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11

Computer Networks and Internets, 5e

By Douglas E. Comer

Lecture PowerPointsAdapted from notes

By Lami Kaya, [email protected]

2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.
mailto:[email protected]:[email protected]


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Chapter 12

Accessand

Interconnection Technologies


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Topics Covered

12.1 Introduction

12.2 Internet Access Technology: Upstream and Downstream

12.3 Narrowband and Broadband Access Technologies

12.4 The Local Loop and ISDN

12.5 Digital Subscriber Line (DSL) Technologies

12.6 Local Loop Characteristics and Adaptation

12.7 The Data Rate of ADSL

12.8 ADSL Installation and Splitters

12.9 Cable Modem Technologies

12.10 The Data Rate of Cable Modems

12.11 Cable Modem Installation

12.12 Hybrid Fiber Coax


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Topics Covered

12.13 Access Technologies That Employ Optical Fiber

12.14 Head-End and Tail-End Modem Terminology

12.15 Wireless Access Technologies

12.16 High-Capacity Connections at the Internet Core

12.17 Circuit Termination, DSU/CSU, and NIU

12.18 Telephone Standards for Digital Circuits

12.19 DS Terminology and Data Rates

12.20 Highest Capacity Circuits (STS Standards)

12.21 Optical Carrier Standards

12.22 The C Suffix

12.23 Synchronous Optical NETwork (SONET)


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12.1 Introduction

This chapter

concludes the discussion of data communications by examining two

facilities used in the Internet

discusses access technologies, such as dialup, DSL, and cable

modems

considers high-capacity digital circuits used in the core of the Internet expands the discussion of the telephone system multiplexing

hierarchy

gives examples of circuits that common carriers offer to businesses

and ISPs

focuses on the data communications aspects of the technologies byconsidering multiplexing and data rates


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12.2 Internet Access Technology:Upstream and Downstream

Internet access technologyrefers to a data communications

system that connects an Internet subscriber to an ISP such as a telephone company(DSL)or cable company

How is access technology designed?

Most Internet users follow an asymmetricpattern

a subscriber receives more data from the Internet than sending a browser sends a URL that comprises a few bytes

in response, a web server sends content

Upstreamto refer to data traveling from a subscriber to an

ISP

Downstreamto refer to data traveling from an ISP in the

Internet to a subscriber

Figure 12.1 illustrates the definitions


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12.2 Internet Access Technology:

Upstream and Downstream

7


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12.3 Narrowband and Broadband AccessTechnologies

A variety of technologies are used for Internet access

They can be divided into two broad categories based on the

data rate they provide

Narrowband

Broadband

In networking terms, network bandwidthrefers to data rate

Thus, the terms narrowbandand broadbandreflect industry

practice

12.3.1 Narrowband Technologies

12.3.2 Broadband Technologies


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12.3.1 Narrowband Technologies

refers to technologies that deliver data at up to 128 Kbps

For example, the maximum data rate for dialup noisy phone lines is

56 Kbps and classified as a narrowband technology

Figure 12.2 (below) summarizes the main narrowband access

technologies


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12.3.2 Broadband Technologies

generally refers to technologies that offer high data rates, but the

exact boundary between broadband and narrowband is blurry

many suggest that broadband technologies deliver more than 1 Mbps

but this is not always the case, and may mean any speed higher than dialup

Figure 12.3 (below) summarizes the main broadband access

technologies


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Narrowband vs Broadband


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Internet connectionnarrow or broadband


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ISP Hierarchy

customer

Local loopor Last mile
http://en.wikipedia.org/wiki/Local_loophttp://en.wikipedia.org/wiki/Local_loop


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Local Loop Technologies

Electric local loop(POTSlines): Voice, ISDN, DSL

Optical local loop: Fiber Optics services such as FiOS

Satellite local loop: communications satelliteand cosmos

Internet connections of satellite television(DVB-S)

