Ciphering is encyption and decyption of user ID in GSM network. It is done inside GSM GPRS component SGSN.
BTS works by regularly sending beacon signal in its coverage range, registration the mobile station in its coverage and as soon as the mobile station invokes service a free channel is assigned to it. MS sends its voice or data signal to BTS and BTS sends it to BSC and BSC sends it to MSC and MSC connects to the other side Mobile Station/PSTN phone/ or connects to SMSC if the service is for SMS or SGSN for internet service. Thus BTS is the first contact for connection or release of a mobile service.