BSIC - Base Station Identity Code
The Base Station Identity Code (BSIC) is a code used in GSM to uniquely identify a base station. The code is needed because it is possible that mobile stations receive the broadcast channel of more than one base station on the same frequency. This is due to frequency re-use in a cellular network.
The BSIC consists of 6 bits of which the first three identify the network (Network Colour Code, NCC). The other 3 bits are used by the operator to uniquely identify base stations within a certain area.
As long as base stations use different frequencies for their broadcasting channel, there is no problem in using the same Base Station Identity Code.
Unique identification of a base station is especiallly important in border areas, where at both sides of the border there is a different operator who might use the same broadcasting channel on the same frequency.