William Stallings Data and Computer Communications Chapter 8

William Stallings Data and Computer Communications Chapter 8 Multiplexing

Multiplexing

Frequency Division Multiplexing z FDM z Useful bandwidth of medium exceeds required bandwidth of channel z Each signal is modulated to a different carrier frequency z Carrier frequencies separated so signals do not overlap (guard bands) z e. g. broadcast radio z Channel allocated even if no data

Frequency Division Multiplexing Diagram

FDM System

FDM of Three Voiceband Signals

Analog Carrier Systems z AT&T (USA) z Hierarchy of FDM schemes z Group y 12 voice channels (4 k. Hz each) = 48 k. Hz y. Range 60 k. Hz to 108 k. Hz z Supergroup y 60 channel y. FDM of 5 group signals on carriers between 420 k. Hz and 612 k. Hz z Mastergroup y 10 supergroups

Synchronous Time Division Multiplexing z Data rate of medium exceeds data rate of digital signal to be transmitted z Multiple digital signals interleaved in time z May be at bit level of blocks z Time slots preassigned to sources and fixed z Time slots allocated even if no data z Time slots do not have to be evenly distributed amongst sources

Time Division Multiplexing

TDM System

TDM Link Control z No headers and tailers z Data link control protocols not needed z Flow control y. Data rate of multiplexed line is fixed y. If one channel receiver can not receive data, the others must carry on y. The corresponding source must be quenched y. This leaves empty slots z Error control y. Errors are detected and handled by individual channel systems

Data Link Control on TDM

Framing z No flag or SYNC characters bracketing TDM frames z Must provide synchronizing mechanism z Added digit framing y. One control bit added to each TDM frame x. Looks like another channel - “control channel” y. Identifiable bit pattern used on control channel ye. g. alternating 0101…unlikely on a data channel y. Can compare incoming bit patterns on each channel with sync pattern

Pulse Stuffing z Problem - Synchronizing data sources z Clocks in different sources drifting z Data rates from different sources not related by simple rational number z Solution - Pulse Stuffing y. Outgoing data rate (excluding framing bits) higher than sum of incoming rates y. Stuff extra dummy bits or pulses into each incoming signal until it matches local clock y. Stuffed pulses inserted at fixed locations in frame and removed at demultiplexer

TDM of Analog and Digital Sources

Digital Carrier Systems z Hierarchy of TDM z USA/Canada/Japan use one system z ITU-T use a similar (but different) system z US system based on DS-1 format z Multiplexes 24 channels z Each frame has 8 bits per channel plus one framing bit z 193 bits per frame

Digital Carrier Systems (2) z For voice each channel contains one word of digitized data (PCM, 8000 samples per sec) y. Data rate 8000 x 193 = 1. 544 Mbps y. Five out of six frames have 8 bit PCM samples y. Sixth frame is 7 bit PCM word plus signaling bit y. Signaling bits form stream for each channel containing control and routing info z Same format for digital data y 23 channels of data x 7 bits per frame plus indicator bit for data or systems control y 24 th channel is sync

Mixed Data z DS-1 can carry mixed voice and data signals z 24 channels used z No sync byte z Can also interleave DS-1 channels y. Ds-2 is four DS-1 giving 6. 312 Mbps

ISDN User Network Interface z ISDN allows multiplexing of devices over single ISDN line z Two interfaces y. Basic ISDN Interface y. Primary ISDN Interface

Basic ISDN Interface (1) z Digital data exchanged between subscriber and NTE - Full Duplex z Separate physical line for each direction z Pseudoternary coding scheme y 1=no voltage, 0=positive or negative 750 m. V +/-10% z Data rate 192 kbps z Basic access is two 64 kbps B channels and one 16 kbps D channel z This gives 144 kbps multiplexed over 192 kbps z Remaining capacity used for framing and sync

Basic ISDN Interface (2) z B channel is basic user channel z Data z PCM voice z Separate logical 64 kbps connections o different destinations z D channel used for control or data y. LAPD frames z Each frame 48 bits long z One frame every 250 s

Frame Structure

Primary ISDN z Point to point z Typically supporting PBX z 1. 544 Mbps y. Based on US DS-1 y. Used on T 1 services y 23 B plus one D channel z 2. 048 Mbps y. Based on European standards y 30 B plus one D channel y. Line coding is AMI using HDB 3

Primary ISDN Frame Formats

Sonet/SDH z Synchronous Optical Network (ANSI) z Synchronous Digital Hierarchy (ITU-T) z Compatible z Signal Hierarchy y. Synchronous Transport Signal level 1 (STS-1) or Optical Carrier level 1 (OC-1) y 51. 84 Mbps y. Carry DS-3 or group of lower rate signals (DS 1 C DS 2) plus ITU-T rates (e. g. 2. 048 Mbps) y. Multiple STS-1 combined into STS-N signal y. ITU-T lowest rate is 155. 52 Mbps (STM-1)

SONET Frame Format

SONET STS-1 Overhead Octets

Statistical TDM z In Synchronous TDM many slots are wasted z Statistical TDM allocates time slots dynamically based on demand z Multiplexer scans input lines and collects data until frame full z Data rate on line lower than aggregate rates of input lines

Statistical TDM Frame Formats

Performance z Output data rate less than aggregate input rates z May cause problems during peak periods y. Buffer inputs y. Keep buffer size to minimum to reduce delay

Buffer Size and Delay

Asymmetrical Digital Subscriber Line z ADSL z Link between subscriber and network y. Local loop z Uses currently installed twisted pair cable y. Can carry broader spectrum y 1 MHz or more

ADSL Design z Asymmetric y. Greater capacity downstream than upstream z Frequency division multiplexing y. Lowest 25 k. Hz for voice x. Plain old telephone service (POTS) y. Use echo cancellation or FDM to give two bands y. Use FDM within bands z Range 5. 5 km

ADSL Channel Configuration

Discrete Multitone z DMT z Multiple carrier signals at different frequencies z Some bits on each channel z 4 k. Hz subchannels z Send test signal and use subchannels with better signal to noise ratio z 256 downstream subchannels at 4 k. Hz (60 kbps) y 15. 36 MHz y. Impairments bring this down to 1. 5 Mbps to 9 Mbps

DMT Transmitter

x. DSL z High data rate DSL z Single line DSL z Very high data rate DSL

Required Reading z Stallings chapter 8 z Web sites on y. ADSL y. SONET