EEL 4930 University of Florida Fall 2003 EEL

EEL 4930 University of Florida Fall 2003 EEL 4930 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. 1

EEL 4930 University of Florida Fall 2003 Multiplexing (1) Common applications in long-haul comm. n e. g. trunks on long-haul networks are high-capacity fiber, coaxial, or microwave links 2

EEL 4930 University of Florida Fall 2003 Multiplexing (2) Two traditional approaches n n Time-Division Multiplexing (TDM) Frequency-Division Multiplexing (FDM) 3

EEL 4930 University of Florida Fall 2003 FDM I can give you some of the bandwidth all of the time. 4

EEL 4930 University of Florida Fall 2003 FDM Motivation n Useful bandwidth of transmission medium exceeds required bandwidth of channel Characteristics n n n Each signal is modulated to a different carrier frequency (subcarrier) – each forms a channel Carrier frequencies separated so signals do not overlap (guard bands) Channel allocated even if no data e. g. broadcast radio, broadcast and cable TV 5

EEL 4930 University of Florida Fall 2003 Example of FDM Channels (a) The original bandwidths. (b) The bandwidths raised in frequency. (c) The multiplexed channel. 6

EEL 4930 University of Florida Fall 2003 FDM System Composite signal may be shifted to another carrier frequency as additional modulation step 7

EEL 4930 University of Florida Fall 2003 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 n n 12 voice channels (4 k. Hz each) combined by FDM to produce group signal with 48 k. Hz bandwidth Spectrum of 60 – 108 k. Hz Supergroup n n 60 channels Each group treated as single signal each with 48 k. Hz bandwidth Supergroup formed by FDM of 5 group signals Spectrum of 312 - 552 k. Hz Mastergroup n 10 supergroups, 600 channels More beyond (e. g. Jumbogroup of 3600 channels) 8

EEL 4930 University of Florida Fall 2003 Wavelength-Division Multiplexing (WDM) 9

EEL 4930 University of Florida Fall 2003 WDM Highlights Basic idea n n Multiple beams of light at different frequencies Carried by optical fiber A form of FDM Each color of light (wavelength) carries separate data channel Landmark for WDM @ Bell Labs in 1997 n n n 100 beams Each at 10 Gbps 1 Terabit per second (Tbps) on a single fiber! Commercial systems of 160 channels each @ 10 Gbps now available Lab systems going farther n n n e. g. 256 channels each @ 39. 8 Gbps 10+ Tbps in total Operating over a span of 100+ km 10

EEL 4930 University of Florida Fall 2003 WDM Operation Same general architecture as other FDM systems n n n No. of sources generating laser beams @ different wavelengths Multiplexer consolidates sources for xmit over single fiber Optical amplifiers amplify all wavelengths Typically tens of kilometers apart n Demux separates channels at destination for multiple receivers e. g. 1550 nm wavelen. (~192 -196 THz) range is common, 50 GHz spacing per channel (i. e. ~. 4 nm per channel) n ITU WDM channel spacing (G. 692) standard accommodates eighty 50 GHz channels! Dense WDM (DWDM) n n No official or standard definition Implies more channels, more closely spaced, than WDM Coarse WDM (CWDM) n Wider spacing, less channels 11

EEL 4930 University of Florida Fall 2003 Synchronous TDM I can give you all of the bandwidth some of the time. 12

EEL 4930 University of Florida Fall 2003 Synchronous TDM Time-Division Multiplexing (TDM) n Generally digital signals carrying digital data Motivation n Data rate of medium exceeds data rate of digital signal to be transmitted Characteristics n n n Multiple digital signals interleaved in time May be at bit level or blocks/characters Time slots pre-assigned to sources and fixed (reason it is called “synchronous” TDM) Time slots allocated even if no data Time slots do not have to be evenly distributed amongst sources Slot length equals transmitter buffer length (bit or character) 13

EEL 4930 University of Florida Fall 2003 Synchronous TDM System 14

EEL 4930 University of Florida Fall 2003 TDM Link Control No headers and trailers in frame TDM data-link protocols not needed But what about flow control? n n Data rate of multiplexed line is fixed If one channel receiver cannot receive data, others must carry on Corresponding source must cease its flow of data This step leaves empty slots But what about error control? n Errors detected and handled by individual channel systems Thus, flow and error control provided as needed on perchannel basis using protocol such as HDLC MUX/De. MUX transparent to stations 15

EEL 4930 University of Florida Fall 2003 Data Link Control on TDM 16

EEL 4930 University of Florida Fall 2003 Framing No flag or SYNC characters to bracket TDM frames But must provide synchronizing mechanism Common mechanism is added-digit framing n One control bit added to each TDM frame Looks like another channel - “control channel” We’ll see it later in DS-1 frame format n n n Identifiable bit pattern, from frame to frame, used on control channel e. g. alternating 1010… unlikely on data channel Receiver can compare incoming bit patterns on each channel with sync pattern If pattern breaks down, receiver enters frame search mode 17

