ning will be in the digital local exchanges should be calculated from a knowledge terms of loudness rating. For this digital network the loudness of the geographical location of the exchanges, the routeing structure ratings will be determined by the local ends as shown in Figure 30.10 and Equations 30.6 and 30.7. Assuming transparent 64kbit/s and the propagation delays of the switching and transmission equip- ment. In practice this assessment may take too long and only paths between local exchanges then the range of values for SLR and RLR will determine the values for OLR. limiting case scenarios may be investigated. This will give an indication of the maximum delay expected for routeing both within the network and also on access routes to other networks. This OLRA-B = SLR 1 + RLR 2 ( in dB (30.6) maximum delay value is now used to make a preliminary assess- OLRB-A = SLR 2 + RLR 1 ( in dB ) (30.7) ment of talker echo performance in Section 30.8.4. DIGITAL EXCHANGE Local Loop Characteristic Impedance Figure 30.11 Local impedance strategy Example transmission plan 30/13 Echo Loss Talker Listener Figure 30.12 Exchange line card losses 30.8.3 Stage 3: Impedance strategy lines using either a computer model or the approximate method outlined in G.111. From Table 30.3 it can be seen that the impedances associated with Calculation of talker echo requires: telephones, local lines and digital exchanges affect a number of parameters including echo, sidetone and loss. The main impedances 1. SLR and RLR values. are shown in Figure 30.11 where it can be seen that for the case of 2. BRL values from 30.8.3. telephone instruments the optimum conditions will be encountered 3. Delay values from 30.8.2. when ZBT = ZIT = ZIE = ZBE = cable characteristic impedance Zo. Such an arrangement will give optimum sidetone performance The method is outlined in G.131 and leads to values for the and minimum reflected levels at the exchange 2-wire to 4-wire probability of customers encountering objectionable echo. converter thus minimising talker