stack in order to establish communication between IPv4 and IPv6 SIP applications. The IMS ALG receives an incoming SIP message from CSCF nodes or from an external IPv4/IPv6 SIP network. It then changes the appropriate SIP/SDP parameters', translating the IPv6 addresses to IPv4 addresses and vice versa. It also requests NA (P) T-PT (Network Address (Port-Multiplexing) Translation-Protocol Translation) to provide the bindings data between different IP addresses upon session establishment and then release the bindings at session release. Translation Gateway (TrGW) can also act as NA (P) T-PT. PDF: The Policy Decision Function (PDF) maybe a logical entity of the P-CSCF or a separate physical node. The PDF is responsible for making policy decisions based on session and media related information obtained from P-CSCF. Charging Entities: IMS CN Subsystem functional elements support both offline and online charging mechanisms. The interface Rf entertains offline charging, whereas Ro interface provides online charg- ing capabilities to support real-time charging models. The IMS charging entity incorporates a flexible structure of the call data records and to integrate with existing back-end business systems and online charging technologies. 9.3.3 IMS Reference Points Table 9.1 lists the main functions of the several reference points of the IMS architecture. 9.4 3GPP IMS and WiMAX WiMAX is defined as a WMAN (Wireless Metropolitan Area Network) access technology. It is an all-IP technology supporting voice, video and data types of traffic. Neither IEEE 802.16 committee nor WiMAX Forum has SO far come up with specifications that can be used by WiMAX systems to support multimedia traffic. Thus, it is widely expected that WiMAX systems will use 3GPP IMS or Common IMS (defined later) architecture to support multimedia traffic. Although IMS is access independent, there is a number of issues that need to be resolved to support multimedia traffic with WiMAX using 3GPP IMS. We will only list these issues in this section, WiMAX and 3