Wimax standards and Security The Wimax

7.1 Introduction
Voice over Internet protocol (VoIP) has been established in the workplace as a transport mechanism for both fixed and wireless infrastructures. Switching voice paths within the existing packet-switched data networks as IP packets means that there is no need for separating voice and data infrastructures, and the traditional private branch exchange (PBX) can be replaced by a single server capable of supporting thousands of IP handsets. These devices look like regular phones but are handled more like personal computers (PCs), carrying their own unique identities with them wherever they connect to the network.
With the demand for wireless access and high bandwidth transmissions, fixed broadband wireless access (BWA) systems such as the local multipoint distribution service (LMDS) are proposed to provide multimedia services to a number of discrete subscriber sites with IP and offer numerous advantages


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over wired IP networks. This is accomplished by using base stations (BSs) to provide network access services to subscriber sites based on IEEE 802.16 WirelessMAN standard [11]. The progress of the standard has been fos- tered by the keen interest of the wireless broadband industry to capture the emerging worldwide interoperability for microwave access (WiMAX) market, the next-wave wireless market that aims to provide wireless broad- band Internet services. The WiMAX Forum, formed in 2003, is promoting the commercialization of IEEE 802.16 and the European Telecommunications Standard Institute’s (ETSI’s) high-performance radio metropolitan area net- works (MANs) (HyperMANs). It provides one of the potential solutions to beyond third generation/4th generation (B3G/4G) architecture [19,22].

IEEE 802.16e standard [16] provides a series of handover procedures for supporting mobility in BWA networks. Three different handover levels of association—Level 0 (L₀), Level 1 (L₁), and Level 2 (L₂)—are investigated for supporting roaming in the WiMAX network. The minimum required handover processing time (also known as service disruption time (DT)) of each levels are evaluated in Ref. 9 and are 280, 230 and 60 ms, respectively. Banerjee and his coauthors [3] analyzed and concluded that a DT of 50 ms is sufficient for media streams, while an interruption of 200 ms is generally acceptable. Meanwhile, it also showed that a DT of 500 ms will cause a percep- tible interruption, which is unacceptable. Hence the present version of IEEE 802.16e is not sufficient for delay-sensitive applications, such as VoIP and video conference, since it will encounter a long handover processing delay due to its long ranging process, reassociation, reauthorization, and network transmission delay.
One feasible solution (to overcome this drawback) to conspicuously reduce
the handover delay time is to proportionally reduce the number of forward- and-back turnaround times. Besides, many other methods were proposed to fulfill this goal in literature. Some of them focused on optimizing the cutoff parameters and appropriate queue sizes that minimize the overall block- ing probability as handover occurs, such as the measurement-based priority scheme (MBPS) [24] and the signal prediction priority queueing (SPPQ) [5]. Also, some researches proposed using special or dedicated channels for han- dover calls, such as guard channel method (GCM) [15]. These methods will significantly reduce the handover failure probability and hence improve the handover performance. In addition, owing to the mobility and fading channel effect, the received signal strength (RSS) will vary with time and dynamically change following various environment conditions. Xhafa and Tonguz [25] demonstrated an analytical framework of handover to analyze the dynamic handover failure probability and estimated the order of handover calls to raise the successful probability of a handover.
Nevertheless, none of the above-mentioned schemes deal with the mech-
anism that preassigns a channel to a mobile subscriber station (MSS) for handover according to the movement of the MSS. Assume that a serving base station (SBS) knows the exact position of the MSS, the SBS could coordinate with the neighboring BSs (nBSs) around the MSS for handover preparation if

the MSS appears in the boundary among the nBSs. The position information of the MSS and its corresponding movement intention could be estimated by observing the moving history of the MSS in recent records. There have been many measured mechanisms proposed for location management in general [1,6,8,20], which studied random mobility model for mobility estimation in wireless networks. Although the above-mentioned mechanism can enhance the successful probability of handover call, none of them aim at speeding up the handover processing time. Thus, in this chapter we will describe how to use measured signal-aware mechanism to aid speeding up the handover procedures. This mechanism can help the WiMAX system to support VoIP in high-speed mobility environment.

2.