Data and Computer Communications Chapter 8 – Multiplexing

Dr. Rami D Halloush Hijjawi Faculty for Engineering Technology

Dr. Rami Dmaithan Halloush

Data and Computer CommunicationsChapter 8 – Multiplexing

Department of Telecommunications EngineeringHijjawi Faculty for Engineering Technology

Yarmouk University


Multiplexing

• Multiple links on 1 physical line• Common on long-haul, high capacity, links


Multiplexing• The widespread use of multiplexing is due– The higher the data rate the more cost effective

transmission facility – Individual data communicating devices requires a

modest data rate support


Multiplexing• Frequency division multiplexing

• Time division multiplexing

• Statistical TDM


Frequency Division Multiplexing• When bandwidth on transmission medium exceeds

required for signals transmission• Number of signals can be carried simultaneously• Each signal is modulated on a different carrier

frequency• Carrier frequencies are sufficiently separated • Freq. bands of signals don’t significantly overlap


Frequency Division Multiplexing• Six signal sources are fed into a mux• Mux modulates each signal onto

different frequency• Each modulated signal requires certain

bandwidth centered on its carrier frequency (channel)

• Channels are separated by guard bands (unused portions of the spectrum)

• Composite signal transmitted is analog • Input may be digital or analog• Digital input passed through modem• Analog signal modulated move it to

appropriate frequency band


Frequency Division Multiplexing

• At sender: n analog or digital signals multiplexed on the same medium

• Each signal is modulated onto a subcarrier fi

• Modulated signals summed to form a composite signal• Composite signal maybe modulated; i.e., shifted as a whole to

another carrier frequency fc by additional modulation step• At receiver: FDM signal is demodulated to retrieve the

composite signals• Composite signal is passed through n bandpass filters each

centered on fi to split the signal into components• Each component is demodulated to recover the original signal


FDMSystem

Overview


FDM Voiceband Example• Three voice signals to be transmitted simultaneously • The bandwidth of the voice signal is 4KHz• Effective spectrum of 300 to 3400 Hz


FDM Voiceband Example• Amplitude modulate at

64KHz carrier frequency (8.5b)

• Modulated signals has 8KHz bandwidth

• Transmit lower sideband for efficient use of bandwidth

• Three voice signals three carrier frequencies 64, 68, and 72KHz (8.5c)


Synchronous Time Division Multiplexing

• Multiple signals can be carried on a single transmission path by interleaving portion of each signal in time

• Interleaving at bit level, byte level or larger quantity• Data organized into frames• Each frame contains a cycle of time slots• In each frame one or more slots are dedicated to each data source• Sequence of slots dedicated to one source from frame to frame is a

channel• At receiver, interleaved data are demultiplexed (routed) to

appropriate destination buffer• For each input source there is an identical destination that will

receive output data at the same rate at which it was generated



• Synchronous because time slots are preassigned to sources and fixed

• Time slot for a source whether there is data or not• This is the case of FDM• Capacity is wasted to achieve implementation simplicity• Although fixed time slot assignment is used, sources with

lower data rates are assigned less number of time slots in each cycle than sources with higher data rates

• TDM device handles sources of different data rates






TDM Link Control• Transmitted data stream does not have headers and trailers• Data link control protocols not needed• Flow control is not needed (from MUX to DEMUX)– 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– Channel in question will carry empty slots– Frames as a whole maintain the same transmission rate


TDM Link Control• Error control– Does not require retransmission of entire TDM frame

because of a channel error– Devices use the other channels don’t need retransmission – Errors detected & handled on individual channel

• Flow and error control are provided on a per channel basis by using data link protocol as HDLC


Data Link Control on TDM


Framing• Link control protocol is not needed to manage overall TDM link• No flag or SYNC chars bracketing TDM frames• Must still provide synchronizing mechanism between src (MUX) and dest (DEMUX)

clocks• If not synchronized data is lost on all channels• “Added digit” framing

– Most commonly used framing mechanism– One control bit added to each TDM frame– Identifiable bit pattern used to control channel– Ex: alternating 01010101…unlikely on a data channel– Demux: Compare incoming bits of one frame position to expected pattern– No match, successive bits searched until pattern persist over multiple frames– Once synch established, receiver (Demux) monitors framing bit channel– If pattern breaks, receiver (Demux) enters framing search mode

Dr. Rami D Halloush Hijjawi Faculty for Engineering Technology 19

Framing


Pulse Stuffing• Motivation:• 1. A problem of synchronizing data sources with different clocks

– Clocks in different sources vary; each source has a different Tb; some sources are fast and some are slow

– Mux and Demux sample using a different local clock, all sources’ clocks must match this clock while keeping the slot size fixed

• 2. Data rates from different sources are not related by simple rational number

• Pulse Stuffing a common solution– 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


Statistical TDM• In synchronous TDM many slots are wasted• Statistical TDM allocates time slots dynamically

based on demand• n input lines of multiplexer, k time slots where k<n • Multiplexer scans input lines and collects data until

frame full• Line data rate lower than aggregate input lines rate • Synchronous and statistical if have the same

multiplexed line data rate (=aggregate input lines data rates) then statistical can support more devices


Statistical TDM

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Data and Computer Communications Chapter 8 – Multiplexing