network to his or her present location. Later it may become possible to connect calls directly. Mobile Communications 5.6.3 Handover or Handoff The main reason to perform handover is to maintain call connection regard- less of the movement of the MS over cell boundaries. The structure of a GSM network requires the possibility to execute handovers at four levels, as shown in Figure 5.13. The BSC is responsible for handover because it occurs most often between two cells under one BSC. The handover process should be as quick as possible SO that communication is not disturbed. To perform handover quickly, the BSC collects measurement data from MSs and BTSs, processes it, and updates ordered candidate cell lists for handover for all the MSs that have an ongoing call. Handover is most often necessary between BTSs of neighboring cells, case (b) in Figure 5.13, which are controlled by the same BSC. The BSC controls the handover and performs the channel switch from an "old" cell to a "new" cell. Sometimes there may be a need to switch communication from one channel to another in the same BTS [case (a) in Figure 5.13]. This may be necessary because of temporary interference. Also this handover is con- trolled and performed by the BSC. The inter-BSC handover, case (c) in Figure 5.13, occurs if an MS moves to a neighboring cell that is controlled by a different BSC. Now the BSC cannot perform switching. Instead, it has to request the MSC to execute the handover switching to the target cell. When a new connection is Anchor connection a) Intra-BTS handover b) Intra-BSC and inter-BTS handover c) Inter-BSC handover d) Inter-MSC handover Anchor MSC is the exchange through which the call was initially connected. It is responsible for the call (charging data and so on), even if it is switched to BSS under another MSC. Handover is not performed between different networks. Figure 5.13 The four different cases of handover. Introduction to Telecommunications Network Engineering established from the MSC to the new BTS and the MS accesse