WiMAX

WiMAX stands for Worldwide Interoperability for Microwave Access and is defined under the IEEE 802.16 working group. Two standards exist for WiMAX—802.16d-2004 for fixed access, and 802.16e-2005 for mobile stations [9]. The WiMAX forum certifies systems for compatibility under these two standards and also defines network architecture for implementing WiMAX-based networks.
WiMAX can be classified as a last-mile access technology similar to DSL, with a typical range of 3 to 10 kilometers and speeds of up to 5 Mbps per user with non-line of sight coverage. WiMAX access networks can operate over licensed or unlicensed spectra in various regions or countries—though licensed spectrum implementations are more common. WiMAX operation is defined over frequencies between 2 and 66 GHz, parts of which may be unlicensed spectrum deployments in some countries. The lower frequencies can operate over longer ranges and penetrate obstacles, so initial network rollouts are in this part of the spectrum—with 2.3-, 2. 5-, and 3.5-GHz frequency bands being common. Channel sizes vary from 3.5, 5, 7, and 10 MHz for 802.16d-2004 and 5, 8.75, and 10 MHz for 802.16e-2005. WiMAX networks are often used to backhaul data from Wi-Fi access points. In fact, they are often envisaged as replacements for the current implementation of metro Wi-Fi networks that use 802.11b/g for client access and 802.11a for backhaul to connect to the other parts of the network.

Technology

The 802.6d-2004 standard uses OFDM similar to 802.16a and 802.16g, whereas 802.16e-2005 uses a technology called Scalable Orthogonal Frequency Division Multiplexed Access (S-OFDMA). This technology is more suited to mobile systems because it uses subcarriers that enable the mobile nodes to concentrate the power on the subcarriers with the best propagation characteristics (because a mobile environment has more dynamic variables). Likewise, the 802.16e radio and signal processing is more complex.
Unlike 802.11, which supports only Time-Division Duplexing (TDD)—where transmit and receive functions occur on the same channel but at different times), 802.16 offers TDD, Frequency-Division Duplexing (FDD) (transmit and receive on different frequencies, which could also be at different times). Another innovation in WiMAX is similar to the scheme in Code Division Multiple Access (CDMA)—subscriber stations are able to adjust their power based on the distance from the base station, unlike the case of client stations in an 802.11 network.
WiMAX base stations use a scheduling algorithm for medium access by the subscriber stations. This access is through an access slot that can be enlarged or contracted (to more or fewer slots) that is assigned to the subscriber stations. Quality-of-Service (QoS) parameters can be controlled through balance of the time-slot assignments among the base stations. The base-station scheduling types can be unsolicited grant service, real-time polling service, non-real time polling service, and best effort. Depending upon the time of traffic and service requested, one of these scheduling types can be used.

WiMAX Network Architecture

The WiMAX network architecture is specified through functional entities (see Figure 4), so you can combine more than one functional entity to reside on a network element. The Mobile Station (MS) connects the Access Service Network (ASN) through the R1 interface—which is based on 802.16d/e. The ASN is composed of one or more base stations (BSs) with one or more ASN gateways to connect to other ASNs and to the Connectivity Service Network (CSN). The CSN provides IP connectivity for WiMAX subscribers and performs functions such as Authentication, Authorization, and Accounting (AAA) [10,11], ASN-CSN tunneling, inter-CSN tunneling for roaming stations, and so on. A critical tenet of the WiMAX Forum network architecture is that the CSN must be independent of the protocols related to the radio protocols of 802.16.

The R3 interface (reference point) is used for the control-plane protocols and bearer traffic between the ASN and CSN for authen-tication, policy enforcement, and mobility management. The base station connects to an ASN gateway to provide the MS with external network access. The R6 interface between the BS and ASN-GW could be open or closed based on the profile—in fact, you could have a co-located base station and ASN gateway (ASN-GW), depending upon the network implementation. The ASN gateway uses the R3 interface to communicate with the AAA services in the visited CSN (that is, the CSN "corresponding" to the ASN). The servers in the visited CSN can communicate with the home CSN (that is, the CSN corresponding to the "home" network of the MS). In the simplest case multiple ASNs (WiMAX networks) connect through ASN gateways to the public Internet (that is, there is only one Network Service Provider (NSP) and the visited and home CSNs are the same). Note that you could implement a WiMAX network with just one ASN and one CSN—in that case, the R3 interface would be completely internal and not exposed.
Three profiles are identified to map ASN functions into ASN-GW and BS functions. These profiles are considered an implementation guideline for how you would build the various devices implementing these functions. Profile A is a strict separation of the BS and ASN-GW functions, where the ASN-GW controls and manages radio resources that are located on the BS and also provides the handover and data-path functions. The R6 interface is exposed in this profile.
Profile B is a more integrated function, where the BS has more functions than in profile A; in fact, the BS might even integrate most of the ASN functions. The R6 interface is a closed interface in this profile. The third profile is profile C, which is similar to profile A except that the base stations to incorporate more functions, including radio resource management and control as well as hand-offs.

