�ݺ�ߣ

Multiplexing in Computer
Networks

Multiplexing
1. Bandwidth : maximum rate of data transfer
2. Bandwidth utilization : ‘wise’ use of bandwidth
3. Link refers to the transmission medium
4. A link is divided into channels
5. When can we share a link?
6. Multiplexing : simultaneous transmission of signals
7. 3 types of multiplexing techniques
8. FDM
9. TDM
10. WDM

1. FDM is an analog technique
2. Applied when bandwidth of link is greater than combined
bandwidths of the signals to be transmitted
3. Signals generated by each sending device modulate different
carrier frequencies
4. Modulated signals are combined into a single composite signal
5. Carrier frequencies are separated by sufficient bandwidth
to accommodate the modulated signal
6. Guard bands are used to prevent signals from overlapping
7. Although an analog multiplexing technique, FDM can be used to
transmit digital signals (How?)
8. Involves both multiplexing (sender site) and demultiplexing
(receiver site)

Assume that a voice channel occupies a bandwidth of 4KHz.
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 FDM, assuming no guard bands

Assume that a voice channel occupies a bandwidth of 4KHz. We need to multiplex 10
voice channels into a link with guard bands of 500 Hz using FDM. Calculate the required
bandwidth
4 444 44 4 444
No. of voice channels = 10
Frequency of each voice channel = 4KHz or 4000 Hz
Total frequency required for voice channels = 4000 * 10 = 40000 Hz
No. of guard bands required to prevent signal overlap = 9 (shown by arrows)
Frequency of a guard band = 500 Hz
Total frequency required by guard bands = 500 * 9 = 4500 Hz
Required bandwidth = 40000 Hz + 4500 Hz = 44500 Hz

Applications of FDM
1. AM and FM radio broadcasting
2. Television broadcasting
3. First generation of cellular telephones use FDM
Implementation
1. Simple
2. No need for physical multiplexer or demultiplexer
3. Base station needs to assign carrier frequency to the telephone user

WDM
1. Designed for optical fiber cable
2. Conceptually same as FDM
3. Analog technique
4. Idea : combine multiple light sources (MUX) and then split them (DMUX)
5. Handled using a prism

Time Division Multiplexing
1. Digital multiplexing technique for combining several low-rate channels into single
high-rate one.
2. Instead of sharing a portion of the bandwidth as in FDM, time is shared
3. Each connection occupies a portion of time in the link
4. Mainly of two types
5. Synchronous TDM
6. Statistical TDM

Important Points for solving numerical
1. If an input time slot is T seconds, output time slot is T/n seconds, where ‘n’ is the number
of input connections to the multiplexer. Thus, a unit in the output connection has a shorter
duration and travels faster.
2. If we have ‘n’ connections, a frame is divided into ‘n’ time slots, and one slot is allocated
for each unit coming from each input line
3. Duration of each input unit = Duration of each frame = Inverse of bit rate = T
4. Duration of each slot in frame = T/n
5. Data rate of output link = n * Data Rate of an input line
6. Duration of a unit on an input line = n * Duration of a unit on the link
7. Number of slots in a frame = ‘n’, where ‘n’ is the number of input lines. Each slot is
allocated to carrying data from a specific input line
8. Time slots are grouped into frames. A frame consists of one complete cycle of time slots,
with one slot dedicated to each sending device

Data Rate Management
1. Disparity in data input rates
2. Assumed to be same
3. If data rates are not same, the following 3 strategies or a
combination of them can be used:
i) multi-level multiplexing
ii) multiple slot allocation
iii) pulse stuffing

Statistical TDM
1. Slots are dynamically allocated to improve efficiency
2. Number of slots in each frame is less than the no. of i/p lines
3. MUX checks each i/p line in round-robin fashion
4. Allocates slot for an i/p line if the line has data to send
5. Addressing
6. Slot Size
7. No synchronization bit
8. Bandwidth

�ݺ�ߣ

Multiplexing

More Related Content

Multiplexing