1. TUNALIData Communication1 Chapter 8 Multiplexing

2. TUNALIData Communication2 Introduction In Chapter 7, it was assumed that the channel capacity is a bottleneck. So, efficient techniques for utilizing data link under load are described. Now, we consider the opposite problem. That is, we will assume that the two communicating stations will not utilize the full capacity of a data link. For efficiency, it should be possible to share that capacity.

3. TUNALIData Communication3 Multiplexing

4. TUNALIData Communication4 Facts affecting the widespread use of multiplexing The higher the data rate, the more cost effective the transmission facility. Most individual data communicating devices require relatively low data rate

5. TUNALIData Communication5 Types of multiplexing FDM: Frequency Division Multiplexing (Analog) TDM: Time Division Multiplexing (Digital) —Synchronous TDM —Statistical (Intelligent) TDM

6. TUNALIData Communication6 Frequency Division Multiplexing FDM Useful bandwidth of medium exceeds required bandwidth of signals to be transmitted Each signal is modulated to a different carrier frequency called subcarrier Carrier frequencies separated so signals do not overlap (guard bands) e.g. broadcast radio Channel allocated even if no data The composite signal transmitted is analog

7. TUNALIData Communication7 Frequency Division Multiplexing Diagram

8. TUNALIData Communication8 FDM Procedure (Transmitter) Each signal is modulated to a subcarrier frequency. Any type of modulation can be used. The resulting modulated signals are summed up to produce the composite baseband signal. The composite baseband signal may be modulated to another carrier frequency. n The resulting signal has bandwidth B>  B i i=1 To prevent interference, the channels are separated by guard bands, which are unused portions of the spectrum

9. TUNALIData Communication9 FDM Procedure (Receiver) FDM signal is demodulated to retrieve the composite baseband signal The composite baseband signal is passed through n bandpass filters each centered on a different subcarrier frequency and having bandwidth B i, i=1,..,n. Each component is demodulated to recover the original signal.

10. TUNALIData Communication10 FDM System

11. TUNALIData Communication11 FDM of Three Voiceband Signals

12. TUNALIData Communication12 Three voiceband signals Higher sidebands are suppressed and only lower sidebands are used. In general FDM has two problems: —Crosstalk: Occurs if spectrum of adjacent component signals overlap significantly. —Intermodulation noise: Due to nonlinearity effects of amplifiers on a signal, signals at unwanted frequencies are produced

13. TUNALIData Communication13 Analog Carrier Systems AT&T (USA) designed a system to transmit voiceband signals Hierarchy of FDM schemes to accommodate transmission systems of various capacities Group —12 voice channels (4kHz each) = 48kHz —Range 60kHz to 108kHz Supergroup —60 channel —FDM of 5 group signals on carriers between 420kHz and 612 kHz Mastergroup —10 supergroups

14. TUNALIData Communication14 Wavelength Division Multiplexing Multiple beams of light at different frequency Carried by optical fiber A form of FDM Each color of lights (wavelengths) carries separate data channel 1997 Bell Labs —100 beams —Each at 10 Gbps —Giving 1 terabit per second (Tbps) Commercial systems of 160 channels of 10 Gbps now available Lab systems (Alcatel) 256 channels at 39.8 Gbps each —10.1 Tbps —Over 100km

15. TUNALIData Communication15 WDM Operation Same general architecture as other FDM Number of sources generating laser beams at different frequencies Multiplexer consolidates sources for transmission over single fiber Optical amplifiers amplify all wavelengths —Typically tens of km apart Demux separates channels at the destination Mostly 1550nm wavelength range Was 200MHz per channel Now 50GHz

16. TUNALIData Communication16 Synchronous Time Division Multiplexing Data rate of medium exceeds data rate of digital signal to be transmitted Multiple digital signals interleaved in time The signals are generally digital signals and carry digital data The incoming data from each source are buffered Each buffer is typically one bit or one character in length Data rate of TDM stream  sum of data rates of individual channels