IP Connectivity and Data Transfer

The MS can be a fixed IP gateway (think of an 802.11 access point that provides connectivity to users in a coffee shop and connects to the IP network of the service provider through WiMAX) or a mobile end node (for example, a laptop with WiMAX connectivity). The IP address used by the gateway on the connection to the WiMAX network is known as the Point of Attachment (PoA) address. A third type of access is nomadic access, where the IP gateway can be moved from one location to another but connects to the network only after it has been relocated.
When the station is mobile, the WiMAX Forum specifies that the Mobile IP (MIP) architecture and protocols should be used. There are two types of Mobile IP possible: Client Mobile IP (CMIP) and Proxy Mobile IP (PMIP). The former involves changes to the MS protocol stack, but the latter does not.
The architecture can support both models. In the P-MIP scenario (see Figure 5), the ASN implements the Foreign Agent (see William Stallings' article in IPJ on Mobile IP [8]), and terminates Mobile IP tunnels for the various mobile stations in the same ASN.

Perspective on WiMAX versus Cellular Services

The WiMAX Forum has specified that the Network Working Group (NWG) architecture should be capable of supporting voice, multimedia services, and priority services such as emergency voice calls. It also supports interfacing with interworking and media gateways. Also, the service permits more than one voice session per subscriber, as well as simultaneous voice and data sessions. Support of IP Broadcast and Multicast services over WiMAX networks is also included. The architecture is also expected to support differentiated QoS levels at a per-MS or -user level (coarse grained) and at a per-service flow (fine-grained) level. It shall also support admission control and bandwidth management.
Initially, WiMAX was touted by some as a replacement for cellular services. An important consideration was using Voice over IP (VoIP) for voice calls—that is, where voice was another service over the data network. This model was in contrast to the existing cellular service where data was an adjunct to the basic service of TDM-based voice. More recently, WiMAX is being positioned as a data-connectivity option for remote locations, especially where it would be difficult to lay new copper or optical cable. Not surprisingly, these options are being pursued aggressively in developing countries.

Common Misperceptions About Wi-Fi, BT, and WiMAX Technologies

We have considered the key aspects of the three technologies—Wi-Fi, BT, and WiMAX—and their position in IP networks. In this section, we will outline and clarify some common perceptions and misperceptions about these technologies.

1. BT and Wi-Fi are competing technologies—Actually, they address a different set of requirements despite operating in the same 2.4-GHz space. BT is a "wire replacement" usually for short distances. Wi-Fi is typically used for data, voice, and video traffic over distances up to 300 meters.
2. WiMAX is Wi-Fi on steroids—To clarify, this statement is an oversimplification used often in the trade press. WiMAX operates in licensed spectra and uses a different network architecture as compared to Wi-Fi, which is in the unlicensed spectrum and uses a simple access point to wired Ethernet architecture. One overlapping function is for backhauling Wi-Fi traffic, which can be done by Wi-Fi (typically 802.11a) or WiMAX.
3. Unlike BT, Wi-Fi cannot be used for voice—This perception is not true because you can send multimedia traffic over Wi-Fi networks implementing 802.11e QoS functions that rely on the access point and stations implementing priority-based traffic transmission and scheduling.
4. Wireless networks are not secure—Although there is some validity to this argument because it is easier to eavesdrop on wireless networks, implementation of security schemes such as Wi-Fi Protected Access (WPA/WPA2) will help alleviate this problem.
5. Wireless and radio technologies consume more power—This statement is often true if the devices transmit continuously or have to increase their power because of the distance between the transmitter and receiver. Noisy channels contribute to this power use also. However, with careful engineering of the wireless implementation and techniques such as power save (in Wi-Fi) and short duty cycle transmissions, the power requirement can be lowered.