GPP IMS inter-working details can be found in the Chapter 11 (End-to-End Networks): Firstly, the inter-working at the session/MAC layer between 3GPP IMS and WiMAX is a key issue in providing efficient interoperability between them. Secondly, differences between WiMAX and 3GPPP QoS mechanisms need to be resolved. Thirdly, the current 3GPP and WLAN inter-working specifications need modification to entertain handoffs between 3GPP, WiMAX and WLAN networks. Lastly, there are some significant differences between 3GPP and WiMAX physical layers which need to be taken into consideration. The WiMAX bearer consists of an access bearer and an IP transmission bearer. The former provides a wireless access service by the IEEE 802.16 mechanism, and the IP bearer deploys differentiated services (DiffServ), and SO on, to guarantee QoS [6]. The QoS request is initiated by CSN (Connectivity Service Node) while the ASN (Access Service Node) implements the QoS negotiation according to the user's profile. UMTS defines four classes of QoS services based on different application requirements: conversational, streaming, interactive, and background. WiMAX on the other hand defines five classes of service: unsolicited grant service (UGS), real-time polling service (rtPS), enhanced real-time variable 3 Session Description Protocol (SDP) describes multimedia sessions for the purposes of session announcement, session invitation, and other forms of multimedia session initiation. IMS, Services and Applications Table 9.1 IMS reference points Reference Point Involved Entities Function CSCF and HSS It is used to communicate between CSCF and HSS. AS and SLF It is used by AS to find a correct HSS in a multi-HSS environment. I-CSCF and SLF It is used to find a correct HSS in a multi-HSS environment. GPRS and IMS It allows operators to exchange charging information between GPRS and IMS. UE and P-CSCF It is used to exchanges SIP signaling messages between UE and CSCFs. GGSN and PDF Allows operators to control QoS in the user plane (GPRS network) and exc