Multiplexing in Communication system

8/11/2019 Multiplexing in Communication system

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Multiplexing in

Communication SystemBHAVIN V KAKANI

IT, NU


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Multiplexing

Multiplexing is the set of techniques thatallows the simultaneous transmission ofmultiple signals across a single data link.

A Multiplexer(MUX) is a device thatcombines several signals into a singlesignal.

A Demultiplexer(DEMUX) is a device thatperforms the inverse operation.


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Categories of Multiplexing


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Frequency-division Multiplexing (FDM)

FDM is an analog technique that can be applied whenthe bandwidth of a link is greater than the combinedbandwidths of the signals to be transmitted.


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In FDM signalsgenerated by eachdevice modulatedifferent carrierfrequencies. These

modulated signals arecombined into asingle compositesignal that can betransported by the

link.

FDM is an analog mul tiplexing technique

that combines signals.


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Modulating (Modulation)

In analog transmission, the sendingdevice produces a high frequency signal(a sine wave) that acts as a basis for the

information signal. This base signal iscalled the carrier signal.

Digital information is then modulated onthe carrier signal by modifying one or

more of its characteristics (amplitude,frequency, phase). This kind ofmodification is called modulation andthe information signal is called amodulating signal.


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Modulation (Amplitude Shift keyingASK)

Time-domain description


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Modulation (Amplitude Shift keyingASK)

Frequency-domain description


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In FDM signals generated by each device modulate

different carrier frequencies. These modulated

signals are combined into a single composite signal

that can be transported by the link.

Carrier frequencies are separated by enoughbandwidth to accommodate the modulated signal.

These bandwidth ranges arte the channels through

which various signals travel.

Channels must separated by strips of unusedbandwidth (guard bands) to prevent signal

overlapping.


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In FDM, signals are modulated onto separatecarrier frequencies using either AM or FMmodulation.


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Example 1

Assume that a voice channel occupies a bandwidth of 4

KHz. We need to combine three voice channels into a link

with a bandwidth of 12 KHz, from 20 to 32 KHz. Show

the configuration using the frequency domain without the

use of guard bands.

Solution

Shift (modulate) each of the three voice channels to adifferent bandwidth, as shown in Figure 6.6.


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Example 1


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Example 2

Five channels, each with a 100-KHz bandwidth, are to be

multiplexed together. What is the minimum bandwidth of

the link if there is a need for a guard band of 10 KHz

between the channels to prevent interference?

Solution

For five channels, we need at least four guard bands.

This means that the required bandwidth is at least

5 x 100 + 4 x 10 = 540 KHz,

as shown in Figure 6.7.


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Example 2


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Wave-division Multiplexing (WDM)

Wave-division multiplexing isconceptually the same as FDM, exceptthat multiplexing and demultiplexing

involve light signals transmitted throughfiber-optic channels.

The purpose is to combine multiple lightsources into one single light at the

multiplexer and do the reverse at thedemultiplexer.

Combining and splitting of light sourcesare easily handled by a prism.


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The only difference with electrical FDM is that onoptical system is completely passive and thushighly reliable.

Due to its enormous bandwidth of around 25KGHz, there is a potential of multiplexing manychannels together over long routes.

Potential application of WDM is in the FTTC (FiberTo The Curb) systems.

Important application of WDM is the SONETnetworks in which multiple optical fiber lines aremultiplexed.


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Time-division Multiplexing (TDM)

Time-division multiplexing (TDM) is a digital process thatcan be applied when the data rate capacity of thetransmission medium is greater than the data raterequired by the sending and receiving devices.


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Data Rate

For T DM, the data rate of the multiplexed signal isalways n times the data rate of the individualsources, where n is the number of sources.

If 3 sources are multiplexed, then the data rate ofthe TDM signal is 3 times higher than the individualdata rate.


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TDM

TDM is a digital mul tiplexing technique to

combine data.


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TDM can be implemented in two ways:synchronous TDM and asynchronous TDM.

In synchronous time-division multiplexing, the

term synchronous means that the multiplexerallocates exactly the same time slot to eachdevice at all times, whether or not a devicehas anything to transmit.

Frames

Time slots are grouped into frames. A frameconsists of a one complete cycle of timeslots, including one or more slots dedicatedto each sending device.


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TDM frames


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Example 5

Four 1-Kbps connections are multiplexed together. A unit

is 1 bit. Find (1) the duration of 1 bit before multiplexing,

(2) the transmission rate of the link, (3) the duration of a

time slot, and (4) the duration of a frame?

Solution

We can answer the questions as follows:

1. The duration of 1 bit is 1/1 Kbps, or 0.001 s (1 ms).2. The rate of the l ink is 4 Kbps.

3. The duration of each time slot 1/4 ms or 250 s.

4. The duration of a frame 1 ms.


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I nter leaving


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Example 6

Four channels are multiplexed using TDM. If each

channel sends 100 bytes/s and we multiplex 1 byte per

channel, show the frame traveling on the link, the size of

the frame, the duration of a frame, the frame rate, and the

bit rate for the link.

