 ASN, a BS may be logically connected to more than one ASN-GW for different MSs. For a given MS, a BS is connected to a single ASN-GW. 2.2 Network reference model Data Path ASN-GW Authentication Function Handover Function Distribution RRM Relay Context Function DHCP Proxy Paging Proxy Mobile Control Service Flow IP Client Authentication Location AAA Client Register Mobile IP FA Data Path Context Radio Resource Function Function Agent Radio Resource Handover Control Authentication Function Relay Paging Agent Service Flow Management Key Receiver FIGURE 2-2 The functional decomposition of ASN The ASN reference model containing multiple ASN-Gateways (ASN-GW) is illustrated in Figure 2-3. An ASN shares R1, R3, and R4 reference points with an MS, a CSN, and with another ASN, respectively. The ASN consists of one or more base stations and one or more ASN-GWs (as shown in Figure 2-3). A BS is logically connected to one or more ASN-GWs. The R4 reference point provides control and bearer-planes for interconnection and interoperability between similar or heterogeneous ASNs. When ASN is composed of multiple ASN-GWs, intra ASN mobility may involve R4 control messages and bearer-plane establishment. 2.2.2 Access Service Network Gateway (ASN-GW) The base stations within an ASN-GW group (i.e., all base stations connected to the same ASN-GW) are connected through R8 reference point. Furthermore, the base stations within an ASN-GW group are separately connected to the ASN-GW via R6 reference point, as shown in Figure 2-3. CHAPTER 2 WiMAX Network Architecture Bearer Path Control Path ASN-GW1 ASN-GW2 FIGURE 2-3 The ASN reference model containing multiple ASN-GW The base station is a logical entity that implements a full instance of Medium Access Control (MAC) and Physical Layer (PHY) protocols, as specified by the IEEE 802.16-2009 standard [12] and includes one or more access functions. A BS instance represents one sector with one frequency assignment. It incorporates scheduler functions for uplink and downlink radio resources, whic