Data Communications and

Networking

Chapter 6:

Multiplexing

Multiplexing

Simultaneous transmission of multiple signals across a single data link.

Can utilize higher capacity links without adding additional lines for each device – better utilization of bandwidth.

Multiplexer (MUX) on sender’s end; Demultiplexer (DEMUX) separates transmission stream and directs signals to intended receiving devices.

Multiplexing

Breaking up a higher speed circuit into several slower (logical) circuits.

Several devices can use it at the same time

Requires two multiplexer: one to combine; one to separate

Main advantage: cost

Fewer network circuits needed

Multiplexing Vs. no Multiplexing

Multiplexing Techniques

3 basic techniques

Frequency-division multiplexing – analog

Wave-division multiplexing – analog

Time-division multiplexing – digital

6.1 Frequency-Division Multiplexing

Analog technique - when bandwidth of link is greater than combined bandwidth of signals to be transmitted.

Signals from each sending device modulate different frequencies.

Modulated signals are combined into a single composite signal.

Bandwidth ranges are channels through which the signals travel, separated by guard bands.

Figure 6.3 FDM

Figure 6.4 FDM Multiplexing Process

Figure 6.5 FDM Demultiplexing Process

Figure 6.6 Example 1

Guardband

Guardbands needed to separate channels

To prevent interference between channels

Unused frequency bands wasted capacity

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?

FDM Example 2

Figure 6.8 Analog Hierarchy of Phone Network

Other Applications of FDM

AM and FM radio broadcasting

Each station uses a different carrier frequency, shifting its signal and multiplexing

Receiver filters (tunes) to the frequency desired

Same concept for TV broadcasting and first generation cell phones

6.2 Wave-Division Multiplexing

Use light signals transmitted through fiber-optic channels.

Very narrow bands of light are combined from several sources to make a wider band of light.

A prism is used to bend the light beams based on the angle of incidence and frequency.

Receiver’s DEMUX separates signals.

Figure 6.9 WDM

Figure 6.10 Prisms in WDM multiplexing and demultiplexing

WDM Applications

Application: SONET network

Multiple optical fiber lines are muxed/demuxed.

DWDM (dense WDM) allows muxing of large numbers of channels by spacing channels closer to one another to achieve greater efficiency.

Over hundred channels per fiber

Each transmitting at a rate of 10 Gbps

Aggregate dat rates in terabit range

6.3 Time-Division Multiplexing (TDM)

Process of combining digital signals from several sources whereby each connection occupies a portion of time in the link.

Link is sectioned by time rather than frequency.

Time Slots and Frames

Data flow of each connection is divided into units

Link combines one unit of each connection to make a frame

n input connections n time slots

Data rate of link must be n times the duration of a time slot to guarantee flow of data

Time slots are grouped into frames; one complete cycle of time slots; each slot dedicated to one device

Figure 6.11 TDM Frames

Interleaving

Process of taking a specific amount of data from each device in a regular order.

May be done by bit, byte, or any other data unit.

Synchronizing

Framing bits are used to alleviate timing inconsistencies that may be introduced.

Usually one or two synchronization bits are added to beginning and end of each frame that allows the DEMUX to synchronize with the incoming stream so it can separate time slots accurately.

Figure 6.15 Framing bits

Comparison of TDM

Time on the circuit shared equally

Each channel getting a specified time slot, (whether it has any data to send or not )

More efficient than FDM

Since TDM doesn’t use guardbands, (entire capacity can be divided up between channels)

Digital Signal (DS) Service

Hierarchy of digital signals

DS-0 – single channel of 64 Kbps

DS-1 – single service or 24 DS-0 channels multiplexed to yield 1.544 Mbps

DS-2 – single service or 4 DS-1 channels or 96 DS-0 channels to yield 6.312 Mbps

DS-3 – single service, 7 DS-2 channels, 28 DS-1 channels, or 672 DS-0 channels to yield 44.376 Mbps

DS-4 – 6 DS-3 channels, 42 DS-2 channels, 168 DS-1 channels, 4032 DS-0 channels to yield 274.176 Mbps

Figure 6.16 DS hierarchy

Table 6.1 DS and T lines rates

Service Line Rate

(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

T Lines

Digital lines designed for digital data, voice, or audio.

May be used for regular analog if sampled then multiplexed using TDM.

Figure 6.17 T-1 line for multiplexing telephone lines

Inverse TDM

Takes data stream from one high-speed line and breaks into portions and sends over several lower-speed lines simultaneously.

Used in bandwidth-on-demand where channels can be used for several applications requiring different transmission rates (i.e. voice, data, video).

Figure 6.21 Multiplexing and inverse multiplexing