Data and Computer Communications

1. Data and Computer Communications Chapter 8 – Multiplexing Ninth Edition by William Stallings Data and Computer Communications, Ninth Edition by William Stallings, (c) Pearson Education - Prentice Hall, 2011

2. Multiplexing It was impossible to get a conversation going, everybody was talking too much. - Yogi Berra

3. Multiplexing • multiple links on 1 physical line • common on long-haul, high capacity, links • have FDM, TDM, STDM (statistical) alternatives

4. Frequency Division Multiplexing (FDM)

5. FDMSystem Overview

6. Questions: • How do you shift a signal around a particularfrequency ? • http://www.cdt21.com/resources/Modulation/modulation_AM.asp • Seeequations and figure in the middle of the page • For the applet, click on image at the bottom • How do youmultiply a signal by another ?

7. FDM Voiceband Example

8. Analog Carrier Systems • long-distance links use an FDM hierarchy • Ref. wikipedia • AT&T (USA) and ITU-T (International Telecommunication Union) variants • Group • 12 voice channels (4kHz each) = 48kHz • range 60kHz to 108kHz • Supergroup • FDM of 5 group signals supports 60 channels • carriers between 312-552 kHz (420kHz and 612 kHz) • Mastergroup • FDM of 10 supergroups supports 600 channels • original signal can be modulated many times

9. North American and International FDM Carrier Standards

10. Wavelength Division Multiplexing (WDM)

11. ITU WDM Channel Spacing (G.692)

12. Synchronous Time Division Multiplexing

13. TDM SystemOverview

14. 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 • corresponding source must be quenched • leaving empty slots • error control • errors detected & handled on individual channel

15. Data Link Control on TDM

16. Framing • no flag or SYNC chars bracketing TDM frames • must still provide synchronizing mechanism between source and destination clocks • added digit framing is most common • one control bit added to each TDM frame • identifiable bit pattern used as control channel • alternating pattern 101010…unlikely to be sustained on a data channel • receivers compare incoming bits of frame position to the expected pattern

17. Pulse Stuffing - problem of synchronizing various data sources - variation among clocks could cause loss of synchronization - issue of data rates from different sources not related by a simple rational number

18. TDM Example 32 bit frame composed of: 16 PCM bits + 8 X 2bits of digital sources

19. Digital Carrier Systems

20. DS-1 Transmission Format

21. North American and International TDM Carrier Standards

22. SONET/SDH • Synchronous Optical Network • ANSI: American national standards institute • Synchronous Digital Hierarchy • ITU-T: International telecommunication union • high speed capability of optical fiber • defines hierarchy of signal rates • Synchronous Transport Signal level 1 (STS-1) or Optical Carrier level 1 (OC-1) is 51.84Mbps • carries one DS-3 or multiple (DS1 DS1C DS2) plus ITU-T rates (e.g., 2.048Mbps) • multiple STS-1 combine into STS-N signal • ITU-T lowest rate is 155.52Mbps (STM-1)

23. SONET/SDH Signal Hierarchy

24. SONET Frame Format(one frame every 125us; transmitted one row at a time)

25. Statistical TDM

26. Example of Statistical TDM Frame Format

27. Single-Server Queues with Constant Service Times and Poisson (Random) Arrivals

28. Cable Modems -dedicate two cable TV channels to data transfer -each channel shared by number of subscribers using statistical TDM

29. Cable Spectrum Division • to support both cable television programming and data channels, the cable spectrum is divided in to three ranges: • user-to-network data (upstream): 5 - 40 MHz • television delivery (downstream): 50 - 550 MHz • network to user data (downstream): 550 - 750 MHz

30. Cable Modem Scheme

31. Cable Modem Configuration

32. Asymmetrical Digital Subscriber Line (ADSL) • link between subscriber and network • uses currently installed twisted pair cable • is Asymmetric - bigger downstream than up • uses Frequency Division Multiplexing • reserve lowest 25kHz for voice (POTS : plain old telephone system) • uses echo cancellation or FDM to give two bands • has a range of up to 5.5km

33. ADSL Channel Configuration

34. Discrete Multitone (DMT) • multiple carrier signals at different frequencies • divide into 4kHz subchannels • test and use subchannels with better SNR • 256 downstream subchannels at 4kHz (60kbps) • in theory 15.36Mbps, in practice 1.5-9Mbps

35. DMT Transmitter

36. (Broadband – Customer Side) • DSL link is between provider and customer • a splitter allows simultaneous telephone and data service • data services use a DSL modem • sometimes referred to as G.DMT modem • DSL data signal can be divided into a video stream and a data stream • the data stream connects the modem to a router which enables a customer to support a wireless local area network

37. (Broadband – Provider Side) a splitter separates telephone from Internet voice traffic is connected to public switched telephone network (PSTN) data traffic connects to a DSL multiplexer (DSLAM) which multiplexes multiple customer DSL connections to a single high-speed ATM line. ATM line connects ATM switches to a router which provides entry to the Internet

38. xDSL • high data rate DSL (HDSL) • 2B1Q coding on dual twisted pairs • up to 2Mbps over 3.7km • single line DSL (SDSL) • 2B1Q coding on single twisted pair (residential) with echo cancelling • up to 2Mbps over 3.7km • very high data rate DSL (VDSL) • DMT/QAM for very high data rates • separate bands for separate services

39. Comparison of xDSL Alternatives VDSL2 : http://en.wikipedia.org/wiki/Very-high-bit-rate_digital_subscriber_line_2

40. Summary • multiplexing multiple channels onsingle link • FDM • analog carrier systems • wavelength division multiplexing • TDM • TDM link control • pulse stuffing • statistical TDM • broadband • ADSL and xDSL