1. Chapter 8: Multiplexing COE 341: Data & Computer Communications (T061) Dr. Radwan E. Abdel-Aal

2. 2 Contents 1.Introduction 2.Two Multiplexing Techniques 1.FDM 2.TDM 1.Synchronous 2.Statistical 3.Application: ADSL (Asymmetric Digital Subscriber Line)

3. 3 Introduction Multiplexing: A generic term used when more than one application or source share the capacity of one link Objective is to achieve better utilization of the link bandwidth (channel capacity) MultiplexerDemultiplexer

4. 4 Motivation High capacity (data rate) links are cost effective. i.e. it is more economical to go for large capacity links But requirements of individual users are usually fairly modest…e.g. 9.6 to 64 kbps for non intensive (graphics, video) applications Solution: Let a number of such users share the high capacity channel (Multiplexing) Example: Long haul trunk traffic:  High capacity links: Optical fiber, terrestrial microwaves, etc. carrying large number of channels between cities over large distances

5. 5 Multiplexing Types Our three resources: Space Time Frequency Our channels must be separated in at least one resource (can overlap in the other two) The resource in which they are separated is “divided” between them:  SDM: Separation in space  TDM: Separation in time  FDM: Separation in frequency Space Frequency Time TDM: Time Division Multiplexing To use the same circuit (line) i.e. sharing space: Use either TDM or FDM FDM: Frequency Division Multiplexing

6. 6 Frequency Division Multiplexing (FDM) (With Analog Signals) Channels exist on the same line (space) at the same time: Must be separated in frequency! f t

7. 7 FDM Useful bandwidth of medium exceeds required bandwidth of a channel Signal of each channel is modulated on a different carrier frequency f c So, channels are shifted from same base band by different f c ’s to occupy different frequency bands Carrier frequencies separated so that channels do not overlap (also include some guard bands) Disadvantage: Channel spectrum is allocated even if no data available for transmission in channel (rigid allocation)

8. 8 FDM Multiplexing Process: Frequency-Domain View at TX All source channels are at (same) base band f f f1f1 f2f2 f3f3 0 4 KHz f1f1 f2f2 f3f3 Channels go on the same Link at the same time… but in different frequency bands

9. 9 FDM De-Multiplexing Process: Frequency-Domain View at RX f1f1 f2f2 f3f3 04 KHz All received channels restored to base band Guard bands prevent channel overlap But represent wasted spectrum f Restoration at RX: 3 different pass-band filters, each bracketing a channel

10. 10 FDM System – Transmitter Subcarriers Main Carrier Any type of modulation: AM, FM, PM Group of channels To meet Transmission Requirements Individual base band channels

11. 11 FDM System – Receiver fcfc Composite base band signal m b (t) recovered Individual base band channels recovered Subcarriers Main Carrier f1f1 f2f2 f3f3

12. 12 FDM of Three Voice band Signals What is the modulation type ? 3 MUXed channel Using lower side band only 3 Subcarriers at: 64, 68, and 72 KHz Channel overlap means crosstalk! Guard bands To reduce channel spectrum overlap BW, allocated: 0 – 4000 Hz BW, actual : 300 – 3400 Hz fcfc Assume we will keep only the lower side band for each channel

13. 13 Analog Carrier Systems: How to MUX 4000 Channels? One-go Vs Hierarchical: (in stages) …. Channel …. Group …. Master super group Super group …. 4000 channels... Modular approach Easier to implement: e.g. fewer sub carriers Also, not all channels may be available at one place Stages ….

14. 14 Analog Carrier Systems Devised by AT&T (USA) Hierarchy of FDM schemes: MUXing in stages Group: AM, Lower Side band  12 voice channels = 12 x 4 kHz = 48 kHz BW  12 sub carriers: 64 kHz – 108 kHz in 4 KHz intervals  Frequency range for group: 60 kHz – 108 kHz = 48 kHz (lower side band) Super group: FM  5 groups = 5 x 48 kHz = 240 kHz BW  5 sub carriers: 420 kHz - 612 kHz at 48 KHz intervals (No GBs bet. groups)  Frequency range: 312 kHz – 552 kHz = 240 kHz Master group: FM  10 super groups = 10 x 240 kHz = 2400 kHz BW  10 sub carriers: 1116 kHz - 3396 kHz (Min of 8 KHz GBs between SGs)  BW of 2.52 MHz (> 10 x 240 KHz = 2.4 MHz due to GB between SGs) Jumbo group: FM  6 master groups  i.e. total of 6 x 10 x 5 x 12 = 3600 voice channels  BW of 16.984 MHz (> 3600 x 4 KHz due to gaud bands between super groups) Each channel is 300 to 3400 = 3100 Hz. 4000 Hz provides 900 Hz guard band

15. 15 Analog Carrier Systems 12 8 KHz Group Channel Super group Master super group

