Page 91 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes DPSK – Differential Phase Shift Keying
QAM – Quadrature Amplitude Modulation
Modems use a combination of the above modulation techniques and compression to achieve
high data transfer rates.

6.4 Modulation of Digital Signal

A digital transmission uses low pass channel with high bandwidth. Similarly, analog
transmission is also possible on band pass channels that require converting of binary data or a
low pass analog signal into a band pass analog signal. This technique is called modulation. Thus,
modulation of binary data or digital to analog modulation is carried over by changing one of
the characteristics of the analog signal in accordance with the information in the digital signal.
The information in the digital signal is always in the form of 0 and 1. The characteristics of
analog signal that are altered are amplitude, frequency and phase of the analog waveform.
Based on the change in one of the characteristics, the digital to analog modulation may be of
amplitude shift keying (ASK), frequency shift keying (FSK) and phase shift keying (PSK) types.
Quadrature amplitude modulation is the fourth category that combines changes in both amplitude
and phase to provide better efficiency.

Data Rate

Bit rate is the number of bits (0 or 1) transmitted during 1 second of time. The number of signal
changes per unit of time to represent the bits is called the data rate of the modem. That rate is
usually expressed in terms of a unit known as a baud. A signal unit may have 1 or more than 1
bits. Therefore, the baud is the number of times per second the line condition can switch from
“1” to “0”. Baud rate and bit rate, which are expressed in bits per second, usually are not the
same, as several bits may be transmitted through the channel by the modem in each signal
change (a few bits can be transmitted as one symbol). The relation between bit rate and baud is
expressed that bit rate equals the baud rate times the number of bits represented by each signal
unit. Bit rate is always more or equal than baud rate. The reason for baud rate is that it determines
the bandwidth required to transmit the signal. The signal may be in the form of pieces or block
that may contain bits. A fewer bandwidth required to move these signal unit with large bits for
an efficient system. To understand the relation between bit and baud rate, we consider an
analogy of car, passengers and highway with signal units, bits and bandwidth respectively.
A car has capacity of carrying 5 passengers maximum at a time. Suppose a highway may support
only 1000 cars per unit time without congestion. When each car on the highway carries 5
passengers, it is considered that the highway is capable of providing services without congestion.
Thus highways services are treated efficient. Consider another case, when all these 5000 passengers
wish to go in separate cars, they require 5000 cars and highway can only support 1000 cars at a
time. The services offered get deteriorated because highway’s capacity is meant only for 1000
cars. It does not bother as to whether these 1000 cars are carrying 1000 passengers or 5000
passengers or more. To support more cars, the highway needs to widen. Similarly, the number
of bauds determines the bandwidth.

Carrier Signal

The carrier signal that is a high frequency signal plays a significant role in the modulation and
data transmission. It is the base signal generated by the sending device whose one of the
characteristics is altered in accordance with the digital signal to be modulated. The modulating
signal or digital signal riding over the carrier signal is transmitted to the receiving device. The
receiving device is tuned to the frequency of the carrier signal. Other advantages of the carrier

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