1. EEL4930University of FloridaFall 2003 1 EEL4930 Computer Networks Dr. George Stallings – Chapter 8 Multiplexing NOTE: Many figures and other materials in this presentation are borrowed from required and reference textbooks cited on the class web page.

2. EEL4930University of FloridaFall 2003 2 Multiplexing (1) Common applications in long-haul comm. e.g. trunks on long-haul networks are high-capacity fiber, coaxial, or microwave links

3. EEL4930University of FloridaFall 2003 3 Multiplexing (2) Two traditional approaches Time-Division Multiplexing (TDM) Time-Division Multiplexing (TDM) Frequency-Division Multiplexing (FDM) Frequency-Division Multiplexing (FDM)

4. EEL4930University of FloridaFall 2003 4 FDM I can give you some of the bandwidth all of the time. I can give you some of the bandwidth all of the time.

5. EEL4930University of FloridaFall 2003 5 FDM Motivation Useful bandwidth of transmission medium exceeds required bandwidth of channel Useful bandwidth of transmission medium exceeds required bandwidth of channelCharacteristics Each signal is modulated to a different carrier frequency (subcarrier) – each forms a channel Each signal is modulated to a different carrier frequency (subcarrier) – each forms a channel Carrier frequencies separated so signals do not overlap (guard bands) Carrier frequencies separated so signals do not overlap (guard bands) Channel allocated even if no data Channel allocated even if no data e.g. broadcast radio, broadcast and cable TV

6. EEL4930University of FloridaFall 2003 6 Example of FDM Channels (a) The original bandwidths. (b) The bandwidths raised in frequency. (c) The multiplexed channel.

7. EEL4930University of FloridaFall 2003 7 FDM System Composite signal may be shifted to another carrier frequency as additional modulation step

8. EEL4930University of FloridaFall 2003 8 Analog Carrier Systems Long-distance carrier system in U.S. and world to transmit voiceband signals over high-capacity trans. links AT&T (USA) designed hierarchy of FDM schemes Group of 12 channels 12 voice channels (4 kHz each) combined by FDM to produce group signal with 48kHz bandwidth 12 voice channels (4 kHz each) combined by FDM to produce group signal with 48kHz bandwidth Spectrum of 60 – 108 kHz Spectrum of 60 – 108 kHzSupergroup 60 channels 60 channels Each group treated as single signal each with 48 kHz bandwidth Each group treated as single signal each with 48 kHz bandwidth Supergroup formed by FDM of 5 group signals Supergroup formed by FDM of 5 group signals Spectrum of 312 - 552 kHz Spectrum of 312 - 552 kHzMastergroup 10 supergroups, 600 channels 10 supergroups, 600 channels More beyond (e.g. Jumbogroup of 3600 channels)

9. EEL4930University of FloridaFall 2003 9 Wavelength-Division Multiplexing (WDM)

10. EEL4930University of FloridaFall 2003 10 WDM Highlights Basic idea Multiple beams of light at different frequencies Multiple beams of light at different frequencies Carried by optical fiber Carried by optical fiber A form of FDM A form of FDM Each color of light (wavelength) carries separate data channel Each color of light (wavelength) carries separate data channel Landmark for WDM @ Bell Labs in 1997 100 beams 100 beams Each at 10 Gbps Each at 10 Gbps 1 Terabit per second (Tbps) on a single fiber! 1 Terabit per second (Tbps) on a single fiber! Commercial systems of 160 channels each @ 10 Gbps now available Lab systems going farther e.g. 256 channels each @ 39.8 Gbps e.g. 256 channels each @ 39.8 Gbps 10+ Tbps in total 10+ Tbps in total Operating over a span of 100+ km Operating over a span of 100+ km