17. TUNALIData Communication17 Time Division Multiplexing

18. TUNALIData Communication18 Synchronous TDM (1) Time slots preassigned to sources and fixed Time slots allocated even if no data The buffer size is equal to the slot length so that the buffer is emptied in each cycle The transmitted data are organized into frames. Each frame contains a cycle of time slots. The sequence of slots dedicated to one source is called a channel

19. TUNALIData Communication19 Synchronous TDM (2) The word ‘synchronous’ is not used to indicate the actual synchronous transmission. It is there to indicate that the time slots pre-assigned to sources are fixed. In general, character interleaving is used for asynchronous devices whereas bit interleaving is used for synchronous devices. Capacity is wasted to achieve simplicity of implementation. Faster devices can be allocated multiple slots per cycle.

20. TUNALIData Communication20 TDM System

21. TUNALIData Communication21 TDM Link Control No headers and trailers 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

22. TUNALIData Communication22 Data Link Control on TDM

23. TUNALIData Communication23 Framing (1) No flag or SYNC characters bracketing TDM frames Must provide synchronizing mechanism Added digit framing —One control bit added to each TDM frame Looks like another channel - “control channel” —Identifiable bit pattern used on control channel —e.g. alternating 01010101…unlikely on a data channel —Can compare incoming bit patterns on each channel with sync pattern

24. TUNALIData Communication24 Framing (2) For synchronization, the receiver monitors the framing bit channel (101010101..) It compares the incoming bits of a frame with the expected pattern (10101010..). If it does not match, successive bits are searched until the pattern is found. During monitoring, if pattern breaks down, the receiver enters to “frame search” mode.

25. TUNALIData Communication25 Pulse Stuffing Problem - Synchronizing data sources Clocks in different sources drifting Data rates from different sources not related by simple rational number Solution - Pulse Stuffing —Outgoing data rate (excluding framing bits) higher than sum of incoming rates —Stuff extra dummy bits or pulses into each incoming signal until it matches local clock —Stuffed pulses inserted at fixed locations in frame and removed at demultiplexer

26. TUNALIData Communication26 TDM of Analog and Digital Sources

27. TUNALIData Communication27 Digital Carrier Systems The long-distance carrier system was designed to transmit voice signals over high capacity transmission links AT&T developed a 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

28. TUNALIData Communication28 Digital Carrier Systems (2) For voice each channel slot contains one word of digitized data (PCM, 8000 samples per sec) —Data rate 8000x193 = 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 data —23 channels of data 7 bits per frame plus indicator bit for data or system control Each frame is repeated 8000 times/sec = 56 kbps —24th channel is sync Reliable reframing following a framing error

29. TUNALIData Communication29 Mixed Data DS-1 can carry mixed voice and data signals 24 channels used No sync byte Can also interleave DS-1 channels —DS-2 is four DS-1 giving 6.312Mbps Subrate multiplexing is used to allow transmit of slower rates. In this case, the first bit is allocated to indicate the subrate multiplexing and the other 6 bits are used to multiplex slower data (E.g. 6X8000=48kbps can be used as 5X9.6=48 kbps

30. TUNALIData Communication30 DS-1 Transmission Format

31. TUNALIData Communication31 SONET/SDH Synchronous Optical Network (ANSI) Synchronous Digital Hierarchy (ITU-T) SONET provides taking advantage of the high speed digital transmission capability of optical fiber Signal Hierarchy —Synchronous Transport Signal level 1 (STS-1) or Optical Carrier level 1 (OC-1) —Data rate: 51.84Mbps —Carry DS-3 or group of lower rate signals (DS1 DS1C DS2) plus ITU-T rates (e.g. 2.048Mbps) —Multiple STS-1 combined into STS-N signal —ITU-T lowest rate is 155.52Mbps (STM-1)

32. TUNALIData Communication32 SONET Frame Format

33. TUNALIData Communication33 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

34. TUNALIData Communication34 Statistical TDM Characteristics Since there are no fixed slots per channel, addressing information is required In general, HDLC is used. HDLC frames contain multiplexing information. Since the frames are not in fixed size, frame length information is required Address and frame fields increase the overhead in Statistical TDM Methods to reduce the size of these fields are devised.

