As we have seen in the preceding sections, the connection between transmitter an the receiver is established through communication channel. We have seen that communication can take place through wirelines, wireless or fibre optic channels. other media such as optical disks, magnetic tapes and disks etc. can also be called as communication channel, because they can also carry data through them. Every communication channel has got some problems. Following are the common problems associated with the channels :
Additive Noise Interference : This noise is generated due to internal solid state devices and resistors etc. used to implement the communication system.
Signal Attenuation : It occurs due to internal resistance of the channel and fading of the signal.
Amplitude and Phase Distortion : The signal is distorted in amplitude and phase because of nonlinear characteristics of the channel.
Multipath Distortion : This distortion occurs mostly in wireless communication channels. Signals comming from different paths tend to interfere with each other. There are two main resources available with the communication channels.
Channel Bandwidth : This is the maximum possible range of frequencies that can be used for transmission. For example, the bandwidth offered by wireline channels is less compared to fibre optic channels.
Power in the Transmitted Signal : This is the power that can be put in the signal being transmitted. The effects of noise can be minimized by increasing the power. But this cannot be increased to very high value because of the equipment and other constraints. For example, the power in the wireline channel is limited because of the cables.
The power and bandwidth limit the data rate of the communication channel. As we know, the fiber optic channel transports light signals from one place to another just like a metallic wire carriers an electric signal. There is no current or metallic conductor in optical fiber.
The optical fibre has following advantages :
- Very large bandwidths are possible.
- Transmission losses are very small.
- Electromagnetic interference is absent.
- They have small size and weight.
- They offer ruggedness and flexibility.
- Optical fibres are low cost and cheap.
Satellites essentially perform wireless communication. Mainly satellites are repeaters. Broad area coverage is the main advantage of satellites. The power requirement is also less, since solar energy is used by satellites. Global communication is very easily possible due to satellite channels. The interference on satellite channels is present but it is minimum.