WiMAX Architecture  

WiMAX Architecture

The IEEE only defined the Physical (PHY) and Media Access Control (MAC) layers in 802.16. This approach has worked well for technologies such as Ethernet and WiFi, which rely on other bodies such as the IETF (Internet Engineering Task Force) to set the standards for higher layer protocols such as TCP/IP, SIP, VoIP and IPSec. In the mobile wireless world, standards bodies such as 3GPP and 3GPP2 set standards over a wide range of interfaces and protocols because they require not only airlink interoperability, but also inter-vendor inter-network interoperability for roaming, multi-vendor access networks, and inter-company billing. Vendors and operators have recognized this issue, and have formed additional working groups to develop standard network reference models for open inter-network interfaces. Two of these are the WiMAX Forum’s Network Working Group, which is focused on creating higher-level networking specifications for fixed, nomadic, portable and mobile WiMAX systems beyond what is defined in the IEEE 802.16 standard, and Service Provider Working Group which helps write requirements and prioritizes them to help drive the work of Network WG.
The Mobile WiMAX End-to-End Network Architecture is based an All-IP platform, all packet technology with no legacy circuit telephony. It offers the advantage of reduced total cost of ownership during the lifecycle of a WiMAX network deployment. The use of All-IP means that a common network core can be used, without the need to maintain both packet and circuit core networks, with all the overhead that goes with it. A further benefit of All-IP is that it places the network on the performance growth curve of general purpose processors and computing devices, often termed “Moore’s Law”. Advances in computer processing occurs much faster than advances in telecommunications equipment because general purpose hardware is not limited to telecommunications equipment cycles, which tend to be long and cumbersome. The end result is a network that continually performs at ever higher capital and operational efficiency, and takes advantage of 3rd party developments from the Internet community. This results in lower cost, high scalability, and rapid deployment since the networking functionality is all primarily software-based services.
In order to deploy successful and operational commercial systems, there is need for support beyond 802.16 (PHY/MAC) air interface specifications. Chief among them is the need to support a core set of networking functions as part of the overall End-to-End WiMAX system architecture. Before delving into some of the details of the architecture, we first note a few basic tenets that have guided the WiMAX architecture development.
1. The architecture is based on a packet-switched framework, including native procedures based on the IEEE 802.16 standard and its amendments, appropriate IETF RFCs and Ethernet standards.
2. The architecture permits decoupling of access architecture (and supported topologies) from connectivity IP service. Network elements of the connectivity system are agnostic to the IEEE 802.16 radio specifics.
3. The architecture allows modularity and flexibility to accommodate a broad range of deployment options such as:
· Small-scale to large-scale (sparse to dense radio coverage and capacity) WiMAX networks
· Urban, suburban, and rural radio propagation environments
· Licensed and/or licensed-exempt frequency bands
· Hierarchical, flat, or mesh topologies, and their variants
· Co-existence of fixed, nomadic, portable and mobile usage models
Support for Services and Applications: The end-to-end architecture includes the support for: a) Voice, multimedia services and other mandated regulatory services such as emergency services and lawful interception, b) Access to a variety of independent Application Service Provider (ASP) networks in an agnostic manner, c) Mobile telephony communications using VoIP, d) Support interfacing with various interworking and media gateways permitting delivery of incumbent/legacy services translated over IP (for example, SMS over IP, MMS, WAP) to WiMAX access networks and e) Support delivery of IP Broadcast and Multicast services over WiMAX access networks.
Interworking and Roaming is another key strength of the End-to-End Network Architecture with support for a number of deployment scenarios. In particular, there will be support of a) Loosely-coupled interworking with existing wireless networks such as 3GPP and 3GPP2 or existing wireline networks such as DSL and MSO, with the interworking interface(s) based on a standard IETF suite of protocols, b) Global roaming across WiMAX operator networks, including support for credential reuse, consistent use of AAA for accounting and billing, and consolidated/common billing and settlement, c) A variety of user authentication credential formats such as username/password, digital certificates, Subscriber Identify Module (SIM), Universal SIM (USIM), and Removable User Identify Module (RUIM).
WiMAX Forum industry participants have identified a WiMAX Network Reference Model (NRM) that is a logical representation of the network architecture. The NRM identifies functional entities and reference points over which interoperability is achieved between functional entities. The architecture has been developed with the objective of providing unified support of functionality needed in a range of network deployment models and usage scenarios (ranging from fixed – nomadic – portable – simple mobility – to fully mobile subscribers).
Figure 15 illustrates the NRM, consisting of the following logical entities: MS, ASN, and CSN and clearly identified reference points for interconnection of the logical entities. The figure depicts the key normative reference points R1-R5. Each of the entities, MS, ASN and CSN represent a grouping of functional entities. Each of these functions may be realized in a single physical device or may be distributed over multiple physical devices. The grouping and distribution of functions into physical devices within a functional entity (such as ASN) is an implementation choice; a manufacturer may choose any physical implementation of functions, either individually or in combination, as long as the implementation meets the functional and interoperability requirements.
The intent of the NRM is to allow multiple implementation options for a given functional entity, and yet achieve interoperability among different realizations of functional entities. Interoperability is based on the definition of communication protocols and data plane treatment between functional entities to achieve an overall end-to-end function, for example, security or mobility management. Thus, the functional entities on either side of a reference point represent a collection of control and bearer plane end-points.

