 and listener echo signals. However in practice it is unlikely that all telephone instruments will have the Calculations according to G.131 will demonstrate whether adequ- same nominal impedance, and 2-wire local lines are unlikely to all ate echo performance can be obtained for the chosen values of have identical values of Z therefore the ideal impedance match loudness and impedances, taking into account the delays on the cannot be achieved. In this case it will be necessary to calculate connections. If a suitable compromise cannot be achieved then the distributions of values for STMR using the known characteristics of use of echo control devices such as echo cancellers may need to be considered. the telephone set and local lines. At the same time, distributions for values of balance return loss at the exchange hybrid can be calcu- Guidance on the use of echo cancellers is given in G.131 which lated. STMR values can then be compared with the sidetone objec- covers aspects such as preferred location and interworking with tives given in Section 30.4.10 and the balance return loss other echo control devices such as echo suppressors. To date it has distribution used later for echo assessment. generally only been large countries such as U.S.A. and Canada that have needed echo control devices on national calls not involving satellites. However increased use of signal processing devices is 30.8.4 Stage 4: Loss apportionment and echo assessment. tending to increase connection delays whilst end to end losses are being reduced by the use of digital transmission and switching. There will therefore be an increasing risk of encountering echo This stage involves an interactive process. The objective is to assign problems in situations where echo has not previously been notice- values to the exchange interface losses A and B in Figure 30.12 such able. that: 1. SLR and RLR meet Section 30.8.1 limits. 30.8.5 Stage 5: Review 2. A and B result in a sufficiently high echo loss value to meet the talker echo objective. Stages 1 to 4 