone) services to be used at the same time. • ADSL requires a special ADSL modem and subscribers must be in close geographical

locations to the provider's central office to receive ADSL service. • Typically this distance is within a radius of 2 to 2.5 miles. • ADSL supports data rates of from 1.5 to 9 Mbps when receiving data (known as the

downstream rate) and from 16 to 640 Kbps when sending data (known as the upstream rate).

ADSL• Similar to 56k modem, it provides higher bit rate in the

downstream direction (from internet to the user) as compared to the bit rate of in the upstream direction(from user to internet) this is why it is called as asymmetrical.

• ADSL divides the bandwidth of a twisted pair cable of 1 MHz into 3 bands as shown in figure.

• The first band is between 0 and 25 Khz. It is used for regular telephone(plain old telephone sets-POTS)

• This service uses only 4 kHz of this band the rest is used as guard band to separate the voice channel from the data channels.

• The second band is from 50kHz to 200 kHz.it is used for upstream communication.

• The third band is from 250Khz to 1 MHz It is used for downstream communication.

• ADSL technology is designed for the residential users. it is not suitable for business applications.

Bands for ADSL

POTS

Upstream Downstream

0 25 50 200 250 1000

Frequency (kHz)

Use of Existing Local Loop• The ADSL uses the existing local loop and still it can support very

high data rates. Let us see how.• Actually the twisted pair local telephone loop can support the

bandwidth upto 1.1 MHz. The bandwidth reduces to 4 kHz due to the filters installed at the end of line.

• If this filter is removed then the bandwidth can again become 1.1 MHz.

• Hence the existing local loop with this modification can have a bandwidth of 1.1 MHz and ADSL can make use of the existing loop.

Adaptive Technology• The bandwidth of 1.1 MHz is not always available practically. It

will depend on various factors such as distance ,cable size,type of signaling etc.

• So ADSL has been designed to operate as an adaptive technology. That means depending on the condition and actually available bandwidth, the data rate of ADSL is adjusted.

• Thus ADSL is an adaptive technology. The data rate of the system is adjusted based on the conditions of the local loop.

Modulation techniques:• Two modulation techniques can be used for

ADSL:1. Carrierless Amplitude/Phase(CAP)2. Discrete Multitone Technique(DMT)

• CAP is modulation technique that is similar to QAM but with one important difference i.e. the carrier signal is eliminated.

Discrete Multitone Techniques (DMT)• DMT is the standard modulation technique for ADSL.; it combines QAM and

FDM.• Typically the available bandwidth of 1.1MHz is divided into 256 channels.

However this is not a rule. The division may change system to system.• Figure shows the block schematic of the DMT system.• Channel 0 is reserved for the voice communication.• Channel 1 to 5 are called as idle channels since they are not used. These

channels create a gap between the voice and data channels.• Channels 6 to 30 (25 channels) are used for the upstream data transfer and

control. These 24 channels are for data transfer and one channel for control.• The downstream bit rate is 224 channels * 4 kHz * 15=13.4Mbps.

Figure Bandwidth division for DMT

• For 24 channels 31 to 255(255 channels) are allotted for downstream data transfer and control. Out of 255 channels, one is reserved for control and remaining 224 are reserved for data.

Actual Bit Rate:• The actual bit rates are normally as follows:• Upstream: 64kbps to 1 mbps• Downstream: 500 kbps to 8 mbpsADSL MODEM• The ADSL modem installed at the customer’s site is shown in figure.• The local loop is connected to the filter which separates voice and

data from each other.• The voice goes to the telephone instrument.• The data goes to ADSL modem which modulates it using DMT and creates the

upstream and downstream channels.

Figure: ADSL modem

Figure : ASDL point-to-point network

(DSL Access Multiplexer)

xDSL(other DSL technologies)• ADSL is one of a number of recent schemes

for providing the high speed digital transmission of the subscribe line.

• Some of the most important new schemes in the field of digital transmission are collectively called as xDSL

• The are as follows:1.HDSL 2. SDSL 3. VDSL

HDSL• High-bit-rate digital subscriber line (HDSL) is a telecommunications

protocol standardized in 1994. • It was the first digital subscriber line (DSL) technology to use a higher

frequency spectrum of copper, twisted pair cables.• HDSL (High bit-rate Digital Subscriber Line), one of the earliest forms of

DSL, is used for wideband digital transmission within a corporate site and between the telephone company and a customer.

• The main characteristic of HDSL is that it is symmetrical: an equal amount of bandwidth is available in both directions. HDSL can carry as much on a single wire of twisted-pair cable as can be carried on a T1 line (up to 1.544 Mbps) in North America or an E1 line (up to 2.048 Mbps) in Europe over a somewhat longer range and is considered an alternative to a T1 or E1 connection.

