The transmission of data and other non-telephone traffic

Abstract

The method of communication used for public telephone networks, whereby independent point-to-point circuits are temporarily linked together by switching centres to establish a through connection between two users who wish to 'converse', is termed circuit switching. The calling and called users must both be available simultaneously for a call to be effective, but do not necessarily have to be human since automatic calling or answering devices may be employed. All circuits and switches involved in the connection must be held for the duration of the call. There is an initial set-up delay (which will worsen with increasing network loading) but, once set-up has been successfully completed, interaction is to all intents and purposes instantaneous and occurs at a transmission speed which is limited by the characteristics of the circuits involved. The conversion of an established analogue public telephone network to an integrated digital network (IDN) is a lengthy and costly undertaking, involving the progressive replacement by digital equipment of all switching centres and transmission systems. Primarily intended for business users, digital transmission access to the IDN will be available as an alternative to analogue local-circuit access at extra cost (see Section 4.1), but it can only be provided where a connection can be established to a digital switching centre. Even though the amount of digital transmission and switching equipment increases each year, therefore, the network will be seen by most users as effectively an analogue transmission network since the normal access to it will still be via analogue local circuits.