ng service. Through the BSS, the MSC re- quests and receives the international mobile equipment identity (IMEI), and it checks the IMEI for validity. Now the mobile telephone sends its setup request along with its dialed digits, and the MSC uses the HLR or VLR parameters to handle the call. The BSS and mobile tele- phone establish a voice channel, they both switch to that channel, and the BSS informs the MSC of the change. Finally, the public switched telephone network (PSTN) is involved in completing the call. Unlike AMPS, the mobile telephone is specifically informed, via signaling mes- sages, when the landline telephone is ringing (network alerting) and when the tele- phone is picked up (connect). When a call comes in from the PSTN, the MSC confirms the mobile telephone iden- tity with the HLR and VLR and has the BSS page the mobile telephone on the paging channel (PCH). The mobile telephone responds on the RACH, and the signaling se- quence from here is similar to the mobile telephone-initiated call, although there are some differences. Once the voice link connection is made, the mobile telephone rings (alerts), sends a network alerting message and then sends a connect message when the subscriber an- swers the phone. There is a similar exchange of messages when a call ends, all carefully prescribed in the GSM specifications. GSM allows for a mobile telephone to power off its nonessential circuitry, putting it in sleep mode during periods where no paging messages will be sent. This feature, dis- 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. TDMA Wireless Telephony (GSM) TDMA Wireless Telephony (GSM) continuous reception (DRx), is supported by having the paging channel divide pages up into paging subchannel groups by the last digits of the mobile telephone number or the international mobile service identity (IMSI). 7.4.2 GSM power contro