11. EEL4930University of FloridaFall 2003 11 WDM Operation Same general architecture as other FDM systems No. of sources generating laser beams @ different wavelengths No. of sources generating laser beams @ different wavelengths Multiplexer consolidates sources for xmit over single fiber Multiplexer consolidates sources for xmit over single fiber Optical amplifiers amplify all wavelengths Optical amplifiers amplify all wavelengths Typically tens of kilometers apart Demux separates channels at destination for multiple receivers Demux separates channels at destination for multiple receivers e.g. 1550nm wavelen. (~192-196THz) range is common, 50GHz spacing per channel (i.e. ~.4nm per channel) ITU WDM channel spacing (G.692) standard accommodates eighty 50GHz channels! ITU WDM channel spacing (G.692) standard accommodates eighty 50GHz channels! Dense WDM (DWDM) No official or standard definition No official or standard definition Implies more channels, more closely spaced, than WDM Implies more channels, more closely spaced, than WDM Coarse WDM (CWDM) Wider spacing, less channels Wider spacing, less channels

12. EEL4930University of FloridaFall 2003 12 Synchronous TDM I can give you all of the bandwidth some of the time. I can give you all of the bandwidth some of the time.

13. EEL4930University of FloridaFall 2003 13 Synchronous TDM Time-Division Multiplexing (TDM) Generally digital signals carrying digital data Generally digital signals carrying digital dataMotivation Data rate of medium exceeds data rate of digital signal to be transmitted Data rate of medium exceeds data rate of digital signal to be transmittedCharacteristics Multiple digital signals interleaved in time Multiple digital signals interleaved in time May be at bit level or blocks/characters May be at bit level or blocks/characters Time slots pre-assigned to sources and fixed (reason it is called “synchronous” TDM) Time slots pre-assigned to sources and fixed (reason it is called “synchronous” TDM) Time slots allocated even if no data Time slots allocated even if no data Time slots do not have to be evenly distributed amongst sources Time slots do not have to be evenly distributed amongst sources Slot length equals transmitter buffer length (bit or character) Slot length equals transmitter buffer length (bit or character)

14. EEL4930University of FloridaFall 2003 14 Synchronous TDM System

15. EEL4930University of FloridaFall 2003 15 TDM Link Control No headers and trailers in frame TDM data-link protocols not needed But what about flow control? Data rate of multiplexed line is fixed Data rate of multiplexed line is fixed If one channel receiver cannot receive data, others must carry on If one channel receiver cannot receive data, others must carry on Corresponding source must cease its flow of data Corresponding source must cease its flow of data This step leaves empty slots This step leaves empty slots But what about error control? Errors detected and handled by individual channel systems Errors detected and handled by individual channel systems Thus, flow and error control provided as needed on per- channel basis using protocol such as HDLC MUX/DeMUX transparent to stations

16. EEL4930University of FloridaFall 2003 16 Data Link Control on TDM

17. EEL4930University of FloridaFall 2003 17 Framing No flag or SYNC characters to bracket TDM frames But must provide synchronizing mechanism Common mechanism is added-digit framing One control bit added to each TDM frame One control bit added to each TDM frame Looks like another channel - “control channel” We’ll see it later in DS-1 frame format Identifiable bit pattern, from frame to frame, used on control channel Identifiable bit pattern, from frame to frame, used on control channel e.g. alternating 10101010… unlikely on data channel e.g. alternating 10101010… unlikely on data channel Receiver can compare incoming bit patterns on each channel with sync pattern Receiver can compare incoming bit patterns on each channel with sync pattern If pattern breaks down, receiver enters frame search mode

18. EEL4930University of FloridaFall 2003 18 Pulse Stuffing Problem Synchronizing data sources Synchronizing data sources Clocks in different sources drifting Clocks in different sources drifting Data rates from different sources not related by simple rational number 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 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 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 Stuffed pulses inserted at fixed locations in frame and removed at demultiplexer

19. EEL4930University of FloridaFall 2003 19 TDM of Analog & Digital Sources

20. EEL4930University of FloridaFall 2003 20 Digital Carrier Systems (1) Hierarchy of Synchronous TDM USA/Canada/Japan use one system USA/Canada/Japan use one system ITU-T use a similar (but different) system ITU-T use a similar (but different) system US (AT&T) system based on DS-1 format Multiplexes 24 channels Multiplexes 24 channels Each frame has 8 bits per channel, plus one framing bit Each frame has 8 bits per channel, plus one framing bit 24 x 8 + 1 = 193 bits per frame 24 x 8 + 1 = 193 bits per frame 125us per frame  8000 frames/second 125us per frame  8000 frames/second

