 connection can be calculated by The mismatch losses are determined by the impedance at the addition of the delays of the component parts. various points of the connection. Calculation of overall transmission loss is best carried out using a transmission and calculation pro- gramme of the type outlined in Supplement 3 to Vol. V (CCITT 30.6.6 Talker-echo 1989b) Use of such a programme will enable the end to end losses to be From Figure 30.5 the loudness rating of the talker echo path TELR calculated for routeings through the intended network structure. is given by Equation 30.2 Such a programme is also used to calculate loudness ratings (SLR, RLR and OLR) of a connection from the characteristics of the TELR = SLR + RLR + Echo loss in dB ) telephone sets and any local lines. In many cases it is convenient to SLR + RLR + Balance Return Loss + + (3.2) calculate SLR and RLR to the first switching point in the connection which may well be a digital highway. Although this point is not The Sound and Receive Loudness Rating (SLR and RLR) of the analogue, the digital code represents encoded analogue signals and local end are determined as outlined in Section 30.6.1. The echo loss a high quality digital to analogue converter would be used to is a function of the nominal 2-wire to 4-wire losses together with the determine the equivalent analogue level. balance return loss of the 2-wire to 4-wire converter. (Balance return loss is the contribution to echo loss from the degree of match between the hybrid balance impedance and the impedance 30.6.2 Noise and interference presented by the 2-wire local access circuit.) Control of talker echo therefore requires adequate specification of The value of noise seen at the output of a connection is the sum of a number of transmission characteristics i.e. loss, impedance and the noise generated by the individual network components. Calcu- delay. lation of the total noise is not straightforward as the various noise sources are attenuated by different amounts as they traverse the 30.6.7 Lis