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 is defined in GSM specification 03.03 section 4.3.2.
Each base-station has its own BSIC, this code is at all times transmitted on the broadcast channel, so the Mobile Stations can distinguish between base stations. The BSIC is composed of a 3-bit Network Color Code (NCC) and a 3-bit Base station Color Code (BCC). The NCC is assigned to each network provider so the MS can sort out which base-stations it is allowed to camp on. The NCC of different providers must be different, also in national border-areas. The BCCHs (Broadcast Control Channel) of each base stations are assigned frequencies by the network operator, and must be assigned such that no neighbor stations have equal BCCH and thus equal BSIC.
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 especially 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
References
3GPP TS 03.03 Numbering, Addressing and Identification http://www.3gpp.org/ftp/Specs/html-info/03-series.htm