oarder Gateway (BG), which can provide the necessary interworking and routing protocols (for example, Border Gateway Protocol [BGP]). It is also foreseeable that GPRS operators will implement quality of service (QoS) mechanisms over the inter-operator network to ensure service level agree- ments (SLAs). The main benefits of the architecture are its flexibility, scal- ablility, interoperability, and roaming. The GPRS network encapsulates all data network protocols into its own encapsulation protocol, called the GPRS Tunneling Protocol (GTP). This is Figure 3-5 The various components for data routing. Inter-PLMN GPRS Backbone Intra-PLMN GPRS Backbone PLMN1 Border Gateway Intra-PLMN GPRS Backbone PLMN2 Packet Data Network (PDN) (for example, Internet, Intranet Router 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 done to ensure security in the backbone network and to simplify the rout- ing mechanism and the delivery of data over the GPRS network. GPRS Mobility Management The operation of the GPRS is partly independent of the GSM network. However, some procedures share the network elements with current GSM functions to increase efficiency and to make optimum use of free GSM resources (such as unallocated time slots). Figure 3-6 shows a reference model for the protocols used between the GPRS components. The model is a relay chart for protocols in the Open Standards Interconnect (OSI) model. A mobile station has three states in the GPRS system: idle, standby, and active. The three-state model represents the nature of packet radio relative to the GSM two-state model (idle or active). Data is transmitted between a mobile station and the GPRS network only when the mobile station is in the active state. In the active state, the SGSN knows the cell location of the mobile station. However, in the standby state, the locatio