t should be designed for enough redun- dancy to meet the quality-of-service (QoS) requirement for our switching systems. 43.6 Auxiliary Equipment Growth We are using the term auxiliary equipment for the home location registers (HLRs), vis- itor location registers (VLRs), signal transfer point (STP), short message service cen- ters (SMSCs), and voice mailbox systems. The growth issues for these machines are similar to the capacity issues in their initial configuration. We discussed those issues in Sec. 34.6. The growth of these items should be planned and managed, whether they are standalone equipment or integrated components of the MSC. As the MSC gains subscribers, the HLR database has to grow as well. This is simply a matter of maintaining the database capacity to follow the increase in subscriber count. The HLR processor will be busier because more subscribers will have more reg- istrations, calls, and parameter downloads. However, if the service mix moves away from voice calls and short messages to high-rate data service, then the HLR could find itself doing less work to serve fewer actual calls. On the other side of the same coin, an increasing trend toward short messages will increase HLR activity. The VLR traffic is going to grow as the roamer community grows, so adding MSCs to a system will increase the number of roamers. Calls that were intra-MSC become 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. Mobile Switching Center (MSC) Growth Increasing Capacity inter-MSC as the system capacity grows and MSC regions become geographically smaller. A shift of subscriber service mix can change the VLR activity level just as it af- fects the HLR. We have to plan for both the throughput and the total database capac- ity of the VLR. The STP required by SS7 handles signal traffic from the PSTN SS7 network plus short message traffic. The two STPs