actice however the transmission plan tends to be developed for the majority use service i.e. telephony. The approach adopted to date by the majority of network operators is to develop a list of Figure 30.2 Overall Loudness Rating (OLR) parameters required for the support of telephony and then consider the impact of the choice on voiceband data transmission. In general analogue transmission plans are not developed for parameters only of relevance to voiceband data services. However, OLR = SLR + CIRCUIT LR + RLR dB (30.1) for the ISDN the approach has been to consider the requirements of the most demanding digital service and develop an appropriate error There are agreed methods of defining and measuring loudness performance plan. Such a plan has generally proved adequate for the ratings, detailed in CCITT recommendations in the P Series (P76, support of telephony services using digital telephones. P78, P79) (CCITT, 1989b). Readers may also see Reference Equi- valents and Corrected Reference Equivalents used in earlier CCITT documentation and the transmission plans of some countries. In End to end requirements outline these were similar to loudness ratings but had some disad- vantages and have now virtually been replaced by loudness ratings. Adopting the approach outlined in the previous section, i.e. develo- If approximate conversion factors are required these can be found ping a list of parameters for telephony and digital services, we arrive in Annex A to CCITT Recommendation G111 (CCITT, 1989c). at the following list of key parameters: Subjective tests carried out in a number of administrations indi- cate the values of loudness rating that are preferred by customers 1. Transmission loss. and also the values that are considered to be too loud or too quiet. 2. Noise/interference. Examples of typical results are shown in Figure 30.3 where it can 3. Loss/frequency distortion. be seen that there is an optimum range for loudness rating on an 4. Non-linear distortion including quantising distortion. overall connection