Downlink Logical Channel in GSM


1. Frequency correction channel (FCCH)
It carries the information for frequency correction for the mobile station. The MS can communicate with a cell and demodulate other information of the same cell just via FCCH. Moreover, the MS can also know whether the carrier is a BCCH carrier via FCCH.
2. Synchronous channel (SCH)
After FCCH decoding, the MS will continue to decode the SCH channel message. This message includes the information for MS frame synchronization and BS identification: Base Station Identifying Code (BSIC). It seizes 6 bits, in which 3 bits are PLMN color codes ranging between 0~7; while the remaining 3 bits are Base Station Color Codes (BCC) ranging between 0~7.
The simplified TDMA frame number (RFN) seizes 22 bits.
3. Broadcast control channel (BCCH)
Generally, there is always a BCCH channel in every cell , which is responsible for broadcasting system information to the mobile station. These system information enable the MS to identify and access network at the idle mode.

4. Paging channel (PCH)
This is a downlink channel which is used to page mobile stations. When the network is to set up communication with a certain MS, it will send paging messages via the PCH channel  to all cells in the LAC area in which the certain MS has  currently registered, and indicates TMSI or IMSI of the certain mobile.
5. Access granted channel (AGCH)
This is a downlink channel used in answering a network access request by the mobile station, i.e. allocation of an SDCCH or a TCH directly.