Principles Digital Communication System & Computer Networks (Charles River Media Computer Engineering)

1.3 TRANSMISSION IMPAIRMENTS

While the electrical signal is traversing over the medium, the signal will be impaired due to various factors. These transmission impairments can be classified into three types:

1. Attenuation distortion

2. Delay distortion

3. Noise

The transmission impairments can be classified into: (a) attenuation distortion; (b) delay distortion; and (c) noise.

The amplitude of the signal wave decreases as the signal travels through the medium. This effect is known as attenuation distortion. Delay distortion occurs as a result of different frequency components arriving at different times in the guided media such as copper wire or coaxial cable. The third type of impairment—noise—can be divided into the following categories:

• Thermal noise

• Intermodulation

• Crosstalk

• Impulse noise

Thermal noise: Thermal noise occurs due to the thermal agitation of electrons in a conductor. This is distributed uniformly across the spectrum and hence called white noise. This noise cannot be eliminated and hence, when designing telecom systems, we need to introduce some method to overcome the ill effects of thermal noise. Thermal noise for a bandwidth of 1 Hz is obtained from the formula:

where No is noise power density, watts per Hz

Thermal noise for a bandwidth of B Hz is given by

If N is expressed in dB (decibels)

Using this formula, thermal noise for a given bandwidth is calculated.

Intermodulation noise: When two signals of different frequencies are sent through the medium, due to nonlinearity of the transmitters, frequency components such as f1 + f2 and f1 – f2 are produced, which are unwanted components and need to be filtered out.

Crosstalk: Unwanted coupling between signal paths is known as crosstalk. In the telephone network, this coupling is quite common. As a result of this, we hear other conversations. Crosstalk needs to be eliminated by using appropriate design techniques.

Impulse noise: This is caused by external electromagnetic disturbances such as lightning. This noise is unpredictable. When the signal is traversing the medium, impulse noise may cause sudden bursts of errors. This may cause a temporary disturbance in voice communication. For data communication, appropriate methods need to be devised whereby the lost data is retransmitted.

Noise is the source of bread and butter for telecom engineers! If there were no noise, there would be no need for telecom engineers—for we can then design perfect communication systems. Telecom engineering is all about overcoming the effects of noise.

Noise can be divided into four categories: (a) thermal noise, (b) intermodulation noise, (c) cross-talk and (d) impulse noise.