above are the most complicated as a number of compromises may be required to meet acceptable values for loud- The two requirements tend to be conflicting as meeting SLR and ness rating, sidetone, talker echo and listener echo. The complexity RLR objectives require low values of A and B whereas echo perfor- arises because the losses and impedances of the telephone, ex- mance is improved by increasing A and B. SLR/RLR can be calcu- change and local area all interact to influence several end to end lated with transmission data on telephone instruments and local parameters as shown in Table 30.3. 30/14 Transmission planning The remaining parameters to be covered are loss/frequency dis- ary is at a digital interface. In this case the network operator will be tortion, noise, balance about earth, crosstalk, errors, non-linear providing a switched network path at, for example, 64kbit/s. If no distortion and quantising distortion (qd). analogue transmission is involved then the network transmission plan will just involve the digital impairments together with propa- 30.8.6 Stage 6: Remaining parameters gation delay. The end to end analogue performance as seen by users will be wholly dependent on the characteristics of the customer Some simplification is possible for the chosen example. The net- equipment e.g. PBXs and digital telephones. It is particularly im- work structure does not incorporate multiple analogue to digital portant that the designers of such equipment appreciate the end to conversions with their associated non-linear and loss/frequency end transmission issues and the significance of the terminal equip- distortion. In addition noise sources are limited to the exchange ment performance. interface, local line and telephone instrument. Providing values in The use of end to end digital connections will also affect the line with the relevant CCITT equipment recommendations (CCITT, present thinking on the use of echo control devices on long delay 1989d; CCITT, 1989e) are adopted then the end to end values will ci