HDSL• High-bit-rate digital subscriber line (HDSL) is a telecommunications

protocol standardized in 1994. It was the first digital subscriber line (DSL) technology to use a higher frequency spectrum of copper, twisted pair cables.

• It is an alternative to the PCM TDM system i.e. T-1 line(1.544Mbps). The T-1 line uses alternate mark inversion(AMI) encoding which is affected by high frequency attenuation.

• This limits the length of T-1 line to 1-km. The HDSL uses 2BIQ encoding method which is less susceptible to attenuation. A data rate of 2 Mbps can be achieved upto a length of 3.6 km.

• The HDSL is a form of DSL providing T-1/E-1 connections over two or three twisted pair copper lines.

• HDSL is not a typical consumer service, it is mostly used to replace traditional T-1/E-1 technology.

• Connections such as connecting PBXes to telephone company (Telco) offices.

• It allows DS-1 signals to be transported over distance of upto 12,000 feet (Approx 3600 mtrs) on copper twisted pair without repeaters.

• It uses full duplex transmission, using echo cancellation with each pair carrying 784 kb/s the lower bit rate allows for a lower frequency range of operation that reduces channel loss and near-end crosstalk.

• HDSL-based hardware can be installed in central office unit as shown in figure. it is a standalone transport system located near DS-1 cross connected frame, in the outside plant on at the customer premises.

• By using the existing copper in the local loop, HDSL technology helps telephone companies for quick and easy installation, provide an intelligent network interface, which reduces maintenance, and deliver revenue-generating high capacity service over copper wires as they fiberize the local loop and as they convert to SONET architecture.

Comparison of T1 and HDSL technology:

SNo T-1 technology HDSL technology1 Line must be pre-conditioned Allows bridge taps and

cable gauge changes.2 Requires T-1 line repeaters. No repeaters for cable

length less than 12,000 feet.

3 Simplex transmission. Full duplex transmission.4 Uses AMI line code Uses 2BIQ line code.5 Affected by high frequency

attenuation.Less susceptible to high frequency attenuation

6 Sensitive to loop swapping and polarity reversal.

Transparent to loop swapping and polarity reversal.

7 Requires more bandwidth Requires less bandwidth.

SDSL• Short for symmetric digital subscriber line, a technology that allows more

data to be sent over existing copper telephone lines (POTS). • SDSL supports data rates up to 3 Mbps. SDSL works by sending digital pulses

in the high-frequency area of telephone wires and can not operate simultaneously with voice connections over the same wires.

• SDSL requires a special SDSL modem. SDSL is called symmetric because it supports the same data rates for upstream and downstream traffic. A similar technology that supports different data rates for upstream and downstream data is called Asymmetric digital subscriber line (ADSL). ADSL is more popular in North America, whereas SDSL is being developed primarily in Europe.

SDSL• It is same as HDSL but uses a single twisted pair cable. In this method echo

cancellation technique is used to achieve the full duplex mode of transmission.

• This technology provides the same bandwidth in both directions, i.e. upsteam and downstream. The same high quality performance is present in the upstream and downstream.

• SDSL provides transmission speeds within a T-1/E-1 range of up to 1.5Mbps at maximum range of 12,000 to 18,000 feet from a central telephone office.

• SDSL is generally used by small and medium sized businesses that have an equal need to download and upload data over the internet. It is also well suited for large file transfers towards and away from users for web site hosting services.

VDSL• Very-high-bit-rate digital subscriber line (VDSL or VHDSL) is a

digital subscriber line (DSL) technology providing data transmission faster than asymmetric digital subscriber line (ADSL) over a single flat untwisted or twisted pair of copper wires (up to 52 Mbit/s downstream and 16 Mbit/s upstream), and on coaxial cable (up to 85 Mbit/s down- and upstream)using the frequency band from 25 kHz to 12 MHz. These rates mean that VDSL is capable of supporting applications such as high-definition television, as well as telephone services (voice over IP) and general Internet access, over a single connection. VDSL is deployed over existing wiring used for analog telephone service and lower-speed DSL connections. This standard was approved by ITU in November 2001.

VDSL• It is similar to ADSL but uses co-axial, fiber optic or twisted pair

cable for short distance of 300 to 1800 meters. • It uses the discrete multitone modulation technique with a bit

rate of 50 to 55 Mbps downstream and 7.5 and 2.5 Mbps upstream.

• The basic features of a VDSL system are:1. Flexible scalability for symmetric and asymmetric transmission.2. Single duplex transmission3. Field programmable bit rates depending on noise environment and

cable length4. Parallel operation of plain old telephone service (POTS) and / or ISDN.

