CSCI Computer Networking: Physical Layer Multiplexing George Blankenship. Physical Layer Multiplexing. George Blankenship 1

Transcription

1 CSCI 6431 Computer Networking: George Blankenship George Blankenship 1

2 Sharing a medium Multiple dialogs Virtual concurrency FDM Copper medium Fiber medium TDM Digital carrier SONET Statistical multiplexing Cbl Cable modem DSL Spread Spectrum Lecture Outline George Blankenship 2

3 George Blankenship 3

4 Frequency Division Useful bandwidth of medium exceeds required bandwidth of individual channels Each signal is modulated to a different carrier frequency Carrier frequencies separated so signals do not overlap (guard bands) Channel allocated even if no data Model is broadcast radio George Blankenship 4

5 Three Voice Band Signals George Blankenship 5

6 Time Division Synchronous Transmission period divided into discrete time slots Channel assigned to a time slot Data demand cannot exceed capacity Statistical Data is divided into discrete packages Package contains channel lidentityi Data demand can exceed capacity George Blankenship 6

7 Synchronous TDM Data rate of medium exceeds data rate of digital signal to be transmitted Multiple digital signals interleaved in time May be at bit level of blocks Time slots preassigned to sources and fixed Time slots allocated even if no data Time slots do not have to be evenly distributed amongst sources George Blankenship 7

8 TDM Link Control No headers and dtrailers Data link control protocols not needed Flow control Data rate of multiplexed line is fixed If one channel receiver can not receive data, the others must carry on The corresponding source must be quenched This leaves empty slots Error control Errors are detected and handled by individual channel systems George Blankenship 8

9 Data Link Control on TDM George Blankenship 9

10 Digital Carrier Systems (Framing) Hierarchy of TDM USA/Canada/Japan use one system ITU-T use a similar (but different) system US system based on DS-1 format Multiplexes 24 channels Each frame has 8 bits per channel plus one framing bit 193 bits per frame George Blankenship 10

11 Digital Carrier Systems (Data Load) For voice each channel contains one word of digitized data (PCM, 8000 samples per sec) Data rate 8000 x 193 = 1.544mbps 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 i ldata 23 channels of data 7 bits per frame plus indicator bit for data or systems control 24th channel is sync George Blankenship 11

12 DS-1 Transmission Format George Blankenship 12

13 Statistical TDM 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 Data rate on line lower than aggregate rates of input lines George Blankenship 13

14 Statistical TDM Frame Formats George Blankenship 14

15 Performance Output data rate less than aggregate input rates May cause problems during peak periods Buffer inputs Keep buffer size to minimum to reduce delay George Blankenship 15

16 Cable Modem Outline Two channels from cable TV provider dedicated to data transfer One in each direction Each channel shared by number of subscribers Scheme needed to allocate capacity Statistical TDM George Blankenship 16

17 Cable Modem Operation Downstream Cable scheduler delivers data in small packets If more than one subscriber active, each gets fraction of downstream capacity May get 500kbps to 1.5Mbps Also used to allocate upstream time slots to subscribers Upstream User requests timeslots on shared upstream channel Dedicated slots for this Headend scheduler sends back assignment of future time slots to subscriber George Blankenship 17

18 Cable Modem Scheme George Blankenship 18

19 Asymmetrical Digital Subscriber Line Link between subscriber and network Local loop Uses currently installed twisted pair cable Can carry broader spectrum 1MHzormore Frequency division multiplexing Lowest 25kHz for voice Use echo cancellation or FDM to give two bands Range 5.5km George Blankenship 19

20 ADSL Channel Configuration George Blankenship 20

21 Spread Spectrum Concept Input tfed into channel encoder Produces narrow bandwidth analogue signal around central frequency Signal modulated using sequence of digits Spreading code/sequence (typically generated by pseudonoise/pseudorandom number generator) Increases bandwidth significantly Spreads spectrum Receiver uses same sequence to demodulate signal Demodulated signal fed into channel decoder George Blankenship 21

22 Gains Free space multiplexing Immunity from various noise and multipath distortioni Including jamming Can hide/encrypt signals Only receiver who knows spreading code can retrieve signal Several users can share same higher bandwidth with little interference Cellular telephones Code division multiplexing g( (CDM) Code division multiple access (CDMA) George Blankenship 22

23 Pseudorandom Numbers Generated by algorithm using initial seed Deterministic algorithm Not actually random If algorithm good, results pass reasonable tests of randomness Need to know algorithm and seed to predict sequence George Blankenship 23

24 Frequency Hopping Spread Spectrum (FHSS) Signal broadcast over seemingly random series of frequencies Receiver hops between frequencies in sync with transmitter Eavesdroppers hear unintelligible blips Jamming on one frequency affects only a few bits George Blankenship 24

25 Basic Operation Typically 2 k carriers frequencies forming 2 k channels Channel spacing corresponds with bandwidth of input Each channel used for fixed interval 300 ms in IEEE Some number of bits transmitted using some encoding scheme Sequence dictated by spreading code George Blankenship 25

26 Frequency Hopping Example George Blankenship 26

27 Direct Sequence Spread Spectrum (DSSS) Each bit represented by multiple bits using spreading code Spreading code spreads signal across wider frequency band In proportion to number of bits used 10 bit spreading code spreads signal across 10 times bandwidth of 1 bit code One method: Combine input with spreading code using XOR Input bit 1 inverts spreading code bit Input zero bit doesn t alter spreading code bit Data rate equal to original spreading code Performance similar to FHSS George Blankenship 27

28 Code Division Multiple Access (CDMA) Technique used with spread spectrum Start with data signal rate D Called bit data rate Break each bit into k chips according to fixed pattern specific to each user User s code New channel has chip data rate kd chips per second k=6, three users (A,B,C) communicating with base receiver R Code for A = <1,-1,-1,1,-1,1> Code for B = <1,1,-1,-1,1,1> Code for C = <1,1,-1,1,1,-1> George Blankenship 28

29 Suggested Reading Stallings chapters 8 and 9 Web sites on ADSL SONET George Blankenship 29