16. 16 Analog FDM Hierarchy 0.24 x 10 Vs 2.52

17. 17 FDM characteristic problems Two potential problems characterize FDM and all broadband applications  Crosstalk: - Due to overlap between channel spectra and the use of non-ideal filters to separate them - Use guard bands to reduce it  Inter modulation noise: - Nonlinearities in amplifiers ‘mix’ channels - This generates spurious frequency components (sum, difference) which fall within channel BWs! - Limits the amount of amplification possible

18. 18 Usually uses synchronous transmission, but asynchronous is also possible Data rate of medium exceeds data rate of digital signals to be transmitted for one channel Digital signals of multiple channels interleaved in time Interleaving may be:  At bit level  At block level (e.g. bytes) Two types:  Synchronous TDM (Fixed rotation on channels)  Statistical or asynchronous TDM (More efficient utilization of the time slots) Time Division Multiplexing (TDM)

19. 19 Time Division Multiplexing 300 Hz Channels occupy the same frequency band (Base band) and go on the same link  Channels must go on the link at different times 3400 Hz

20. 20 Time Division Multiplexing (TDM) All Baseband Signals- share same Frequency band T s = 1/2f max Channel sampling interval Note: MUXing and DeMUXing are transparent to the end stations. Each pair ‘think’ they have a dedicated link ! Time Rotate At the channel sampling rate

21. 21 TDM Frames Channel sampling interval = 3T For each channel, data rate is 1 sample/3T 1 2 3 4 5 6 123456 …. Sample Number On the link: Data is sent at a rate of 1 sample/T Data rate is 3 times the channel data rate Sampling Interval Channel data rate = R Link data rate = NR N Channels

22. 22 Synchronous TDM: (Fixed channel scan arrangement)  Time slots pre-assigned to sources and fixed  Disadvantage: Time slots allocated even if no data available (channel capacity waste, as with BW waste in FDM)  But simple to implement, e.g. No need to send ID of source channel  We could assign more than time slot per scan for faster sources- but on a permanent basis Time Division Multiplexing (TDM)

23. 23 Synchronous TDM – Transmitter Scanning and link data rate: high enough to prevent channel buffers overflowing N channels, Sampling rate R sample/s Minimum link capacity = N R sample/s Analog Signal Digital Signal or Transmitted frames consist of interleaved channel data Time Slot, T Channel dwell time T Should be enough to empty a channel buffer Channel 2 Bit stream Channel Buffers Sample data fills buffer

24. 24 TDM System – Receiver TDM signal

25. 25 Data Link Control with Sync TDM Data rate on the link (multiplexed line) is fixed and MUX and DEMUX must operate at it Data link control protocol not needed on the MUXED line Flow control Channel based  If one channel receiver is not ready to receive data, other channels will carry on  Channel-based flow control would then halt corresponding source channel  This causes transmission of empty slots for that channel in the MUXED data Error control Channel based  Errors are detected and handled by individual channel systems

26. 26 Framing in TDM So far, no flag or SYNC characters bracketing composite (MUXED) TDM frames on the link Must provide frame synchronization to allow RX to keep ‘in step’ with TX Two approaches:  Frame-by-Frame: A synch pattern at the beginning of each assembled frame (similar to the preamble flag)  Frame-to-Frame: Additional control channel with a unique frame-to-frame pattern that can be easily identified by RX (can be just 1 bit, and extends across frames, so less overhead) This is called “added digit framing”

27. 27 Framing in TDM Added digit framing  One control bit added to each TDM frame as an additional “control channel”  Carries an identifiable known bit pattern in time (frame to frame) e.g. alternating 01010101…unlikely to occur on a normal data channel  RX searches frame-to-frame for this pattern until it finds it. This establishes frame sync. Will keep locked to it

28. 28 Frame-to-Frame Sync (added digit framing) C 1 2 3 4 C 1 2 3 4 C 1 01 0 Four data channels Control Channel, C 010101…. ….. A data Channel Unlikely to have 010101…. over successive frames Once the position of this control channel is established, RX knows where the channel sequence starts and sync is established with TX ….. MUXed frame RX knows the size of the MUXed frame It can check each frame bit frame-to-frame for the special pattern until it finds it!

29. 29 Digital Carrier Systems Hierarchy for TDM (as with FDM!) US system based on DS format, for example DS-1 (similar to a group in FDM):  Multiplexes 24 PCM voice channels digitized with n = 8 bits + a framing bit (a control channel for frame-to- frame synchronization)  Frame takes a sample of each channel  So, frame size is 24 x 8 +1 = 193 bits  Channels must be sampled at 2 x 4000 = 8000 sample/s  This gives a data rate = 8000 x 193 = 1.544 Mbps for DS-1 Note: FDM Group needed 48 KHz for 12 channels

30. 30 The DS Hierarchy 42 x 96 = 4032 DS-0 (4032 voice channels) DS-0 is a PCM voice channel: 8000 sample/s x 8 b/sample = 64 kbps Transmission lines used should support the progressively increasing data rate (channel capacity) requirement FDM Jumbo group: 16.984 MHz for 3600 channels Which one uses BW more efficiently ?