The ASN defines a logical boundary and represents a convenient way to describe aggregation of functional entities and corresponding message flows associated with the access services. The ASN represents a boundary for functional interoperability with WiMAX clients, WiMAX connectivity service functions and aggregation of functions embodied by different vendors. Mapping of functional entities to logical entities within ASNs as depicted in the NRM may be performed in different ways. The WiMAX Forum is in the process of network specifications in a manner that would allow a variety of vendor implementations that are interoperable and suited for a wide diversity of deployment requirements.
Connectivity Service Network (CSN) is defined as a set of network functions that provide IP connectivity services to the WiMAX subscriber(s). A CSN may comprise network elements such as routers, AAA proxy/servers, user databases and Interworking gateway devices. A CSN may be deployed as part of a Greenfield WiMAX Network Service Provider (NSP) or as part of an incumbent WiMAX NSP.
Figure 16 provides a more basic view of the many entities within the functional groupings of ASN and CSN.

The network specifications for WiMAX-based systems are based on several basic network architecture tenets, including those listed below.
Some general tenets have guided the development of Mobile WiMAX Network Architecture and include the following: a) Provision of logical separation between such procedures and IP addressing, routing and connectivity management procedures and protocols to enable use of the access architecture primitives in standalone and inter-working deployment scenarios, b) Support for sharing of ASN(s) of a Network Access Provider (NAP) among multiple NSPs, c) Support of a single NSP providing service over multiple ASN(s) – managed by one or more NAPs, d) Support for the discovery and selection of accessible NSPs by an MS or SS, e) Support of NAPs that employ one or more ASN topologies, f) Support of access to incumbent operator services through internetworking functions as needed, g) Specification of open and well-defined reference points between various groups of network functional entities (within an ASN, between ASNs, between an ASN and a CSN, and between CSNs), and in particular between an MS, ASN and CSN to enable multi-vendor interoperability, h) Support for evolution paths between the various usage models subject to reasonable technical assumptions and constraints, i) Enabling different vendor implementations based on different combinations of functional entities on physical network entities, as long as these implementations comply with the normative protocols and procedures across applicable reference points, as defined in the network specifications and j) Support for the most trivial scenario of a single operator deploying an ASN together with a limited set of CSN functions, so that the operator can offer basic Internet access service without consideration for roaming or interworking.
The WIMAX architecture also allows both IP and Ethernet services, in a standard mobile IP compliant network. The flexibility and interoperability supported by the WiMAX network provides operators with a multi-vendor low cost implementation of a WiMAX network even with a mixed deployment of distributed and centralized ASN’s in the network. The WiMAX network has the following major features:
The end-to-end WiMAX Network Architecture is based on a security framework that is agnostic to the operator type and ASN topology and applies consistently across Greenfield and internetworking deployment models and usage scenarios. In particular there is support for: a) Strong mutual device authentication between an MS and the WiMAX network, based on the IEEE 802.16 security framework, b) All commonly deployed authentication mechanisms and authentication in home and visited operator network scenarios based on a consistent and extensible authentication framework, c) Data integrity, replay protection, confidentiality and non-repudiation using applicable key lengths, d) Use of MS initiated/terminated security mechanisms such as Virtual Private Networks (VPNs), e) Standard secure IP address management mechanisms between the MS/SS and its home or visited NSP.
Mobility and Handovers
The end-to-end WiMAX Network Architecture has extensive capability to support mobility and handovers. It will: a) Include vertical or inter-technology handovers— e.g., to Wi-Fi, 3GPP, 3GPP2, DSL, or MSO – when such capability is enabled in multi-mode MS, b) Support IPv4 or IPv6 based mobility management. Within this framework, and as applicable, the architecture SHALL accommodate MS with multiple IP addresses and simultaneous IPv4 and IPv6 connections, c) Support roaming between NSPs, d) Utilize mechanisms to support seamless handovers at up to vehicular speeds— satisfying well-defined (within WiMAX Forum) bounds of service disruption. Some of the additional capabilities in support of mobility include the support of: i) Dynamic and static home address configurations, ii) Dynamic assignment of the Home Agent in the service provider network as a form of route optimization, as well as in the home IP network as a form of load balancing and iii) Dynamic assignment of the Home Agent based on policies.