Cable local loop: Cablemodem

Wireless local loop(WLL): LMDS, WiMAX, GPRS, HSDPA,

DECT

12 4 Th L l L d ISDN
http://en.wikipedia.org/wiki/Local_loophttp://en.wikipedia.org/wiki/Plain_old_telephone_servicehttp://en.wikipedia.org/wiki/FiOShttp://en.wikipedia.org/wiki/Communications_satellitehttp://en.wikipedia.org/wiki/Satellite_televisionhttp://en.wikipedia.org/wiki/Digital_Video_Broadcastinghttp://en.wikipedia.org/wiki/Cablemodemhttp://en.wikipedia.org/wiki/Wireless_local_loophttp://en.wikipedia.org/wiki/LMDShttp://en.wikipedia.org/wiki/WiMAXhttp://en.wikipedia.org/wiki/GPRShttp://en.wikipedia.org/wiki/HSDPAhttp://en.wikipedia.org/wiki/DECThttp://en.wikipedia.org/wiki/DECThttp://en.wikipedia.org/wiki/HSDPAhttp://en.wikipedia.org/wiki/GPRShttp://en.wikipedia.org/wiki/WiMAXhttp://en.wikipedia.org/wiki/LMDShttp://en.wikipedia.org/wiki/Wireless_local_loophttp://en.wikipedia.org/wiki/Cablemodemhttp://en.wikipedia.org/wiki/Digital_Video_Broadcastinghttp://en.wikipedia.org/wiki/Satellite_televisionhttp://en.wikipedia.org/wiki/Communications_satellitehttp://en.wikipedia.org/wiki/FiOShttp://en.wikipedia.org/wiki/Plain_old_telephone_servicehttp://en.wikipedia.org/wiki/Local_loop


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12.4 The Local Loop and ISDN Local loop describes the physical connection between a

telephone company Central Office (CO) and a subscriber

consists of twisted pair and dialup call with4KHz of bandwidth

It often has much higher bandwidth; a subscriber close to a CO may be able

to handle frequencies above 1MHz

Integrated Services Digital Network(ISDN)

ISDN offers three separate digital channels

designated B, B, and D(usually written 2B + D)

The 2Bchannels (each 64Kbps) are intended to carry

digitized voice, data, or compressed video

Both of the Bchannels can be combined or bonded to produce a

single channel with an effective data rate of 128 Kbps

The Dchannel (16Kbps) is used as a control channel

Newer local loop technologies provide higher data rates at

lower cost, relegatingISDN to a few special cases

ISDN has little niche market in US, pretty popular in Japan &Europe


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12.5 Digital Subscriber Line (DSL)

Technologies DSLis one of the main technologies used to provide high-speed data

communication services over a local loop Figure 12.4 (below) lists DSL variants

Because the names differ only in the first word, the set is collectively referredto by the acronym xDSL

Currently, ADSL is most popular


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12.5 Digital Subscriber Line (DSL)

Technologies ADSL is the most widely deployed variant

and the one that most residential customers use

ADSL uses FDM to divide the bandwidth of the local loop

into three regions

one of the regions corresponds to traditional analog phone service,

which is known as Plain Old Telephone Service (POTS) and two regions provide data communication

Figure 12.5 (below) illustrates how ADSL divides bandwidth


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12.6 Local Loop Characteristics and

Adaptation ADSL technology is complex

because no two local loops have identical electrical characteristics

ADSL is adaptive

That is, when a pair of ADSL modems are powered on, they probe

the line between them to find its characteristics

agree to communicate using techniques that are optimal for the line

ADSL uses Discrete Multi Tone modulation (DMT)

that combines frequency division multiplexing andinverse

multiplexingtechniques

FDM in DMT is implemented by dividing the bandwidth into286separate frequencies called sub-channels

255sub-channels allocated for downstreamdata transmission

31allocated for upstreamdata transmission

12 6 L l L Ch i i d


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12.6 Local Loop Characteristics and