21. EEL4930University of FloridaFall 2003 21 Digital Carrier Systems (2) For voice Analog voice signal digitized with PCM at 8000 samples/sec Analog voice signal digitized with PCM at 8000 samples/sec Thus, each channel slot and thus each frame must repeat same Thus, each channel slot and thus each frame must repeat same  Data rate = 8000 frames/sec x 193 bits/frame = 1.544 Mbps  Data rate = 8000 frames/sec x 193 bits/frame = 1.544 Mbps Five out of six frames have 8-bit PCM samples per channel Five out of six frames have 8-bit PCM samples per channel Sixth frame, each channel is 7-bit PCM word plus signaling bit Sixth frame, each channel is 7-bit PCM word plus signaling bit Signaling bits form stream for each voice channel containing control and routing info (e.g. call connection or termination) Signaling bits form stream for each voice channel containing control and routing info (e.g. call connection or termination) Same DS-1 format also for digital data 23 channels of data 23 channels of data 7 bits per frame plus indicator bit for data or systems control Thus, 7 x 8000 = 56 kbps data rate per channel 24th channel reserved for special sync byte 24th channel reserved for special sync byte Allows faster and more reliable reframing after a framing error

22. EEL4930University of FloridaFall 2003 22 Mixed Data DS-1 can carry mixed voice and data signals 24 channels used 24 channels used No sync byte No sync byte Higher-level multiplexing Interleave DS-1 channels Interleave DS-1 channels e.g. DS-2 is four DS-1s giving 6.312Mbps e.g. DS-2 is four DS-1s giving 6.312Mbps 1.544 Mbps x 4 = 6.176 Mbps Remaining is for framing and control bits

23. EEL4930University of FloridaFall 2003 23 T1/DS-1 Carrier Standard (1.544 Mbps) Note T-1 (a.k.a.T1) is digital carrier facility of 24 channels used to transmit DS-1 (a.k.a. DS1) formatted digital signals Similarly for others T-2/DS-2 @ 6.312 Mbps w/ 96 channels T-3/DS-3 @ 44.736 Mbps w/ 672 channels T-4/DS-4 @ 274.176 Mbps w/ 4032 channels) Hyphens are optional

24. EEL4930University of FloridaFall 2003 24 Multiplexing T1 streams onto higher carriers

25. EEL4930University of FloridaFall 2003 25 SONET/SDH Synchronous Optical Network (SONET) Optical transmission interface Optical transmission interface Developed by BellCore, standardized by ANSI Developed by BellCore, standardized by ANSI Synchronous Digital Hierarchy (SDH) ITU-T standard compatible with SONET ITU-T standard compatible with SONET Signal Hierarchy OC-x rate is optical equivalent of STS-x electrical signal OC-x rate is optical equivalent of STS-x electrical signal Synch Transport Signal level 1 (STS-1) or Optical Carrier level 1 (OC-1) Synch Transport Signal level 1 (STS-1) or Optical Carrier level 1 (OC-1) 51.84 Mbps data rate, 50.112 Mbps payload rate Could carry DS-3 or group of lower rate signals (e.g. DS1 or DS2) plus ITU-T TDM carrier rates (e.g. Level-1 at 2.048 Mbps) Multiple STS-1 are combined into STS-N signal; some common ones Multiple STS-1 are combined into STS-N signal; some common ones OC-3 has 155.52 Mbps data rate, 150.336 payload rate OC-12 has 622.08 Mbps data rate, 601.344 payload rate OC-48 has 2488.32 Mbps data rate, 2405.376 payload rate OC-192 has 9953.28 Mbps data rate, 9621.504 payload rate ITU-T lowest rate is STM-1 = STS-3 = OC-3 ITU-T lowest rate is STM-1 = STS-3 = OC-3

26. EEL4930University of FloridaFall 2003 26 SONET system Consists of switches, multiplexers, and repeaters, all connected by fiber Example path from source to destination shown below Fiber from one device to another is a section Fiber from one device to another is a section A run between two MUXs (perhaps with repeaters in middle) is a line A run between two MUXs (perhaps with repeaters in middle) is a line Connection between source and destination (perhaps with MUXs and repeaters in middle) is a path Connection between source and destination (perhaps with MUXs and repeaters in middle) is a path Topology can be a mesh, but often a dual ring Topology can be a mesh, but often a dual ring

