Control channels
Common channels can be accessed both by idle mode and dedicated mode mobiles. The common channels are used by idle mode mobiles to exchange the signalling information required to change to dedicated mode. Mobiles already in dedicated mode monitor the surrounding base stations for handover and other information. The common channels are defined within a 51-frame multiframe, so that dedicated mobiles using the 26-frame multiframe TCH structure can still monitor control channels. The common channels include:
Dedicated mode means a mobile is in use. Dedicated to service. Control and common channels seem to be synonymous terms. Speaking of terms, don't try to memorize these channel names and functions. You will remember them soon, especially when we go over call processing in GSM. Bookmark or make this page a favorite so you can come back later. The GSM standard covers more than 5,000 pages so expect this kind of complexity. But keep reading the discussion. I think after you've glanced at this table you will stay interested in the article. BTW, these are just some of the channels . . .
| Control Channels | Channel Types | Usage |
| Broadcast Control Channel (BCCH) |
Broadcast downlink (Base station to mobile) |
Continually broadcasts, on the downlink, information including base station identity, frequency allocations, and frequency-hopping sequences. |
| Frequency Correction Channel (FCCH) | Broadcast downlink | Used to synchronise the mobile to the time slot structure of a cell by defining the boundaries of burst periods, and the time slot numbering. Every cell in a GSM network broadcasts exactly one FCCH and one SCH, which are by definition on time slot number 0 (within a TDMA frame). |
| Synchronisation Channel (SCH) | Broadcast downlink | |
| Random Access Channel (RACH) |
Common uplink (Mobile to base station) |
Slotted Aloha channel used by the mobile to request access to the network. (p.s. I love that term "Aloha"; appropriate and to the point) |
| Paging Channel (PCH) |
Common downlink (Base station to mobile) |
Used to alert the mobile station of an incoming call. |
| Access Grant Channel (AGCH) | Broadcast downlink | Used to allocate an SDCCH to a mobile for signalling (in order to obtain a dedicated channel), following a request on the RACH. |
| Slow Associated Control Channel (SACCH) | Uplink and downlink | In every traffic channel. Used for low rate, non critical signaling. |
| Fast Associated Control Channel (FACCH) | Uplink and downlink | "A high rate signaling channel, used during call establisment, subscriber authentication, and for handover comands." Macario |