Which control channels does a GSM cell use before a call exists, and in which direction does each run?
A handful of control channels carry the "switch-on" signalling: the cell broadcasts FCCH, SCH and BCCH downlink; the phone reaches up on the RACH; and once a link exists, SDCCH/SACCH/FACCH carry signalling both ways.
* GSM control channels grouped by transmission direction. *
The phone needs these channels — not the voice traffic channel — to find a cell, register, and be paged. They split cleanly by direction, and that direction is itself a fact worth knowing (it's why a phone can listen to a cell long before it may transmit to it):
| Direction | Channel | Carries |
|---|---|---|
| Downlink (BS → MS) | FCCH (Frequency Correction) | a pure tone the phone tunes its frequency to |
| Downlink | SCH (Synchronisation) | frame timing + the base station identity (BSIC) |
| Downlink | BCCH (Broadcast Control) | the System Information: MCC/MNC, Cell-ID, access rules |
| Downlink | PCH / AGCH (Paging + Access Grant) | "there's a call for you" / "here's your dedicated channel" |
| Uplink (MS → BS) | RACH (Random Access) | the phone's "raise your hand" channel-request bursts |
| Bidirectional (MS ↔ BS) | SDCCH (Standalone Dedicated Control) | the registration/SMS signalling dialogue (Location Update rides here) |
| Bidirectional | SACCH (Slow Associated Control) | background measurements + power-control alongside a connection |
| Bidirectional | FACCH (Fast Associated Control) | urgent in-call signalling (e.g. handover), stolen from the traffic channel |
The asymmetry is the point: the cell broadcasts (FCCH/SCH/BCCH) continuously so any phone can synchronise and read it without permission; the only way "up" before a dedicated channel exists is the shared, collision-prone RACH. The network answers on the AGCH by assigning an SDCCH, and only then does a private two-way dialogue (the Location Update) become possible.
Tip: Map them to the power-on story — SCH/FCCH (sync) → BCCH (identify the cell) → RACH (ask in) → AGCH (get an SDCCH) → SDCCH (register). Every step is a different channel.
Go deeper:
Um interface — GSM logical channels (Wikipedia) — the air-interface channel catalogue: traffic vs dedicated vs common control channels, and which direction each runs.- Broadcast control channel (Wikipedia) — the downlink-only beacon that must transmit continuously, the anchor of the whole control-channel set.
- GSM logical & physical channels (Electronics Notes) — how the BCCH, RACH, PCH/AGCH and SDCCH map onto physical timeslots and bursts.