Network Functionalities | WiMAX Architechture

This section describes the main functions achieved by the WiMAX end-to-end system.

1 Network Discovery and Selection

This function is required for nomadic, portable and mobile WiMAX services where in the same geographical area the MS may have radio coverage access to an ASN managed by a single NAP and shared by several NSPs or coverage access to several ASNs managed by several NAP/NSPs. To perform network discovery and selection, the MS (which has been pre-provisioned with configuration information at subscription) performs a four-step process:

§  NAP discovery. During the scanning of the DCD of the different BSs in the coverage reach from the MS, the MS detects the ‘operator ID’ in the BSID field.

§  NSP discovery. The MS discovers the available NSP through the list NSP ID which is broadcast by the ASN as part of the system information identity message. NSP discovery is also possible via solicited request/response messages.

§  NSP enumeration and selection. Based on the dynamic information obtained in the coverage area and the configuration information from the subscription, the MS selects the appropriate NSP. Manual configuration of the NSP may also be available in the case of a visited NSP.

§  ASN attachment. After selection of the NSP and associated ASN, the MS indicates its NSP selection by sending an NAI (Network Access Identifier) message used by the ASN to determine the next hop AAA where an MS AAA packet should be routed to.

2 IP Addressing

WiMAX networks support IPv4 and IPv6 addressing mechanisms. At the end of the procedure, a PoA (Point of Attachment) IP address is delivered to the MS. The IP allocation address modes depend on the WiMAX access service types.

In the case of IPv4, the dynamic PoA configuration is based on DHCP. The DHCP proxy may reside in the ASN and the DHCP server in the CSN. In the case of IPv6, stateful IP address allocation is based on DHCPv6. The DHCP server resides in the CSN and the DHCP proxy may reside in the ASN. For the stateless CoA (Care of Address), IP address allocations RFC 2462 and RFC 3041  are used.

3 AAA Framework

The AAA framework follows the IETF specifications and includes the following services:

§  authentication: device, user or combined user/device authentication;

§  authorisation: user profile information delivery for sessions, mobility and QoS;

§  accounting: delivery of information for pre-paid/post-paid services.

Authentication and authorisation procedures are based on the EAP (Extensible Authentication Protocol). Between the MS and the ASN (the authenticator function), EAP runs over PKMv2, which enables both user and device authentication. Between the AAA server and the ASN, the EAP runs over RADIUS.

Accounting is also based on RADIUS messages. WiMAX Release 1 offers both offline (post-paid) and online (pre-paid) accounting. In the case of offline accounting, the ASN generates UDRs (Usage Data Records), which are a combination of radio-specific parameters and IP-specific parameters. The serving ASN then sends the UDR to the RADIUS server.

4 Mobility

The mobility procedures are divided into two mobility levels:

§  ASN anchored mobility procedures. This refers to MS mobility where no CoA address update is needed, also known as micromobility.

§  CSN anchored mobility procedures. The macromobility between the ASN and CSN is based on mobile IP protocols running across the R3 interface.

CSN anchored mobility implies that, in the case of IPv4, the MS changes to a new anchor FA (Foreign Agent). WiMAX systems must support at least one of the following mobile IP schemes:

§  Proxy-MIP. In this case, the MS is unaware of CSN mobility management activities and there is no additional signalling/overhead over the air to complete the CSN mobility.

§  Client MIP (CMIPv4). In this case, the MIP client in the MS participates in inter-ASN mobility.

5 End-to-End Quality of Service

The IEEE standard defines the QoS framework for the air interface. The WiMAX architecture specifications extend the QoS framework to the complete network where many alternatives for enforcing the QoS on Layer 2 or Layer 3 may exist.

The end-to-end QoS framework relies on functions implemented in the CSN (PF (Policy Function) and AF (Application Function)) and in the ASN (SFM (Service Flow Management) and SFA (Service Flow Authorisation)). In the CSN, the AF triggers a service flow trigger to the PF based on the information sent by the MS with whom it communicates. The PF then evaluates service requests against a policy database in the NSP. In the ASN, the SFA communicates with the PF and is responsible for evaluating the service request against user QoS profiles. The SFM (located in the BS and responsible for creation, admission, modification and release of 802.16 service flows) mainly consists of an admission control function, which decides, based on available radio resource and other local information, whether a radio link can be created.