r delays of only a few minutes. SMS does have a function for prioritization of message handling. A message marked high priority is moved to the top of the queue when it arrives at the SMSC and is handled before low-priority messages, even those which arrived sooner. This reduces latency for messages, such as dispatch messages for utility repairmen, which may be relatively urgent. The messages themselves have to be stored until they are picked up. These mes- sages are small, no more than 160 bytes each, but the SMSC has to keep not only the 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. Capacity Calculations for Cellular Networks Capacity and Quality Principles message but the status of the message. The SMSC has to store messages from the time they are left until the time they are picked up. There are significant daily and weekly cycles in SMS usage, as well as bursty usage for unexpected local events. The SMSC queue for incoming messages and the storage for messages waiting for pickup have to leave enough room in the SMSC for the work. The rate of lost messages in the SMSC is the combined loss of messages lost in the in- coming message queue and messages lost because there is not enough storage for them. 26.2.3 Voice-mail capacity A voice-mail system is an entity separate from the wireless telephone system, not a base station, not an MSC. The voice messages have to get from the system to the voice- mail system when they are being left and have to get to their destinations when they are fetched. There must be enough full-duplex voice channels for both functions: leav- ing voice-mail messages and picking up voice-mail messages. This is a traffic engi- neering problem, an exercise in making sure that there are enough channels between the wireless telephone system and the voice-mail system that the blocking rate for voice mail stays low. Voice 