, usually called the overall loudness rating or 5. Propagation delay. OLR. CCITT recommend in G.111 that the long term objective 6. Talker-echo. value of OLR is in the range 10dB to 12dB and the maximum value 7. Listener-echo. should not exceed 29dB for an average sized country. This informa- 8. Stability. 9. Error performance. 10. Sidetone. 11. Crosstalk. These are the essential parameters to be considered for any trans- Customer mission plan for an evolving network capable of supporting tele- Opinion phony and digital services. Such a network is also likely to be able to support the majority of voiceband data services. Required end to end values for these parameters are given in the following sections. 30.4.1 Transmission loss and loudness rating Overall Loudness Rating (OLR) dB The exact definition of transmission loss depends on the service offered. In the case of telephony where telephone instruments are included in the service offering then the end to end transmission loss Figure 30.3 Customer opinion against OLR is the ratio of the acoustic power at the telephone microphone to the End to end requirements 30/5 tion is of key importance to transmission planners as it sets the shown that the distortion equivalent to 14qdu represents the upper bounds for overall loss planning. limit for acceptable transmission and this is used as the basis for the end to planning rule as detailed in CCITT Recommendation G.113. 30.4.2 Noise and interference Tests have been done to assign values of qdu to a number of different coding or digit processing systems e.g. 32kbit/s ADPCM and digital Noise in telephone systems has a major impact on customer opinion. pads hence the transmission planner can add the various qdu con- Noise is characterised by its spectrum, amplitude distribution and tributions to check compliance with the overall limit. absolute power level. It is generally measured using an instrument At the present time there is some uncertainty as to whether the qdu called a psophometer which includes a frequency weighti