How a GSM phone keeps in touch with the network between calls

Idle mode refers to when the handset is on "standby" and not in a call. It is a bit of a misnomer, because it isn't entirely idle!

It listens to several common (shared) channels:

Broadcast Channels:

The Broadcast Control Channel (BCCH) sends information including the identity of the base station, its frequency allocations, and the frequency-hopping sequences it uses as well as a list of neighbouring cells that the mobile might like to consider.

The Frequency Correction Channel (FCCH) and Synchronisation Channel (SCH) send out a marker that allows the mobile to precisely synchronise the frequencies it transmits on, and identifies precisely when each timeslot sequence begins. Every cell in a GSM network broadcasts one FCCH and one SCH, in time slot number 0 (the first of a TDMA frame).

Common Control Channels

The Random Access Channel (RACH) is used by mobiles to request a connection, so it is not used for anything else. Mobiles don't actually listen to this one, but it is a "common channel"

The Paging Channel (PCH) alerts the mobile of an incoming call. This is divided into subchannels. To save power, handsets can only listen to "their" sub-channel, and doze for the rest of the time.

The Access Grant Channel (AGCH) is used to allocate an SDCCH (a slow data channel) to a mobile so that it can request a channel to carry a call.

For an outgoing call, a mobile uses the RACH to ask to make the call, and is given a SDCCH by the AGCH so that it can make a request for Traffic Channel. For details of what happens in a call, see the Air Interface - In a Call page.

As well as listening for incoming calls on the PCH, the mobile also checks on the status of neighbouring cells in case one of them is more suitable to use. For details of this, see the Air Interface - Handoffs page

See also [ In a Call ] : [ Air Interface ] : [ Handoffs ]

< previous • site map • ^ up • MobileShop.com, mobile phones online • home • glossary • next >