Because mobile phones move around, they don't stay in coverage of one cell. As a result, there has to be a mechanism to transfer calls from cell to cell without interrupting the call.
When in Idle Mode, the mobile only reports when it transfers to another VLR by doing a Location Update.
Every so often (controlled by an interval time setting the Network chooses) each mobile reports its position by sending a Location Update, just in case the network has mislaid it through a database or signalling error. The mobiles decide when to do this, so that they don't all report in at once.
In practice, you may suddenly get old SMS messages or be told of long-waiting Voicemail when a Location Update occurs.
When the mobile is switched off, it signals a log-off (known as an IMSI Detach) to the network so that it won't try to search for a switched-off mobile. It is possible that this doesn't happen (if switched off out of coverage, for example). In such a case, the network won't notice until the next scheduled Location Update has been missed.
During a Call
When a call is in progress, during the time between sending and receiving data, the handset monitors the signal it gets from the 16 nearby cells listed in the current cell's Neighbour List, and every second it reports the signal level of the best six of them to the BSC, using a Slow Access Control Channel (SACCH).
How the decision to switch cells is made can vary, but generally the idea is to switch to the cell with the best signal to economise on power in the mobile, but the alternative of staying put till the signal quality fades is sometimes used.
To trigger and coordinate a handoff is a time-critical function, so the Fast Access Control Channel (FACCH) needed to do this "commandeers" an entire databurst on the control channel to do this.
The decision to switch to another cell can be made by the mobile or by the BTS: the latter usually because it is getting too busy. Occasionally, the handoff fails, and the mobile has to start again, scanning for a network for a fresh start. This can happen when unusual signal propagation has led it to register on a far distant cell, over the hilltops, which has a neighbour list of cells nowhere near the mobile!
Types of Handoffs
There are five types of handoff, but only four are supported in the GSM standard.
(1) From one time slot to another in the same cell. This is managed by the BSC and reported to the MSC.
(2) From one cell to another under the control of the same BSC. This is managed by the BSC and reported to the MSC.
(3) From one BSC area to another, but still under the control of the same MSC. The MSC manages this transfer.
(4) From one MSC area to another. This leaves the original MSC in charge of the call, but the new MSC manages any new handoffs.
(5) From one network to another: this is the one you can't do! Cross a national boundary, or move into coverage of a different network when roaming, and you'll have to redial to continue the call on the new network. Note that some partner networks have made special arrangements for this to happen, but it's exceptional.
back to 'How Mobiles Work'