Adaptation Two of the upstream channels are reserved for control

information There is a separate modemrunning on each sub-channel,

which has its own modulated carrier Carriers are spaced at 4.1325KHz intervals to keep the signals from

interfering with one another

To guarantee that its transmissions do not interfere withanalog phone signals ADSL avoids using the bandwidth below 26KHz

Two ends assess the signal quality at each frequency

Use the quality to select a modulation scheme If a particular frequency has a highsignal-to-noise ratio

ADSL selects a modulation scheme that encodes many bits per baud

If the quality on a given frequency is low ADSL selects a modulation scheme that encodes fewer bits per baud


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12.7 The Data Rate of ADSL

How fast can ADSL operate? ADSL can achieve

a downstream rate of 8.448Mbps on short local loops

and an upstream rate of 640Kbps

Network control channel requires 64Kbps

The effective upstream rate for user data is 576Kbps

ADSL2can download at close to 20Mbps

Adaptation has an interesting property ADSL does not guarantee a data rate

ADSL can only guarantee to do as well as line conditions allow

Those farther(physical distance) from a CO (or local loop passes near

sources of interference) lower data rates than subscribers who live near

the CO (or a local loop does not pass near sources of interference) thus

the downstream rate varies from 32Kbps to 8.448Mbps

the upstream rate varies from 32 to 640 Kbps


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Analog phones operate at frequencies below4 KHz

lifting a receiver can generate noise that interferes with DSL signals

ADSL uses an FDM device known as a splitter

It divides the bandwidth by passing low frequencies to one output

and high frequencies to another

A splitter is passive; it does not require power A splitter is usually installed at the location where the local loop

enters a residence or business

Figure 12.6 illustrates the connection

A variation of ADSL wiring (DSL-lite) has become popular it does not require a splitter to be installed on the incoming line

a subscriber can install DSL by plugging a splitter into a wall jack

and plugging a telephone into the splitter


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22


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12.9 Cable Modem Technologies

A variety of wireless and wired technologies have been

developed for use in the local loop

An alternative access technology that uses the wiring

already in place for cable television

It is also known as Community Antenna TeleVision (CATV)

It uses FDM to deliver TV signals over coaxial cable

CATV is not available in all countries

Coaxial cable has high bandwidth and is less susceptible to

electromagnetic interference than twisted pair

CATV systems use FDM to deliver many channels

In CATV the bandwidth is insufficient to handle a FDM scheme that

extends a channel to each user

Using a separate channel per subscriber does not scale


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Cablemodem connection
http://images.google.com/imgres?imgurl=http://www.cpsc.gov/cpscpub/prerel/prhtml07/07011a.jpg&imgrefurl=http://www.cpsc.gov/cpscpub/prerel/prhtml07/07011.html&h=600&w=800&sz=64&hl=en&start=4&sig2=5watG_R8i8RuYZiFxgoDzQ&um=1&tbnid=0PwyDwJJ2D45hM:&tbnh=107&tbnw=143&ei=R0ikSPDXCZSypgSb0rTODQ&prev=/images%3Fq%3Dcomputer%2Bimage%26um%3D1%26hl%3Den%26rls%3Dcom.microsoft:en-us:IE-SearchBox%26rlz%3D1I7GGIC%26sa%3DNhttp://images.google.com/imgres?imgurl=http://www.cpsc.gov/cpscpub/prerel/prhtml07/07011a.jpg&imgrefurl=http://www.cpsc.gov/cpscpub/prerel/prhtml07/07011.html&h=600&w=800&sz=64&hl=en&start=4&sig2=5watG_R8i8RuYZiFxgoDzQ&um=1&tbnid=0PwyDwJJ2D45hM:&tbnh=107&tbnw=143&ei=R0ikSPDXCZSypgSb0rTODQ&prev=/images%3Fq%3Dcomputer%2Bimage%26um%3D1%26hl%3Den%26rls%3Dcom.microsoft:en-us:IE-SearchBox%26rlz%3D1I7GGIC%26sa%3DNhttp://images.google.com/imgres?imgurl=http://i14.photobucket.com/albums/a315/ezaviar/OldTv.jpg&imgrefurl=http://www.gadgetreview.com/2007/05&h=380&w=308&sz=46&hl=en&start=7&sig2=8UupIojig_0Ls2wOOImL1A&um=1&tbnid=fCu9uXB273NflM:&tbnh=123&tbnw=100&ei=gUSkSKTTPIWmpASEosS-DQ&prev=/images%3Fq%3Dtv%26um%3D1%26hl%3Den%26rls%3Dcom.microsoft:en-us:IE-SearchBox%26rlz%3D1I7GGIC%26sa%3DNhttp://images.google.com/imgres?imgurl=http://www.ncsind.com/usrimage/sb5101_right_nb_large.gif&imgrefurl=http://www.ncsind.com/cat98_1.htm&h=300&w=245&sz=15&hl=en&start=313&sig2=5FXj0ww-iZakkBsrdpPlsQ&um=1&tbnid=MrfRW2gUS3DBiM:&tbnh=116&tbnw=95&ei=H5SjSNPiHoGaoQSQuvDJDw&prev=/images%3Fq%3Dcablemodem%26start%3D300%26ndsp%3D20%26um%3D1%26hl%3Den%26rlz%3D1T4GGIC_enUS279US280%26sa%3DN


