tered allowing transmission over long lines. The definition of a signal may take a variety of forms: 2. Connecting wires are not necessary for the whole of a telecom- munications circuit. 1. The conveying of information through a medium. 2. The physical embodiment of a message. 10.1.1 Bits and bauds 3. A media manifestation conveying information or direction from one end of a transmission medium to another Bits are defined as the number of binary digits that are transmitted through a transmission medium per second. The baud is the unit of Two further definitions associated with a signal are : modulation rate. When a modulation method is used that has binary values then the line rate for the modulation directly corresponds 1. Signal frequency, the frequency of the carrier wave upon which with the transmission of bits, and the bit rate and baud rate are equal. the signal information has been impressed. When a multi-level modulation method is used each modulation can 2. Signalling, the use of a signal to convey coded direction or represent more than one bit. If one modulation equals two bits, each instructions to a person or piece of apparatus at a distance, such modulation represents a dibit. A tribit is when each modulation a signal normally being associated with the establishment, represents three bits. In the latter case the bit rate is three times the servicing or breaking of a connection. modulation rate. An example of the dibit operation is found in CCITT V.22 recom- Signals that follow the instantaneous variation of the original mendations which uses four phase changes for modulation, as in information energy are defined as analogue. A digital signal is in the Table 10.1. The baud rate is 600 whilst the bit rate is 1200 per form of a pre-determined code of pulses or variations, which repre- second. sent symbols taken from a selected set of symbols. Table 10.1 Example of a dibit operation A direct current signal is when the flow of current is only in one direction. However, the strength of the current may be 