ll-digital network. In most cases the local serving switch is the point of analog-to-digital interface. Digital transmission and switching are discussed in Chapter 6. However, there are still locations in less-developed parts of the world where digital conversion takes place deeper in the PSTN, perhaps at a tandem exchange. Local service inside the local serving area in these cases remains analog, and local trunks (junctions) may consist of wire pairs carrying the analog signals. Analog wire pair trunks are even more prevalent outside of North America. In this chapter we will define the analog voice channel and describe its more common impairments. The subscriber subset's functions are reviewed as well as the sound to electrical signal conversion which takes place in that subset. We then discuss subscriber loop and analog trunk design. Since the publication of the first edition of this text, the traffic intensity of voice versus data have changed places. Whereas voice traffic was predominant in the PSTN, data traffic intensity currently is far greater. Some sources state that only some 5% of the network traffic is voice and the remaining 95% is now data in one form or another. Much of the remainder being data traffic in one form or another. One such form is voice over IP which has the potential for astronomic jumps in traffic intensity. Do we count that as voice or data? It should also be kept in mind that the connectivity to the PSTN- that is, the portion from the user's subset to the local serving exchange-will remain analog for 1 PABX stands for private automatic branch exchange. It is found in the office or factory environment, and it is used to switch local telephone calls and to connect calls to a nearby local serving exchange. In North America and in many other places a PABX is privately owned. Fundamentals of Telecommunications, Second Edition, by Roger L. Freeman ISBN 0-471-71045-8 Copyright © 2005 by Roger L. Freeman TRANSMISSION ASPECTS OF VOICE TELEPHONY some time into the future. Here again we may ha