 voice-mail messages if the voice-mail function is not supported on the system where the user is currently registered. A user terminal's ROAM indicator will go on when the SID of the MSC where it is registered is different from the home system's SID. At the largest level, one could consider the entire continental network owned by one service provider to be the home system and roaming to be the state where a user ter- minal is being served by a different service provider. From the customer's perspective, this is the case for Sprint PCS customers in North America. All home rates apply when- ever the user terminal is using the Sprint PCS North American Network. If a Sprint PCS base station is not within range, the cellular telephone attempts to find first an alternate digital provider and then an analog provider. If it succeeds, the subscriber is notified of the digital roam or analog roam state and warned of roaming charges that will apply, which may exceed 40 cents per minute. 9Here is an example of a potential glare condition (described in Sec. 12.3): The subscriber can initiate a call while the user terminal is being paged for an incoming call. The MSC software has to resolve this. 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. The PSTN and Telephone Switching Key Telephony Concepts In the following technical discussion we will be referring to the ANSI-41 definition of roaming as what happens when the user terminal is registered at any MSC other than the home MSC unless we specify otherwise. 11.4.3.2 Roaming registration (HLR and VLR). When a subscriber terminal regis- ters with the system, it sends its mobile identification number (MIN) and its electronic serial number (ESN) to the network. The MIN is a unique 10 decimal digits (40 bits). 10 The ESN is a 32-bit binary number encoded during the manufacturing process. The serving MSC uses t