EEL 4930 University of Florida Fall 2003 Pulse Stuffing Problem n n n Synchronizing data sources Clocks in different sources drifting Data rates from different sources not related by simple rational number Solution: Pulse Stuffing n n n 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 18

EEL 4930 University of Florida Fall 2003 TDM of Analog & Digital Sources 19

EEL 4930 University of Florida Fall 2003 Digital Carrier Systems (1) Hierarchy of Synchronous TDM n n USA/Canada/Japan use one system ITU-T use a similar (but different) system US (AT&T) system based on DS-1 format n n Multiplexes 24 channels Each frame has 8 bits per channel, plus one framing bit 24 x 8 + 1 = 193 bits per frame 125 us per frame 8000 frames/second 20

EEL 4930 University of Florida Fall 2003 Digital Carrier Systems (2) For voice n n n Analog voice signal digitized with PCM at 8000 samples/sec Thus, each channel slot and thus each frame must repeat same Data rate = 8000 frames/sec x 193 bits/frame = 1. 544 Mbps Five out of six frames have 8 -bit PCM samples per channel 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) Same DS-1 format also for digital data n 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 n 24 th channel reserved for special sync byte Allows faster and more reliable reframing after a framing error 21

EEL 4930 University of Florida Fall 2003 Mixed Data DS-1 can carry mixed voice and data signals n n 24 channels used No sync byte Higher-level multiplexing n n Interleave DS-1 channels e. g. DS-2 is four DS-1 s giving 6. 312 Mbps 1. 544 Mbps x 4 = 6. 176 Mbps Remaining is for framing and control bits 22

EEL 4930 University of Florida Fall 2003 T 1/DS-1 Carrier Standard (1. 544 Mbps) Note n n T-1 (a. k. a. T 1) is digital carrier facility of 24 channels used to transmit DS-1 (a. k. a. DS 1) 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) n Hyphens are optional 23

EEL 4930 University of Florida Fall 2003 Multiplexing T 1 streams onto higher carriers 24

EEL 4930 University of Florida Fall 2003 SONET/SDH Synchronous Optical Network (SONET) n n Optical transmission interface Developed by Bell. Core, standardized by ANSI Synchronous Digital Hierarchy (SDH) n ITU-T standard compatible with SONET Signal Hierarchy n n 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) 51. 84 Mbps data rate, 50. 112 Mbps payload rate Could carry DS-3 or group of lower rate signals (e. g. DS 1 or DS 2) plus ITU-T TDM carrier rates (e. g. Level-1 at 2. 048 Mbps) n 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 n ITU-T lowest rate is STM-1 = STS-3 = OC-3 25

EEL 4930 University of Florida Fall 2003 SONET system Consists of switches, multiplexers, and repeaters, all connected by fiber Example path from source to destination shown below n n Fiber from one device to another is a section 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 Topology can be a mesh, but often a dual ring 26

EEL 4930 University of Florida Fall 2003 SONET physical layer 27

EEL 4930 University of Florida Fall 2003 SONET frame format Building block is STS-1 = OC-1 frame n 810 octets transmitted once every 125 us 810 x 8000 frames/s = 6. 48 MB/s = 51. 84 Mb/s n Viewed as matrix of 9 rows of 90 octets each First 3 columns (27 octets) are overhead n n 9 of 27 for section-related overhead Generated/checked at start/end of each section 18 of 27 for line-related overhead Generated/checked at start/end of each line Remainder is payload n n 9 x 87 = 783 octets 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 28

EEL 4930 University of Florida Fall 2003 SONET Frame Format Transmitted row by row (Left Right, Top Bottom) • 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 Lowest rate in ITU-T SDH is STM-1 @ 155. 52 Mb/s; thus STM-1 OC-3, STM-4 OC 12, etc. 29

EEL 4930 University of Florida Fall 2003 SONET STS-1 Overhead Octets 30

EEL 4930 University of Florida Fall 2003 Two back-to-back SONET frames SPE can begin anywhere within frame n n n 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) 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 31

EEL 4930 University of Florida Fall 2003 Multiplexing in SONET To prevent long runs of 0 s or 1 s 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. ) 32

EEL 4930 University of Florida Fall 2003 SONET and SDH multiplex rates Other interesting ones of late: OC-192 (9. 95328 Gb/s gross), OC-768 (39. 81312 Gb/s) 33

EEL 4930 University of Florida Fall 2003 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” n n e. g. OC-3 c, OC-12 c e. g. ATM on OC-3 c from single computer at 155. 52 Mbps Amount of actual user data in concatenated stream is slightly higher than in multiplexed stream n Example With OC-3 c 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 34

