Apparatus and method for reselecting a public land mobile network in a mobile communication terminal

1. A method for reselecting a Public Land Mobile Network (PLMN) in a mobile communication terminal camped on a first PLMN, the method comprising the steps of:

determining to request an updating of a location registration from a second PLMN;

storing system information about the first PLMN to which the mobile communication terminal is registered;

transmitting a location registration update request to the second PLMN;

determining whether a Subscriber Identify Module (SIM) card is valid, when the second PLMN fails to update the location registration; and

when the SIM card is invalid, resetting the SIM card to a previous state as when the mobile communication terminal transmitted a location registration update request to the first PLMN, and transmitting a location registration update request to the first PLMN using the stored system information.

2. The method of claim 1 , further comprising the steps of:

when the second PLMN fails to update the location registration in the first PLMN, recording a number of times of failing to update the location registration;

updating a PLMN list in an order where the second PLMN that failed to update the location registration comes last;

executing a timer for limiting the updating of the location registration in accordance with the number of times of failing to update the location registration; and

canceling limitation on the updating of the location registration when the timer for limiting the updating of the location registration is terminated.

3. The method of claim 2 , wherein after the second PLMN fails to update the location registration in the first PLMN, the first PLMN is selected in accordance with an updated priority, and updating the location registration is then requested.

4. The method of claim 1 , wherein the system information includes at least one of information on the first PLMN, a Radio Access Technology (RAT) and an Absolute Radio Frequency Channel Number (ARFCN).

5. An apparatus for reselecting a Public Land Mobile Network (PLMN) in a mobile communication terminal that is camped on a first PLMN, the apparatus comprising:

a transmitter for transmitting a location registration update request;

a receiver for receiving a location registration update response message for a transmitted location registration update request; and

a PLMN update unit for:

determining to request an updating of a location registration from a second PLMN,

storing system information about the first PLMN to which the mobile communication terminal is registered,

commanding the transmitter to transmit the location registration update request to the second PLMN,

when the location registration update response message from the second PLMN indicates denial of the request, determining if a SIM card is invalid, and

when the SIM card is invalid, resetting the SIM card to a previous state as when the mobile communication terminal transmitted a location registration update request to the first PLMN and commanding the transmitter to transmit a location registration update request to the first PLMN using the stored system information.

6. The apparatus of claim 5 , wherein the PLMN update unit records a number of times of failing to update the location registration; updates a PLMN list in an order where the second PLMN that failed to update the location registration comes last; executes a timer for limiting the updating of the location registration in accordance with the number of times of failing to update the location registration; and cancels the limitation on the updating of the location registration when the timer for limiting the updating of the location registration is terminated.

7. The apparatus of claim 6 , wherein after the second PLMN fails to update the location registration, the PLMN update unit selects the first PLMN in accordance with the updated priority and then requests the updating of the location registration.

8. The apparatus of claim 5 , wherein the system information comprises at least one of:

information on the first PLMN;

a Radio Access Technology (RAT); and

an Absolute Radio Frequency Channel Number (ARFCN).