rant message. The data is transmitted on the reserved channels. The main reasons for the standby state are to reduce the load in the GPRS network caused by cell-based routing update messages and to conserve the mobile station battery. When a mobile station is in the standby state, the SGSN does not need to be informed of every cell change-only of every routing area change. The operator can define the size of the routing area and, in this way, adjust the number of routing update messages. In the idle state, the mobile station does not have a logical GPRS context activated or any Packet-Switched Public Data Network (PSPDN) addresses allocated. In this state, the MS can receive only those multicast messages that can be received by any GPRS mobile station. Because the GPRS net- work infrastructure does not know the location of the mobile station, it is impossible to send messages to the mobile station from external data net- works. A cell-based routing update procedure is invoked when an active mobile station enters a new cell. In this case, the mobile station sends a short message containing information about its move (the message contains the identity of the mobile station and its new location) through GPRS chan- nels to its current SGSN. This procedure is used only when the mobile sta- tion is in the active state. 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. System Architecture Chapter 3 Network Architecture- New Interfaces The GPRS backbone network will permit point-to-point GPRS calls, inter- working with the BSS, HLR, MSC, SMSC, and the Internet. A new set of interfaces has been developed for GPRS. These interfaces all are labeled with a Gx where the x stands for a variety of interfaces, as discussed in the following section and in Figure 3-7. These services will be supported via the following interfaces: Gb Between the PCUSN and SGSN, 