oise and other disturbances in the network may cause errors in the trans- mitted data. If errors occur, some amount of data may be lost. Voice and video transmission services are used by human beings, and they can tolerate accidental short disturbances. In computer communications a single errone- ous bit usually destroys a whole data frame, which may contain a large amount of data. The loss of one frame destroys the transmission of a large file that is transferred in multiple frames. Most of the data communication sys- tems are able to retransmit data frames in error. Systems designed for voice or video transmission do not use retransmission schemes because temporary retransmission delay is even more disturbing for human users than the loss of some data. Fixed Delay Tolerance When communication is interactive, as voice communication usually is, the two-way transmission delay should be very short for good quality. In the case of voice it should be of the order of some tens of milliseconds. Otherwise, we feel that quality is degraded because the response from the other party is Introduction to Telecommunications Network Engineering delayed. We tolerate much longer delays in the case of ordinary data applica- tions when we are waiting for a response to our "click" command. Variable Delay Tolerance Voice and video information is traditionally transmitted as samples at regular periods of time. The reconstruction of images and voice requires that all sam- ple values be received sequentially and suffer the same delay. Conventional data networks recover from errors with the help of retransmission of the frames in error. This is a very efficient error recovery scheme, but it intro- duces some additional and variable delay. For voice applications this variable delay is often a worse solution than that of losing some data. Peak Information Rate Encoding of analog voice and video often produces a constant information data rate. Values of the samples with constant length contain voice or video information and they are transmi