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12.10 The Data Rate of Cable Modems

How fast can a cable modem operate?

In theory, a cable system can support data rates of 52Mbps

downstream and 512Kbps upstream.

In practice, the rate can be much less

The data rate of a cable modem only pertains to

communication between the local cable office and thesubscriber's site

The bandwidth is shared among a set of Nsubscribers (the

size of the set is controlled by the cable provider)

sharing the bandwidth with other subscribers can be a disadvantage because the effective data rate available to each individual subscriber

varies over time

if Nsubscribers share a single frequency

the amount of capacity available to an individual subscriber will be 1/N


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Cable Infrastructure
http://en.wikibooks.org/wiki/Image:Spec.jpghttp://en.wikibooks.org/wiki/Image:Inoc.jpg


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12.11 Cable Modem Installation

Cable modem installation is straightforward

Cable modems attach to the cable wiring directly

The FDM hardware in existing cable boxes and cable

modems guarantees that data and entertainment channels

will not interfere with one another


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12.12 Hybrid Fiber Coax (HFC)

HFC can provide high-speed data communications

a HFC system uses a combination of optical fibers and coaxial cables

fiber used for the central facilities and coax used for connections to

individual subscribers

An HFC system is hierarchical

It uses fiber optics for the portions that require the highest bandwidth and it uses coax for parts that can tolerate lower data rates

Trunkto refer to the high-capacity connections between the

cable office and each neighborhood area

Feeder circuit to refer to the connection to an individualsubscriber

Trunk connections can be up to 15miles long

Feeder circuits are usually less than a mile

12 12 H b id Fib C


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12.12 Hybrid Fiber Coax

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12 13 Access Technologies That Employ


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12.13 Access Technologies That Employ

Optical Fiber There are available a variety of technologies that either

employ optical fiber in a hybrid system or deploy optical fiberall the way to each subscriber

Figure 12.8 summarizes names of key technologies

12 13 Access Technologies That Employ


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12.13 Access Technologies That Employ

Optical Fiber Fiber To The Curb (FTTC)

it uses optical fiber for high capacity trunks

the idea is to run optical fiber close to the end subscriber

and then use copper for the feeder circuits

it uses two media in each feeder circuit to allow the cable system to

provide an additional service, such as voice Fiber To The Building (FTTB)

it will use optical fiber to allow high upstream data rates for businesses

Fiber To The Home (FTTH)

uses optical fiber to deliver higher downstream data rates toresidential subscribers

The emphasis is on many channels of entertainment and video

Fiber To The Premises (FTTP)

