The transmission of telephone traffic
Public telephone network local circuits, as introduced in Chapter 1, together with the cables etc. which carry them, constitute the local network, and the switching centre to which they are connected is referred to as a local switching centre. Each local circuit will usually serve one customer and must normally consist of a pair of electrically conducting wires for speech transmission pur poses. These same two wires are used to signal the calling condition and convey the dialled or keyed digits to the local switching centre, functions to which the generic term signalling applies (where a telephone is an extension on a PBX, additional wires may be provided for this purpose). The explanation of the transmission of telephone traffic given in the following paragraphs is particular to a metallic-pair local network, but radio or optical-fibre transmission systems may be appropriate in particular circumstances and can be expected to become an increasingly acceptable alternative in the future.
The transmission of telephone traffic, Page 1 of 2
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