1

Ch 12 Long-Distance and Local Loop Digital

Technologies

2

MotivationMotivation

Connect computers acrossLarge geographic distancePublic right-of-way

StreetsBuildingsRailroads

3

Digital TelephoneDigital TelephoneAnalog signals degrade as they pass over

copper wiresAmplifier along a path distorts the signal

slightly and introduce noiseDigitalization: convert an analog signal to

digital formAnalog-to-digital (AD) converter

4

Designed for use in voice systemAnalog audio from use’s telephone converted to

digital formatDigital format sent across networkDigital format converted back to analog audio

Digital TelephoneDigital Telephone

5

Illustration of Digitized Signal

Illustration of Digitized Signal

Pick neatest digital value for each sample

coded digitally as 000-010-100-100-111-001-001-001-001-001…

6

Illustration of Digitized Signal

Illustration of Digitized Signal

Nyquist’s Sampling Theorem If a continuous signal is sampled at a rate greater than

twice the highest significant frequency, the original signal can be reconstructed from the samples

Human voice up to 4k HzTelephone standard known as Pulse Code

Modulation (PCM)Sample rate 8k HzSample value in range 0 to 255

7

Long-Distance Transmission Technologies

Long-Distance Transmission TechnologiesGeneral solution: lease transmission facilities

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

availableCustomer chooses analog or digital

8

Equipment for Leased Connections

Equipment for Leased Connections

Analog circuitModem required at each end

Digital CircuitDSU / CSU required at each end

9

Digital Circuit and DSU/CSUs

Digital Circuit and DSU/CSUs

Needed because telephone industry digital encoding differs from computer industry digital encoding

10

Digital Circuit and DSU/CSUs

Digital Circuit and DSU/CSUs

Channel Service Unit (CSU) portionHandles line termination and diagnostics

Line connection testLoop-back capacity when installing and testingProhibits excessive consecutive 1’s (non-zero voltage for

1’s) to prevent excessive current over the copperBalanced encoding scheme: success 1 bits alternating

between +3V and -3VData Service Unit (DSU) portion

Translates data between two encodings

11

Cost of digital circuit depends onDistanceCapacity

Telephone StandardsTelephone Standards

12

Common Data RatesCommon Data Rates

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

Also availableFractional T1 (e.g., 56 Kbps circuit) since T1 is

too expensive to private individual The phone company uses TDM to subdivide a T1

circuit

13

Inverse MultiplexingInverse MultiplexingCombines two or more circuits to produce

intermediate capacity circuitA DSU/CSU is required at the ends of each T1 circuitHow to efficiently use the bandwidth?

ProxyLoad balancing

14

Highest Capacity Digital Circuits

Highest Capacity Digital Circuits

Also available from phone company as the major trunk connections across the country

Use optical fiberElectrical standards called Synchronous

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

15

Highest Capacity Digital Circuits

Highest Capacity Digital Circuits

16

Local Subscriber LoopLocal Subscriber Loop

Telephone terminologyRefers to connection between residences /

businesses and the phone company central office (CO)

Crosses public right-of-wayOriginally analog

17

Local Subscriber LoopLocal Subscriber Loop

18

Digital Local Loop Technologies

Digital Local Loop Technologies

Integrated Services Digital Network (ISDN)Provides digitalized voice and data over

conventional TP local loop wiresProvides simultaneous voice / data servicesBasic Rate Interface (BRI) offers three separate

(2B+D) digital channelsB channel: 64k bps for voice/dataD channel: 16k bps for controlling purpose

ISDN now is an expensive alternative that offers little throughput

19

Digital Local Loop Technologies

Digital Local Loop Technologies

Asymmetric Digital Subscriber Line (ADSL)Runs over conventional POTS wiringProvides simultaneous voice / data servicesHigher capacity, but asymmetric

Downstream can reach 6.14 MbpsUpstream can reach 640 Kbps

Optimized for typical users who receive much more information than they send

20

Illustration of ADSL WiringIllustration of ADSL Wiring

21

Illustration of ADSL WiringIllustration of ADSL Wiring

22

Illustration of ADSL BillingIllustration of ADSL Billing

23

Other DSL TechnologiesOther DSL TechnologiesSymmetric Digital Subscriber Line (SDSL)

Uses a different encoding scheme than ADSLCan operate over local loops for which ADSL is

inappropriate Business policy

High-Rate Digital Subscriber Line (HDSL)Provides 1.544M bps in two directionsShort distance limitationRequires two independent TPs, but can tolerate

failure gracefully (i.e., operate at one-half rate)

24

Source: IEC On-Line Education

Cable Modem TechnologyCable Modem Technology

25

Cable Modem TechnologyCable Modem Technology

Wiring (e.g., TPs) places an upper bound on how fast data can be transferred

Deliver over existing CATV coaxial cableCable system has unused bandwidth Group of subscribers in neighborhood share

the downstream bandwidth (up to 36 Mbps) by FDM

26

Cable Modem TechnologyCable Modem Technology

Upstream communicationDual path approach in dial-up modem without

changing the existing cable systemModification to the basic infrastructure (e.g.,

HFC)VOD services beyond computer network connections

27

Hybrid Fiber Coax (HFC)Hybrid Fiber Coax (HFC)Neighborhood area

Trunk

TDMFDM

28

Hybrid Fiber Coax (HFC)Hybrid Fiber Coax (HFC)Allow 2-way communication across a cable systemOptical fiber

Highest bandwidthReplaces trunk from CO to neighborhood concentration

pointsCoaxial cable

Less bandwidthExtends from neighborhood concentration point to

individual subscribers (feeder circuit)Replaces all amplifier / modem with 2-way devices

29

Hybrid Fiber Coax (HFC)Hybrid Fiber Coax (HFC)Combination of FDM & TDM

5-50 MHz : upstream50-450 MHz : analog TV(6M/ch) 450-750 MHz : downstream

The group shares one carrier frequency with TDM

30

Fiber To The Curb (FTTC)Fiber To The Curb (FTTC)

Source: Bell Labs Technique Journal

( 人行道的 ) 路緣

31

Fiber To The Curb (FTTC)Fiber To The Curb (FTTC)Differs from HFC

Run OF closer to the end subscribersUses two media in each feeder circuit to provide an

additional service

32

Broadband Access Technologies

Broadband Access Technologies

Source: TEN-Telecom Workshop

33

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

34

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