rcuits such as provided by satellites. At present the echo control be significantly inside the Section 30.4 objectives and detailed device will be associated with the international part of the connec- calculations are not required. tion for economic reasons. However echo control devices are, by The chosen example covers the telephony service which is rela- definition, signal processing devices which destroy bit integrity tively tolerant of the digital impairments such as error performance. therefore they could not be used on circuits where a 64kbit/s trans- The error contributions potentially come from the digital line sys- parent path was required. Any telephony service over a satellite items, multiplexors and switches. Compliance with the appropriate system without echo control is likely to give major problems hence CCITT values is likely to result in acceptable end to end perfor- the user would not be able to use his transparent 64kbit/s path for mance for telephony as the additional noise contribution due to speech. The solution to this problem is to migrate the echo control errors will not be significant. to the terminal equipment which is consistent with the G.131 objec- tive of locating echo control devices close to the point of reflection. 30.8.7 Stage 7: Additional areas It seems possible therefore that future digital terminal equipment will need to incorporate an echo control function in the same way Stages 1 to 6 have addressed the relevant end to end parameters and as is proposed for the European digital cellular system GSM. Such indicated a procedure for arriving at a plan. There are a few addi- an arrangement would minimise the problems of incorporating new tional areas which, if not considered, may have an effect on perfor- technologies like point to multipoint radio access systems and ATM mance i.e.: switching, both of which result in increased delay. 1. DC current feed. The digital exchange must supply the appro- priate minimum DC current on the highest resistance local loop. This ensures correct