Control Channels

Control channels are communication channels used in a system (such as a radio control channel), which are dedicated to the sending and/or receiving of command messages between devices (such as a base station and a mobile radio). On the GSM system, the control channel sends messages that include paging (alerting), access control (channel assignment) and system broadcast information (access parameters and system identification).
Beacon (Broadcast) Channels (BCH) - are used to transfer system information such as timing references and synchronization information. The broadcast channel provides system information, system configuration information (such a paging channel sleep groups), and lists of neighboring radio channels to all mobile devices operating within its radio coverage area.
Common Control Channels (CCCH) - are communication channels that are used to coordinate the control of mobile devices operating within its cell radio coverage area. GSM control channels include the random access channel (RACH), paging channel (PCH), and access grant channel (AGCH).
Dedicated Control Channels - are communication channels that transfer signaling messages to specific devices.

This diagram shows that the TDMA physical channel is divided into a control channel (time slot 0) and a traffic channel (time slot 4 in this example). The forward logical control channels include the frequency correction channel, synchronization channel, broadcast channel, paging channel, and access grant channel and the reverse logical control channel includes an access request channel. The traffic channel carries user data in both directions. This example shows that while on the traffic channel, fast control channel messages (FACCH) and slow control channel messages (SACCH) can be sent.