a channels in the two directions which could oscillate or 'sing'. The local-exchange area, or the number of links that may be used in terminating set prevents this. If the balance impedance exactly tandem for a trunk connection. These place restrictions on the matches the impedance of the two-wire line, the hybrid transformer routings that may be employed. provides transmission between the two-wire lines and the four-wire The national plans for numbering, charging, transmission and circuit (with 3dB loss), but there is infinite loss across it between the signalling thus form a set of standards that govern planning both for Transmission 29/9 listener echo talker echo singing path talker echo listener echo Figure 29.9 Four-wire circuit two channels of the four-wire circuit. Thus, it is theoretically stability margin) and each exchange introduces a loss of, say, 1dB, possible for a four-wire circuit to provide zero loss, or even gain, a connection of n tandem links will have a loss of 4ndB. The number between its two-wire ends. However, in practice, line balances are of links that can be connected in tandem is thus severely limited. In not perfect, so the losses across the hybrid transformers are not the BT analogue trunk network, GSCs used 2-wire switching, as infinite. If the trans-hybrid loss is too low, or the gain (G4) in either shown in Figure 29.7. Consequently, connections between GSCs direction is too high, the nett gain around the loop will be greater which did not use the transit network could be routed through only than zero and the circuit can still sing. In practice, there is a one other intermediate GSC (Tobin, 1960). tolerance on G and line balances cannot exactly match the two- If 4-wire switching is used, terminating sets are required only at wire lines. Consequently, the gains G must be set to provide an the ends of the connection, as shown in Figure 29.10(b). This avoids adequate stability margin to cater for these variations. As a result, 3dB loss for every intermediate switching node. Consequen