Multiplexing 3 9 2009 Multiplexing Multiple links on 1 physical line Common on long haul high capacity links More cost effective when multiplexing is used Include frequency division multiplexing time division multiplexing code division multiplexing Multiplexing Techniques FDM TDM CDM Advantages and Disadvantages Frequency Division Multiplexing FDM Analog signaling is used to transmits the signals Examples Broadcast radio and television cable television the AMPS cellular phone systems The oldest multiplexing technique It involves analog signaling it is more susceptible to noise FDM Analog Signals Summed Summed Basic Operation Subcarriers Multiple signals can be modulated using different frequencies Signals with different frequencies can be Multiplexed together The multiplexed signal has a center frequency and bandwidth Larger than total BW of all multiplexed signals Shifted in frequency No overlap B Total BW FDM Example Transmitted TV signal Total BW is 6 MHZ Audio carrier operating at fca at f0 5 75 Color subcarrier operating at fcc at f0 4 799545 MHz Video subcarrier operating at fvc at f0 1 25 MHz CATV has a bandwidth of about 500 MHZ Many channels can be multiplexed together FDM Multiplexing three voice signals Voice signal has a range of 300 3400 KHz Recall FM Carrier Freq 1 x t cos 2 f c t x t cos 2 f mt cos 2 f ct cos 2 f c f m t cos 2 f c f m t Lower Sideband Fc Bw Upper Sideband Fc Bw s1 Analog Carrier Systems long distance links use an FDM hierarchy AT T USA and ITU T International variants Group Supergroup FDM of 5 group signals supports 60 channels on carriers between 420kHz and 612 kHz Mastergroup 12 voice channels 4kHz each 48kHz in range 60kHz to 108kHz FDM of 10 supergroups supports 600 channels so original signal can be modulated many times Analog Carrier Systems 4KHz each Range Wavelength Division Multiplexing FDM with multiple beams of light at different frequency carried over optical fiber links architecture similar to other FDM systems commercial systems with 160 channels of 10 Gbps lab demo of 256 channels 39 8 Gbps multiplexer consolidates laser sources 1550nm for transmission over single fiber Optical amplifiers amplify all wavelengths Demux separates channels at the destination also have Dense Wavelength Division Multiplexing DWDM Synchronous Time Division Multiplexing The original time division multiplexing The multiplexor accepts input from attached devices in a round robin fashion and transmit the data in a never ending pattern T 1 and ISDN telephone lines are common examples of synchronous time division multiplexing Synchronous Time Division Multiplexing TDM TDM Basic Operation 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 corresponding source must be quenched leaving empty slots error control errors detected handled on individual channel Example Only one station is transmitting Another Example Byte interleaving Characters are shuffled The receiver reassembles each channel Pulse Stuffing Reasons synchronizing data sources having clocks in different sources drifting having data rates from different sources not related by simple rational number Pulse Stuffing a common solution have 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 Example TDM of analog and digital sources with different transmission rates Analog Analog Sampler A D Converter 1 Example TDM of analog and digital sources with different transmission rates Conversing Analog To Digital Analog Sampler A D Converter 1 Example TDM of analog and digital sources with different transmission rates Digital R Digital Digital Signal Pulse Stuffing Digita l R Example TDM of analog and digital sources with different transmission rates 16 Bits 32 Bits 2 Bit Each Note Voice signal hence minimum BW requirement is 4KHz per bit Digital Carrier Systems long distance links use an TDM hierarchy AT T USA and ITU T 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 digitized data PCM 8000 samples per sec same format for 56kbps digital data can interleave DS 1 channels for higher rates e g DS 2 is four DS 1 at 6 312Mbps DS 1 Transmission Format Bit 8 indicated voice or data 6x8KHz or 5x9 6KHz or 10x4 8 KHz The first bit is used to indicate the subrate Statistical TDM in Synch TDM many slots are wasted Statistical TDM allocates time slots dynamically based on demand multiplexer scans input lines and collects data until frame full line data rate lower than aggregate input line rates may have problems during peak periods must buffer inputs Comparing Synchronous and Statistical TDM Frame Format Each slot has channel ID and possibly message length information http www trendcomms com multimedia training broadband 20networks web main m2 temari seccio9 t1e1 htm Performance of Statistical TDM Refer to your notes Asymmetrical Digital Subscriber Line ADSL link between subscriber and network uses currently installed twisted pair cable is Asymmetric bigger downstream than up uses Frequency division multiplexing has a range of up to 5 5km Its underlying technology is Discrete Multitone DMT http www cs tut fi tlt stuff adsl node6 html kuspektri High bit rate Digital Subscriber Line HDSL Connecting the subscriber to the PSTN http www thenetworkencyclopedia com d2 asp ref 872 Discrete Multitone DMT A multiplexing technique commonly found in digital subscriber line DSL systems The basic idea of DMT is to split the available bandwidth into a large number of subchannels DMT then combines hundreds of different signals or subchannels into one stream Each subchannel is quadrature amplitude modulated recall eight phase angles four with double amplitudes Theoretically 256 subchannels each transmitting 60 kbps yields 15 36 Mbps Unfortunately there is noise Discrete Multitone DMT Idea If some subchannel can not carry any data it can be turned off and the use of available bandwidth is optimized High attenuation at higher frequencies http www cs tut fi tlt stuff adsl node22 html kudmtex Discrete Multitone DMT How On initialization the DMT modem sends test signals to each subchannel to