ECOM 4314 Data Communications Fall September, 2010

Data Communication2 Chapter 6 Outline  Frequency-Division Multiplexing  Wavelength-Division Multiplexing  Synchronous Time-Division Multiplexing  Statistical Time-Division Multiplexing

Data Communication3 Bandwidth 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.

Data Communication4 Bandwidth 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

Data Communication5Multiplexing Figure 6.1 Dividing a link into channels

Data Communication6Multiplexing Figure 6.2 Categories of multiplexing

Data Communication7 Frequency Division Multiplexing (FDM) FDM is an analog multiplexing technique that combines analog signals. It uses the concept of modulation discussed in Ch 5.

Data Communication8FDM Figure 6.3 Frequency-division multiplexing (FDM)

Data Communication9FDM Figure 6.4 FDM process

Data Communication10FDM Figure 6.5 FDM demultiplexing example

Data Communication11Example 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. Solution We 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.

Data Communication12Example

Data Communication13Example 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 × × 10 = 540 kHz, as shown in Figure 6.7.

Data Communication14Example

Data Communication15 Wavelength-division multiplexing WDM is an analog multiplexing technique to combine optical signals.

Data Communication16WDM Figure 6.10 Wavelength-division multiplexing (WDM)

Data Communication17 Figure 6.11 Prisms in wavelength-division multiplexing and demultiplexing

Data Communication18 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

Data Communication19 Figure 6.12 Time Division Multiplexing ( TDM)

Data Communication20 Synchronous 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

Data Communication21 Synchronous TDM Figure 6.13 Synchronous time-division multiplexing

Data Communication22 Synchronous TDM

Data Communication23Interleaving  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.

Data Communication24Interleaving Figure 6.15 Interleaving

Data Communication25 Data 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

Data Communication26 Data 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.

Data Communication27 Multilevel multiplexing Figure 6.19 Multilevel multiplexing

Data Communication28 Multiple-slot multiplexing Figure 6.20 Multiple-slot multiplexing

Data Communication29 Pulse stuffing Figure 6.21 Pulse stuffing

Data Communication30Synchronization  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.

Data Communication31Synchronization Figure 6.22 Framing bits

Data Communication32 Figure 6.23 Digital hierarchy

Data Communication33 T-1 line for multiplexing Figure 6.24 T-1 line for multiplexing telephone lines

Data Communication34 Inefficient 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

Data Communication35 Empty slots Figure 6.18 Empty slots

Data Communication36 TDM slot comparison Figure 6.26 TDM slot comparison

Data Communication37 SPREAD 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)

Data Communication38 Spread 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.

Data Communication39 Spread Spectrum Figure 6.27 Spread spectrum

Data Communication40 Frequency hopping spread spectrum (FHSS)

Data Communication41 Frequency selection in FHSS Figure 6.29 Frequency selection in FHSS

Data Communication42 Figure 6.30 FHSS cycles

Data Communication43 Direct Sequence Spread Spectrum Figure 6.32 DSSS

Data Communication44DSSS Figure 6.33 DSSS example

Data Communication45References  Ayman, Maliha, “Data Communication Lectures”, IUG.  BehrouzA. Forouzan, “Data Communications and Networking”, 4rdEdition, Chapter6, 2007

Data Communication46Thanks