RELATIVE MERITS OF CHANNELS OF COMMUNICATION COM. CHANNEL FEATURES

Wire-pairs - HF signals lose their energy over short distances - High attenuation due to heating caused by electrical resistance of wires and partly due to radiation (radio waves) since wire s act as

aerials - Signal requires regular amplification - Wires act as aerials hence they pick up any electromagnetic waves - A lot of noise that cause deterioration of the signal - Wire-pairs close to each other pick up each other’s signals. This effect is called cross-talk or cross-linking. - Poor security and can be tapped easily - Used mainly for short-distance communication. - Have limited bandwidth

Coaxial cables Reduced attenuation compared to wire-pairs Repeater amplifiers can be further apart on coaxial cables Has a larger bandwidth of about 50MHz More information can be carried compared to wire-pairs Less noisy More secure More costly Used to connect e.g. an aerial and a television

Radio and microwave links

Microwaves are electromagnetic waves that can be considered alongside other radio waves. Have a frequency range of 3GHz to 30GHz Generally used for point-to-point communication Reflecting parabolic dishes are used so that the transmission is in the form of a parallel beam so that as much high power as

possible can be focused onto the receiving aerial. Has bandwidth of the order of 1GHz and thus transmit a larger capacity of information Security is achieved by encoding the information Their mode of use and range depend on frequency

Optic fibres Pulses of light or infra-red radiation travel along the fibre as a result of total internal reflection. The radiation pulses are provided by lasers (Light Amplification Stimulated by Emission of Radiation/s) and have very high

frequencies (order of 108MHz). A single pulse lasts 10-14s and hence not easy to control although it is governed by the frequency of switching on and off. Have high bandwidth, giving rise to large transmission capacity Much lower costs than metal wires. Diameter of the cable is much less than metal cable, hence easier to handling and storage. Much less signal attenuation, so far fewer regenerator amplifiers are required, reducing the cost of installation. Do not pick up electromagnetic interference, so very high security and negligible cross-talk. Can be laid alongside existing routes such as electric railway lines and power lines.