ed by the MSC for billing and for sys- tem analysis. While these functions are not part of call processing, we put them here because the workload they represent for the MSC processor is also proportional to the number of calls made. 24.1.2 Short message service (SMS) SMS is another work item for the MSC main processor. Each SMS message may rep- resent a workload comparable to a voice call. An outgoing message comes from the base station and becomes a Signaling System 7 (SS7) signaling message to the SMS message center (SMSC). The MSC processor has to make sure that the message goes to the correct SMSC for the user terminal, SO it has to process some subscriber information. And it has to maintain accounting statistics for billing and analysis. Incoming messages are more complicated. They involve locating the user terminal, which means that the MSC processor is managing communication with base stations. As we will discuss in Sec. 25.4, SMS is a wildcard because short messages use about as much MSC resources as a voice telephone call but almost no radio resources, so the SMS traffic can grow considerably and tie up switches and signaling without exceed- ing the air-interface capacity. 2 Airport planners can figure that the number of takeoffs and landings will be more or less equal over time. What goes up must come down. 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. Switching Capacity Switching Capacity 24.1.3 Handoffs Handoffs are a big item in MSC processing usage. The base station does most of the work, but the MSC still performs the switching and manages the resources for the handoffs. A great deal of design effort went into keeping the handoff workload at the base sta- tion rather than at the MSC. Measurements for handoffs are done at the base stations, and the MSC is little more than a message switch to send handoff measurements from one