ECOM 4314 Data Communications Fall September, 2010

ECOM 4314

Data CommunicationsFall September, 2010

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Chapter 6 OutlineChapter 6 Outline

Frequency-Division Multiplexing Wavelength-Division Multiplexing Synchronous Time-Division Multiplexing Statistical Time-Division Multiplexing

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Bandwidth utilizationBandwidth utilization

Bandwidth utilization is the wise use of available bandwidth to achieve

specific goals.

Efficiency can be achieved by multiplexing; i.e., sharing of the

bandwidth between multiple users.

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Bandwidth utilizationBandwidth utilization

Whenever the bandwidth of a medium linking two devices is greater than the bandwidth needs of the devices, the link can be shared.

Multiplexing is the set of techniques that allows the (simultaneous) transmission of multiple signals across a single data link.

As data and telecommunications use increases, so does traffic

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MultiplexingMultiplexing

Figure 6.1 Dividing a link into channels

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MultiplexingMultiplexing

Figure 6.2 Categories of multiplexing

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Frequency Division Multiplexing (FDM)Frequency Division Multiplexing (FDM)

FDM is an analog multiplexing technique that combines analog signals.

It uses the concept of modulation discussed in Ch 5.

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FDMFDM

Figure 6.3 Frequency-division multiplexing (FDM)

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FDMFDM

Figure 6.4 FDM process

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FDMFDM

Figure 6.5 FDM demultiplexing example

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ExampleExample

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. Assume there are no guard bands.

SolutionWe shift (modulate) each of the three voice channels to a different bandwidth, as shown in Figure 6.6. We use the 20- to 24-kHz bandwidth for the first channel, the 24- to 28-kHz bandwidth for the second channel, and the 28- to 32-kHz bandwidth for the third one. Then we combine them as shown in Figure 6.6.

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ExampleExample

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ExampleExample

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?

SolutionFor five channels, we need at least four guard bands. This means that the required bandwidth is at least

5 × 100 + 4 × 10 = 540 kHz, as shown in Figure 6.7.

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ExampleExample

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Wavelength-division multiplexingWavelength-division multiplexing

WDM is an analog multiplexing technique to combine optical signals.

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WDMWDM

Figure 6.10 Wavelength-division multiplexing (WDM)

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Figure 6.11 Prisms in wavelength-division multiplexing and demultiplexing

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Time Division Multiplexing (TDM)Time Division Multiplexing (TDM)

TDM is a digital multiplexing technique for combining several low-rate digital channels into one high-rate one.

Instead of sharing banswidth as in FDM , TDM share time

TDM has two different schemes Synchronous Statistical

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Figure 6.12 Time Division Multiplexing (TDM)

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Synchronous TDMSynchronous TDM

The data flow of each input connection is divided into units, where each input occupies one input time slot.

A unit can one bit , one charcter, one block

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Synchronous TDMSynchronous TDM

Figure 6.13 Synchronous time-division multiplexing

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Synchronous TDMSynchronous TDM

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InterleavingInterleaving

The process of taking a group of bits from each input line for multiplexing is called interleaving.

We interleave bits (1 - n) from each input onto one output.

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InterleavingInterleaving

Figure 6.15 Interleaving

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Data Rate ManagementData Rate Management

Not all input links maybe have the same data rate.

Some links maybe slower. There maybe several different input link speeds

There are three strategies that can be used to overcome the data rate mismatch: multilevel, multislot and pulse stuffing

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Data Rate ManagementData Rate Management

Multilevel: used when the data rate of the input links are multiples of each other.

Multislot: used when there is a GCD between the data rates. The higher bit rate channels are allocated more slots per frame, and the output frame rate is a multiple of each input link.

Pulse Stuffing: used when there is no GCD between the links. The slowest speed link will be brought up to the speed of the other links by bit insertion, this is called pulse stuffing.

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Multilevel multiplexingMultilevel multiplexing

Figure 6.19 Multilevel multiplexing

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Multiple-slot multiplexingMultiple-slot multiplexing

Figure 6.20 Multiple-slot multiplexing

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Pulse stuffingPulse stuffing

Figure 6.21 Pulse stuffing

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SynchronizationSynchronization

To ensure that the receiver correctly reads the incoming bits, i.e., knows the incoming bit boundaries to interpret a “1” and a “0”, a known bit pattern is used between the frames.

The receiver looks for the anticipated bit and starts counting bits till the end of the frame.

Then it starts over again with the reception of another known bit.

These bits (or bit patterns) are called synchronization bit(s).

They are part of the overhead of transmission.

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SynchronizationSynchronization

Figure 6.22 Framing bits

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Figure 6.23 Digital hierarchy

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T-1 line for multiplexingT-1 line for multiplexing

Figure 6.24 T-1 line for multiplexing telephone lines

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Inefficient use of BandwidthInefficient use of Bandwidth

Sometimes an input link may have no data to transmit.

When that happens, one or more slots on the output link will go unused.

That is wasteful of bandwidth

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Empty slotsEmpty slots

Figure 6.18 Empty slots

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TDM slot comparisonTDM slot comparison

Figure 6.26 TDM slot comparison

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SPREAD SPECTRUMSPREAD SPECTRUM

In spread spectrum (SS), we combine signals from different sources to fit into a larger bandwidth, but our goals are to prevent eavesdropping and jamming.

To achieve these goals, spread spectrum techniques add redundancy.

Frequency Hopping Spread Spectrum (FHSS)

Direct Sequence Spread Spectrum (DSSS)

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Spread SpectrumSpread Spectrum

A signal that occupies a bandwidth of B, is spread out to occupy a bandwidth of Bss

All signals are spread to occupy the same bandwidth Bss

Signals are spread with different codes so that they can be separated at the receivers.

Signals can be spread in the frequency domain or in the time domain.

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Spread SpectrumSpread Spectrum

Figure 6.27 Spread spectrum

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Frequency hopping spread Frequency hopping spread spectrum (FHSS)spectrum (FHSS)

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Frequency selection in FHSSFrequency selection in FHSS

Figure 6.29 Frequency selection in FHSS

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Figure 6.30 FHSS cycles

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Direct Sequence Spread SpectrumDirect Sequence Spread Spectrum

Figure 6.32 DSSS

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DSSSDSSS

Figure 6.33 DSSS example

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ReferencesReferences

Ayman, Maliha, “Data Communication Lectures”, IUG.

BehrouzA. Forouzan , “Data Communications and Networking”, 4rdEdition, Chapter6, 2007

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