t specify how these tasks are done. These functions are defined in the SMS standard (see Chap. 20). Many SMEs, including broadcast paging services, are on data networks. ANSI-41 only defines the functional components and processes that al- low SMS messages to move through the cellular network. 15.3.2 Message centers Message centers (MCs) are the ANSI-41 store-and-forward hubs for most mobile- originated short messages and for all mobile-terminated short messages. These are usually physically separate machines but may be combined with other functional enti- ties. Each SMS subscriber is associated with a home MC in the subscriber's home sys- tem. The MC should be able to forward short messages to the user terminal SME, store short messages for unavailable user terminal SMEs, and perform other administrative services as required. For roamers, the serving MC coordinates with the HLR for location update and user terminal state. To receive a message, the user terminal must be located, be in an active state, and be authorized to receive short messages in the serving system. 15.3.3 Short message processing Here is a typical short message scenario from mobile A to mobile B: Mobile subscriber A sends a short message. Serving MSC A uses ANSI-41 and SS7 to route the message to MCA MC A sends acknowledgment to serving MSC A. Serving MSC A sends acknowledgment to user terminal A. MC A sends the message using the Transmission Control Protocol/Internet Protocol (TCP/IP) to MC B. MC B uses ANSI-41 and SS7 to route the message to serving MSC B. Serving MSC B sends the message to user terminal B. User terminal B sends acknowledgment to serving MSC B. Serving MSC B sends acknowledgment to serving MC B. 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. ANSI-41 ANSI-41 The message transfer is complete. It is up to subscriber B to respond to subscriber A. 15.4 Opera