The CBA bit is a parameter for the network operator to set. Usually all the cells are allowed to be accessed by MS, so the bit is set NO. However, in special cases, the telecom operator may want to assign a certain cells for handover service only, then the bit can be set YES. Suppose we have commerical are in a large city, we usally adopt the micro cellular mode to increase the access rate of the area via the limited frequency resources. We often use the double layer network concept, i.e. to set up a micro cell(its capacity can be smaller) to cover the whole Area and a macro cell providing outer coverage. Here micro cell will provide the capacity and macro is for coverage. Here we can use Cell Barred in the macro cell and enable Rapid HO for the fast moving vehicles in that are.
Also we can use other parameter CBQ to entirely bar a cell and to set reselection prioirty. Below table may let u understand:
CBQ CBA CellSelectionPriority CellReselectionState
NO NO Normal Normal
NO YES Barred Barred
YES NO Low Normal
YES YES Low Normal