A generic term, FTTP, encompasses both FTTB and FTTH

12 14 Head End and Tail End Modem


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12.14 Head-End and Tail-End ModemTerminology

An access technology requires a pair of modems with one at the subscriber's site and one at the provider's site

Head-end modem to refer to a modem used at the CO

Head-end modems are not individual devices

Instead, a large set of modems is built as a unit that can beconfigured, monitored, and controlled together

A set of head-end modems used by a cable provider is known as a

Cable Modem Termination System (CMTS)

Tail-end modem to refer to a modem used at the subscriber

Data Over Cable System Interface Specifications (DOCSIS)

specifies both the format of data that can be sent as well as the

messages that are used to request services (e.g., pay-per-view)


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How to provide access in rural areas?

Imagine a farm or remote village many miles from the nearest city

The twisted pair wiring used to deliver telephone service to such

locations exceeds the maximum distance for technologies like ADSL

Rural areas are least likely to have cable television service

Even in suburban areas, technologies like ADSL may havetechnical restrictions on the type of line they can use

it may be impossible to use high frequencies on telephone lines that

contain loading coils, bridge taps, or repeaters

Local loop technology may not work on all lines

To handle special cases, a variety of wireless access technologies

have been explored

Figure 12.9 lists a few examples

12 15 Wi l A T h l i


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35


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S t llit ti


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Satellite connection

12 16 High Capacity Connections at the


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12.16 High-Capacity Connections at theInternet Core

Access technologies handle the last mile(sometimes called

as First mile)problem

where the last mile is defined as the connection to a typical

residential subscriber or a small business

An access technology provides sufficient capacity for a

residential subscriber or a small business the term Small Office Home Office (SOHO) is used to refer them

Connections to large businesses or connections among

providers require substantially more bandwidth

Corerefers to connections at the backbone of Internet Core technologies refers to high-speed technologies

Figure 12.10 shows the aggregate traffic from the Internet to

the provider



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12.16 High-Capacity Connections at the

Internet Core

39



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12.16 High-Capacity Connections at theInternet Core

What technology can a provider use to move data a long

distance at a rate of 10Gbps? The answer lies in a point-to-pointdigital circuitleased from a

telephone company

High-capacity digital circuits are available for a monthly fee, and can

be used to transfer data

Telephone companies have the authority to install wiring

that crosses municipal streets

A circuit can extend between two buildings, across a city, or

from a location in one city to a location in another

The fee charged depends on the data rate of the circuit and the

distance spanned


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One of International Backbones
http://www.answers.com/topic/internet-backbonehttp://www.answers.com/topic/internet-backbone


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One of International Backbones

12 17 Circuit Termination DSU/CSU
http://www.answers.com/topic/internet-backbonehttp://www.answers.com/topic/internet-backbone


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12.17 Circuit Termination, DSU/CSU,

and NIU To use a leased digital circuit, one must agree to follow the

rules of the telephone system including adhering to the standards that were designed for

transmitting digitized voice

Computer industry and the telephone industry developed

independently Standards for telephone system digital circuits differ from those used

in the computer industry

A special piece of hardware is needed to interface a computer to a

digital circuit provided by a telephone company

Known as a Data Service Unit/Channel Service Unit (DSU/CSU) Device contains two functional parts, usually combined into a single chassis

The CSU portion of the DSU/CSU device handles line termination

and diagnostics


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CSU/DSU for T1 connection


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CSU/DSU for T1 connection

(T1)

12 18 Telephone Standards for Digital


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12.18 Telephone Standards for Digital

Circuits A digital circuit leased from a telco follows the same digital

transmission standards that the telco uses to transportdigital phone calls

In the USA, standards for digital telephone circuits were

given names that consist of the letter Tfollowed by a

number One of the most popular is known as T1

Many small businesses use a T1circuit to carry data

T-standards are not universal

Japan adopted a modified version of the T-series standards Europe chose a slightly different scheme; it uses the letter E

Figure 12.11 lists the data rates of several digital circuit

standards

12 18 Telephone Standards for Digital


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12.18 Telephone Standards for Digital

