In Telecommunication, transmission can be divided into two broad categories: Guided and Unguided. Guided Media includes twisted pair, Coaxial cables, and fiber-optic cable. Unguided media is free space.

(i) Coaxial Cables:

Characteristics:

• It has a large bandwidth and low losses.
• This cable is suitable for point to point or multipoint applications. In fact this is the most widely used medium for local area network.
• Due to shield provided, this cable has excellent noise immunity.

(ii) Twisted Pair Cables:

A twisted pair consists of two conductors (normally copper), each with its own plastic insulation, twisted together as shown in above figure. One of the wires is used to carry signals to the receiver and other is used only as a ground reference. The receiver uses the difference between the two. In addition to the signal sent by the sender on one of the wires, interference (noise) and crosstalk may affect both wires and create unwanted signals.

(iii) Optical Fiber cables:

Advantages:

• Small size and light weight: The size of the optical fibres is very small. Therefore a large number of optical fibres can fit into a cable of small diameters.
• Easy availability and low cost: The material used for the manufacturing of optical fibres is the “Silica glass”. This material is easily available.
• Large Bandwidth: As the light rays have a very high frequency in the GHz range, the bandwidth of the optical fibre is extremely large.

Transmission impairments:

In any communication system, the received signal is never is identical to the transmitted one due to some transmission impairments. The quality of analog signals will deteriorate due to transmission impairments as given above.

Attenuation:

The strength of a signal decrease with the increase in distance travelled over a medium. Attenuation means loss of energy. When any signal travels over a medium or channel, it loses some of its energy in the form of heat in the resistance of the medium. Attenuation decides the signal to noise ratio hence the quality of received signal. Attenuation is given in decibels as:

Attenuation(dB)= 10log10 (Pout/Pin)

Where, Pin= Power at the sending end

Pout= Power at the receiving end

Distortion (Harmonic):

Another meaning of distortion is change in shape of the signal. This type of distortion is observed for the composite signals made by different frequencies. If the medium is not perfect, then all the frequency components present at the input will not only be equally attenuated and will not be proportionally delayed.

Noise:

When the data travels over a transmission medium, noise gets added to it. Noise is a major limiting factor in communication system performance. Noise can be categorized into four types as follows:

(i) Thermal noise (ii) Intermodulation noise (iii) Crosstalk (iv) Impulse noise