27. EEL4930University of FloridaFall 2003 27 SONET physical layer

28. EEL4930University of FloridaFall 2003 28 SONET frame format Building block is STS-1 = OC-1 frame 810 octets transmitted once every 125us 810 octets transmitted once every 125us 810 x 8000 frames/s = 6.48 MB/s = 51.84 Mb/s Viewed as matrix of 9 rows of 90 octets each Viewed as matrix of 9 rows of 90 octets each First 3 columns (27 octets) are overhead 9 of 27 for section-related overhead 9 of 27 for section-related overhead Generated/checked at start/end of each section 18 of 27 for line-related overhead 18 of 27 for line-related overhead Generated/checked at start/end of each line Remainder is payload 9 x 87 = 783 octets 9 x 87 = 783 octets Except one column of path overhead Except one column of path overhead Injected somewhere in payload Line-related overhead contains pointer to its location Indicates start of Synchronous Payload Envelope (i.e. user data) More on next two slides

29. EEL4930University of FloridaFall 2003 29 SONET Frame Format Section overhead: for single point-to-point fiber run Line overhead: for multiplexing multiple data streams (tributaries) onto single line and demultiplexing at other end Path overhead: for end-to-end issues Transmitted row by row (Left  Right, Top  Bottom) Lowest rate in ITU-T SDH is STM-1 @ 155.52 Mb/s; thus STM-1  OC-3, STM-4  OC12, etc.

30. EEL4930University of FloridaFall 2003 30 SONET STS-1 Overhead Octets

31. EEL4930University of FloridaFall 2003 31 Two back-to-back SONET frames SPE can begin anywhere within frame As we said, pointer to first byte contained in first row of line overhead As we said, pointer to first byte contained in first row of line overhead First column of SPE is path overhead (i.e. header for end-to-end path sublayer protocol) First column of SPE is path overhead (i.e. header for end-to-end path sublayer protocol) SPE may begin anywhere within frame and even span two frames for flexibility SPE may begin anywhere within frame and even span two frames for flexibility e.g. If payload arrives at source while dummy SONET frame being constructed, can be inserted into current frame instead of waiting for next

32. EEL4930University of FloridaFall 2003 32 Multiplexing in SONET To prevent long runs of 0s or 1s Multiplexing done on a byte-by-byte basis (e.g. when three STS-1 tributaries merged into one STS-3 stream, MUX first outputs one octet from tributary #1, then one from #2, then one from #3, then one from #1, etc.)

33. EEL4930University of FloridaFall 2003 33 SONET and SDH multiplex rates Other interesting ones of late: OC-192 (9.95328 Gb/s gross), OC-768 (39.81312 Gb/s)

34. EEL4930University of FloridaFall 2003 34 SONET odds and ends Higher rates need not necessarily be multiplexed When carrier not multiplexed, it carries data from single source, and its designation is appended with “c” for “concatenated” e.g. OC-3c, OC-12c e.g. OC-3c, OC-12c e.g. ATM on OC-3c from single computer at 155.52 Mbps e.g. ATM on OC-3c from single computer at 155.52 Mbps Amount of actual user data in concatenated stream is slightly higher than in multiplexed stream Example Example With OC-3c stream, only one path overhead column per SPE With OC-3 stream, three path overhead columns, one from each of three independent OC-1 streams

35. EEL4930University of FloridaFall 2003 35 Statistical TDM I can give you all of the bandwidth some of the time. How often/much depends on how busy you get. I can give you all of the bandwidth some of the time. How often/much depends on how busy you get.

