Practical Industrial Data Networks:Design, Installation and ...

318 Practical Industrial Data Networks: Design, Installation and TroubleshootingInput signal = 1kHzOutput signal, channelbandwidth = 10kHzOutput signal, channelbandwidth = 1kHzOutput signal, channelbandwidth = 3kHzFigure B.2Effect of channel bandwidth on a digital signalB.1.6Channel capacityThe channel capacity i.e. the maximum data transfer rate of the transmission channel canbe determined from its bandwidth, by use of the following formula derived by Shannon.C = 2Blog 2M bpswhere C is the channel capacity, B is the bandwidth of the channel in hertz and Mdiscrete levels are used for each signaling element.In the special case where only two levels, ‘ON’ and ‘OFF’ or ‘HIGH’ and ‘LOW’ areused (binary), M = 2. Thus C = 2 Blog 22 but log 22 = 1, therefore C = 2 B. As anexample, the maximum data transfer rate for a PSTN channel of bandwidth 3200 hertzcarrying a binary signal would theoretically be 2 × 3200 = 6400 Bps. In practice thisfigure is largely reduced by other factors such as the presence of noise on the channel toapproximately 4800 Bps.B.2 Simplex, half-duplex and full-duplexB.2.1SimplexA simplex channel is unidirectional and allows data to flow in one direction only, asshown in Figure B.3. Public radio broadcasting is an example of a simplex transmission.The radio station transmits the broadcast program but does not receive any signals backfrom the radio receiver.