MULTIPLEXING.doc

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MULTIPLEXING

AIM OF MULTIPLEXING:

1. To share an expensive resource that is bandwaidth.

2. To maximize the utilization of channel

MULTIPLEXING:

Multiplexing means combining multiple streams of information for transmission over a

shared medium. Or Multiplexing is sending multiple signals or streams of information ona carrier at the same time in the form of a single, complex signaland then recovering the

separate signals at the receiving end.

In analogtransmission, signals are commonly multiplexed using frequency-division

multiplexing (FM!, in "hich the carrierband"idthis divided into subchannels of

different frequency "idths, each carrying a signal at the same time in parallel.

In digitaltransmission, signals are commonly multiplexed using time-division

multiplexing (#M!, in "hich the multiple signals are carried over the same channel in

alternating time slots. In some optical fiber net"or$s, multiple signals are carried togetheras separate "avelengths of light in a multiplexed signal using dense "avelength division

multiplexing(%M!.

Figure &' Mux and emux
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multiplexeris the multiplexing device that combines the signals from the transmitters

and sends them over the high-speed channel. demultiplexeris the multiplexing device

via "hich the receivers are connected to the high-speed channel.

TYPES OF MULTIPLEXING:

A. FREQUENCY DIVISION MULTIPLEXING:

Frequency-division multiplexing, also calledFDM, ma$es it possible to share the

available frequency band on the high-speed channel by dividing it into a series of

narro"er-band channels so as to be able to continuously send signals coming from the

different lo"-speed channels over the high-speed channel.

#his process is used, in particular, on telephone lines and t"isted-pair physical

connections to increase the data rate.

Figure: basic concept of FDM


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Figure )' assignment of frequecies to channel "ith a guardband bet"een ad*acent

channels

Figure +' frequency domain plot of the channel allocation "ith a guardband visible

bet"een channels.

If the channels are very close to one another, it leads to inter-channel cross tal$. hannels

must be separated by strips of unused band"idth to prevent inter channel cross tal$.

#hese unsued channel bet"een each successful channel are $no"n as guardband. FM

are commonly used in radio broadcast and # net"or$s

APPLICATIONS:

&. FM is used for .M. adio / adio broadcasting.

). 001 234 band"idth is assign to each station.

+. FM is used for #. broadcasting.

5. In this frequency range is 61 7 &16 M34.

0. In this each channel is of 811 Mh4.

ADVANTAGES:

&. 9imple

). Inexpensive

+. :opular "ith adio, #, able #


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5. ll the receivers, cellular telephones, need not to be at the same location

0. It is not sensitive to propagation delays.

;. It allo"s maximum transmission lin$ usage.

DISADVANTAGES:

&. In FM there is need of filters, "hich are very expensive and complicated to construct

and design.

). nalog signal only having limited frequency range.

+. 9ometimes, it is necessary to use more complex linear amplifiers in FM system

B. TIME DIVISION MULTIPLEXING:

In time-division multiplexing, also calledTDM, the signals from the different lo"-speed

channels are sampled and transmitted successively on the high-speed channel by

allocating each channel in turn all of the band"idth, even if it does not have any data to

transmit.

Figure 5' #M

a. SYNCHRONOUS TIME DIVISION MULTIPLEXING:

In 9ynchronous #M many slots are "asted. 9tatistical #M allocates time slots

dynamically based on demand. Multiplexer scans input lines and collects data until frame

full. ata rate on line lo"er than aggregate rates of input lines


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Figure 0' 9ynchrous #M system "ith 5 senders

If one device generates data at a faster rate than other devices, then the multiplexor musteither sample the incoming data stream from that device more often than it samples the

other devices, or buffer the faster incoming stream.

If a device has nothing to transmit, the multiplexor must still insert a piece of data from

that device into the multiplexed stream.

9ynchronous #M is called synchronous mainly because each time slot is preassigned to

a fixed source. #he time slots are transmitted irrespective of "hether the sources have any

data to send or not. 3ence, for the sa$e of simplicity of implementation, channel capacity

is "asted. lthough fixed assignment is used #M, devices can handle sources of

different data rates.

Figure ;' synchronous #M system having slots unfilled "hen sources doesn


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b. STATISTICAL TIME DIVISION MULTIPLEXING:

One dra"bac$ of the #M approach is that m in the frame are "asted. It is because, if a

particular terminal has no data to transmit at particular instant of time, an empty time slot

"ill be transmitted. n efficient alternative to this synchronous #M is statistical #M,also $no"n as asynchronous #M or intelligent #M. It dynamically allocates the time

slots on demand to separate input channels, thus saving the channel capacity. s "ith

9ynchronous #M, statistical multiplexers also have many I=O lines "ith a buffer

associated to each of them. uring the input, the multiplexers scans the input buffers,

collecting data until the frame is filled and send the frame. t the receiving end, the

demultiplexer receives the frame and distributes the data to the appropriate buffers.

Figure 8' 9tatistical #M to avoid unfilled slots and ta$es less time to send data

9tatistical multiplexing is similar to time-division multiplexing except that it only

transmits lo"-speed channels that actually have data on the high-speed channel. #he

name of this type of multiplexing comes from the fact that the multiplexers base their

behavior on statistics concerning the data rate of each lo"-speed channel.

9ince the high-speed line does not transmit the empty channels, performance is better

than "ith time-division multiplexing.

statistical multiplexor transmits only the data from active "or$stations.

If a "or$station is not active, no space is "asted on the multiplexed stream.

statistical multiplexor accepts the incoming data streams and creates a frame containing

only the data to be transmitted.


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