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8. Multiplexing8. Multiplexing

2Prof. Sang-Jo Yoo

ContentsContents

Concept of Multiplexing Frequency-Division Multiplexing Synchronous Time-Division Multiplexing Statistical Time-Division Multiplexing

3Prof. Sang-Jo Yoo

Concept of MultiplexingConcept of Multiplexing

Multiplexing integrate n inputs into one signal shares limited media by multiplexing inputs. should not incur interferences

The higher the data rate, the more cost-effective the transmission facility.

MuxDe

muxn inputs n outputs

4Prof. Sang-Jo Yoo

Frequency-Division Frequency-Division Multiplexing(FDM)Multiplexing(FDM)

Each signal is modulated on a different carrier frequency The carrier frequencies are sufficiently separated that the

bandwidths of the signals do not overlap. To prevent interference, the channels are separated by guard bands.

chan

nel 1

chan

nel 2

chan

nel 3

chan

nel 4

chan

nel 5

chan

nel 6

FrequencyTimef1

f2

f3

f4

f5

f6

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Frequency-Division MultiplexingFrequency-Division Multiplexing

General Block Diagram for FDM

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FDM of Three Voiceband SignalsFDM of Three Voiceband Signals

Problems of FDM systems•Crosstalk: Significant signal overlap•Intermodulation noise: Sum of f1 and f2, produce noise in f1+f2 frequency.

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Synchronous Synchronous Time-Division Multiplexing(TDM)Time-Division Multiplexing(TDM)

Multiple digital signals can be carried on a single transmission path by interleaving portions of each signal in time (time slot). Interleaving can be bit, byte, or blocks of bytes.

8Prof. Sang-Jo Yoo

Synchronous Synchronous Time-Division Multiplexing(TDM)Time-Division Multiplexing(TDM)

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Synchronous TDMSynchronous TDM

TDM link control Link control as channel-wise

Input 1

Input 2

Output 1

Output 2

Input 1

Input 2

F1 f1 f1 d1 d1 d1 C1 A1 F1 f1 f1 d1 d1 d1 C1 A1 F1

F2 f2 f2 d2 d2 d2 d2 C2 A2 F2 f2 f2 d2 d2 d2 d2 C2 A2 F2

Input Data Stream

Multiplexed Data Stream

F = flag field d = one octet of data field A = address fieldf = one octet FCS field C = control field

f2 F1 d2 f1 d2 f1 d2 d1 d2 d1 C2 d1 A2 C1 F2 A1 f2 F1 f2 f1 d2 f1 d2 d1 d2 d1 d2 d1 C2 C1 A2 A1 F2 F1

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Synchronous TDMSynchronous TDM

Framing Need to synchronize frames. Add one control bit to each TDM frame. Typical example is 10101010… pattern of control bit Receiver searches for the pattern and makes a sync. for frames.

Pulse Stuffing Each source has a different data rate. To multiplex the sources efficiently, dummy pulse stuffing is don

e to each data stream.

1 0 1 0

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Synchronous TDMSynchronous TDM

Example of TDM

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Synchronous TDMSynchronous TDM

Digital Carrier Systems Two standards

originally from AT&T, used in North America and Japan from ITU-T

For American Case(DS-1) , 24 channels in 1 frame 8 bits for each channel 1 framing bit for 1 frame so, 1 frame = 24*8+1 = 193 bits 8000 samples/sec rate (PCM for voice) --> 8000*193=1.544 Mbps

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Synchronous TDMSynchronous TDMNorth American ITU-T

DigitalSignal

Number

Numberof voicechannels

Data rate(Mbps)

LevelNumber

Number ofvoice

channelsData rate(Mbps)

DS-1 24 1.544 1 30 2.048

DS-1C 48 3.152 2 120 8.448

DS-2 96 6.312 3 480 34.368

DS-3 672 44.736 4 1920 139.264

DS-4 4032 274.176 5 7680 565.148

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Digital Carrier SystemsDigital Carrier Systems

For voice each channel contains one word of digitized data Five out of six frames have 8 bit PCM samples Sixth frame is 7 bit PCM word plus signaling bit Signaling bits form stream for each channel containing control and

routing info

Same format for digital data 23 channels of data (56Kbps)

