operating the intervening links at zero loss. Alternatively, every link and is thus even more troublesome. It is therefore necessary to may have a small loss, typically 0.5dB. The former policy was used ensure that echoes are attenuated sufficiently to make these effects in the BT analogue trunk network shown in Figure 29.7 and the unobjectionable. latter policy in the North American network (Andrews, 1971) shown in Figure 29.6 and the international network (Munday, 1967) 29.4.3 Trunk transmission plans shown in Figure 29.8. The annoying effect of echo increases with its delay. The longer A complete connection through a trunk network consists of several the distance, the greater the round-trip delay and SO the greater the four-wire circuits connected in tandem at exchanges. The exchanges echo attenuation required. This can be achieved by making the may use either 2-wire switching or 4-wire switching. If 2-wire overall loss increase with distance, as shown in Figures 29.6 and switching is used, 4-wire circuits must be terminated at every 29.8. However, there is a limit to the extent to which echo can be switching node, as shown in Figure 29.10(a). If each 4-wire circuit controlled by increasing overall loss. This is usually reached when is lined up to provide, say, a loss of 3dB (to provide an adequate the round-trip delay is about 40ms. This delay is exceeded on long 29/10 Telephone networks EXCHANGE EXCHANGE EXCHANGE EXCHANGE Figure 29.10 Switching of four-wire circuits: (a) 2-wire switching; (b) 4-wire switching trans-continental and inter-continental circuits, particularly if these vision switching is employed to interconnect circuits provided by use satellite links. It is then impossible to obtain both an adequately transmission systems using FDM or TDM, it is necessary to install low transmission loss and an adequately high echo attenuation. On channelling equipment at each end of every link in order to demulti- such circuits, it is necessary to control echo by fitting echo sup- plex every channel to audio f