Multiplexing: Carrying Multiple Signals On One Medium Is Called Multiplexing

Multiplexing

Carrying multiple signals on one medium is called multiplexing 1

Need for Multiplexing ❚ The higher the data rate, the more costeffective the transmission facility. That is for a given application and for a given distance, the cost per kbps declines with an increased in the data rate of the transmission facility. Similarly, the cost of transmission and receiving equipment, per kbps, also declines with increasing data rate. ❚ Most individual data-communicating devices require relatively modest data-rate support. For example, a data rate of between 9600 2

Benefits of Multiplexing ❚ Multiplexing in data communication tries to share a link between multiple stations so each one gets to use the link according to its need. Multiplexing tries to share the capacity of a link. ❚ Multiplexing results in more efficient utilization of data communication facilities. Naturally there is complexity associated with 3

Terminal Configurations

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Frequency Division Multiplexing - FDM ❚ The oldest used technique used for multiplexing. ❚ Possible when the useful bandwidth of the medium exceeds that of the signals it has to carry. ❚ Each signal is modulated on a different carrier frequency. This results in shifting the spectrum of the signal around the carrier frequency. ❚ Sufficient guard-band is given so those neighboring signals do not overlap in the 5

Frequency Division Multiplexing - FDM ❚ At the receiving end each signal is extracted by first passing it through a band-pass filter and then demodulating with the same carrier frequency that was used to modulate the signal. ❚ The signals carried using FDM may be analog signals or may be analog signals representing digital data. However FDM is mostly a technique from the era of analog communications. ❚ In FDM a device uses some of the channel all of the time. ❚ FDM is used in radio and television broadcasting. FDM is also used in high 6

Frequency Division Multiplexing - FDM ❚ Frequency division multiplexing (FDM) achieves multiplexing by using different carrier frequencies ❚ Receiver can "tune" to specific frequency and extract modulation for that one channel ❚ Frequencies must be separated to avoid interference - “Wastes” potential signal

bandwidth for guard channels ❚ Only useful in media that can carry multiple signals with different frequencies - high-

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Frequency Division Multiplexing ❚ The standard of the analog telephone network ❚ The standard in radio broadcasting ❚ The standard for video ❙ broadcast ❙ cable ❙ satellite

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Frequency Division Multiplexing Diagram

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FDM Process

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Allocation of Frequency Bands for Broadcast Television

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Time Division Multiplexing TDM ❚ Time division multiplexing is more suitable for digital data. ❚ TDM can be used when the data rate available on a communication link exceeds the data rate required by any one of the sources. ❚ In TDM each source that is to use the link fills up a buffer with data. A TDM multiplexer scans the buffers in some predetermined order and transmits bits from each source one after the other. 12

Synchronous Time Division Multiplexing (TDM) ❚ requires digital signaling & transmission ❚ requires data rate = sum of inputs + framing ❚ data rate much higher than equivalent analog bandwidth uses ❚ separates data streams in time not frequency ❚ the standard of the modern digital telephone system ❙ US, Canada, Japan: DS-0, DS-1 (T-1), DS-3 (T-3), ... ❙ Europe, elsewhere: E-1, E3, ... 13

Time Division Multiplexing TDM ❚ The sources are either pure digital (i. e. computers) or analog (voice or video) converted into digital using PCM. ❚ In TDM a device uses all of the channel some of the time. ❚ Time division multiplexing uses a single carrier and sends data streams sequentially ❚ Transmitter/receiver pairs share single channel ❚ Basis for most computer networks using shared media

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Synchronous Time Division Multiplexing ❚ Data rate of medium exceeds data rate of digital signal to be transmitted ❚ Multiple digital signals interleaved in time ❚ May be at bit level of blocks ❚ Time slots preassigned to sources and fixed ❚ Time slots allocated even if no data ❚ Time slots do not have to be evenly 15

Time Division Multiplexing

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TDM Process

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Digital Multiplexing Hierarchy ❚ This method could be further used for increasing the number of channels yet again from 32 channels to 4*32 channels and so on. Each increase is of course accompanied by a suitable increase in the bit rate of the line. ❚ Well, we succeeded in sending 32 channels over a single line, but how will the receiving end (the demultiplexer) know which bit 18

Synchronisation ❚ Special bits in the bit stream are used for synchronisation. These bits tell the demultiplexer where a new 32 byte group starts so it will know how to divide the following bits between the channels. No synchronisation is needed for distinguishing between each of the 32 channels. ❚ If we multiplex several 32 channels together, more synchronisation bits are added for distinguishing between 19 the different groups.

Digital Data and Video ❚ The upside for transmitting digital data or video is that no analog to digital conversion is needed. Instead, the bit stream in directly inserted into the multiplexer. Video, which needs a much higher bit rate than 64 Kbps is usually inserted directly into the second level multiplexer, thus allowing a bit rate of 1.5-2 Mbps. 20

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 ❙ The corresponding source must be quenched ❙ This leaves empty slots

❚ Error control ❙ Errors are detected and handled by

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Data Link Control on TDM

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Framing ❚ No flag or SYNC characters bracketing TDM frames ❚ Must provide synchronizing mechanism ❚ Added digit framing ❙ One control bit added to each TDM frame ❘ Looks like another channel - “control channel”

❙ Identifiable bit pattern used on control channel ❙ e.g. alternating 01010101…unlikely on a 23 data channel

Pulse Stuffing ❚ Problem - Synchronizing data sources ❚ Clocks in different sources drifting ❚ Data rates from different sources not related by simple rational number ❚ Solution - Pulse Stuffing ❙ 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

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Basic ISDN Interface (1) ❚ Digital data exchanged between subscriber and NTE - Full Duplex ❚ Separate physical line for each direction ❚ Pseudoternary coding scheme ❙ 1=no voltage, 0=positive or negative 750mV +/-10%

❚ Data rate 192kbps ❚ Basic access is two 64kbps B channels and one 16kbps D channel ❚ This gives 144kbps multiplexed over 192kbps ❚ Remaining capacity used for framing 25

Basic ISDN Interface (2) B channel is basic user channel Data PCM voice Separate logical 64kbps connections o different destinations ❚ D channel used for control or data ❚ ❚ ❚ ❚

❙ LAPD frames

❚ Each frame 48 bits long ❚ One frame every 250µs 26

Frame Structure

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Primary ISDN ❚ Point to point ❚ Typically supporting PBX ❚ 1.544Mbps ❙ Based on US DS-1 ❙ Used on T1 services ❙ 23 B plus one D channel

❚ 2.048Mbps ❙ Based on European standards ❙ 30 B plus one D channel ❙ Line coding is AMI using HDB3 28

Statistical TDM ❚ The TDM described above is called synchronous TDM. ❚ In synchronous TDM time slots are dedicated to sources in advance even if they have no data to send in the time slot. This might be wasteful. ❚ Statistical TDM allocates time slots on demand and uses extra framing, synchronisation, addressing bits (HDLC) so that each frame can be sent to the proper receiving end 29

Statistical Time Division Multiplexing ❚ requires digital signaling & transmission ❚ data rate capacity required can be well below the sum of connected capacity ❚ same concepts as synchronous TDM ❚ uses memory buffers to hold excess input ❚ widely used for remote communications with multiple terminals ❚ similar to the medium-sharing done

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