A mobile station (MS) can be in one of the following states:
· Detached - MS is powered off or the SIM card is deactivated
· Attached - MS power is on and the SIM card is activated. An attached MS can be:
1. Idle - MS has no dedicated channel allocated. It just listens to base stations broadcasted signals (called beacon signal) to remain attached
2. Active (dedicated) - MS has a dedicated connection to the network
There are some communications activities between the MS and the network when an MS changes its state. When it is switched-off its data in the VLR and the HLR get updated (detached state) so that no incoming call will try to page the MS. When an MS is switched-on it starts the initialization procedure in order to find the network and synchronizes itself to a right frequency (beacon frequency) and TDMA (Time Division Multiple Access) slot. After that the MS gets through the association procedure in order to anchor itself to the base-station (idle state). When an MS is idle it still listens the paging channels for an incoming call, periodically re-synchronize itself and measures the signal strength of its own as well as its neighboring cells (for the purpose of location update).
An MS enters an active state when it requests for a service. At first it gets a dedicated data channel for a variety of authentication and security procedure. This channel is also used for SMS transfer (send/receive). If it is a voice call request then a dedicated voice channel is allocated before releasing the dedicated data channel.
The following list includes most of the categories of signaling on the GSM radio interface. Each of them is discussed in separate sections of this document.
1) MS initialization after switched-on
2) Service request
3) Authentication and security
4) MS attach and detach
5) Location update
6) Outgoing voice call
7) Incoming voice call
8) SMS and special services
9) Call hand over
