The BSIC is an identifier of the cell, so that MS can distinguish between two cells using the same frequencies.
About the channel request, it’s a little weird. A cell is always tagged with a certain a BSIC, so a MS is always going to send a channel request in a cell which is broadcasting a BSIC. Your situation is impossible 🙂
If a cell doesn’t have a BSIC, the world would come to an end.