 are accommodated in the reference network design. Furthermore, WiMAX network supports an MS with multiple IP addresses and simultaneous IPv4 and IPv6 connections. CHAPTER 2 WiMAX Network Architecture 3. The network design does not preclude roaming between NSPs. It allows a single NAP to serve multiple MSs using different private and public IP domains owned by different NSPs. The NSP may be one operator or a group of operators. 4. Seamless and robust handover at different user mobility conditions are supported by dynamic and static home address configurations. The WiMAX network allows dynamic assignment of the mobile IP Home Agent (HA) in the service provider network as a form of routing optimization, as well as in the home IP network as a form of load balancing. 6. The architecture allows dynamic assignment of the HA in Home CSN (H-CSN) or Visited CSN (V-CSN) based on policies. The Quality of Service (QoS) considerations for network operation are as follows: 1. The network design supports differentiated levels of QoS, admission control, iv and bandwidth assignment in order to flexibly support simultaneous use of a diverse set of IP services. 2. The network protocols support implementation of policies as defined by various operators for QoS based on their SLAs, including policy enforcement per user and user group, etc., where the QoS policies may be coordinated among operators depending on subscriber SLAs. 3. The reference design employs standard IETF mechanisms for managing policy definition and policy enforcement between operators. Other considerations for network operation including scalability, extensibility, coverage, etc., can be summarized as follows. The architecture enables a user to manually or automatically select from available NAPs and NSPs. The architecture enables ASN and CSN system designs that readily up-scale or down-scale in terms of coverage, range, or capacity while accommodating a variety of ASN designs. It further facilitates a variety of backhaul links, both wireline and wireless with diff