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Chapter 12

Long-Distance and Local Loop

Digital Connection Technologies

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MotivationMotivation

Connect computers acrossLarge geographic distancePublic right-of-way

StreetsBuildingsRailroads

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Long-Distance Transmission Technologies

Long-Distance Transmission TechnologiesGeneral solution: lease transmission facilities

from telephone (or network) companyPoint-to-point topologyNOT part of conventional telephone systemCopper, fiber microwave, or satellite channels

availableCustomer chooses analog or digital

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Equipment for Leased Connections

Equipment for Leased Connections

Analog CircuitModem required at each end

Digital CircuitDSU / CSU (Data Service Unit/Channel Service

Unit) required at each end

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Digital Circuit TechnologyDigital Circuit Technology

Developed by telephone companiesDesigned for use in voice system

Analog audio from user’s telephone converted to digital format

Digital format sent across networkDigital format converted back to analog audio

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Illustration of Digitized Signal

Illustration of Digitized Signal

Pick nearest digital value for each sampleTelephone standard known as Pulse Code

Modulation (PCM), 8000 samples/s, 8 bits

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DSU / CSUDSU / CSU

Perform two functions; usually a single “box”Needed because telephone industry digital encoding

differs from computer industry digital encodingDSU (Data Service Unit) portion

Translates between two encodingsCSU (Channel Service Unit) portion

Terminates lineAllows for maintenance

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Illustration of DSU / CSUIllustration of DSU / CSUCost of digital

circuit depends on

- Distance- Capacity

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Telephone Standards For Digital Circuits

Telephone Standards For Digital Circuits

Specified by the telephone industry in each country

Differ around the worldAre known by two-character standard nameNote: engineers refer to circuit capacity as

“speed”

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Example Circuit CapacitiesExample Circuit Capacities

A T1 may carry 24 independent voice calls, each at 64 kbps.

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Common Digital Circuit Terminology

Common Digital Circuit Terminology

Most common in North AmericaT1 circuitT3 circuit (28 times T1)

Also availableFractional T1 (e.g., 56 Kbps circuit)TDM used to divide the total bandwidth

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Inverse MultiplexingInverse Multiplexing

Combines two or more circuitsProduces intermediate capacity circuitSpecial hardware required

Needed at each endCalled inverse multiplexor

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Example of Inverse Multiplexing

Example of Inverse Multiplexing

Can alternate between circuits forEvery other bitEvery other byte

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High-Capacity Digital Circuits

High-Capacity Digital Circuits

Also available from phone companyUse optical fiberElectrical standards called Synchronous

Transport Signal (STS)Optical standards called Optical Carrier (OC)

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High-Capacity CircuitsHigh-Capacity Circuits

STS- is standard for electrical signalsOC- is standard for optical signalsEngineers usually use OC- terminology for

everythingOC-3 popular (compare with ATM!)

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SONET/SDHSONET/SDH

Standard for sending digital transmissions on STS-links

DefinesFrame formatMultiplexingSynchronization

Can be used to build ring networks

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Local LoopLocal Loop

Telephone terminologyRefers to connection between residence /

business and central officeCrosses public right-of-wayOriginally for analog POTS (Plain Old

Telephone Service)

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Digital Local Loop Technologies

Digital Local Loop Technologies

Integrated Services Digital Network (ISDN)Handles voice and dataExtends to longer distanceWidely available in Europe

Digital Subscriber Line (DSL)Newer technologyHigher speedSeveral variants exist

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Asymmetric Digital Subscriber Line (ADSL)

Asymmetric Digital Subscriber Line (ADSL)

Popular DSL variantRuns over conventional POTS wiringHigher capacity downstreamUses frequencies above POTS

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ADSL combined with POTSADSL combined with POTS

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Illustration of ADSL WiringIllustration of ADSL Wiring

Downstream can reach 6.4 MbpsUpstream can reach 640 Kbps

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Cable ModemsCable Modems

Send / receive over CATV (Community Antenna Television) wiring

Use FDMGroup of subscribers in neighbourhood

share bandwidth

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Hybrid Fiber Coax (HFC)Hybrid Fiber Coax (HFC)

Wiring scheme for cable to allow digital accessOptical fiber

Highest bandwidthExtends from central office to neighborhood

concentration pointsCoaxial cable

Less bandwidthExtends from neighborhood concentration point to

individual subscribers (e.g., residence)

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SummarySummary

Technologies exist that span long distancesLeased analog lines (require modems)Leased digital circuits (require DSU / CSUs)

Digital circuitsAvailable from phone companyCost depends on distance and capacityPopular capacities called T1 and T3Fractional T1 also available

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Summary (continued)Summary (continued)

High capacity circuits availablePopular capacities known as OC-3, OC-12

Local loop refers to connection between central office and subscriber

Local loop technologies includeDSL (especially ADSL)Cable modems