among the MSCs and the borders between the MSC regions are not high-traffic areas. Keeping the inter-MSC borders in low-traffic regions is probably more important than maintaining a convex shape of a given MSC region. It certainly would be better to have an inter-MSC boundary follow a winding river with few bridges than to have it follow a straight line near the river. For a more complete discussion of inter-MSC border areas, see Sec. 36.1. 34.6 Auxiliary Component Planning Subscriber service depends on the home location registers (HLRs) and the visitor loca- tion registers (VLRs) having enough capacity to do their jobs. Also, SMS and voice mail are important features to wireless subscribers, so the short message service centers (SMSCs) and voice mailboxes have to have enough capacity. These auxiliary components may not be glamorous to radio engineers who are used to solving highly mathematical optimizations of CDMA carrier capacity, but a shortfall in the capacity of an auxiliary component is going to cause a loss of service just as cer- tainly as a poorly designed radio antenna. Part of the auxiliary component design is deciding how to distribute their services. While each MSC is assigned an HLR, a VLR, an SMSC, and a voice mailbox, these functions may be aggregated into common equipment. We may have a single machine to be the HLRs and VLRs for an entire wireless system, or we may decide that each MSC has a colocated machine that does all four of these auxiliary functions. When we are done planning the auxiliary systems, we will have an equipment and capacity plan for the HLRs, VLRs, SMSCs, and voice mailboxes. 34.6.1 Home location register (HLR) planning The HLR should have a database large enough for all the subscribers. There should be no surprises in the HLR database because we know the number of subscribers associ- ated with each MSC. The processing capacity of the HLR depends on the amount of activity per subscriber. This depends on the number of registrations, calls, and parameter downloads. This means t