36. EEL4930University of FloridaFall 2003 36 Statistical TDM (1) Movitation In Synchronous TDM, static allocation implies that many slots may be wasted In Synchronous TDM, static allocation implies that many slots may be wasted Not all attached devices may be transmitting all the time Not all attached devices may be transmitting all the timeCharacteristics Statistical TDM allocates time slots dynamically based on demand Statistical TDM allocates time slots dynamically based on demand Multiplexer scans input lines and collects data until frame full Multiplexer scans input lines and collects data until frame full Does not send empty slots if there is data to send from any source Does not send empty slots if there is data to send from any source Data rate on line is lower than aggregate of the rates of input lines Data rate on line is lower than aggregate of the rates of input lines We have n I/O lines into our statistical multiplexer, but only k time slots available where k < n We have n I/O lines into our statistical multiplexer, but only k time slots available where k < n Of course, data arrives and must be distributed to I/O lines unpredictably here Of course, data arrives and must be distributed to I/O lines unpredictably here Address info. required to ensure proper delivery Implies more overhead per slot than with synch. TDM

37. EEL4930University of FloridaFall 2003 37 Statistical TDM (2) “On-demand” TDM Function of MUX to scan input buffers, collect data until a frame is filled, then send frame on link Function of MUX to scan input buffers, collect data until a frame is filled, then send frame on link Uses synchronous protocol such as HDLC Uses synchronous protocol such as HDLC …

38. EEL4930University of FloridaFall 2003 38 Statistical TDM Frame Formats Two options with Statistical TDM frame First case (b), only one source of data included per frame; length of data is variable; works well under light load. Second case (c), multiple sources per frame; more overhead but more efficient for heavier loads.

39. EEL4930University of FloridaFall 2003 39 Performance Data rate on line is lower than aggregate of the rates of inputs (@ peak; o/w on average) Buffer data contending for the link Buffer data contending for the link Buffer (queue) overflow a symptom of congestion Buffer (queue) overflow a symptom of congestion Pros and cons of lower data rate on line Good in getting more for less Good in getting more for less Bad during peak periods Bad during peak periods Tradeoff between buffer size and line data rate Reduction in one requires increase in other! Reduction in one requires increase in other! Why would we want to limit buffer size given how cheap memory technology has become? Why would we want to limit buffer size given how cheap memory technology has become? More buffering implies longer delay Thus, real tradeoff is system response time versus speed of multiplexed line

40. EEL4930University of FloridaFall 2003 40 Buffer Size and Delay Example shown here w/ random arrivals Data in 1000-bit frames M is effective capacity of muxed line As offered load (from sum of mux inputs) approaches muxed line capacity, avg. # of frames buffered rises quickly. This increase in turn causes quick rise in delay experienced.

41. EEL4930University of FloridaFall 2003 41 Example: ADSL Asymmetrical Digital Subscriber Line (ADSL) Problem In high-speed, wide-area, public digital networks, key challenge is link between subscriber and network In high-speed, wide-area, public digital networks, key challenge is link between subscriber and network Digital subscriber line High expense to run new lines to homes and businesses High expense to run new lines to homes and businessesSolution Exploit installed base of twisted-pair wire already present from telephone service Exploit installed base of twisted-pair wire already present from telephone service Can carry broader spectrum than just 4 kHz voice Can carry broader spectrum than just 4 kHz voice 1 MHz bandwidth or more 1 MHz bandwidth or more

42. EEL4930University of FloridaFall 2003 42 ADSL Design Asymmetric Greater capacity downstream than upstream Greater capacity downstream than upstream Novel use of FDM Exploits 1 MHz capacity of twisted-pair wire Exploits 1 MHz capacity of twisted-pair wire Reserve lowest 25 kHz for voice Reserve lowest 25 kHz for voice Plain old telephone service (POTS) Additional bandwidth included here to prevent crosstalk between voice and data channels Use echo cancellation or FDM to allocate two bands (upstream and downstream) or channels Use echo cancellation or FDM to allocate two bands (upstream and downstream) or channels Then use FDM within these two bands Then use FDM within these two bands Serial bit stream split into multiple parallel bit streams Each portion carried on separate freq. band (subchannel) Range up to 5.5 km depending on quality of cable Range up to 5.5 km depending on quality of cable

43. EEL4930University of FloridaFall 2003 43 ADSL Channel Configuration Two alternatives FDM for POTS and two data bands (a) FDM for POTS and two data bands (a) Echo cancellation permits upstream to overlap lower portion of downstream (b) Echo cancellation permits upstream to overlap lower portion of downstream (b) Echo cancellation: signal processing technique for simultaneous transmission in both directions on single line Echo cancellation: signal processing technique for simultaneous transmission in both directions on single line Transmitter subtracts echo of its own transmission from incoming signal to recover signal sent by other side Transmitter subtracts echo of its own transmission from incoming signal to recover signal sent by other side

