With any communications system it must be recognized that the received signal will differ from the transmitted signal due to various transmission impairments.
On any telecommunication link, there will be noise and distortion of some degrees they will introduce random modifications in analog signals and bit errors in digital signal.
Traismission impairments can be broadly classified in two categories systematic and fortuitous.
Systematic distortion: is that which occurs every time as we transmit a given signal over a given channel.
Knowing the channel, we can predict what is going to occur.
The pulses may always be distorted in a certain way.
This type of distortion is something which might be possibly compensated electronically so that its effects are eliminated.
Fortuitous distortion is something which occurs at random, so it is not predictable
except in terms of probability.
Fortuitous distortion is more difficult to compensate for, though steps can be taken to minimize its effects and repair the damage it does.
This type of distortion occasionally produce an extra large noise burst or impulse which destroys or creates one more bits at random.
Transmission impairments can be broadly divided into three types:
- Attenuation
- Distoriton
- Noise
Attenuation: (Suppose a signal travels through a transmission medium and its power is reduced to one-half.
This means that P2 is (1/2)P1.
In this case, the attenuation (loss of power) can be calculated as
A loss of 3 dB (–3 dB) is equivalent to losing one-half the power.)
The strength of a signal falls off with distance over any transmission medium.
For guided media, this reduction in strength or attenuation is generally logarithmic and thus typically expressed as a constant number of decibels per unit distance.
For unguided media, attenuation is a more complex function of distance and of the make-up of the atmosphere.
Amplifierrs or repeaters are inserted at intervals along the medium to improve the received signhal quality as closed to its origin level.
Atenuation and amplification are measured in (dB).
Attenuation = 101og10(P1/132)dB
Amplification = 1Olog10(P2/P1)dB
Delay Distortion: It defines as the signal changes in its form or shapes.
The phase of the signal also is not transmitted linearly.
This is referred to as phase frequency distortion or delay distortion.
Signals on wire pairs are propagated with some different speeds at different frequencies.
Delay distortion is particularly critical for digital data, Consider that a sequence of bits is being transmitted, using either analog or digital signals. Because of delay distortion, some of the signal components of one bit position will spill over into other bit position.
Which is a major limitation to maximum bit rate over a transmission control.
Velocity of propagation of a signal through a guided medium varies with frequency
Signal components of one bit position will spill over into other bit position.
Result limit max, bit rate transmission
Solving : equalizing
Noise: Noise the third impairment. It can be defined as unwanted energy from sources other than the transmitter.
Thermal noise is caused by the random motion of the electrons in a wire.
Types of Noise
Atmospheric Noise
- Lightning: static discharge of clouds
- Solar noise : sun's Ionized gases
- Cosmic noise : distant stars radiate high frequency signal
Gaussian Noise
- Thermal noise : generated by random motion of free electrons
Crosstalk
- NEXT (near-end crosstalk): Interference in a wire at the transmuting end of signal sent on a different wire.
- FEXT (far-end crosstalk): Interference in a wire at the receiving end of a signal sent on a different wire.
Impulse Noise: Sudden bursts of irregulary pulses, Its a signal of high energ) in a very short time.
Noise: Noise the third impairment. It can be defined as unwanted energy from sources other than the transmitter.Thermal noise is caused by the random motion of the electrons in a wire.Types of NoiseAtmospheric Noise