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Computer Networks Set 7 Multiplexing Multiplexing FDM •TDM •Statistical TDM •CDM •Example: DSL • FDM System Analog Carrier Systems Ñ AT&T (USA) Ñ Hierarchy of FDM schemes Ñ Group Ñ 12 voice channels (4kHz each) = 48kHz Ñ Range 60kHz to 108kHz Ñ Supergroup Ñ 60 channel Ñ FDM of 5 group signals on carriers between 420kHz and 612 kHz Ñ Mastergroup Ñ 10 supergroups TDM System TDM Link Control Ñ No headers and tailers Ñ Data link control protocols not needed Ñ Flow control Ñ Data rate of multiplexed line is fixed Ñ If one channel receiver can not receive data, the others must carry on Ñ The corresponding source must be quenched Ñ This leaves empty slots Ñ Error control Ñ Errors are detected and handled by individual channel systems Framing Ñ No flag or SYNC characters bracketing TDM frames Ñ Must provide synchronizing mechanism Ñ Added digit framing Ñ One control bit added to each TDM frame Ñ Looks like another channel - ®control channel” Ñ Identifiable bit pattern used on control channel Ñ e.g. alternating 01010101… unlikely on a data channel Ñ Can compare incoming bit patterns on each channel with sync pattern Pulse Stuffing Ñ Problem - Synchronizing data sources Ñ Clocks in different sources drifting Ñ Data rates from different sources not related by simple rational number Ñ Solution - Pulse Stuffing Ñ Outgoing data rate (excluding framing bits) higher than sum of incoming rates Ñ Stuff extra dummy bits or pulses into each incoming signal until it matches local clock Ñ Stuffed pulses inserted at fixed locations in frame and removed at demultiplexer TDM of Analog and Digital Sources Digital Carrier Systems Ñ Hierarchy of TDM Ñ USA/Canada/Japan use one system Ñ ITU-T use a similar (but different) system Ñ US system based on DS-1 format Ñ Multiplexes 24 channels Ñ Each frame has 8 bits per channel plus one framing bit Ñ 193 bits per frame Digital Carrier Systems (2) Ñ For voice each channel contains one word of digitized data (PCM, 8000 samples per sec) Ñ Data rate 8000x193 = 1.544Mbps Ñ Five out of six frames have 8 bit PCM samples Ñ Sixth frame is 7 bit PCM word plus signaling bit Ñ Signaling bits form stream for each channel containing control and routing info Ñ Same format for digital data Ñ 23 channels of data Ñ 7 bits per frame plus indicator bit for data or systems control Ñ 24th channel is sync Mixed Data Ñ DS-1 can carry mixed voice and data signals Ñ 24 channels used Ñ No sync byte Ñ Can also interleave DS-1 channels Ñ Ds-2 is four DS-1 giving 6.312Mbps ISDN User Network Interface Ñ ISDN allows multiplexing of devices over single ISDN line Ñ Two interfaces Ñ Basic ISDN Interface Ñ Primary ISDN Interface Basic ISDN Interface (1) Ñ Digital data exchanged between subscriber and NTE - Full Duplex Ñ Separate physical line for each direction Ñ Pseudoternary coding scheme Ñ 1=no voltage, 0=positive or negative 750mV +/-10% Ñ Data rate 192kbps Ñ Basic access is two 64kbps B channels and one 16kbps D channel Ñ This gives 144kbps multiplexed over 192kbps Ñ Remaining capacity used for framing and sync Basic ISDN Interface (2) Ñ B channel is basic iser channel Ñ Data Ñ PCM voice Ñ Separate logical 64kbps connections o different destinations Ñ D channel used for control or data Ñ LAPD frames Ñ Each frame 48 bits long Ñ One frame every 250ms Frame Structure Primary ISDN Ñ Point to point Ñ Typically supporting PBX Ñ 1.544Mbps Ñ Based on US DS-1 Ñ Used on T1 services Ñ 23 B plus one D channel Ñ 2.048Mbps Ñ Based on European standards Ñ 30 B plus one D channel Ñ Line coding is AMI usingHDB3 Sonet/SDH Ñ Synchronous Optical Network (ANSI) Ñ Synchronous Digital Hierarchy (ITU-T) Ñ Compatible Ñ Signal Hierarchy Ñ Synchronous Transport Signal level 1 (STS-1) or Optical Carrier level 1 (OC-1) Ñ 51.84Mbps Ñ Carry DS-3 or group of lower rate signals (DS1 DS1C DS2) plus ITU-T rates (e.g. 2.048Mbps) Ñ Multiple STS-1 combined into STS-N signal Ñ ITU-T lowest rate is 155.52Mbps (STM-1) Statistical TDM Ñ In Synchronous TDM many slots are wasted Ñ Statistical TDM allocates time slots dynamically based on demand Ñ Multiplexer scans input lines and collects data until frame full Ñ Data rate on line lower than aggregate rates of input lines Statistical TDM Frame Formats Performance Ñ Output data rate less than aggregate input rates Ñ May cause problems during peak periods Ñ Buffer inputs Ñ Keep buffer size to minimum to reduce delay Buffer Size and Delay Code Division Multiplexing Ñ Rather than assign specific time slots or frequency bands to users, spreading codes are ditributed. Ñ By spreading the spectrum of the users, the energy from each user over a freq. band is small and can be super-imposed Ñ Inherently takes advantage of statistical distribution of input Ñ Allows for dynamic resource allocation Asymmetrical Digital Subscriber Line Ñ ADSL Ñ Link between subscriber and network Ñ Local loop Ñ Uses currently installed twisted pair cable Ñ Can carry broader spectrum Ñ 1 MHz or more ADSL Design Ñ Asymmetric Ñ Greater capacity downstream than upstream Ñ Frequency division multiplexing Ñ Lowest 25kHz for voice Ñ Plain old telephone service (POTS) Ñ Use echo cancellation or FDM to give two bands Ñ Use FDM within bands Ñ Range 5.5km ADSL Channel Configuration Discrete Multitone Ñ DMT Ñ Multiple carrier signals at different frequencies Ñ Some bits on each channel Ñ 4kHz subchannels Ñ Send test signal and use subchannels with better signal to noise ratio Ñ 256 downstream subchannels at 4kHz (60kbps) Ñ 15.36MHz Ñ Impairments bring this down to 1.5Mbps to 9Mbps DMT Transmitter xDSL Ñ High data rate DSL Ñ Single line DSL Ñ Very high data rate DSL Required Reading Ñ Stallings chapter 8 Ñ Web sites on Ñ ADSL Ñ SONET