When Mobile is turned ON

Initial Process When Mobile is turned ON :-

Network Attachment

Network attachment is the process of selecting an appropriate cell (radio frequency) by the mobile station to provide the available services, and making its location known to the network. The process starts when the mobile station is switched on, and ends when the mobile station enters the idle mode. In idle mode the mobile station does not have a traffic channel allocated to make or receive a call, but the Public Land Mobile Network (PLMN) is aware of the existence of the mobile station within the chosen cell.

The network attachment process consists of the following tasks:

Cell identification - When a mobile station is switched on, it attempts to make contact with a GSM PLMN by performing the following actions:

• Measure the BCCH (Broadcast Control Channel) channels.

• Search for a suitable cell.

The mobile station measures the signal strength of the BCCH channels received. It stores in list information about 30 of these BCCH channels, such as the signal strength and the frequency corresponding to these BCCH channels.

PLMN selection - A suitable PLMN is chosen.

PLMN Area - This area is the geographical area in which a particular PLMN operator provides land mobile communication services to the public. From any position within a PLMN area, the mobile user can set up calls to another user of the same network, or to a user of another network. The other network may be a fixed network, another GSM PLMN, or another type of PLMN. Other network users, and users of other networks, can also call a mobile user who is active in the PLMN area.

When there are several PLMN operators, the geographical areas covered by their networks may overlap. National borders normally limit the extent of a PLMN area.

Cell selection - Cell selection is the process of selecting an appropriate cell (radio frequency) by the mobile station to provide the available services.

Location update - In order to initiate a call or to receive a call, the mobile station tunes to the control channel (BCCH plus CCCH – Common Control Channel) of the chosen cell. Then, it registers its presence in this cell (registration process) by means of a location updating procedure.

No suitable cell found - If the mobile station is unable to find a suitable cell to access, it attempts to access a cell irrespective of the PLMN identity, and enters a "limited service" state in which it can only attempt to make emergency calls.

PLMN selection mechanism - The particular PLMN to be contacted can be selected either in one of the following modes:

• Automatic mode - In automatic mode, the mobile station will choose which PLMNs to try all by it. The automatic mode is based on the existence of the preferred list, which is stored in a non-volatile memory in the SIM. This list includes a number of PLMN identities in order of preference and is under control of the user. The most preferred is usually the home PLMN. The list is filled in by the user through a mechanism to be specified by the mobile station manufacturer.

• Manual mode - In manual mode, the user is presented a list containing all found PLMNs. The user chooses one of the PLMNs from the list.

Cell selection criteria - The mobile station attempts to find a suitable cell by passing through the list in descending order of received signal strength; the first BCCH channel which satisfies a set of requirements is selected. The requirements that a cell must satisfy before a mobile station can provide service from it, are:

• It should be a cell of the selected PLMN - The mobile station checks whether the cell is part of the selected PLMN.

• It should not be "barred" - The PLMN operator may decide not to allow mobile stations to access certain cells. These cells may, for example, only be used for handover traffic. Barred cell information is broadcast on the BCCH to instruct mobile stations not to access these cells.

• The radio path loss - This loss between the mobile station and the selected BTS must be below a threshold set by the PLMN operator.

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