31. 31 DS & T Lines Rates ServiceLine Data Rate (Mbps) No. of Voice Channels DS-1T-11.54424 DS-2T-2 6.312 6.31296 DS-3T-3 44.736 44.736 672 672 DS-4T-4274.1764032 Transmission line that supports it Corresponding Channel Capacity Relevant Standards: SONET: Synchronous Optical Network (ANSI) SDH: Synchronous Digital Hierarchy (ITU-T)

32. 32 DS-1 Digital Carrier Systems For voice, each channel contains one byte of digitized data (PCM, 8000 samples per sec)  Data rate 8000 MUXed frames/s x (24x8+1) bits/frame = 1.544Mbps  Five out of every six frames have 8 bit PCM user data samples for each channel  Sixth frame has (7 bit PCM user data + 1 signaling bit) for each channel  Signaling bits form a stream for each channel containing control (e.g. error and flow) and routing info Same format for digital data  23 channels of data 7 bits per frame plus indicator bit for data or systems control  24th channel is for signaling DS-1 can carry mixed voice and data signals

33. 33 DS-1 Transmission Format Frame (frame-to-frame) 125/ 193 (8000 x 7 bits = 56 kbps)

34. 34 Asymmetric Digital Subscriber Line (ADSL) ADSL is an asymmetric communication technology designed for residential users over ordinary telephone twisted pair wires High speed digital data transmission Existing subscriber lines (local loops) were installed for base band speech (0 – 4 kHz), but can actually provide bandwidths of up to 1 MHz (short distances) ADSL is an adaptive technology, using different data rates based on the condition of the local loop line Ranges up to 5.5 km (95% of subscriber lines in USA) Two main technologies: - Multi-level encoding, e.g. QAM - Discrete Multitone (DMT) by FDM Shorter distance, Higher data rates Q. What is the BE for 2.5 km lines?

35. 35 ADSL Design Asymmetric: Providing higher capacity down stream (to customer) than upstream (from customer) Originally targeting the video-on-demand market Now being used for Internet traffic Uses Frequency Division Multiplexing (FDM) in a novel way to utilize the 1 MHz BW of twisted pair wires Service Provider Video, graphics Voice, e-mail ADSL Subscriber Downstream (download) Upstream (upload)

36. 36 FDM is used at two levels: a. Use FDM to obtain three major bands: 1. POTS band: “Plain Old Telephone Service!” 0 - 20 kHz (includes 16 KHz Guard band) 2. Upstream band: 25 – 200 kHz 3. Downstream band: 250 – 1000 KHz 4 b. Discrete Multitoning (DMT) : Further FDM inside the upstream and the downstream bands: Single fast bit stream is split into multiple bit streams traveling at lower data rates in parallel (simultaneously) in subchannels at different subbands within the upstream and downstream bands.

37. 37 ADSL Hardware Home Telephone Exchange Subscriber Loop Telephone channel operates even is ADSL line is down or unplugged

38. 38 ADSL Frequency Bands and DMT Channels Guard bands Between voice and data Channel # 256 x 4 kHz  1 MHz 1 MHz - 256 4KHz sub channels - DMT distributes data rate load on sub channels, non uniformly

39. 39 Discrete Multitoning (DMT) Multiple subchannels (each 4 KHz wide) within the upstream and downstream bands Subchannels are modulated with subcarriers of different frequencies (FDM) (hence “multi-tone”) Bit stream to be transmitted is split into a number of streams that travel in parallel (simultaneously) at a lower data rate on a number of these limited BW subchannels 1 1011010011 Serial to Parallel Converter Subchannel 1 Subchannel 5.... 1 0 1 1 0 1 0 0 1 1 Data rate for each channel: R/5 bps Overall data rate: R bps Data rate R bps (Each: 4 kHz BW) 1 5

40. 40 Discrete Multitoning (DMT): Adaptive ADSL adaptive property: Not all subchannels run at the same data rate! Each subchannel can carry data rate from 0 - 60 kbps In general, channels towards the higher frequency end would carry lower data rates (larger attenuation  lower SNR  lower channel capacity!) Moreover, DMT modem can send out test signals on various subchannels to determine SNR (expected lower for subchannels located at higher frequencies due to larger attenuation) Then faster data rates are assigned to subchannels having better signal transmission conditions 1 1

41. 41 Discrete Multitoning (DMT) Uses QAM (Quadrature Amplitude Modulation) multilevel modulation allowing up to 15 bits/baud (L = 15 bits/signal level) (4 KHz B  D = 4 kbauds (if filtering coefft. r = 0)  R max = 4 kbauds x 15 = 60 kbps per channel) Ideally, 256 x 60 kbps = 15.36 Mbps maximum (if uniform) But rate is not uniform over all sub channels, and maximum is not achievable in practice due to various transmission impairments Practical system operate at 1.5 to 9 Mbps depending on distance and line quality

42. 42 DMT  i  : data rate distributing factors (<1) over available sub channels (in either the Upstream or the Downstream bands) R = Aggregate (total) data rate R =  i R +  i R + …. +  n R

43. 43 DMT Demodulators Modulators