mail is not a queuing system. The voice-mail system has to have the capability to support all the channels between it and the wireless system. This includes the pro- cessing capacity to support the channels and several internal capacity and speed design issues as well. The system must have sufficient storage space for all messages. The amount of storage needed may be hard to calculate as any given message may remain on the system anywhere from just a few minutes up to the maximum allowed, often 15 days. Each voice-mail message is stored in the voice-mail system as it is being left, and it re- mains in the system until it is deleted by the subscriber or exceeds the allowed maximum storage time. The system also must have a sufficiently rapid retrieval mechanism for messages when users call to pick them up. We view these as equipment design issues. Voice mail is an attribute of an incoming call, so local events such as traffic jams and late flights do not generate the same local workload as they do for SMS. This is so be- cause the recipients of all the calls being made are typically spread over many voice- mail service areas. However, there are still day-of-week and time-of-day cycles in voice- mail activity. One important issue is that business messages left on Friday afternoon typically remain in the system until Monday morning. Voice-mail system will have a burst of usage when other components fail in the wire- less telephone system. A base station outage or a facility failure will generate a surge of voice-mail activity for those subscribers. 26.3 Cellular Network Issues Dedicated transport between MSCs can save significant cost over using the public switched telephone network (PSTN) for those calls. Two areas where dedicated facili- ties can save money are roamer-terminated calls and mobile-to-mobile calls from one MSC to another. We can calculate the traffic loads for these functions and do some traf- fic engineering to determine how much dedicated transport is required. However, the PSTN is always there as a