EEL 4930 University of Florida Fall 2003 Statistical TDM I can give you all of the bandwidth some of the time. How often/much depends on how busy you get. 35

EEL 4930 University of Florida Fall 2003 Statistical TDM (1) Movitation n n In Synchronous TDM, static allocation implies that many slots may be wasted Not all attached devices may be transmitting all the time Characteristics n n n Statistical TDM allocates time slots dynamically based on demand Multiplexer scans input lines and collects data until frame full 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 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 Address info. required to ensure proper delivery Implies more overhead per slot than with synch. TDM 36

EEL 4930 University of Florida Fall 2003 Statistical TDM (2) “On-demand” TDM n n 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 … 37

EEL 4930 University of Florida Fall 2003 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. 38

EEL 4930 University of Florida Fall 2003 Performance Data rate on line is lower than aggregate of the rates of inputs (@ peak; o/w on average) n n Buffer data contending for the link Buffer (queue) overflow a symptom of congestion Pros and cons of lower data rate on line n n Good in getting more for less Bad during peak periods Tradeoff between buffer size and line data rate n n Reduction in one requires increase in other! 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 39

EEL 4930 Buffer Size and Delay University of Florida Fall 2003 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. Example shown here w/ random arrivals Data in 1000 -bit frames M is effective capacity of muxed line 40

EEL 4930 University of Florida Fall 2003 Example: ADSL Asymmetrical Digital Subscriber Line (ADSL) Problem n In high-speed, wide-area, public digital networks, key challenge is link between subscriber and network Digital subscriber line n High expense to run new lines to homes and businesses Solution n Exploit installed base of twisted-pair wire already present from telephone service Can carry broader spectrum than just 4 k. Hz voice 1 MHz bandwidth or more 41

EEL 4930 University of Florida Fall 2003 ADSL Design Asymmetric n Greater capacity downstream than upstream Novel use of FDM n n Exploits 1 MHz capacity of twisted-pair wire Reserve lowest 25 k. Hz for voice Plain old telephone service (POTS) Additional bandwidth included here to prevent crosstalk between voice and data channels n n Use echo cancellation or FDM to allocate two bands (upstream and downstream) or channels Then use FDM within these two bands Serial bit stream split into multiple parallel bit streams Each portion carried on separate freq. band (subchannel) n Range up to 5. 5 km depending on quality of cable 42

EEL 4930 University of Florida Fall 2003 ADSL Channel Configuration Two alternatives n n FDM for POTS and two data bands (a) 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 Transmitter subtracts echo of its own transmission from incoming signal to recover signal sent by other side 43

EEL 4930 University of Florida Fall 2003 DMT (1) Discrete Multitone (DMT) Modulation Multiple carrier signals at different frequencies n n Transmission band divided into 4 k. Hz subchannels Sending bits on each subchannel Initialization n n DMT modem sends test signal on each subchannel to determine SNR Assigns more bits to subchannels with better SNR, less to poorer ones Each subchannel carries data at rate from 0 to 60 kbps Typical example below (increasing attenuation at higher frequencies) 44

EEL 4930 University of Florida Fall 2003 DMT (2) DMT Transmission n 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 k. Hz freq. band Sum of substream data rates equals total data rate 45

EEL 4930 University of Florida Fall 2003 ADSL/DMT Data Rate Common ADSL/DMT designs employ 256 subchannels on downstream channel n n 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 Depends on line distance and quality e. g. Bell. South “Fast. Access” DSL n Residential service Up to 1. 5 Mbps downstream Up to 256 kbps upstream 46

EEL 4930 University of Florida Fall 2003 Example: Cable Modem Based on cable TV, FDM of analog signals n Two FDM channels dedicated, one for upstream and one for downstream Each channel shared by number of subscribers n Statistical TDM used to allocate capacity Topology looks like a tree n n Head-end controller at provider is the root Cable modems at customers are the leaves Role of cable modem n n IP packets modulated by provider at head-end controller then sent downstream Cable modem demodulates signals back to IP packets 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 Thus, performance depends on several factors n n Bandwidth of channels Provider constraints e. g. local provider may connect to Internet via T-1 n Load from fellow local subscribers 47

EEL 4930 University of Florida Fall 2003 Cable Modem Operation Downstream n n Cable scheduler delivers data in small packets If more than one subscriber active, each gets fraction of downstream capacity e. g. may get 500 kbps to 1. 5 Mbps n Also used to allocate upstream time slots to subscribers Upstream n User requests timeslots on shared upstream channel Dedicated slots for this n Headend scheduler sends back assignment of future time slots to subscriber 48

EEL 4930 University of Florida Fall 2003 Cable Modem Scheme 49