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Understanding Multiplexing Techniques in Computer Networks

This lesson explores multiplexing in computer networks, highlighting its motivations such as cost reduction and efficient use of existing facilities. We focus on Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM), explaining their principles, advantages, and implementations. FDM separates multiple signals using carrier frequencies to prevent overlap, while TDM interleaves digital signals in time slots. We also discuss digital carrier systems, including DS-1 and SONET, outlining their structures and benefits in terms of reliability and performance.

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Understanding Multiplexing Techniques in Computer Networks

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  1. CISC450 Computer Networks Lesson 6 Multiplexing Slides courtesy of William Stallings and Cisco Systems

  2. Multiplexing Motivations: Cost reduction, leverage existing facilities

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

  4. Frequency Division MultiplexingDiagram

  5. FDM System

  6. FDM of Three Voiceband Signals

  7. Analog Carrier Systems • AT&T (USA) • Hierarchy of FDM schemes • 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

  8. L Carrier Systems

  9. L Carrier Layout

  10. L4 and L5 Coaxial Cable Systems

  11. National L Carrier System

  12. Synchronous Time Division Multiplexing • 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 pre-assigned to sources and fixed • Time slots allocated even if no data • Time slots do not have to be evenly distributed amongst sources

  13. Time Division Multiplexing

  14. TDM System Byte-interleaved Vs Bit-interleaved

  15. 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

  16. Data Link Control on TDM

  17. Framing • 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

  18. 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

  19. Advantages of Digital Communications • Reduction of noise, distortion, and other impairments • Regeneration of the signal is easier • Encryption and compression is easier • Easier to handle diverse channel types • VLSI

  20. Digital Carrier Systems • 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

  21. Digital Carrier Systems (2) • For voice each channel 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 systems control • 24th channel is sync

  22. Digital Hierarchies

  23. 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

  24. Primary ISDN • Point to point • Typically supporting PBX • 1.544Mbps • Based on US DS-1 • Used on T1 services • 23 B plus one D channel • 2.048Mbps • Based on European standards • 30 B plus one D channel • Line coding is AMI usingHDB3

  25. Primary ISDN Frame Formats

  26. Introduction to SONET • Synchronous Optical NETwork • Provides a rich built-in capacity for advanced network management and maintenance • Supports new types of customer service signals (ATM, TCP/IP, SNA, X.25, frame relay, etc.)

  27. Sonet/SDH • Synchronous Optical Network (ANSI) • Synchronous Digital Hierarchy (ITU-T) • Compatible • Signal Hierarchy • Synchronous Transport Signal level 1 (STS-1) or Optical Carrier level 1 (OC-1) • 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)

  28. SONET Advantages • Reliability • electronics/optics redundancy • path diversity • performance monitoring • circuit provisioning online • Vendor-neutral standard • leads to less expensive hardware • Ease of engineering

  29. SONET Hierarchy

  30. SONET Frame Format

  31. SONET STS-1 Overhead Octets

  32. 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

  33. Statistical TDM Frame Formats

  34. 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

  35. Buffer Size and Delay

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

  37. 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 bands • Range 5.5km (or better)

  38. Defining DSL: Digital Subscriber Loop Customer DSL Value-Added Packet Network Copper Loop DSL “Modem” DSL “Modem” • Like dial, cable, wireless, and T1, DSL is a Transmission Technology, NOT a full end-to-end solution • Users don’t “buy” DSL, they “buy” services, such as high-speed Internet, leased line, VPN, and Video on Demand

  39. DSL Modem Technology DSL Service Max. Data Rate Down/Uplink (bps) Line Coding Technology Analog Voice Support Max. Reach (km-feet) VDSL–Very High Bit Rate DSL 25M/1.6M or 8M/8M ??? Yes .9–3,000 Residential SOHO Business ADSL–Asymmetric DSL 8M/1M CAP & DMT 5.5–18,000 Yes IDSL–ISDN DSL 144K/144K 2B1Q No 5.5–18,000 SDSL–Symmetric DSL 768K/768K 2B1Q / CAP No 6.9–22,000 HDSL2– High Bit Rate DSL 1.5M–2.0M/1.5M–2.0M Optis No 4.6–15,000 • Trade-off is Reach vs. Bandwidth • Reach numbers imply “Clean Copper” • Different layer 1 transmission technologies, need a common upper protocol layer to tie them together

  40. RADSL (Rate Adaptive ADSL) • Mass deployment technology • Good Reach • Preserves Baseband POTS • Rate adjusts to local loop conditions • Good Spectral Compatibility • Competing line code variations - CAP, DMT, G.Lite

  41. POTS Upstream Downstream 4 kHz f1 f2 Carrierless Amplitude and Phase One Supplier (GlobeSpan) Utilizes CAP encoding within upstream and downstream multiplexed carriers Rate adapts by modifying #bits/cycle (Constellation size) + Carrier baud rate 136 kHz 340 kHz 680 kHz 952 kHz 1080 kHz 136 kHz

  42. POTS Upstream Downstream 4 kHz f1 f2 Discrete Multi-Tone ANSI T1.413 Standard for ADSL Multiple Suppliers Utilizes QAM line code within many multiplexed bands or “tones” Rate adapts by changing or zeroing-out bits/tone

  43. Downstream POTS Upstream 4 kHz f1 f2 G.Lite • Sub-rate (<1.5Mbps), splitterless, consumer-oriented standard • Promoted by UAWG (Intel, Microsoft, Compaq) • Based on DMT standard (lower 128 tones) • Should interoperate with “full-rate” DMT (g.hs used to signal at startup

  44. ISDN 2B1Q RADSL Downstream (1088 baud) RADSL Upstream T1 AMI HDSL 2B1Q Frequency Spectrum Utilization

  45. Customer Upstream DSL “Modem” Downstream DSLAM Crosstalk • Downstream power is highest at the DSLAM and lowest at the CPE. • Upstream power is lowest at the DSLAM and highest at the CPE. • If these signals are in different frequency spectrums then they will not crosstalk, otherwise there will be interference from one signal to the other.

  46. Crosstalk - ADSL and T1 Repeated T1 Customer DSL “Modem” DSLAM Upstream Downstream • Downstream power is highest at the DSLAM and lowest at the CPE. • Upstream power is lowest at the DSLAM and highest at the CPE. • If these signals are in different frequency spectrums then they will not crosstalk, otherwise there will be interference from one signal to the other.

  47. ATM DSL Transmission Technology ATM over xDSL • Multiple connection multiplexing • Built in QoS / CoS for newer services

  48. HDLC / Frame DSL Transmission Technology IDSL • Legacy CPE requires that the access link use HDLC or Frame links. • ITU-C on DSLAM will interwork from Frame to ATM

  49. DSL Transmission Technology SDSL - 633 CPE • SDSL will use ATM over the access link. The 633 SDSL CPE will interwork Frame to ATM in order to connect existing routers. Frame to ATM

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