WIMAX NETWORK ARCHITECTURE

In Figure 1, we present the overall network architecture of a WiMAX network. The network can be logically partitioned into three components, user terminals, ASN, and CSN. User terminals capture the data origination points, could be using the fixed, mobile, or portable WiMAX technology. All the three variations can be supported using a common air interface. ASN spans the BS and the ASN-GW. BS receives the transmitted signal, processes it, and converts into an IP packet and sends to the GW on the outgoing IP transport link. GW receives and upon processing determines the destination on the network side and sends the packet. BS and GW are connected to each other using an IP transport. Typical implementations would have BS located in the field/coverage area and the GW will be centrally located in the switch centers. Therefore, the IP link between BS and GW forms the transport backhaul network. CSN contains many different commercial off-the shelf (COTS) components, which provide connectivity services to the WiMAX subscribers. Addressing, authentication, and availability (AAA) servers, mobile IP home agent (MIP HA), IP multimedia services (IMS), content services, etc. provide support for seamless services to subscribers. AAA servers ensure that a user is uniquely identified and authenticated as legitimate customer. MIP HA ensures that roaming across IP networks is handled and accurate routing of data packets is ensured. Call processing related services are provided by IMS entity. Billing and operational support systems help in managing the overall network.

 
Figure 1: Logical network architecture of a WiMAX network.

In Figure 2, we present typical implementation of a WiMAX network in a market. For example, say a carrier plans to lay down WiMAX network in Washington D.C. market. Typically, we would have more than 100 BSs connecting to a GW location, based on the anticipated traffic, each GW location might require a cluster of servers providing the functions of the GW. Each IP transport link would be leased from the local carrier and provisioned. Based upon the cost points and required capacity, the carrier can choose to directly lease a TDM segment, Ethernet link, fiber connectivity, etc. Components of the CSN located at each switch center might also be implemented using clusters and would have enough capacity to support the entire market. Switch centers could be connected to each other using a high speed IP network running on an OC-192 (or higher) SONET ring leased from local exchange carrier. Actual network would also include connectivity to the other markets, trunking with public switched telephone network (PSTN) via the end office (EO), tandem connections with other wireless carriers, etc.

 
Figure 2: Physical network architecture of a WiMAX network.

For most WiMAX networks, it is unlikely that the carriers would provision the IP transport based on the capacity of the WiMAX air interface. According to WiMAX forum, air interface built on 10 MHz channel with 2 × 2 MIMO can support peak downlink rate of 63 Mbps and peak uplink rate of 28 Mbps per sector. Assuming three sectors per BS, this would translate into close to 200 Mbps of backhaul transport for each BS. When we share the symbols 3:1 between DL and UL, it could provide data rates of 46 Mbps DL and 8 Mbps UL per sector. Even then it would require about 150 Mbps of capacity between BS and GW. Such a requirement would lead to an unmanageable backhaul cost, which might become a road block in the large-scale adoption of the WiMAX technology.

Our contention is that the service providers will only provision based on the anticipated demand. For example, they might provision just enough capacity for N voice calls, Mvideo calls, and few more Mbps for best effort. This would ensure that the initial cost of building the network is manageable, and as the users grow, more backhaul can be added to ensure acceptable QoS for the subscribers.

CORE SERVICES NETWORK | WIMAX NETWORK ARCHITECTURE

The CSN is the transport, authentication, and switching part of the network. It represents the core network in WiMAX. It consists of the home agent (HA) and the AAA system and also contains the IP servers, gateways to other networks, i.e., public switched telephone network (PSTN), and 3G.

WiMAX has five main open interfaces which include reference points R1, R2, R3, R4, and R5 interface. The R1 interface interconnects the subscriber to the BS in the ASN and is the air interface defined on the physical layer and Medium Access Control (MAC) sublayer. The R2 is the logical interface between the mobile subscriber and the CSN. It is associated with authorization, IP host configuration management, services management, and mobility management. The R3 is the interface between the ASN and CSN and supports AAA, policy enforcement, and mobility management capabilities. The R4 is an interface between two ASNs. It is mainly concerned with coordinating mobility of MSs between different ASNs. The R5 is an interface between two CSNs and is concerned with internetworking between two CSNs. It is through this interface that activities such as roaming are carried out.

The unbundling of WiMAX divides the network based on functionality. The ASN falls under the network access provider (NAP). The NAP is a business entity that provides WiMAX network access to a network service provider (NSP). The NSP is a business entity that provides core network services to the WiMAX network and consists of the CSN. The Applications services fall under the applications service provider (ASP).

If network operator wants to reap the full benefits that WiMAX and its all-IP architecture can deliver, they need to carefully select the ASN and CSN solutions that best suit their requirements and provide all the functionality required while avoiding unnecessary complexity in their network.

ACCESS SERVICES NETWORK | WIMAX NETWORK ARCHITECTURE

The ASN is the access network of WiMAX and it provides the interface between the user and the core service network. Mandatory functions as defined by the WiMAX forum include the following:

• Handover
• Authentication through the proxy authentication, authorization, and accounting (AAA) server
• Radio resource management
• Interoperability with other ASN’s
• Relay of functionality between CSN and mobile station (MS), e.g., IP address allocation

Base station (BS): The cell equipment comprises the basic BS equipment, radio equipment, and BS link to the backbone network. The BS is what actually provides the interface between the mobile user and the WiMAX network. The coverage radius of a typical BS in urban areas is around 500–900 m. In rural areas the operators are planning cells with a radius of 4 km. This is quite a realistic number now and quite similar to the coverage areas of GSM and UMTS high-speed downlink packet access (HSDPA) BSs today.

Deployment is driven either by the bandwidth required to meet demand, or by the geographic coverage required to cover the area. Based on the cell planning of other previous technologies, urban and suburban segments cell deployment will likely be driven by capacity. Rural segment deployment will likely be driven by the cell radius. For BTS systems, the emphasis is more on performance than on cost and size, although there still is an interest in low cost because WiMAX is a new deployment.

ASN gateway: The ASN gateway performs functions of connection and mobility management and interservice provider network boundaries through processing of subscriber control and bearer data traffic. It also serves as an Extensible Authentication Protocol (EAP) authenticator for subscriber identity and acts as a Remote Authentication Dial-In User Service (RADIUS) client to the operator’s AAA servers.