Scalability, Extensibility, Coverage and Operator Selection
The end-to-end WiMAX Network Architecture has extensive support for scalable, extensible operation and flexibility in operator selection. In particular, it will: a) enable a user to manually or automatically select from available NAPs and NSPs, b) Enable ASN and CSN system designs that easily scale upward and downward – in terms of coverage, range or capacity, c) Accommodate a variety of ASN topologies - including hub-and-spoke, hierarchical, and/or multi-hop interconnects, d) Accommodate a variety of backhaul links, both wireline and wireless with different latency and throughput characteristics, e) Support incremental infrastructure deployment, f) Support phased introduction of IP services that in turn scale with increasing number of active users and concurrent IP services per user, g) Support the integration of base stations of varying coverage and capacity - for example, pico, micro, and macro base stations and e) Support flexible decomposition and integration of ASN functions in ASN network deployments in order to enable use of load balancing schemes for efficient use of radio spectrum and network resources.
Additional features pertaining to manageability and performance of WiMAX Network Architecture include: a) Support a variety of online and offline client provisioning, enrollment, and management schemes based on open, broadly deployable, IP-based, industry standards, b) Accommodation of Over-The-Air (OTA) services for MS terminal provisioning and software upgrades, and c) Accommodation of use of header compression/suppression and/or payload compression for efficient use of the WiMAX radio resources.
Multi-Vendor Interoperability
Another key aspect of the WiMAX Network Architecture is the support of interoperability between equipment from different manufacturers within an ASN and across ASNs. Such interoperability will include interoperability between: a) BS and backhaul equipment within an ASN, and b) Various ASN elements (possibly from different vendors) and CSN, with minimal or no degradation in functionality or capability of the ASN.
The IEEE 802.16 standard defines multiple convergence sub-layers. The WiMAX Network Architecture framework supports a variety of CS types including: Ethernet CS, IPv4 CS and IPv6 CS.
Quality of Service
The WiMAX Network Architecuture has provisions for support of QoS mechanisms. In particular, it enables flexible support of simultaneous use of a diverse set of IP services. The architecture supports: a) Differentiated levels of QoS - coarse-grained (per user/terminal) and/or fine-grained (per service flow per user/terminal), b) Admission control, c) Bandwidth management and d) Implementation of policies as defined by various operators for QoS-based on their SLAs (including policy enforcement per user and user group as well as factors such as location, time of day, etc.). Extensive use is made of standard IETF mechanisms for managing policy definition and policy enforcement between operators.
The flexible WiMAX network specifications allows different implementations of Access Service Network (ASN) configurations namely ASN profiles, including both distributed/collapsed as well as centralized architectures. Furthermore, the WiMAX forum is developing an interoperability framework in which intra-ASN and inter-ASN interoperability across different vendors is ensured.
Network Architecture Specifications Timeline

The WiMAX Forum Network Working Group has divided the scope of its work on WiMAX features into 3 releases. Each release has 3 stages. Stage 1 is on requirements that are set by the Service Provider Working Group. The second and third stages correspond to architecture development and detailed protocol specifications. The work on Stage 2 of Release 1 was completed in 4Q-05 and currently Stage 3 work is in progress. It is expected to finish by 3Q-06. Meanwhile, the Network Working Group will initiate stage 2 work on the next set of features (unofficially called Release 1.5) in 2Q-06. It is anticipated Release 1.5 features will be completed by 4Q-06. As the work progresses, the WiMAX Forum Service Provider Working Group will set requirements based on operator priorities for subsequent releases and the Network Working Group will scope its specification work based on inputs from the Service Provider Working Group

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