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Chapter 6. Multiplexing

1. FDM

2. WDM

3. TDM

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Multiplexing: Dividing a Link into Channels

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Categories of Multiplexing

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Frequency Division Multiplexing

• Signals modulate different carrier frequencies

• Modulated signals are combined into a composite signal

• Channel - Bandwidth range to accommodate a modulated signal

• Channels must be separated by strips of unused bandwidth (guard band) to prevent overlapping

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FDM Process

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FDM Demultiplexing Example

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FDM: Example 1

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FDM: Example 2

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FDM: Example 3

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Analog Hierarchy

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Wave Division Multiplexing

• Conceptually the same as FDM• Light signals transmitted through fiber optic channels• Combining different signals of different frequencies

(wavelengths)

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Prisms in WDM

• Combining and splitting of light sources are easily handled by a prism

• Prism bends a light beam based on the incidence angle and the frequency

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Time Division Multiplexing

• Portions of signals occupy the link sequentially

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TDM

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TDM: Example 1

• 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?

1. The duration of 1 bit is 1/1 Kbps, or 0.001 s (1 ms).

2. The rate of the link is 4 Kbps.

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

4. The duration of a frame 1 ms.

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Interleaving

• Interleaving can be done by bit, by byte, or by any other data unit

• The interleaved unit is of the same size in a given system

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TDM: Example 2

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TDM: Example 3

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Framing Bits

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TDM: Example 4

• We have four sources, each creating 250 characters per second. 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.

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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Bit Padding

• The time slot length is fixed

• Different data rates must be integer multiples each other

• When the speeds are not integer multiples of each other,

bit padding is used

• In bit padding, the multiplexer adds extra bits to a device’s

source stream

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DS Hierarchy

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DS and T Line Rates

Service LineRate

(Mbps)Voice

Channels

DS-1DS-1 T-1T-1 1.5441.544 2424

DS-2DS-2 T-2T-2 6.3126.312 9696

DS-3DS-3 T-3T-3 44.73644.736 672672

DS-4DS-4 T-4T-4 274.176274.176 40324032

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T-1 Line for Multiplexing Telephone Lines

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T-1 Frame Structure

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E Line Rates

E LineRate

(Mbps)Voice

Channels

E-1E-1 2.0482.048 3030

E-2E-2 8.4488.448 120120

E-3E-3 34.36834.368 480480

E-4E-4 139.264139.264 19201920

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Inverse TDM

• Breaks one high-speed line into several lower-speed lines• High-rate data (video, for example) can be inversely

multiplexed over multiple lines