e MSC and the base station, SO these calculations already should be re- flected in the allocation of computational resources in the MSC. The HLR has a significant load in service validation as well. Every time a subscriber sends or receives a short message via SMS or uses some vertical service, the HLR consulted to see if the subscriber can use the service. This process has to be fast so that the subscriber does not have to wait too long for service. When an external event occurs, a traffic accident, a ball game, or even the landing of a Boeing 747 with 400 passengers, a single VLR sees the full workload of registering all the subscribers and setting up all the calls. The total HLR workload may be just as great, but it is distributed over many HLRs in many places. Thus the HLR traffic is compara- tively smooth compared with the bursty workload at the VLR where an event occurs. 26.2.2 SMS message center (SMSC) capacity In addition to the signaling load, the SMS message center (SMSC) has to process the messages and store them. The workload of the SMSC depends on the activity of SMS subscribers, leaving messages and picking them up. The rate of message arrivals de- pends on subscribers using SMS, and SMS usage is highly dependent on events. Peo- ple leave messages when they feel like it, but they also leave messages when something happens. An unexpected local event, a traffic jam or a late flight, stimulates a wave of SMS messages arriving at the SMSC all begging to be processed. The SMSC workload picking messages up is far smoother because most subscribers pick up their messages when they turn on their cell phones. The incoming message workload is a queue. Messages come into the SMSC, and they are processed and sometimes they can back up. The SMSC must have enough queue storage for the workload it cannot process immediately. The storage issue is important because SMS subscribers do not like to lose their messages. The time issue is less im- portant because SMS messages usually are not time-critical, at least not fo