changes between the user equipment and the radio access network. For the purpose of determining handover interruption time, the interactions with the core network (i.e., network entities beyond the radio access network) are assumed to occur in zero time. It is also assumed that all necessary attributes of the target channel, i.e., downlink synchronization is achieved and uplink access procedures are successfully completed, are known at initiation of the handover from the serving BS to the target BS. As shown in Figure 12-10, the handover can be initiated by either the MS or the serving BS. The handover process is initiated when the MS issues a handover request to the serving BS, or when the serving BS issues a handover command to the MS. The handover request/command is issued following mobile-assisted handover procedures. The MS acquires the network topology through either serving BS broadcasts or unicast messages. The serving BS also provides the MS with the relevant trigger conditions to initiate or cancel neighbor-cell measurements, measurement reporting, and handover requests. Scanning intervals are provided for neighbor-cell measurements by the serving BS unilat- erally or at the request of the MS. The handover request/command is an outcome of these measure- ments and reporting. The handover procedure is divided into four steps: handover initiation; handover preparation; handover execution; and handover cancellation procedure to allow the MS to cancel a handover procedure. The following assumptions were made in the calculation of the handover interruption time: (1) the handover is of the intra-ASN type, i.e., the serving and target base stations belong to the same ASN; (2) this is a mobile-assisted handover; (3) the handover is a mobile-initiated type, which in terms of latency is the worst-case compared to BS-initiated type; (4) this is an optimized hard handover, which means that for intra-frequency, the MS is frame-synchronized with the serving BS and the target BS, and the MS context including security c