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 3

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 – Basic Operation Analog Signals Summed Summed 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. Lower Sideband Upper Sideband s1 Fc-Bw Fc+Bw

Analog Carrier Systems 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 Supergroup FDM of 5 group signals supports 60 channels on carriers between 420kHz and 612 kHz Mastergroup 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 commercial systems with 160 channels of 10 Gbps lab demo of 256 channels 39.8 Gbps architecture similar to other FDM systems 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 Pulse Stuffing a common solution 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 1 Analog Sampler A/D Converter

Example – TDM of analog and digital sources with different transmission rates Conversing Analog To Digital 1 Analog Sampler A/D Converter

Example – TDM of analog and digital sources with different transmission rates Digital Signal Pulse Stuffing Digital

Note: Voice signal, hence minimum BW requirement is 4KHz per bit 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 determine its S/N Channels with higher S/N receive more bits Each subchannel is quadrature amplitude modulated (recall - eight phase angles, four with double amplitudes) Change alfa depending on the availability of the noise