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Transmission Medium
the data communication media are terestrial , microwave and satellite

A. terrestrial cables used three most commonly techniques
1. terrestrial wire pair: a twisted pair consists of two insulated copper wires. it is widely used in place of parallel wires to minimise the risk of cross talk

2. coaxial cable : coaxial cables consists of an inner copper conductor held in postion by cicular spacer. the cable can carry digital signal at very high rates of 500 mega bits per second

3. optical fibre : it support data rate of 2 giga bits per second. fiber optic provide high quality (low error date rate ) transmission of signals at very high speeds. fiber optics transmission not affacted by electromegnetic interference.
the transfer of data is through a very thin glass or plastic fibers with a beam of light.

B. microwave systems : these use very high frequancy signals used in telephone systems and television transmissions.

C. communication satellite : these are now becoming very populer for data communication between computers. the speed of the sattellite in orbit equals the speed of rotation of the earth and thus the sattellite is stationary relative to earth.

In india, govt agencies like NIC and VSNL are provide satellite based communication services.

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