44. EEL4930University of FloridaFall 2003 44 DMT (1) Discrete Multitone (DMT) Modulation Multiple carrier signals at different frequencies Transmission band divided into 4 kHz subchannels Transmission band divided into 4 kHz subchannels Sending bits on each subchannel Sending bits on each subchannelInitialization DMT modem sends test signal on each subchannel to determine SNR DMT modem sends test signal on each subchannel to determine SNR Assigns more bits to subchannels with better SNR, less to poorer ones Assigns more bits to subchannels with better SNR, less to poorer ones Each subchannel carries data at rate from 0 to 60 kbps Each subchannel carries data at rate from 0 to 60 kbps Typical example below (increasing attenuation at higher frequencies) Typical example below (increasing attenuation at higher frequencies)

45. EEL4930University of FloridaFall 2003 45 DMT (2) DMT Transmission After init., bit stream divided into substreams (one per subchannel) After init., bit stream divided into substreams (one per subchannel) Each substream converted to analog signal using quadrature amplitude modulation (QAM); each QAM signal occupies distinct 4 kHz freq. band Each substream converted to analog signal using quadrature amplitude modulation (QAM); each QAM signal occupies distinct 4 kHz freq. band Sum of substream data rates equals total data rate Sum of substream data rates equals total data rate

46. EEL4930University of FloridaFall 2003 46 ADSL/DMT Data Rate Common ADSL/DMT designs employ 256 subchannels on downstream channel In theory, at 60 kbps per subchannel, can achieve total data rate of 60 kbps x 256 = 15.36 Mbps In theory, at 60 kbps per subchannel, can achieve total data rate of 60 kbps x 256 = 15.36 Mbps However, in practice, impairments limit implementations with range from 1.5 to 9 Mbps However, in practice, impairments limit implementations with range from 1.5 to 9 Mbps Depends on line distance and quality e.g. BellSouth “FastAccess” DSL Residential service Residential service Up to 1.5 Mbps downstream Up to 256 kbps upstream

47. EEL4930University of FloridaFall 2003 47 Example: Cable Modem Based on cable TV, FDM of analog signals Two FDM channels dedicated, one for upstream and one for downstream Two FDM channels dedicated, one for upstream and one for downstream Each channel shared by number of subscribers Statistical TDM used to allocate capacity Statistical TDM used to allocate capacity Topology looks like a tree Head-end controller at provider is the root Head-end controller at provider is the root Cable modems at customers are the leaves Cable modems at customers are the leaves Role of cable modem IP packets modulated by provider at head-end controller then sent downstream IP packets modulated by provider at head-end controller then sent downstream Cable modem demodulates signals back to IP packets Cable modem demodulates signals back to IP packets Downstream data received by all cable modems; total bandwidth shared; modem filters for local use Downstream data received by all cable modems; total bandwidth shared; modem filters for local use Upstream data sent in bursts typically via statistical TDM, via reserved and contention slots Upstream data sent in bursts typically via statistical TDM, via reserved and contention slots Thus, performance depends on several factors Bandwidth of channels Bandwidth of channels Provider constraints Provider constraints e.g. local provider may connect to Internet via T-1 Load from fellow local subscribers Load from fellow local subscribers

48. EEL4930University of FloridaFall 2003 48 Cable Modem Operation Downstream Cable scheduler delivers data in small packets Cable scheduler delivers data in small packets If more than one subscriber active, each gets fraction of downstream capacity If more than one subscriber active, each gets fraction of downstream capacity e.g. may get 500 kbps to 1.5 Mbps Also used to allocate upstream time slots to subscribers Also used to allocate upstream time slots to subscribersUpstream User requests timeslots on shared upstream channel User requests timeslots on shared upstream channel Dedicated slots for this Headend scheduler sends back assignment of future time slots to subscriber Headend scheduler sends back assignment of future time slots to subscriber

49. EEL4930University of FloridaFall 2003 49 Cable Modem Scheme