he BS Switching Timer (i.e., the minimum time the MS requires between transmission of handover indication message at the serving BS until it is able to receive fast ranging information element at the target BS) [1]. The serving BS indicates the time of the fast (i.e., non-contention-based) ranging opportunity negotiated with the potential target base stations via an information field in the handover request/response message. The network re-entry procedures are illustrated in Figure 4-14. Some of the steps may be skipped or shortened as described earlier. The MS ID* shown in Figure 4-14 is a hash value (scrambled) of the real MS ID (MAC address of the mobile station) that is temporarily used prior to establishment of a secure connection between the MS and the BS, in order to avoid revealing the user identity over the air interface [2]. Deregistration with Content Retention (DCR) is a mode in IEEE 802.16m where an MS can deregister from the network while its context is maintained in a network entity until the Context Retention Timer is valid to expedite the mobile station's network re-entry. In that case, the network assigns a 72-bit Context Retention Identifier (CRID) to each MS during network entry. The MS is identified by the CRID in coverage loss recovery and DCR mode, where the CRID allows the network to retrieve MS context. The network may assign the MS a new CRID if necessary. The CRID is used to uniquely identify the DCR mode mobile stations. The DCR mode may only be terminated on MS re- entry to the network or on expiration of the Context Retention Timer. 4.3 STATE TRANSITIONS AND MOBILITY Handover (HO) is defined as the migration of a mobile station between the air interfaces of different base stations or cells. The BS associated with the MS before the handover is called the serving BS, 4.3 State transitions and mobility DL Synchronization AAI_RNG-REQ (MS ID* is transmitted over the air) AAI_RNG-RSP (TSTID is assigned by the BS) Basic Capability Negotiation MS Authentication and Authorization Key Exchange