ent (SLA) established with WiMAX subscribers. To provide these services, an NSP sets up contractual agreements with one or more NAPs. Additionally, an NSP also establishes roaming agreements with other NSPs, and contractual agreements with third- party application providers for providing WiMAX services to subscribers. From a WiMAX subscriber perspective, an NSP can be classified as Home NSP (H-NSP) or Visited NSP (V-NSP). The WiMAX network architecture supports access to operator's services through inter- networking functions and specifies open and standard reference points between various groups of network functional entities, in particular between an MS, ASN, and CSN to enable multi- vendor interoperability. It is sufficiently flexible to accommodate future enhancements/exten- sions to the IEEE 802.16 suite of standards, and can accommodate documented geo-specific constraints. The WiMAX network architecture supports evolution paths between the various usage models subject to reasonable technical assumptions and constraints. It does not preclude multi-vendor implementations based on different combinations of functional network entities as long as these implementations comply with the normative protocols and procedures across applicable reference points. The structural design supports a simple usage model where a single operator can deploy an 2.1 Design principles of WiMAX network architecture ASN together with a limited set of CSN functions, SO that the operator can offer basic Internet access without consideration for roaming or interworking. The service and application design requirement for mobile WiMAX network architecture are as follows: 1. Support voice, multimedia services, and other mandated regulatory services such as emergency services and lawful interception; 2. Enable access to a variety of independent Application Service Provider (ASP) networks; 3. Support mobile communications using VoIP in applicable roaming scenarios, further supporting inter-operator policy definition, distribution and enforceme