Ch 12 Long-Distance and Local Loop Digital Technologies
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Transcript of Ch 12 Long-Distance and Local Loop Digital Technologies
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Ch 12 Long-Distance and Local Loop Digital
Technologies
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MotivationMotivation
Connect computers acrossLarge geographic distancePublic right-of-way
StreetsBuildingsRailroads
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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
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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
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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…
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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
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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
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Equipment for Leased Connections
Equipment for Leased Connections
Analog circuitModem required at each end
Digital CircuitDSU / CSU required at each end
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Digital Circuit and DSU/CSUs
Digital Circuit and DSU/CSUs
Needed because telephone industry digital encoding differs from computer industry digital encoding
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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
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Cost of digital circuit depends onDistanceCapacity
Telephone StandardsTelephone Standards
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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
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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
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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)
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Highest Capacity Digital Circuits
Highest Capacity Digital Circuits
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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
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Local Subscriber LoopLocal Subscriber Loop
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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
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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
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Illustration of ADSL WiringIllustration of ADSL Wiring
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Illustration of ADSL WiringIllustration of ADSL Wiring
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Illustration of ADSL BillingIllustration of ADSL Billing
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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)
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Source: IEC On-Line Education
Cable Modem TechnologyCable Modem Technology
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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
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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
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Hybrid Fiber Coax (HFC)Hybrid Fiber Coax (HFC)Neighborhood area
Trunk
TDMFDM
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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
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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
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Fiber To The Curb (FTTC)Fiber To The Curb (FTTC)
Source: Bell Labs Technique Journal
( 人行道的 ) 路緣
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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
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Broadband Access Technologies
Broadband Access Technologies
Source: TEN-Telecom Workshop
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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