7 bits per frame plus an indicator bit Indicator bit indicates whether the channel, for that frame, contains

user data or system control data. 24th channel is sync

For faster and more reliable reframing

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Digital Carrier SystemsDigital Carrier Systems

DS-1 can carry mixed voice and data signals 24 channels used No sync byte

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ISDN User-Network Interface (TDM)ISDN User-Network Interface (TDM) ISDN User-Network Interface (UNI)

Basic ISDN interface Primary ISDN interface

Basic ISDN interface Multiplex 2 B-channels (each 64 kbps), 1 D-channel (16 kbps) 3 channels in one frame, 48 bits for one frame. Frame duration is 250*10-6 sec.

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ISDN User-Network Interface (TDM)ISDN User-Network Interface (TDM) Primary ISDN interface

Provides a synchronous TDM facility for access to ISDN Two data rates are defined

1.544 Mbps(American) : 24 B +1D(64Kbps) 2.048 Mbps(European) : 30B + 1D(64Kbps)

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SONET/SDH (TDM)SONET/SDH (TDM)

SONET/SDH Due to the appearance of an opti

cal fiber, it needs to specify the higher-speed digital transmission

SONET(Synchronous Optical Network) is proposed by BellCore and standardized by ANSI.

A compatible version, referred to as Synchronous Digital Hierarchy(SDH) has been published by ITU-T in Recommendations G.707, G.708, G.709.

Why hierarchy?

Signal HierarchySONETdesignation

CCITTdesignation

Data rate(Mbps)

Payload rate(Mbps)

STS-1/OC-1 51.84 50.112

STS-3/OC-3 STM-1 155.52 150.336

STS-9/OC-9 STM-3 466.56 451.008

STS-12/OC-12 STM-4 622.08 601.344

STS-18/OC-18 STM-6 933.12 902.016

STS-24/OC-24 STM-8 1244.16 1202.688

STS-36/OC-36 STM-12 1866.24 1804.032

STS-48/OC-48 STM-16 2488.32 2405.376

STS : Synchronous Transport SignalOC : Optical Carrier

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SONET Frame FormatSONET Frame Format

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Statistical Time-Division MultiplexingStatistical Time-Division Multiplexing

Characteristics In Synchronous TDM many slots are wasted Statistical TDM allocates time slots dynamically based on demand Multiplexer scans input lines and collects data until frame full First-come, first-served basis. The data rate on the multiplexed line is less than the sum of the dat

a rates of he attached devices.

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Statistical Time-Division MultiplexingStatistical Time-Division Multiplexing

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Statistical TDM PerformanceStatistical TDM Performance

Example of Performance (with buffering)

Capacity = 5000 bps Capacity = 7000 bps

input output backlog output backlog

6 5 1 6 0

9 5 5 7 2

3 5 3 5 0

7 5 5 7 0

2 5 2 2 0

2 4 0 2 0

10 5 5 7 3

8 5 8 7 1

4 5 7 5 0

6 5 8 6 0

1 5 4 1 0

* 10 sourceseach with1000 bps

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PerformancePerformance

Output data rate less than aggregate input rates May cause problems during peak periods

Buffer inputs Keep buffer size to minimum to reduce delay

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Asymmetrical Digital Subscriber LineAsymmetrical Digital Subscriber Line

ADSL Originally targeted for VoD and related services.

Link between subscriber and network Use FDM

Uses currently installed twisted pair cable To carry voice-grade signals (0-4KHz). Can carry broader spectrum 1 MHz or more

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ADSL DesignADSL Design

Asymmetric Greater capacity downstream than upstream

Frequency division multiplexing Lowest 25kHz for voice

Plain old telephone service (POTS) Voice : 0-4KHz, others: to prevent crosstalk between voice and data.

Use echo cancellation or FDM to give two bands

Range 5.5km

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ADSL Channel ConfigurationADSL Channel Configuration

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xDSLxDSL

High data rate DSL: symmetric : 1.544Mbps (2.048) Single line DSL: symmetric : 1.544Mbps (2.048) Very high data rate DSL : asymmetric:

13 Mbps – 52 Mbps (downstream) 1.5Mbps – 2.3 Mbps (upstream) Distance : 1.4 Km