Multiplexing and Spreading Circuit Switching and Telephone Network

In this lecture we will cover the following topics:
6. Multiplexing and Spreading
6.1 Multiplexing
6.2 Spread spectrum
6.3 Summary (part 6)
7. Circuit Switching and Telephone Network
7.1 Circuit-switched networks
7.2 Datagram networks
7.3 Virtual circuit networks
7.4 Structure of a switch
7.5 Telephone network
7.6 Dial-up modem
7.7 Digital subscriber line
7.8 Summary (part 7)

Bandwidth utilization is the wise use of
available bandwidth to achieve
specific goals.
Efficiency can be achieved by multiplexing; privacy
and anti-jamming can be achieved by spreading


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.

Topics discussed in this section:
Frequency Division Multiplexing (FDM)
Wavelength Division Multiplexing (WDM)
Time Division Multiplexing (TDM)
Statistical Time Division Multiplexing (Stat TDM)

Dividing a link into channels

Categories of multiplexing

Frequency Division Multiplexing (FDM)

FDM can be used when the BW of a link is greater than the combined BW of signals to be transmitted.
Signals generated by each sending device modulate different carrier frequencies, which are then combined into a single composite signal
Guard bands are used to prevent signals from overlapping


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

Wavelength Division Multiplexing (WDM)

WDM is designed to use the high data rate capability of fiber optic
• Using a fiber-optic cable for one single line wastes the available
bandwidth. Multiplexing allows us to connect several lines into one.
• WDM is conceptually the same as FDM, except that the multiplexing
and demultiplexing involve optical signals

Time Division Multiplexing (TDM)

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

Synchronous Time Division Multiplexing

Note: In synchronous TDM, the data rate of the link is n times faster, and the unit duration is n times shorter


Figure in next slide shows synchronous TDM with a data stream for each
input and one data stream for the output. The unit of data is 1 bit. Find (a)
the input bit duration, (b) the output bit duration, (c) the output bit rate, and
(d) the output frame rate.
a. The input bit duration is the inverse of the bit rate: 1/1 Mbps = 1 μs.
b. The output bit duration is one-fourth of the input bit duration, or 0.25 μs.
c. The output bit rate is the inverse of the output bit duration or 1/(4μs) or
4 Mbps. This can also be deduced from the fact that the output rate is 4
times as fast as any input rate; so the output rate = 4 × 1 Mbps = 4
d. The frame rate is always the same as any input rate. So the frame rate is
1,000,000 frames per second. Because we are sending 4 bits in each
frame, we can verify the result of the previous question by multiplying
the frame rate by the number of bits per frame.


TDM can be visualized as two fast rotating switches, one on the MUX
side and the other on the DEMUX side. The switches are synchronized
and rotate at the same speed but in opposite directions. On the MUX
side, as the switch opens in front of a connection, that connection has
the opportunity to send a unit onto the path. This process is called

Digital hierarchy

Telephone companies implement TDM through a hierarchy of digital signals, called Digital Signal (DS) Service.
The following figure shows the data rates supported by each level:

T-1 line for multiplexing telephone lines

T-1 frame structure

DS-1 requires 8 kbps overhead due to the synchronization bit:
T1 line = 24 slots x 8 bits + 1 bit for synchronization = 193 bits x 8kbps
= 1.544Mbps

E line rates


In spread spectrum (SS),, we combine signals from different
sources tto fit into a larger bandwidth,, butt our goals are to
prevent eavesdropping and jamming.. To achieve these
goals,, spread spectrum techniques add redundancy.

Topics discussed in this section:
Frequency Hopping Spread Spectrum (FHSS)
Direct Sequence Spread Spectrum (DSSS)

Spread spectrum

Input is fed into a channel encoder that produces an analog signal
with a relatively narrow BW around a center frequency.
• This signal is further modulated using a sequence of digits known as a
spreading code or spreading sequence.
• The effect of this modulation is to increase significantly the BW
(spread the spectrum) of the signal to be transmitted.
• On the receiving end, the same digit sequence is used to demodulate
the spread spectrum signal.

Frequency hopping spread spectrum (FHSS)

With FHSS the signal is broadcast over a random series of
radio frequencies, hopping from frequency to frequency at
fixed intervals.
A receiver, hopping between frequencies in synchronization with the transmitter picks up the message.

For transmission, binary data are fed into a modulator using FSK or
PSK. The resulting signal is entered on a base frequency. A
pseudorandom code generator serves as an index into a table of
frequencies (spreading code). Each k bits of the spreading sequence
specifies one of the 2k carrier frequencies. At each k-bit interval a new
carrier frequency is selected.

This frequency is then
modulated to produce a
new signal with the same
shape but centered on the
selected carrier frequency.

Direct Sequence Spread Spectrum (DSSS)

With DSSS each bit in the original signal is represented by multiple bits in the transmitted signal using a spreading code.
The spreading code spreads the signal across a wider frequency band in direct proportion to the number of bits used. eg. a 10-bit spreading code spreads the signal across a frequency band that is 10 times greater than a 1-bit spreading code.

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