Transmission ImpairmentsSamiul Ehsan

What is Data Transmission?

The Communication of data between two nodes by the propagation and processing signals.

The two terms “analog” and “digital” are used frequently in this context as there are two types of data transmission.

While there are two types, in both cases signals received at receiver are not same as the ones that sent. This is called transmission impairment.

Transmission Impairments

For analog signal, these impairments cause various modifications that degrade the signal quality.

For digital signal, due to bit error a binary 1 maybe changed into binary 0 and vice versa.

There are three types of transmission impairments

Attenuation

• At Attenuation, signal strength falls off with distance. It happens exponentially with the travelled distance.

• Attenuation affects the propagation of waves and signals in electrical circuits, in optical fibers, as well as in air.

• If the signal attenuates too much, it becomes unintelligible, which is why most networks require repeaters at regular intervals. Attenuation is measured in decibels(DB).

• Attenuation is often an increasing function of frequency. This leads to attenuation distortion.

• It is particularly noticeable for analog signals: the attenuation varies as a function of frequency, therefore the received signal is distorted.

Distortion

• Distortion is known as the alternation of the original signal. This may happen due to

the properties of the medium. When the distortion occurs, shape of waveform is

changed.

• Distortion only happens in mediums like cables, wires, fibers, etc.

• There are many types of distortion such as amplitude distortion, harmonic distortion,

and phase distortion.

• Distortion is critical for digital data since

bits change into other bit.

• Can be solved by equalizing circuits.

Some Distortions

Noise

• The random and unpredicted electrical signal (coming from both internal or external portion of the system) which interfere the reception of actual required signal is called- noise.

• Noise is a major limiting factor in communications system performance.

• Four categories of noise:• Thermal noise• Induced noise• Crosstalk• Impulse noise

Effects of noise

Noise

• Thermal noise (also known as white noise)• Due to thermal agitation of electrons• It is present in all electronic devices and

transmission media, and is a function of temperature.

• Cannot be eliminated, and therefore places an upper bound on communications system performance.

• Thermal noise increases with Temperature and Bandwidth.

Noise

Intermodulation noise• When signals at different frequencies share the

same transmission medium, the result may be intermodulation noise.

• Signals at a frequency that is the sum or difference of original frequencies or multiples of those frequencies will be produced.

• the mixing of signals at f1 and f2 might produce energy at frequency f1 + f2. This derived signal could interfere with an intended signal at the frequency f1 + f2.

• Comes from motors and other appliances.

Noise

Crosstalk

It is an unwanted coupling between signal paths. It can occur by electrical coupling between nearby twisted pairs.

Typically, crosstalk is of the same order of magnitude as, or less than, thermal noise.

Happens in both wired and wireless mediums.

Two types of crosstalk

NEXT(Near-end Crosstalk): Interference in a wire at the transmitting end of a 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.

Noise

Impulse noise Impulse noise is non-continuous, consisting of irregular

pulses or noise spikes of short duration and of relatively high amplitude.

It is generated from a variety of cause, e.g., external electromagnetic disturbances such as lightning, power lines.

It is generally only a minor annoyance for analog data.

But it is the primary source of error in digital data communication.

A small impulse can corrupt many bits.

A median filter is used to remove impulse noises.