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Unformatted text preview: Data and Computer Communications Communications
Chapter 8 – Multiplexing Eighth Edition by William Stallings Lecture slides by Lawrie Brown Multiplexing Multiplexing
It was impossible to get a conversation going, everybody was talking too much. Yogi Berra Multiplexing Multiplexing multiple links on 1 physical line common on long-haul, high capacity, links have FDM, TDM, STDM alternatives Frequency Division Multiplexing Multiplexing FDM FDM System System Overview Overview FDM Voiceband Example FDM Analog Carrier Systems Analog long-distance links use an FDM hierarchy AT&T (USA) and ITU-T (International) variants Group 12 voice channels (4kHz each) = 48kHz in range 60kHz to 108kHz FDM of 5 group signals supports 60 channels on carriers between 420kHz and 612 kHz FDM of 10 supergroups supports 600 channels Supergroup Mastergroup so original signal can be modulated many times Wavelength Division Multiplexing Multiplexing FDM with multiple beams of light at different freq carried over optical fiber links commercial systems with 160 channels of 10 Gbps lab demo of 256 channels 39.8 Gbps multiplexer consolidates laser sources (1550nm) for multiplexer transmission over single fiber transmission Optical amplifiers amplify all wavelengths Demux separates channels at the destination architecture similar to other FDM systems also have Dense Wavelength Division also Multiplexing (DWDM) Multiplexing Synchronous Time Division Multiplexing Multiplexing TDM System System Overview TDM Link Control TDM no headers and trailers data link control protocols not needed flow control data rate of multiplexed line is fixed iif one channel receiver can not receive data, the f others must carry on others corresponding source must be quenched leaving empty slots errors detected & handled on individual channel error control Data Link Control on TDM Data Framing Framing no flag or SYNC chars bracketing TDM frames must still provide synchronizing mechanism must between src and dest clocks between added digit framing one control bit added to each TDM frame identifiable bit pattern used on control channel eg. alternating 01010101…unlikely on a data channel compare incoming bit patterns on each channel with compare known sync pattern known Pulse Stuffing Pulse have problem of synchronizing data sources with clocks in different sources drifting also issue of data rates from different sources also not related by simple rational number not Pulse Stuffing a common solution have outgoing data rate (excluding framing bits) have higher than sum of incoming rates higher stuff extra dummy bits or pulses into each incoming stuff signal until it matches local clock signal stuffed pulses inserted at fixed locations in frame and stuffed removed at demultiplexer removed TDM Example TDM Digital Carrier Systems Digital long-distance links use an TDM hierarchy AT&T (USA) and ITU-T (International) variants US system based on DS-1 format can carry mixed voice and data signals 24 channels used for total data rate 1.544Mbps each voice channel contains one word of each digitized data (PCM, 8000 samples per sec) digitized same format for 56kbps digital data can interleave DS-1 channels for higher rates DS-2 is four DS-1 at 6.312Mbps DS-1 Transmission Format DS-1 SONET/SDH SONET/SDH Synchronous Optical Network (ANSI) Synchronous Digital Hierarchy (ITU-T) have hierarchy of signal rates Synchronous Transport Signal level 1 (STS-1) Synchronous or Optical Carrier level 1 (OC-1) is 51.84Mbps or carries one DS-3 or multiple (DS1 DS1C DS2) carries plus ITU-T rates (eg. 2.048Mbps) plus multiple STS-1 combine into STS-N signal ITU-T lowest rate is 155.52Mbps (STM-1) SONET Frame Format SONET Statistical TDM Statistical in Synch TDM many slots are wasted Statistical TDM allocates time slots Statistical dynamically based on demand dynamically multiplexer scans input lines and collects multiplexer data until frame full data line data rate lower than aggregate input line line rates may have problems during peak periods must buffer inputs Statistical TDM Frame Format Statistical Cable Modems Cable dedicate two cable TV channels to data transfer each channel shared by number of subscribers, each using statistical TDM using Downstream cable scheduler delivers data in small packets active subscribers share downstream capacity also allocates upstream time slots to subscribers user requests timeslots on shared upstream channel Headend scheduler notifies subscriber of slots to use Upstream Cable Modem Scheme Cable Asymmetrical Digital Subscriber Line (ADSL) Subscriber 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) uses echo cancellation or FDM to give two bands has a range of up to 5.5km ADSL Channel Configuration ADSL Discrete Multitone (DMT) Discrete 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 DMT Transmitter DMT xDSL xDSL High data rate DSL (HDSL) 2B1Q coding on dual twisted pairs up to 2Mbps over 3.7km 2B1Q coding on single twisted pair 2B1Q (residential) with echo cancelling (residential) up to 2Mbps over 3.7km DMT/QAM for very high data rates over separate bands for separate services Single line DSL Very high data rate DSL Summary Summary looked at multiplexing multiple channels looked on a single link on FDM TDM Statistical TDM ADSL and xDSL ...
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This note was uploaded on 04/06/2011 for the course EE 5363 taught by Professor Kang during the Spring '09 term at NYU Poly.
- Spring '09