GSM CALL MANAGEMENT
Mobile Terminated Call
When a call destined for a mobile is made, the main problem for the network is that it has no idea which cell the mobile is in. However, due to the location updating procedure the VLR does know which location area the mobile is in, and so it can use the paging channel to broadcast paging messages into every cell in the whole location area in the hope that the mobile will respond. This approach could be said to be an inefficient use of PCH resources, but like many aspects of GSM it is a trade off; if the VLR had to keep track of every mobile right down to individual cell level, the amount of location updating would be unmanageable.
There can be (and usually are) several different paging channels in each cell, so the mobile must know which one to listen to in case any paging messages are transmitted. In fact the mobile calculates the correct paging group to listen to itself. This is another of the few examples of the mobile working autonomously, albeit using information supplied by the BTS via the BCCH.
When the mobile receives a page on its nominated PCH, responds with a RACH which includes a unique code (known as the cause value) to indicate that the mobile is responding to a page rather than, for example, making a call of its own or handing over. The effect of the RACH is to ask the network to provide a channel (usually an SDCCH) on which the mobile and the network can communicate to set the call up.
The BTS decides if it has a suitable channel available, and replies to the mobile via the AGCH. Assuming there is an SDCCH available, then the AGCH message defines it in terms of frequency, timeslot, timing advance (calculated using the “lateness” of the access burst) and power required. The mobile then moves through frequency and time to the SDCCH, on which it makes contact with the network.
Once the call set-up process is complete, the final message on the downlink side of the SDCCH defines the traffic channel in similar terms as above, and the mobile moves again. Once on the traffic channel, the traffic part of the call can commence.
Mobile Originated Call
If a mobile originates a call, the process is quite similar to the mobile terminated call but happens in a different sequence.
The first message transmitted by the mobile is the channel request on the RACH. Upon receipt of this, the BTS decides whether it has an available SDCCH, and responds on the AGGH. The mobile moves to its SDCCH and completes the call set-up, including signalling all the way up to the MSC to obtain a terrestrial circuit for the terrestrial part of the call. When this is complete, the mobile is told which traffic channel to use, and re-tunes to it. gsmarena
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