simple and full mobility. Figure 4.10 represents the NRM with its logical entities and reference points for interconnection of the logical entities. The logical entities are SS/MS (Subscriber Station for fixed users and Mobile Station for mobile users), ASN (Access Service Node) and CSN (Connectivity Service Node). Each of the entities, MS/SS, ASN and CSN represent a grouping of functional entities. The ASN consists of base stations and ASN Gateways connected over an IP cloud. The ASN represents a boundary for functional interoperability with WiMAX clients and WiMAX connectivity service functions. CSN is defined as a set of network functions that provide IP connectivity services to the WiMAX subscribers. A CSN comprises of network elements such as routers and servers, AAA (Authentication, Authorization, and Accounting), Home Agent, DHCP (Dynamic Host Configuration Protocol), DNS (Domain Name System), and SO on. WiMAX architecture allows IP and Ethernet services in a standard mobile IP compliant network [5, 10]. WiMAX architecture also consists of a number of reference points. A reference point is a conceptual link that connects two groups of functions that reside in different functional entities of an ASN, CSN, or 4 64-QAM is only supported in downlink. 5 In Chase combining every retransmission contains the same information (data and parity bits). One could think of every retransmission adding extra "energy" to the received transmission. In Incremental redundancy every retrans- mission contains different information than the previous one. At every retransmission the receiver gains knowledge of extra information. Next Generation Mobile Communications Ecosystem Visited NSP Home NSP ASP Network OR ASP Network OR Another ASN Internet Internet legend of lines bearer plane control plane Figure 4.10 WiMAX network reference model. Reproduced from C 2009 WiMAX [10] MS. It is not necessarily a physical interface. A reference point only becomes a physical interface when the functional entities on either side of it are conta