aving in backhaul expense. Turning a base station into an IP node still means connecting it to the nearest POP on the IP network. This may be easier and cheaper than using a bigger pipe all the way back to the MSC. 35.5 Reliability Engineering A fan-out network, with or without a backbone, has only one route from each base station to the MSC. This makes the backhaul link a single point of failure, raising a 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. Backhaul Planning Planning for CDMA Capacity reliability issue. We have given some thought to reliability in base station design and MSC equipment planning. The backhaul network deserves some reliability planning as well. The backbone network itself may have the same lack of redundancy. Part of a service- level agreement (SLA) for any leased transport should include the level of reliability and the level of redundant facilities. Any facilities of our own design, dark fiber or mi- crowave links, for example, should be evaluated for their reliability. If all the transport facilities are rock solid with negligible failure rates, then we have done our reliability homework for the backhaul network. If, however, there are some questionable links, then we have to think about the consequences if those links stop working. The basic scheme to make a transport network more reliable is to have diverse rout- ing, redundant paths from one place to another. Not only is transport diversity insur- ance against equipment failure, it is also protection against environmental damage. The environmental damage could be a flood or a tornado, but it also could be the result of human actions. More than one T-carrier has been disconnected by a road crew dig- ging a ditch, and underwater cables also have been severed. 2 A letter of apology is small consolation to subscribers who are left without service. The bad news i