• To reach a large number of customers (subscribers) with the existing copper wire twisted pair distribution network the VDSL system should use only one twisted pair and coexist with POTS and ISDN.

• To take care of the coexistence with POTS and ISDNN the VDSL spectrum should meet the frequency plan as shown in fig 12.10.2.

• Besides this, depending on the means for separating downstream and upstream transmission, any possible configuration of symmetric and asymmetric system should not interface with one another.

SNo Parameter ADSL HDSL SDSL VDSL

1 Mode Asymmetric symmetric symmetric asymmetric

2 Signaling Analog digital digital analog3 Frequency 1 to 5 MHz 196kHz 196kHz 10MHz

4 Copper pairs 1 2 1 15 Bits/second 1.5 to 9 Mbps

downstream 16 to 640 kbps upstream

1.544 or 2.048 Mbps

1.544 or 2.048 Mbps

13 to 52Mbps down stream 1.5 to 2.3Mbps upstream.

6 Bits/cycle variable 4 4 variable

Comparison of xDSL Schemes

Comparing DSL TypesThere are several variations on DSL technology. In fact, there are so many that you will often see the term xDSL, where x is a variable, when the discussion is about DSL in general.Asymmetric DSL (ADSL) - It is called "asymmetric" because the download speed is greater than the upload speed. ADSL works this way because most Internet users look at, or download, much more information than they send, or upload.High bit-rate DSL (HDSL) - Providing transfer rates comparable to a T1 line (about 1.5 Mbps), HDSL receives and sends data at the same speed, but it requires two lines that are separate from your normal phone line.ISDN DSL (ISDL) - Geared primarily toward existing users ofIntegrated Services Digital Network (ISDN), ISDL is slower than most other forms of DSL, operating at fixed rate of 144 Kbps in both directions. The advantage for ISDN customers is that they can use their existing equipment, but the actual speed gain is typically only 16 Kbps (ISDN runs at 128 Kbps).Multirate Symmetric DSL (MSDSL) - This is Symmetric DSL that is capable of more than one transfer rate. The transfer rate is set by the service provider, typically based on the service (price) level.

Rate Adaptive DSL (RADSL) - This is a popular variation of ADSL that allows the modem to adjust the speed of the connection depending on the length and quality of the line.Symmetric DSL (SDSL) - Like HDSL, this version receives and sends data at the same speed. While SDSL also requires a separate line from your phone, it uses only a single line instead of the two used by HDSL.Very high bit-rate DSL (VDSL) - An extremely fast connection, VDSL is asymmetric, but only works over a short distance using standard copper phone wiring.Voice-over DSL (VoDSL) - A type of IP telephony, VoDSL allows multiple phone lines to be combined into a single phone line that also includes data-transmission capabilities.The chart below provides a comparison of the various DSL technologies:As you can see, VDSL provides a significant performance boost over any other version. But for VDSL to become widely available, it must be standardized.

Cable Modem• Now a days, the cable companies have to come forward to

provide high speed access to the internet• Due to DSL technology, it has become possible to provide

the high data rate connection for the residential user on the local loop

• But DSL technology uses the existing unshielded twisted pair cable which does not perform well in the presence of external interference or noise.

• So the data rate on such cables cannot be increased beyond a certain limit.

• A solution to this problem is to use cable TV network.

14.97

CABLE NETWORKCABLE NETWORK

The cable networks became popular with people who just wanted a better signal. In addition, cable networks enabled access to remote broadcasting stations via microwave connections. Cable TV also found a good market in Internet access provision, using some ofthe channels originally designed for video.

14.98

Traditional Cable NetworksTraditional Cable Networks

Cable TV started to distribute broadcast video signals to locations with poor or no reception in the late 1940s. It was called community antenna television (CATV) because an antenna at the top of a tall hill or building received the signals from the TVstations and distributed them, via coaxial cables, to the community. Figure shows a schematic diagram of a traditional cable TV network.

Figure Traditional cable TV network

• The receiving antenna is generally set up on the top of a tall building.• It receives the video signals from the broadcasting stations and supplies

them to the cable TV office called the head end.• The head end processes these signals and feeds them into coaxial cable.• Amplifiers are inserted at regular intervals in order to amplify the signal

passing from the head end to the user’s house.• There could be unto 35 amplifiers between the head end the user’s

premises.• The splitters are used to split the signal from one to many paths. Then

the taps and the drop cables make the connection to the users residences.

Disadvantages of traditional cable system:• A large number of amplifiers are required• The bandwidth of the coaxial cable is limited.• Due to the use of amplifiers the system

becomes unidirectional. Signal can travel only from head end to the user’s residence.

• The biggest disadvantage of the traditional cable TV network is that this network is unidirectional.

