ined in different physical mobile stations. The key reference points are as follows: R1: Between SS/MS and ASN R2: Between SS/MS and CSN (Home and Visited Network) R3: Between ASN and CSN R4: Between ASNs R5: Between CSNs (Home and Visited Network) R6: Base station and ASN-Gateway (not shown) R7: ASN-GW (decision point) and (enforcement point) ASN-GW (not shown) R8: Between base stations (not shown). 4.4.1 WiMAX and WiFi Network Architecture Figure 4.11 shows WiMAX and WiFi combo architecture where WiMAX coverage is extended by WiFi access points. The WiFi access point (AP) is connected to the WiMAX base station which is in turn linked to WAG (wireless access gateway) and core network. One of the key challenges of this architecture is the delivery of end-to-end QoS. The QoS mechanism and the associated traffic classes are defined differently in WiMAX and WiFi technologies. There are eight main traffic classes defined in WiFi 802. 11e standard and eight categories of QoS are described in WiMAX 802.16e standard but not for the same type of application. IEEE WiMAX ((0)) WIMAX CPE Ethernet Cable WiFi AP Access Internet Controller Base Station ((0)) WIMAX CPE Ethernet Cable WiFi AP Figure 4.11 WiMAX and WiFi integrated network architecture In the case of WiMAX and WiFi hybrid networks when a WiMAX user is utilizing any application which requires other than "best effort" service, the user will not be successful in getting that class of QoS. For instance (Figure 4.12), a WiMAX user A (in a WiMAX network) who is sending VoIP packets via MGW (Media Gateway) utilizing WiMAX UGS quality of service to a WiMAX user B who is in a WiMAX/WiFi hybrid network. The user B's WiFi Access Point does not have a mechanism that could assist it in mapping user A WiMAX UGS VoIP traffic to WiFi QoS method and then finally delivering the correct QoS to the user B. Similarly, a laptop user (in a hybrid network) cannot communicate the correct QoS for a video application via WiFi access point to an outdoor WiMAX CPE (Customer Premise Equipment)