Wimax standards and Security The Wimax

Legacy IEEE 802.16e Handover Procedures

To begin with the introduction of the proposed mechanism, we first review the architecture of the legacy IEEE 802.16e standard. The architecture of IEEE 802.16e is based on the Internet connecting several BSs through wired package-switched network as shown in Figure 7.1(1). An MSS communicates with a BS in an active set by using WiMAX technology through the air interface as shown in Figure 7.1(2).
Association of handover is an optional initial ranging procedure, which can be selected by the MSS. There are three handover levels of associations—L₀, L₁, and L₂—in IEEE 802.16e standard.

Buffer
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4 ^L0
9 ^L1

nonserving
14 ^L2
nBS

Internet
⁸ L₁
12 L_2
1 IP-based package- switched network
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serving

SBS
6 ^L1

2 WiMAX
11 ^L2
3 L₀

PART I
MS ^{7 L1}
10 L₂

PART II
5 ^L0

B_..u_..f_.fer nonserving
13 ^L2
TBS

FIGURE 7.1
The architecture of IEEE 802.16e.

1. 
   Association L₀
   

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Association L₀ (scan and associate without coordination) is a contention-based ranging without coordination of handover. In general, the SBS allocates peri- odic intervals to MSS as shown in Figure 7.1(3). Therefore, the MSS may choose a ranging code arbitrarily to perform the initial ranging to all nBS as shown in Figure 7.1(4), which may include target base station (TBS) as shown in Figure 7.1(5). This ranging code is a contention-based resolution, which is based on the random backoff mechanism with an initial backoff countdown interval CW 2ⁿ⁺⁵, n [0, 5] and the the maximum contention window size CW_max 1024.
After the TBS successfully receives ranging code and sends ranging response (RNG-RSP) message with ranging status success, it will provide uplink allocation of adequate size for the MSS to transmit ranging request (RNG-REQ) message with type-length-value (TLV) parameters (SBS ID, MSS MAC address) related to the association ranging. The average time required for RNG-REQ message is denoted as T_RNG. In all cases, the MSS should syn- chronize with the new downlink first after the connection has been released by the SBS. The average time required to frame synchronization with the new downlink is denoted as T_SYN. The average time required during hand- over for reauthorization is denoted as T_RA (full authentication is assumed, where only 3-way handshake is performed instead of full authentication). The average time required for reregistration during handover is denoted as T_RR. The average time required for contention-based ranging (T_CR) process can be expressed as
T_CR = T₁P_s + T₂P_s(1 − P_s) + ··· + T_nP_s(1 − P_s)ⁿ⁻¹ (7.1)

= −

= ∈
where T_n represents the mean contention window of the nth ranging and T_n CW_n/2, n [0, 5]. P_s is the successful ranging probability, which can be calculated by S (CW 1/CW)^N, where N is the number of MSS. The service disruption time is defined as starting from the time the SBS or MSS sends a handover request to the time the MSS completes frame synchronization with the TBS. Therefore, we can get T_L0 , the service DT for L₀ scheme during the handover process by
T_L0 = T_CR + T_RA + T_RNG + T_RR + T_SYN (7.2)

2.