35. TUNALIData Communication35 Statistical v Synchronous TDM Fig 8.14

36. TUNALIData Communication36 Statistical TDM Frame Formats

37. TUNALIData Communication37 Improving Efficiency The address field can be reduced by using relative addressing —Each address specifies the current address relative to previous address Refinement in length field —A value of 00, 01, 10 corresponds to a data field of 1,2 or 3 bytes —11 indicates that a length field is included

38. TUNALIData Communication38 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 There is a tradeoff between system response time and the speed of the multiplexed line

39. TUNALIData Communication39 Performance Math (1) Let —I = number of input sources —R = data rate of each source, bps —M = effective capacity of multiplexed line, bps —  = mean fraction of time each source is transmitting, 0 <  < 1 —K = M / I R ratio of multiplexed line capacity to total maximum input —Then the value of K is bounded by  < K < 1 –If K = 1 then synchronous TDM –If K <  then input exceed multiplexing capacity

40. TUNALIData Communication40 Performance Math (2) Define further the following: — = average arrival rate in bps — T s = time it takes to transmit one bit —  = fraction of total link capacity being used —N = amount of buffer space being used —T r = average delay encountered by an input source Then, using queuing theory, we have, — =  I R —T s = 1 / M —  = T s = / M —N =  2 / (2(1-  )) +  —T r = T s (2 -  ) / (2 (1 -  ))

41. TUNALIData Communication41 Buffer Size and Delay

42. TUNALIData Communication42 Probability of Overflow as a Function of Buffer Size Fig 8-17

43. TUNALIData Communication43 Performance Comments The figures suggest that the utilization should not go above 80 percent since buffer requirements and delay increase There is always a finite probability that the buffer will overflow. For constant utilization, average delay decreases as line capacity increases

44. TUNALIData Communication44 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

45. TUNALIData Communication45 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

46. TUNALIData Communication46 Cable Modem Scheme

47. TUNALIData Communication47 Asymmetrical Digital Subscriber Line ADSL Link between subscriber and network Uses currently installed twisted pair cable —Can carry broader spectrum —Links installed to carry voice grade signals in a bandwidth from 0 to 4 kHz —1 MHz or more

48. TUNALIData Communication48 ADSL Design Asymmetric —Greater capacity downstream than upstream Frequency division multiplexing —Lowest 25kHz for voice Plain old telephone service (POTS) —Use echo cancellation or FDM to give two bands —Use FDM within the downstream and upstream bands

49. TUNALIData Communication49 ADSL Channel Configuration

50. TUNALIData Communication50 FDM v Echo Cancellation in ADSL Higher frequency has higher attenuation. With echo cancellation, more of the downstream bandwidth is in good portion of the spectrum as compared to FDM In Echo Cancellation, upstream capacity can be expanded Echo Cancellation is more complicated

51. TUNALIData Communication51 Discrete Multitone DMT Multiple carrier signals at different frequencies Some bits on each channel 4kHz subchannels Send test signal and use subchannels with better signal to noise ratio 256 downstream subchannels at 4kHz (60kbps) —15.36MHz —Impairments bring this down to 1.5Mbps to 9Mbps

52. TUNALIData Communication52 DTM Bits Per Channel Allocation

53. TUNALIData Communication53 DMT Transmitter

54. TUNALIData Communication54 xDSL High data rate DSL —Data rate 1.544 or 2.048 Mbps (with different coding schemes) Single line DSL —HDSL requires two twisted pair —SDSL was developed to provide the same type of service as HDSL Very high data rate DSL —Provides a scheme similar to ADSL at a much higher data rate by sacrificing distance

55. TUNALIData Communication55 Required Reading Stallings chapter 8 Web sites on —ADSL —SONET