gator IP backbone Base Station Servers Base Station Network Management System Figure 11.3 Mobile WiMAX architecture R6 reference point. This flexibility reduces mobility-related signaling as the same ASN-Gateway can serve the user's active IP session while the user is moving across several different base stations. The R8 reference point facilitates the context transfer and handover optimization when the user switches base stations. The authentication is carried out via EAP as stated in the IEEE 802.16 specifications. The EAP exchanges are transported between the MS and BS within Privacy Key Management (PKMv2) MAC messages. QoS in Mobile WiMAX is handled via multiple service flows to enable differentiated services. To enable dynamic QoS authorization and enforcement, the ASN-GW and WiMAX Home Agent could make use of a policy exchange framework that is enabled by 3GPP's policy and charging control (PCC) architecture. WiMAX uses PCC (policy and charging control) to facilitate the use of the existent IMS framework specified by 3GPP. The policy and charging framework designed to allow an access network to connect to a policy and charging rules function (PCRF) for the purposes of QoS and charging authorization. The PCC uses the PCRF to receive the authorization envelope for associated QoS parameters that are enforced in the ASN-GW via policy charging enforcement functions (PCEF) in the ASN-GW (A-PCEF) and/or the HA (C-PCEF). The A-PCEF can be relocated during the lifetime of a bearer due to user mobility. To hide this relocation, a policy decision function (PDF) in the PCRF introduced in the WiMAX PCC architecture. The PDF is a WiMAX-specific logical entity on the path between PCRF and PECF(s). It could be a standalone network entity or integrated with the PCRF [2]. 11.4 Mobility Management Mobility Management (MM) covers a lot of ground and it has been exhaustively discussed by the wireless telecom industry at large. Mobility Management is one of the major functions of cellular systems that allow mobile phones to work