WIMAX NETWORK ARCHITECTURE

The mobile WiMAX end-to-end network architecture is based on an All-Internet Protocol (IP) platform, all packet technology, and no circuit switch telephony. The end-to-end architecture makes the greatest possible use of IETF and IEEE standards and protocols along with the adoption of commonly available standard equipment.

The open IP architecture gives network operators great flexibility when selecting solutions that work with legacy networks or that use the most advanced technologies, and in determining what functionality they want their network to support. They can choose from a vertically integrated vendor that provides a turnkey solution or they can pick and choose from a dense ecosystem of best-of-breed players with a more narrow focus. The architecture allows modularity and flexibility to accommodate a broad range of deployment options such as small scale to large scale, urban, suburban, and rural coverage, mesh topologies, flat, hierarchical and their variant, and finally, coexistance of fixed, nomadic portable and mobile usage models.

Mobile WiMAX adds both the mobility and multiple-input multiple-output (MIMO) functionalities to the IEEE 802.16-2005 standard. It is one of two standards adopted by the WiMAX forum with the other one being the IEEE 802.16-2004. Mobile WiMAX network architecture mainly has three components. These include the access services network (ASN), the core services network (CSN), and the application services (AS) network. Figure 12.1 illustrates the interconnection of these networks. The WiMAX network supports the following key functions:

• All-IP access and core service networks
• Support for fixed, nomadic, and mobile access
• Interoperability with existing networks via internetworking functions
• Open interfaces between ASNs and between the ASN and the CSN
• Support for differential quality of service (QoS) depending on the application
• Unbundling of the access, core, and application service networks

 

Figure 1: WiMAX network architecture.

WiMAX—Architecture, Planning, and Business Model

The fixed/portable broadband wireless access is becoming a necessity for many residential and business subscribers worldwide. The demand is exploding as the pricing of broadband services is rapidly decreasing. The worldwide interoperability for microwave access (WiMAX) technology is an integral part of the portfolio by complementing 2G/3G mobile access, Digital Subscriber Line (DSL) broadband fixed access, and Wireless Fidelity (WiFi) hotspot access. An extended overview of WiMAX and its applications in higher generation wireless networks as a cost-effective solution to answering the challenges posed by the digital divide is presented. Technology behind WiMAX and its network architecture, design, and deployment are examined in addition to factors that impact WiMAX planning and performance. A WiMAX radio coverage simulation and analysis at different frequency bands for different demographic is presented. Furthermore, the WiMAX business models and a comparison with two enhanced third-generation (3G) technologies that are potential competitors to WiMAX are explored. 

Telecommunications has grown at a tremendous rate in the last ten to twenty years. Improved semiconductor and electronics manufacturing technology, and the growth of the Internet and mobile telecommunications have been some of the factors which have fueled this growth in telecommunications. The deployment of state-of-the-art telecommunications infrastructure and services has, however, been restricted to the developed world. The least developed countries have been left in the technological dark ages with few or none of the next-generation networks installed. Developed countries now boast high-speed connections with a large percentage of homes having access to the Internet and broadband services at an affordable fee. The underdeveloped countries are yet to enjoy such facilities. This is referred to as the digital divide. During the first World Summit on the Information Society (WSIS) held in Geneva in December 2003, the Digital Divide was defined as the unequal access to information and communication technologies (ICTs), where the least developed countries are separated from the developed countries because of a lack of technology particularly ICT.

The digital divide has persisted due to the relatively high cost of putting up modern telecommunications infrastructure. This is compounded by the fact that there are a number of different services available and each service requires its own technology and network. Therefore, existing technologies such as Wireless Fidelity (WiFi), Digital Subscriber Line (DSL), Global System for Mobile communications (GSM), Integrated Services Digital Network (ISDN), and the relatively new 3G technologies have not been able to provide a total solution to closing the digital divide. Figure 1 illustrates the main network types and the prevalent technologies associated with each, mapped against usage models and access modes.

 

Figure 1: Wireless network type and range. MAN—metropolitan area network (citywide, rural area), LAN—local area network (office, home, campus), and WAN—wide area network (countrywide, international).

WiMAX will boost today’s fragmented broadband wireless access market and mobile WiMAX promises to offer a solution to closing the existing digital divide. WiMAX can address the fixed wireless access and portable Internet market, complementing other broadband wireless technologies. Government initiatives to reduce the digital divide are making gains for broadband wireless countries such as Australia, South Korea, Taiwan, and the United States have programs in place today, and there has been a push by the European Commission for more flexible spectrum policies.

WiMAX access can be easily integrated within both fixed and mobile architectures, enabling operators to integrate it within a single converged core network, thereby providing new capabilities for a user-centric broadband world.

