work of the country of the called subscriber. be totally unacceptable. Fortunately, most calls do not follow the most adverse routing permitted by the transmission plan. By means of statistical studies of routings and of the various transmission Network standards impairments, it is possible to design a network which is economic and yet provides an acceptably small proportion of unsatisfactory In designing a public switched telecommunication network connections. (PSTN), consideration must be given to the following factors: 29.4.2 Four-wire circuits 1. The transmission plan. 2. The numbering plan. Long-distance circuits use amplifiers to overcome their high attenu- 3. The charging plan. ation. Amplifiers are usually unilateral devices. An amplified circuit 4. The routing plan. therefore has a separate channel for transmission in each direction, 5. The signalling plan. as shown in Figure 29.9. Such a circuit is called a four-wire circuit, 6. The grade of service required (end to end). although the channels may be provided in high-capacity multiplex 7. The capabilities of switching systems. systems over optical fibres or microwave radio instead of wires. A 8. The economics of direct and indirect routing. connection made up of such circuits will be joined at each end to two-wire circuits leading to the subscribers' stations. A four-wire These considerations are inter-related. For example, charging, circuit is connected to its two-wire terminations by means of a routing and numbering are closely related. Both the route for a call four-wire to two-wire terminating set containing a hybrid transfor- and its charging rate are determined by the directory number of the mer and a line-balance impedance. called station. The flexibility with which these can be handled is If the four-wire circuit were to be directly connected to the governed by the capabilities of the switching equipment employed. two-wire circuit at each end, this would complete a loop round the Either transmission or signalling standards may limit the size of 