(SCA), and password call acceptance (PCA). The HLR sends a message to the Signaling System 7 (SS7) service control point (SCP), where it retrieves data on the subscriber's call screening. Then the HLR contacts the serving MSC with the screen- ing information so that the user terminal can be paged and the subscriber can be queried. Do not disturb (DND) is just what it sounds like; terminating calls are denied as if the user terminal were turned off. Mobile access hunting (MAH) goes through a group of mobile telephone numbers just like a hunt group of landline telephones, where the local office rings the first nonbusy line. In the wireless case, each user terminal in the sequence is checked for an active location and paged if it has one. Flexible alerting (FA) allows several wireless tele- phones all to alert when a call is placed to a designated telephone number. These two features place more calls to user terminals and, therefore, may increase the ANSI-41 message load. We suspect that use of these features will be low enough not to be a ma- jor issue. What is the bottom line for call processing? Calls generate work for the wireless tele- phone system, obviously, and they also generate ANSI-41 message for the SS7 signal- ing network. 25.4 Short Message Service (SMS) SMS is the signaling wildcard. SMS makes demands of signaling capacity without in- creasing the demand for voice channels on the network and while demanding very lit- tle capacity on the air interface. Generally, calls start and stop in predictable fashion, so we can predict the number of ANSI-41 messages that are related to calls per minute 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 Signaling Capacity ANSI-41 Signaling Capacity of air-interface use. SMS adds to the demand for ANSI-41 at a much higher ratio of sig- nals per capacity demanded than voice calls do. We ex