 tion is described by its loudness rating or reference equivalent. This is determined by the performances of the two telephones (which 0.5dB 0.5dB 0.5dB partly depend on the DC feeding currents they receive from their local exchanges) and on the losses (attenuations) of all intervening circuits in the connection through the PSTN (Richards, 1973). 0.5dB These circuits comprise the subscribers' lines to their local ex- changes, together with any junctions and trunk circuits used in the connection. A national transmission plan should allocate the permissible over- all attenuation SO as to obtain the most economical design. Sub- scribers' exchange lines are most numerous and contribute very significantly to the total cost of a network. In order to minimise this National Network National Network cost, subscribers' lines use the minimum size of cable conductors. Consequently, the transmission plan should allocate as much loss as possible to the subscribers' lines at the two ends of a connection. Figure 29.8 International network Junction circuits between local exchanges and between them and primary trunk switching centres are much less numerous. It is thus economic to use cables with larger conductors which have lower losses. Trunk circuits can justify still higher cost, SO they contain 1. From the calling subscriber through the national network of the amplifiers and have very small losses. country to a gateway exchange (CT3). In an analogue PSTN, the economic necessity of allocating a large 2. An international chain, which may consist of up to five interna- limiting attenuation to subscribers' lines, together with the loss tional circuits (or exceptionally six). These are interconnected on a variations of other circuits, results in some connections which are 4-wire basis via international transit centres (CT1 and CT2) be- unsatisfactory. If present limiting standards were consistently en- tween the gateways (CT3) of the two countries. countered, the performance obtained on complex trunk calls would 3. The national net