BSIC=NCC+BCC
In GSM system, each BTS is allocated with a color code, which is called BSIC. MS can identify two cells with the same BCCH by the help of BSIC. In network planning, effort should be made to make sure that BCCH of neighbor cells are different from the serving cell’s BCCH to reduce the interference.
Practically it is still possible that a same BCCH is re-used in the surrounding cells. For cells using the same BCCH in a relevant near distance, their BSIC must be different so that MS can identify two neighbor  cells with same BCCH.

BSIC is transmitted on Synchronous Channel (SCH) of each cell. Its functions are as below:

If MS have read SCH, it is considered as being synchronous with that cell. However, to correctly read the information on the downlink common signaling channel, MS must get the TSC (Training Sequent Code) that is adopted by the common signaling channel. According to GSM specification, TS (Training Sequent) has eight fixed formats, which are represented by TSC ranged 0~7 respectively.

TSC number adopted by common signaling channel of each cell is just the BCC of the cell. So one of the functions of BSIC is to inform MS of the TSC adopted by the common signaling channel of the cell.

Since BSIC attends the coding process of information bits in random access burst, it can be used to prevent the BTS from accepting a RACH transmitted from MS in a neighbor cell as the access signal from the MS of the serving cell.

When MS is in dedicated mode, it must measure the BCCH level of the neighbor cells and report it to BTS according to BA2 that is sent on SACCH, including their respective BSIC. In special circumstance, when there are two or more cells using the same BCCH in the neighbor cells, BSC can use BSIC to distinguish these cells and avoid wrong handover or even handover failure.

MS must measure the BCCH signals of neighbor cells in dedicated mode, and report the results to the network. Since MS sends measurement report which contain the contents of a maximum of 6 neighbor cells each time, it is necessary to control MS to report  only the cells which have neighbor relationships with the serving cell.

The NCC is used for the above purpose. Network operators can use parameter “ NCC Permitted” to control MS to report  the neighbor cells with NCC permitted in the serving cell only.