WiMAX addresses the following needs which may answer the question of closing the digital divide:

• Cost effective
• Offers high data rates
• Supports fixed, nomadic, and mobile applications thereby converging the fixed and mobile networks
• Easy to deploy and has flexible network architectures
• Supports interoperability with other networks
• Aimed at being the first truly a global wireless broadband network

WiMAX is a standard that is championed by the WiMAX forum which was formed in June 2001 to promote conformance to IEEE 802.16 standard. The WiMAX forum currently has more than 470 members comprising the majority of operators, component, and equipment companies in the communications ecosystem. The WiMAX forum promotes interoperability by working closely with IEEE and other standards groups such as the European Telecommunications Standards Institute (ETSI) which have their own versions of broadband wireless. Along these lines, the WiMAX forum works closely with service providers and regulators to ensure that WiMAX forum certified systems meet customer and government requirements.

The original WiMAX standard only catered for fixed and nomadic services. It was reviewed to address full mobility applications; hence, the mobile WiMAX standard, defined under the IEEE 802.16e specification was created. Mobile WiMAX supports full mobility, nomadic, and fixed systems to compete against DSL to cover isolated areas such as rural hot spots, private campus networks, and remote neighborhoods. Mobile WiMAX is more promising to be deployed as a cellular network that offers ubiquitous broadband services to mobile users to over large geographical areas. It can be deployed as a central office bypass to avoid using existing wired infrastructure for competitive local exchange carriers and wireless Internet service provider. Figure 2 shows the standard history for 802.16.

 

Figure 2: 802.16 standard evolution.

WiMAX HANDOFF

IEEE 802.16e standard defined the required procedures and functions that should be implemented at the physical and MAC layers to perform handoff. The mobile WiMAX version inherits from the IEEE 802.16e standard, but it also defines the protocols that should be implemented at the higher layers to support intra-/inter-ASN handoff, roaming, seamless handoff, and micro-/macro-mobility.

1: SUPPORTED ARCHITECTURE

As described earlier, an ASN includes at least one ASN GW responsible for communicating with the CSN and a BS managing the connections to the MSs in its coverage. An ASN GW may be associated with one or more BSs while a BS can be managed by one or more ASN GWs so that multiple vendors can simultaneously interoperate within the same ASN. The BS may be a serving BS or a target BS depending on its task during the handoff process. In fact, the serving BS is the BS related to the MS before handoff while the target BS is the BS associated with the MS after handoff. On the other hand, we distinguish the serving ASN GW, the target ASN GW, and the anchor ASN GW. The serving ASN GW is the ASN GW corresponding to the serving BS; the target ASN GW is the ASN GW connected to the target BS while the anchor ASN GW is the ASN GW receiving the CSN data addressed to the MS and relaying them to the serving ASN GW. Thanks to the anchoring ASN GW, the MSs mobility is transparent to the CSN that does not need to know which ASN GW is managing the BS that is serving the MS. Therefore, the anchoring function prevents the CSN from frequently changing IP addresses. If the serving ASN GW is directly receiving data from the CSN, it is also considered as the anchor ASN GW. Nevertheless, the anchor ASN GW does not need to be a serving ASN GW or a target ASN GW. The intra-ASN handoff is processed between BSs within the same ASN; it does not induce important delay and minimizes data loss. Besides, intra-ASN handoff does not result in a change of the MSs IP address because the mobility is transparent to the outside of the ASN. Contrarily, an inter-ASN handoff is processed between BSs belonging to different ASNs and involves ASN GWs associated with separate ASNs. These ASN GWs need to coordinate their actions by adopting either anchoring or reanchoring to make the handoff smooth to the MS.

2: FUNCTIONAL DECOMPOSITION

The ASN-anchored mobility management is defined as mobility of an MS not involving a change in the CoA; it applies to mobility in networks not based on MIP. The specifications identify three functions responsible for the handoff, the MS context, and the data delivery control. More specifically, the Handoff function, which is implemented on the serving, the relaying, and the target peers, manages the signaling messages exchange and takes decisions associated with the handover. Figure 1 illustrates a possible handoff scenario. First, the serving-handoff function sends a handoff request (HO_Req) and waits for the corresponding reply. That HO_Req should include at least the MS_ID identifying the MS that requests the handoff, the list of the candidate target BS identifiers (IDs), possibly the MS/Session information content, and the first requested bicast SDU sequence number. The relaying handoff function relays the HO_Req to multiple target handoff functions, which are in charge of analyzing the request, formulating, and sending the correspondent handoff responses (HO_Rsp). The HO_Rsp primitive includes at least the MS_ID and the list of the recommended target BS IDs; it may also carry other optional information. The received responses are forwarded by the handoff-relaying function to the serving-handoff function. The latter should send back a handoff confirmation (HO_Cnf) to the chosen target stating the final handoff action that may either be an initiation, a cancellation, or a handoff rejection. The HO_Cnf should at least indicate the MS_ID, the DL ARQ synchronization information per service flow describing the context necessary to restore communication from the point it has been interrupted and the UL ARQ synchronization information per service flow describing the context necessary to restore communication from the point it has been interrupted.