HFC network• The improve version of cable network is called as hybrid fiber-coaxial

(HFC) network.• This is the second generation of the cable network and it uses a

combination of fiber optic and coaxial cable for the transmission of signal.

• Figure shows the block schematic of the HFC network.• The antenna placed on a tall building receives the broadcast signals

and passes them on to a block known as distribution hub.• The transmission medium from the antenna to the box called fiber

node is the optical cable as shown in figure

• However from the fiber node to user residence the coaxial cables are being used.

• the regional cable head (RCH) is generally capable of serving upto 400,00users

• the RCH feeds into the distribution hubs each one of which can serve upto 40,000 users.

• The distribution hub carries out two important operations, namely modulation and distribution of signals.

• These signal are then fed to the fiber nodes through the optical fiber cables as shown in figure.

• The fiber node splits the analog signals so as to send the same signal to each coaxial cable.

• Hence the number of amplifiers are required to be used is reduced as compared to the conventional cable network.

• Typically unto eight amplifiers are required to be used, as compared to 35 in the conventional cable network

• The HFC cable TV network is a bidirectional network. This is its biggest advantage.

Bandwidth of HFC network:

• Since the last part of HFC network uses coaxial cable,the bandwidth of this network is seceded by the bandwidth of the coaxial cable.

• So the bandwidth range approximately from 5MHz to 750MHz as shown in figure.

• This bandwidth is divided into three bands as shown in table given.

• Table shows HFC network bands

Band name Frequency range

Upstream data 5MHz-42MHz

Video band 54MHz-550MHz

Downstream data 550mhZ-750MHz

Figure HFC network

Figure Coaxial cable bands

Video band• This band extends from 54MHZ to 550 MHz.it basically

carries all the TV channels.• If each TV channel needs a bandwidth of 6 MHz then the

video band can accommodate about 83 TV channels.Downstream Data Band• Downstream data travels from the internet to the

subscriber as shown in figure, the dawn stream data band extends from 550MHZ to 750 MHz

• This band is also divided into 6MHz channels.

Modulation technique• The modulation technique used for downstream data band

is either 64-QAM or 256QAM.Data Rate• With 64-qam,we can receive the downstream data at the

rate upto 30Mbps.but since the cable modem is connected to the computer through a 10 base T cable the data rate is limited only to 10 Mbps.

• So theoretical downstream data rate is 30 Mbps but practically it is restricted to 10Mbps.

UPSTREAM DATA BAND:• The upstream data travel from the subscriber

to the internet. As shown in figure, the upstream band extends from 5MHz to 42 MHz.

• This band is also divided into 6MHz channels • Modulation technique used is QPSK• Theoretically the upstream data rate is 12 Mbps

but practically this rate cannot be achieved.

Cable network for data transmission• in order to utilize the cable network for data transmission we

have to use two device namely CM(cable modem) and CMTS(cable modem transmission system).

Cable Modem(CM)• Given figure shows the block schematic of cable modem it is

similar to ADSL modem and it is installed at the user premises.• The filter separates out video and data in the incoming signal

from the cable.• The video signal is applied to TV whereas the data signal is

applied to the computer via the Cable Modem.

Figure Cable modem

Cable Modem Transmission System(CMTS)• The cable companies install the CMTS inside the distribution hub

as shown in figure.• the data from internet is passed through the CMTS to the

combines the video signal from the haed and also is applied to the combiner.

• The combiner output contains video as well as data from the internet. this signal is put on the optical fiber for routing it to the subscribers.

• The data from the subscribers coming via the optical fiber4 is applied to the CMTS, and the CMTS passes it to the internet.

Figure CMTS

Extra Information

BRIEF OF CIRCUIT SWITCHING

Echo in Transmission• In computer telecommunications, echo is the display or return of

sent data at or to the sending end of a transmission. Echo can be either local echo, where the sending device itself displays the sent data, or remote echo, where the receiving device returns the sent data that it receives to the sender (which is of course simply no local echo from the point of view of the sending device itself). That latter, when used as a form of error detection to determine that data received at the remote end of a communications line are the same as data sent, is also known as echoplex, echo check, or loop check. When two modems are communicating in echoplex mode, for example, the remote modem echoes whatever it receives from the local modem

SONET• Synchronous Optical Networking (SONET) and Synchronous

Digital Hierarchy (SDH) are standardized protocols that transfer multiple digital bit streams synchronously over optical fiber using lasers or highly coherent light from light-emitting diodes (LEDs). At low transmission rates data can also be transferred via an electrical interface. The method was developed to replace the Plesiochronous Digital Hierarchy (PDH) system for transporting large amounts of telephone calls and data traffic over the same fiber without synchronization problems.