uits were the same size and that the average length of telephone calls was unlikely to change very much. Then we will discuss the implications of data usage and data services and how they require new models. After that, we will discuss the traffic engineering of packet networks. 23.1 Voice Calls Assuming Fixed-Size Channels The most fundamental traffic engineering job in telephony is the assignment of trans- port facilities to serve fixed-size voice channels. There is a demand of voice telephone calls to be served, some community of subscribers making calls. While telephone usage patterns are not random to the subscribers themselves, so far as the telephone network is concerned, the demand is a random statistical process, a distribution of calls over time. When each call attempt is made, the telephone network serves the call if it has fa- cilities to do so, or it blocks the call. Our measure of network performance in traffic en- gineering is the fraction of calls that are blocked. The fraction of calls that are blocked can be viewed as the probability that any one call will be blocked. If the blocking prob- ability is low enough, then the network is adequately designed. Each voice channel uses a single DS-0 or E-0, and it is the traffic engineer's job to ensure that the network has enough telephone lines to keep the blocking rate low enough during the busy period. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Traffic Engineering for Voice and Data Capacity and Quality Principles 23.1.1 Erlangs, the units of demand The units of telephony demand are erlangs, named after Danish telephone pioneer Ag- ner Krarup Erlang. Roughly speaking, the demand in erlangs is the average number of calls that would be in progress if there were no blocking. We say that a call is blocked it is not served because there are not enough resources to serve it. The d