ailure for a community of subscribers. It is some consolation for the ca- pacity engineer that there will be some favorable traffic engineering in a network with alternate pathways. Most of the time all the facilities will be working properly, and the system will enjoy favorable blocking probabilities. And when a facility fails, the extra expense incurred for reliability pays off. Our focus has been on the big job of traffic engineering, calculating the facilities needed to support circuit-switched or packet-switched transport for voice calls on circuits or vir- tual circuits and data bandwidth on packet networks. However, the principles of traffic 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 engineering also apply to calculating the facilities needed for signaling or other pur- poses. However, when the capacity calculation is done, the actual implementation of physical transport capacity for Signaling System 7 (SS7) networks is quite different from the implementation of transport capacity for the calls themselves. For transport capacity, we seek to provide capacity to produce an acceptably low blocking rate at low cost. For the SS7 network, we define our expected maximum capacity and then build a network that can handle that load when running at 40 percent capacity. To ensure re- liability, SS7 is built with hot spares and redundant transport capacity. If any one link fails, its hot spare takes over and should be running at a maximum of 80 percent capacity even during the time of the infrastructure failure. 23.7 Conclusion In this chapter we have explored the traffic engineering principles first developed for telephony and then expanded for use in planning hybrid voice-data circuit-switched networks as well as packet pipes and packet networks. Downloaded from Digital 