Call re-establishment

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Call re-establishment enables the mobile station to resume the contact with the cellular network when the connection to a particular VBTS is suddenly broken. This may happen because of a brutal propagation loss, due to obstacles such as bridges and tunnels.
Call re-establishment is a GSM feature that can be enabled or disabled on a per cell basis.

   Call re-establishment process : After the communication has been lost, the mobile station selects the cell with the highest signal strength from the neighbor cell list. The neighbor cell list contains the cell identifiers to which a handover is allowed. It is kept in the BSC of a particular cell. The list is transferred to the mobile station in the BCCH during the registration phase of a wireless call. The mobile station uses the neighbor cell list by only measuring the signals from the BTSs located in the cells that are on the list.

The selected cell identifier is used to re-establish the connection to that particular cell by following the normal access procedures. Actually, it sends an access request on the RACH (Random Access Channel) of the particular frequency channel.
At the moment of time the communication is lost, a timer is initiated in the serving MSC. When the timer expires, it is not possible anymore for the mobile station to re-establish the call. A typical value for the timer is 4 seconds

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