in one switch to do the job. We will run out of main processor capacity, transport capacity, cir- cuit switching, speech coding, or packet switching to the point where we can no longer just add more equipment and make a bigger switch. At this point we need to add a switch. We have to decide how to divide the base stations among the new MSCs. We had m MSCs before the addition, and we have m + 1 MSCs afterward, so we have to redis- tribute hundreds of base stations. There are two separate but related issues in MSC assignment. One is the assignment of base stations to MSCs, and the other is the assignment of user home MSCs in the home location register (HLR). The management issues in HLR reassignment depend on the logical and physical relationships between the HLR or HLRs and the MSC switches, and this varies from implementation to implementation. In cases where MSC switches are located at different physical sites, there is an additional issue: Variations in backhaul cost become a significant component in deciding which base stations to as- sign to the new MSC. We could do a global reassignment of base stations to MSCs. Such a reassignment could end up changing the MSC assignment of many base stations. This could mean new HLRs and short message service centers (SMSCs) for a huge number of user ter- minals depending on the logical and physical relationships between HLRs and MSCs and how closely we have assigned subscriber home service to a particular base station. At the very least, reassigning a lot of base stations is a lot of administrative work and raises the risk that most of the system might not work if something goes wrong. When adding MSCs to a growing system, we can think of the MSC regions as giant cells, as we did in Sec. 34.5. The base stations are assigned to the MSC giant cells, and we have to make changes in that assignment when we add another MSC and another giant cell. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Compan