Figure 1: Handoff function network transaction.

On the other hand, the context function manages the MS context and related information while handling their exchange in the backbone to set up any state or retrieve any state in network elements. For instance, the MSs context in the context function associated with the serving/anchor handoff function needs to be updated. More specifically, the MSs context in the context function associated with the serving handoff-function will be transferred to the context function associated with the target handoff function. Context information transfer may be triggered to populate a new MSs security context at a target BS, inform the network of an MSs initial network entry, or inform the network of the MSs idle mode behavior. The specifications identify relaying context functions, context functions acting as context servers, and context functions acting as context clients. The relaying context function mediates information delivery between context-client and context-server functions. The context-server function stores the most updated session context information for the MS while the context-client function, which is associated with the functional entity having the 802.16 physical link, retrieves session context information stored at the context-server level during handoff processes.

Finally, the data path (DP) function, also referred to as the bearer function, establishes the routes and manages the current data packets transmission between two functional entities. More specifically, the DP function controls the setup of the bearer plane between two BSs, two gateways, or a gateway and a BS; it may implement the setup of tunnels and support multicast and broadcast. The specifications distinguish four DP functions with respect to their roles in the handoff process. First, the anchor DP function anchors the data path associated with the MS across handoffs by forwarding the received data packets toward the serving DP function; it may buffer some of the packets and maintain some state information regarding bearer for the MS during handoffs. Second, the serving DP function is implemented at the end of the DP and associated with the serving PHY(physical)/MAC function (e.g., the serving BS) to handle the transmission of all data packets destined to the MS. Third, the target DP function is associated with a target BS that has been selected as the target of the handoff; it communicates with the anchor DP function to establish the DP that will replace the current path after the termination of the handoff. If the handoff succeeds, the target DP function becomes the serving DP function. Fourth, the relaying DP function mediates message exchange between serving, target, and anchor DP functions.

MOBILE WIMAX END-TO-END ARCHITECTURE

IEEE 802.16 standards family designed the physical and the MAC layers to provide broadband services access at the metropolitan scale for fixed, nomadic, portable, and mobile subscribers. The WiAMX forum, whose mission is to promote the interoperability of the broadband wireless access equipments implementing the IEEE 802.16 and ETSI HIPERMAN standards, has created a Network Working Group and a Service Provider Working Group to address higher-layer specifications such as intervendor inter-network interoperability for roaming, multivendor access networks, and intercompany billing. The result of these standardization efforts is the mobile WiMAX end-to-end architecture. The architecture is based on an all-IP platform implementing packet switching; it defines an access network and a common core network while decoupling the access from connectivity IP service. Moreover, the end-to-end architecture is designed to support loosely coupled interworking with existing wireless networks such as UMTS and existing wired networks such as DSL. Besides, a global roaming across WiAMX operator networks is achieved through the support for credential reuse, common billing and settlement, and consistent use of authentication, authorization, and accounting (AAA) services. The WiMAX forum designed the WiMAX network reference model (NRM), which aims at achieving interoperability through identifying the functional entities and reference points and the corresponding communication protocols and data plane treatment within a logical representation of the network architecture. As depicted in Figure 1, the MS, the access service network (ASN), and the connectivity service network (CSN) represent a set of functional entities that may be implemented by a single physical device or by different physical devices. The ASN is formed by at least one BS and one ASN gateway (ASN GW); it implements the access services and represents a boundary for functional interoperability with WiMAX clients and WiAMX connectivity service functions. For instance, the BS manages the MSs in its coverage while the ASN GW relays data to the CSN. On the other hand, the CSN may be defined as a network of Internet gateways, user databases, routers, servers, and proxies providing IP connectivity services to WiMAX subscribers.

Figure 1: The WiMAX NRM.

Mobile WiMAX end-to-end architecture is based on a security framework which provides basic security services and particularly authentication and confidentiality. In fact, each MS authenticates itself to the WiMAX network while the WiMAX network authenticates itself to the MS by implementing consistent and extensible authentication mechanisms. Besides, data confidentiality and integrity, replay protection and nonrepudiation services are guaranteed using applicable key length. Last but not least, MSs have the possibility to initiate and terminate specific security mechanisms such as virtual private networks (VPNs). The end-to-end architecture supports advanced IPv4-or IPv6-based mobility management mechanisms. For instance, vertical handovers can occur with wireless LANs or third generation wireless networks while roaming between different network service providers (NSPs) is supported. Seamless handover is also supported at up to vehicular speed while optimizing the overall network resources through the implementation of dynamic and static home address configurations, dynamic assignment of the home agent in the service provider network, and in the home IP network based on policies, etc. WiMAX mobility management will be further detailed.