1
Chapter 12
Long-Distance and Local Loop
Digital Connection Technologies
2
MotivationMotivation
Connect computers acrossLarge geographic distancePublic right-of-way
StreetsBuildingsRailroads
3
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
4
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
5
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
6
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
7
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
8
Illustration of DSU / CSUIllustration of DSU / CSUCost of digital
circuit depends on
- Distance- Capacity
9
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”
10
Example Circuit CapacitiesExample Circuit Capacities
A T1 may carry 24 independent voice calls, each at 64 kbps.
11
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
12
Inverse MultiplexingInverse Multiplexing
Combines two or more circuitsProduces intermediate capacity circuitSpecial hardware required
Needed at each endCalled inverse multiplexor
13
Example of Inverse Multiplexing
Example of Inverse Multiplexing
Can alternate between circuits forEvery other bitEvery other byte
14
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)
15
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!)
16
SONET/SDHSONET/SDH
Standard for sending digital transmissions on STS-links
DefinesFrame formatMultiplexingSynchronization
Can be used to build ring networks
17
Local LoopLocal Loop
Telephone terminologyRefers to connection between residence /
business and central officeCrosses public right-of-wayOriginally for analog POTS (Plain Old
Telephone Service)
18
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
19
Asymmetric Digital Subscriber Line (ADSL)
Asymmetric Digital Subscriber Line (ADSL)
Popular DSL variantRuns over conventional POTS wiringHigher capacity downstreamUses frequencies above POTS
20
ADSL combined with POTSADSL combined with POTS
21
Illustration of ADSL WiringIllustration of ADSL Wiring
Downstream can reach 6.4 MbpsUpstream can reach 640 Kbps
22
Cable ModemsCable Modems
Send / receive over CATV (Community Antenna Television) wiring
Use FDMGroup of subscribers in neighbourhood
share bandwidth
23
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)
24
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
25
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
Top Related