(1) Field of the Invention
The present invention relates to a mobile phone, and especially to processing performed by a mobile phone upon writing a PRL (Preferred Roaming List).
(2) Description of the Related Art
Some telecommunications carriers have a mutual agreement to provide a service called roaming, which is the extending of connectivity in another carrier's service area. The roaming service allows users to use their mobile phones via a communication system (i.e. base station) of a carrier other than the carrier with which the users are registered. A list called PRL indicates which of available communication systems the mobile phone should use by priority, and also indicates other information regarding the respective communication systems.
With the use of PRL, the users are allowed to set up a selection condition, such as a communication mode, that needs to be satisfied by a communication system to be selectable. The mobile phone selects a communication system to be used, based on the PRL stored therein and the selection condition set by the user. That is, by setting up a selection condition specifying a specific communication mode, the user can ensure that the mobile phone conducts communications with a communication system in the desired communication mode.
For example, the user may set up such a selection condition that the mobile phone does not use any communication system available via a roaming service. Use of a roaming communication system generally incurs extra charges. By setting a selection condition for not using a roaming service, the user can save such extra charges.
The PRL stored in a mobile phone needs to be updated in the case where the telecommunications carriers build a new base station or remove an existing base station. In such a case, information contained in the PRL may no longer be consistent with the current state of communication systems. In order to maintain the accuracy of the PRL, each time a change is made to the communication systems, the mobile phone newly acquires a PRL reflecting the change, and selects a communication system to use based on the newly acquired PRL. A method for updating data stored in a mobile device from an external source is disclosed in JP Patent Application Publication No. 2001-217847.
It should be noted, however, that the newly acquired PRL may not contain any communication system satisfying the selection condition set by the user.
Suppose, for example, the user specifies an analog communication mode as a selection condition, whereas all the communication systems contained in the newly acquired PRL are capable of digital communications and not capable of analog communications.
In the case where, as described above, the PRL does not contain any communication system satisfying the user-set selection condition, the mobile phone cannot select any communication system. As a consequence, the mobile phone will be unable to carry out communications.
In order to avoid such an undesirable occurrence, upon acquisition of a new PRL, a state-of-the-art mobile phone automatically resets the selection condition set by the user, so that all the communication systems are made selectable. This arrangement serves to prevent that the mobile phone will be unable to conduct communications.
It should be noted, however, according to the above arrangement, each time a new PRL is acquired, the user-set selection condition is automatically discarded and changed to a predetermined state. This change of the selection condition takes place without informing the user or without user's consent.
According to OTAPA (Over-the-air Parameter Administration), the mobile phone automatically acquires a new PRL from an external source without requiring any user operation. Thus, the PRL acquisition and the change of selection condition take place while the user is not aware of it. As a result, it is possible that while the user believes that the mobile phone sets up communications using a communication system satisfying the selection condition set by the user, a communication system actually used is the one not desired by the user.