separate pipes. However, the cost saving of designing an efficient backhaul network can be quite large. There are economies of scale in transport: Doubling the capacity of a pipe between two points of- ten costs far less than twice as much. A well-designed packet network for backhaul can take advantage of these economies. In this section we explain the mathematical model for network optimization. We expect the minimum-cost backhaul network to be a fan-out network where each base station has a single path to the MSC and these paths join like tributaries in a river to 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. Capacity Calculations for Cellular Networks Capacity and Quality Principles the mouth at the MSC. This kind of network is shown in Fig. 26.1. We will explain the modeling assumptions and the mathematical reasons why we expect a fan-out network to provide backhaul at the lowest cost. 26.1.1 Backhaul traffic engineering For each carrier on each cell sector, there is a two-way stream of data between the base station and the MSC. These data represent the voice calls, dedicated data sessions, shared packet data channels, short message service (SMS) messages, and all the pag- ing and access overhead. The base station collects all these for all its antenna faces and CDMA carriers and forms a single stream of packet data. Each manufacturer deter- mines what protocols are used and how efficiently these data are packetized, but the result is a two-way packet stream between each base station and its serving MSC. It is important that this backhaul link not be a bottleneck. Delays in signaling in- formation can degrade system performance even though all the radio and switch equip- ment is up to the job. It is also important that circuit-switched voice links have mini- mum delay to provide adequate sound quality. For time-sensitive service such a