Solution

The multiplexer is shown in Figure 6.15.

E l 6


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Example 6


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Example 7

A multiplexer combines four 100-Kbps channels using a

time slot of 2 bits. Show the output with four arbitrary

inputs. What is the frame rate? What is the frame

duration? What is the bit rate? What is the bit duration?

Solution

Figure 6.16 shows the output for four arbitrary inputs.

E l 7


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Example 7


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Framing Bits

Because the time slot order in a synchronous

TDM system does not vary from frame to

frame, very little overhead information needs

to be included in each frame. However, one

or more synchronization bits are usually

added to the beginning of each frame.

These bits, called framing bits, allows thedemultiplexer to synchronize with the

incoming stream so that it can separate the

time slot accurately.

F i bit


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Framing bits


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Example

Suppose that we have four input devices on a synchronous TDM

link, where the transmissions are interleaved by character. If

each device is generating 250 characters per second, and each

frame is carrying 1 character from each device, what is the

minimum data rate of this link?


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The link must be able to carry250 frames per second.

If we assume that eachcharacter consists of 8bits, then each frame has4x8 + 1= 33 bits ( 32 bits forthe four characters plus 1framing bit).

On the other hand, eachdevice is creating2000bps, because 250characters per second x 8bits =2000 bits per second,and the link is carrying8250 bps, because 250frames per second x33 bitsis 8250 bps.


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Example 8

We have four sources, each creating 250 characters persecond. If the interleaved unit is a character and 1

synchronizing bit is added to each frame, find (1) the data

rate of each source, (2) the duration of each character in

each source, (3) the frame rate, (4) the duration of each

frame, (5) the number of bits in each frame, and (6) the

data rate of the link.

Solution

See next slide.


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Solution (continued)

We can answer the questions as follows:

1. The data rate of each source is 2000 bps = 2 Kbps.

2. The duration of a character is 1/250 s, or 4 ms.

3. The link needs to send 250 frames per second.4. The duration of each frame is 1/250 s, or 4 ms.

5. Each frame is 4 x 8 + 1 = 33 bits.

6. The data rate of the link is 250 x 33, or 8250 bps.


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Example 9

Two channels, one with a bit rate of 100 Kbps and

another with a bit rate of 200 Kbps, are to be multiplexed.How this can be achieved? What is the frame rate? What

is the frame duration? What is the bit rate of the link?

Solution

We can allocate one slot to the first channel and two slots

to the second channel. Each frame carries 3 bits. The

frame rate is 100,000 frames per second because itcarries 1 bit from the first channel. The frame duration is

1/100,000 s, or 10 ms. The bit rate is 100,000 frames/s x

3 bits/frame, or 300 Kbps.

DS hierarchy


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DS hierarchy


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Table 6.1 DS and T l ines rates

Service LineRate

(Mbps)

Voice

Channels

DS-1 T-1 1.544 24

DS-2 T-2 6.312 96

DS-3 T-3 44.736 672

DS-4 T-4 274.176 4032


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Table 6.2 E line rates

E LineRate

(Mbps)

Voice

Channels

E-1 2.048 30

E-2 8.448 120

E-3 34.368 480

E-4 139.264 1920


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Asynchronous TDM

Synchronous TDM does not guarantee that the full

capacity of a link is used. Because the time slots arepreassigned and fixed, whenever a connected device

is not transmitting, the corresponding slot is empty.

Asynchronous time-division multiplexing, or statistical

time-division multiplexing, is designed to avoid thistype of waste.

Like synchronous TDM, asynchronous TDM allows a

number of lower-speed input lines to be multiplexed

to a single higher-speed line. However, in

asynchronous TDM the total speed of the input lines

can be greater than the capacity of the link.


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In an asynchronous system,

if we have n input lines, the

frame contains no more than

m slots, with m less than n.

The number of time slots in

an asynchronous TDM

frame (m) is based onstatistical analysis of the

number of input lines that

are likely to be transmitting

at any given time.

In this case any slot is

available to any of the

attached input lines that has

data to send.


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Asynchronous TDM

Addressing and Overhead

In asynchronous TDM each time slot mustcarry an address telling the demultiplexerhow direct the data. This address, forlocal use only, is attached by themultiplexer and discarded by the

demultiplexer once it has been read. Asynchronous TDM is efficient only when

the size of the time slots kept relativelylarge.


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Inverse Multiplexing

Inverse multiplexing takesthe data stream from onehigh-speed line andbreaks it into portions thatcan be sent across severallower-speed linessimultaneously, with noloss in the collective datarate.

Figure 6.21 Multiplexing and inverse mul tiplexing


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Multiplexing in Communication system

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