Page 89 - DCAP207_NETWORKS_DCAP406_COMPUTER_NETWORKS

Computer Networks/Networks

Notes Code Division Multiplexing assigns each channel its own code to make them separate from each
other. These unique underlying codes, which when decoded restore the original desired signal
while totally removing the effect of the other coded channels. Guard spaces are realized by
using codes with orthogonal codes. In case of TDM and FDM, channels are isolated by separate
time or frequency slots, which are occupied in common by all users. Figure 6.6 explains how all
channels Ci use the same frequency at the same time for transmission.

Figure 6.6: Code Division Multiplexing (CDM)

C

f

t

A single bit may be transmitted by modulating a series of signal elements at different frequencies
in some particular order. These numbers of different frequencies per bit are called as the chip
rate. If one or more bits are transmitted at the same frequency, it is called as frequency hopping.
This will happen only when the chip rate is less than one because chip rate is the ratio of
frequency and bit. At the receiving side, receiver decodes a 0 or a 1 bit by checking these
frequencies in the correct order.
A disadvantage of CDM is that each user’s transmitted bandwidth is larger than the digital data
rate of the source. The result is an occupied bandwidth approximately equal to the coded rate.
Therefore CDM and spread spectrum are used interchangeably. The transmitter and receiver
require a complex electronics circuitry. The main advantage of CDM is protection from
interference and tapping because only the sender the receiver knows the spreading code.

6.2.4 Wavelength Division Multiplexing (WDM)

Fiber optic technology is considered to meet the ever-increasing demand of bandwidth for the
exchange of information and WDM provides solutions for the ever-increasing demand of
bandwidth through optical networks. In optical communications, the analog of FDM is referred
to as wavelength-division multiplexing (WDM).
WDM may be defined as the fiber-optic transmission technique that employs two or more
optical signals having different wavelengths to transmit data simultaneously in the same
direction over one fiber, and later on is separated by wavelength at the distant end. WDM allows
transmission of analog or digital signals up to a few GHz or Gbits/s on a carrier’s very high
frequency around 190 THz (infrared). In fact, using several carrier waves that are propagating
without significant interaction on the same cable can increase the bit rate further. These carrier
waves correspond to different wavelengths. This is the reason it is called Wavelength Division
Multiplexing (WDM). The relationship between frequency and wavelength is given as follows:
l = c/f where c and f are the velocity and frequency of the signal in the medium.

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