Circuits

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12 19 DS T i l d D t R t


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12.19 DS Terminology and Data Rates

The data rates of Tstandards have been chosen so they

can each handle multiple voice calls

Note that the capacity of circuits does not increase linearly with their

numbers

For example, the T3standard defines a circuit with much more than

three times the capacity of T1

Telcos may lease circuits with lower capacity than thoselisted in the figure

they are known as fractional T1 circuits

To multiplex multiple phone calls onto a single connection

known as Digital Signal Level standards or DS standards

For example, DS1denotes a service that can multiplex 24

phone calls onto a single circuit

12 20 Highest Capacity Circuits


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12.20 Highest Capacity Circuits

(STS Standards)

Telephone companies use the term trunk to denote a high-capacity circuit, and have created a series of

standards for digital trunk circuits

Synchronous Transport Signal(STS) standards specify the

details of high-speed connections

Figure 12.12 summarizes the data rates associated with

various STS standards

All data rates in the table are given in Mbps, for easy comparison

Note that data rates for STS-24and above are greater than 1Gbps

12 20 Hi h t C it Ci it


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12.20 Highest Capacity Circuits

(STS Standards)

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12 21 O ti l C i St d d


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12.21 Optical Carrier Standards

Telcos define an equivalent set of Optical Carrier (OC)

standards

Figure 12.12 gives the names for optical standards as well

as for copper standards

One should observe a distinction between the STS and OC

terminology: the STS standards refer to the electrical signals used in the digital

circuit interface (i.e., over copper)

the OC standards refer to the optical signals that propagate across

the fiber

12 22 Th C S ffi


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12.22 The C Suffix

The STC and OC terminology described above has one

additional feature not shown in Figure 12.12 an optional suffix of the letter C, which stands for concatenated

The suffix denotes a circuit with no inverse multiplexing

an OC-3 circuit can consist of three OC-1 circuits operating at 51.840

Mbps each

or it can consist of a single OC-3C (STS-3C) circuit that operates at

155.520Mbps

Is a single circuit operating at full speed better than multiple

circuits operating at lower rates?

The answer depends on how the circuit is being used

In general, having a single circuit operating at full capacity

provides more flexibility

and eliminates the need for inverse multiplexing equipment

12.23 Synchronous Optical NETwork


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(SONET) Telcos defined a broad set of standards for digital

transmission In North America, the standards are known by the term Synchronous

Optical NETwork (SONET)

In Europe they are known as the Synchronous Digital Hierarchy (SDH)

SONET specifies some details, such as how data is framed

how lower-capacity circuits are multiplexed into a high-capacity circuit

how synchronous clock information is sent along with data

Figure 12.13 shows the SONET frame format used on anSTS-1 circuit

12 23 Synchronous Optical NETwork


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(SONET)

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SONET
http://en.wikipedia.org/wiki/SONEThttp://en.wikipedia.org/wiki/SONET


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SONET
http://en.wikipedia.org/wiki/SONEThttp://images.google.com/imgres?imgurl=http://upload.wikimedia.org/wikipedia/commons/thumb/6/6b/Add-drop-multiplexer-SONET-SDH.jpg/450px-Add-drop-multiplexer-SONET-SDH.jpg&imgrefurl=http://commons.wikimedia.org/wiki/Image:Add-drop-multiplexer-SONET-SDH.jpg&h=600&w=450&sz=58&hl=en&start=4&sig2=bYvebrWH8aZeQVZcsAN4pA&um=1&tbnid=amaOZj9T23oBKM:&tbnh=135&tbnw=101&ei=xGikSJ77GYSuoQTBhuTHDQ&prev=/images%3Fq%3Dsonet%26um%3D1%26hl%3Den%26rls%3Dcom.microsoft:en-us:IE-SearchBox%26rlz%3D1I7GGIC%26sa%3DNhttp://en.wikipedia.org/wiki/SONET

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