nt (RP) represent a collection of control or bearer-plane end- points. The mobile station is a communication device providing radio connectivity between a user terminal and a WiMAX Base Station (BS) that is compliant with WiMAX Forum mobile system profiles [11]. The ASN is defined as a complete set of network functions required to provide radio access to a terminal. The ASN provides the following functions: Layer 2 connectivity with the MS; Transfer of AAA messages to the subscriber's Home Network Service Provider (H-NSP) for authentication, authorization, and session accounting for subscriber sessions; Network discovery and selection of the subscriber's preferred NSP; Relay functionality for establishing Layer 3 connectivity or IP address assignment to an MS; Radio resource management (RRM). CHAPTER 2 WiMAX Network Architecture Visited NSP Home NSP Visited Control Plane Bearer Plane ASP Network/ ASP Network/ Internet Internet FIGURE 2-1 The WiMAX network reference model In addition to the above functions, and to support mobility, an ASN supports the following functions: ASN-anchored mobility; CSN-anchored mobility; Paging and Idle State operation; ASN-CSN tunneling. The ASN comprises network elements such as one or more base stations and one or more ASN Gateways (ASN-GW). An ASN may be shared by more than one Connectivity Service Network (CSN). The radio resource control functions in the BS would allow Radio Resource Management (RRM) within the BS. The CSN is defined as a set of functions that provide IP connectivity to user terminals. In the following sections, more detailed description of network elements and their interconnections will be provided. 2.2.1 Access Service Network (ASN) The ASN is defined as a logical set of functional entities and corresponding signaling and message flows. The ASN represents an interface for functional interoperability with user terminals, on the one hand, and connectivity service functions, on the other hand. The functional decomposition of ASN is shown in Figure 2-2. Within an