WO2001005169A2

WO2001005169A2 - Method for the administration of forbidden plmn list by a mobile station

Info

Publication number: WO2001005169A2
Application number: PCT/US2000/040374
Authority: WO
Inventors: Sami Jutila
Original assignee: Nokia Corporation; Nokia Inc.
Priority date: 1999-07-14
Filing date: 2000-07-13
Publication date: 2001-01-18
Also published as: EP1195069A2; AU7386100A; WO2001005169A3

Abstract

A method for erasing a forbidden PLMN list stored in the SIM of a mobile station, controlled by ERASURE TYPE and ERASURE TRIGGER indicators stored alongside. The ERASURE TRIGGER specifies the criterion for deciding when elements of the list should b erased, and may include a predetermined moment; or a succesful user manual registration to a network which belonged to the fobidden list; or when roaming only forbidden are found upon powering the station; or when returning to the home network; or when leaving the home network. The ERASURE TYPE indicator indicates which part of the forbidden list should be erased, and may specify part or all of the list, including the current country visited or third countries.

Description

MOBILE STATION CONTROLLED FORBIDDEN PLMN LIST ERASURE

FIELD OF THE INVENTION:

This invention relates to mobile stations such as cellular telephones and personal communicators and, more particularly, relates to methods and apparatus for managing a forbidden Public Land Mobile Network (PLMN) list in a mobile station.

BACKGROUND OF THE INVENTION:

New GSM and other types of mobile networks are continually being deployed. A new operator typically has no roaming agreements with other operators, or at best just a few, when a new network is established. Also, existing roaming agreements between operators can change over time. However, the GSM standards require that once a network has indicated "PLMN Not Allowed", that operator code must be stored within a Subscriber Identity Module (SIM) in a forbidden PLMN list.

Currently, there are but a few techniques to remove a network that has been written to the SIM forbidden PLMN list. These techniques are as follows:

1) the user manually selects a forbidden network and obtains service there;

2) the list of forbidden networks becomes full (e.g., four networks) and then the network which was written first in list is removed; or

3) over-the-air updating of the forbidden PLMN list. This can be accomplished using known methods, such as with the help of short messages to update datafields in the SIM.

The techniques 1) and 2) are generally difficult ways to remove a network that has been placed in the forbidden PLMN list, while the technique 3) will not work at all if the mobile station is in an area where there are only forbidden networks available. For example, assume that a user has a Finnish SIM and is located in Ghana. Upon arrival in Ghana the user finds that the network there is forbidden. By the time of the user's next trip to Ghana a roaming agreement between the Finnish operator and the Ghana operator has been put in place. However, since the Ghana network is in the mobile station's forbidden PLMN list, the mobile station does not register there and, as a result, over-the- air updating of the forbidden PLMN list in the SIM is impossible to perform.

Thus, each of three techniques discussed above have drawbacks which may lead to a situation where the mobile station cannot acquire service, even when there exist allowed networks from which to chose.

OBJECTS AND ADVANTAGES OF THE INVENTION:

It is a first object and advantage of this invention to provide an improved technique for managing the forbidden PLMN list in a mobile station.

It is a further object and advantage of this invention to provide a technique to selectively erase all or some networks from the forbidden PLMN list, under control of the mobile station.

SUMMARY OF THE INVENTION

The foregoing and other problems are overcome and the objects and advantages are realized by methods and apparatus in accordance with embodiments of this invention.

A feature of the teachings of this invention is that forbidden PLMN list erasure (or modification) by a mobile station is accomplished so that, without user action or over-the-air interaction, the forbidden PLMN list is modified by the mobile station.

Another feature of the teachings of this invention is that the operator issuing the SIM to a customer is given an option either to enable or disable the forbidden PLMN list management feature, and possibly also to affect how the erasure operates .

One relatively simple method to manage the forbidden PLMN list is timer-based, where the mobile station erases the entire forbidden PLMN list at periodic intervals, such as once a month. In this embodiment it is preferred that the erasure of the forbidden PLMN lists is distributed evenly or substantially evenly over the population of mobile stations, so that not all mobile stations erase their respective forbidden PLMN lists on the same day. In this invention methods for ensuring the even distribution of forbidden PLMN list erasure are provided.

A more complex method has the mobile station analyze the list of forbidden networks and select the one(s) to be erased. As this type of autonomous mobile equipment behavior may not be desirable to some network operators, the network operator can be given the prerogative as to whether to enable the use of this method.

A presently preferred method employs the timer-based erasure of the forbidden PLMN list, but other methods where the mobile station decides automatically, without user intervention, to remove forbidden PLMNs from the SIM are also within the scope of the teachings of this invention.

The disclosed embodiments provide at least the following advantages over the conventional techniques for managing the forbidden PLMN list.

(A) First, new roaming agreements can be updated in the SIM without requiring over-the-air programming and the associated signaling.

(B) Second, the user does not have to manually select the networks that once were forbidden, but no longer are, in order to obtain service .

(C) By maintaining the behavior of the forbidden PLMN list in the SIM essentially the same as it currently is defined, e.g., the mobile equipment does not attempt to access the forbidden networks, the networks do not receive a large number of unnecessary registration attempts. For example, the time-based method, when activated once a month, provides an adequate solution to the unnecessary registration attempt problem.

The timer-based method (e.g., once a month) reduces greatly the opportunity to attempt to access forbidden networks . Furthermore, if an ERASURE TYPE argument is provided, such as "only erase network where the country is different from the home network", then unnecessary registration attempts may be removed altogether. This technique also does not require any user interaction.

Through the use of these teachings roaming agreements may be updated immediately (this solution can be accomplished also by correctly defining TRIGGERS for erasure) . User flexibility is also provided, as the user is enabled to control the network selection process according to his or her desires. For example, it becomes possible to place an expensive network into the forbidden PLMN list, even when there actually is a roaming agreement in place.

A mobile station is disclosed, as is a method for operating the mobile station. The mobile station has a forbidden PLMN list stored in a memory that is accessible to a mobile station processor. The mobile station processor determines whether at least one predetermined criterion is met and, if the at least one predetermined criterion is determined to be met, erases all or a part of the forbidden PLMN list.

In a preferred embodiment the memory is contained in a SIM having an ERASURE TYPE stored within for specifying at least one of: erase the entire forbidden PLMN list; erase only networks that are from a different country than a home network; erase only networks from the country the mobile station is visiting or roaming in; and erase only networks from countries other than where the mobile station is currently roaming. In the preferred embodiment the SIM also has an ERASURE TRIGGER stored within for specifying at least one of: a timer-based erasure trigger; erasure when, after switch-on, only forbidden networks are found and the mobile station is visiting another country than its home country; erasure when, after switch-on, only forbidden networks are found regardless of the country; erasure when returning to the home network, after roaming; erase when roaming after being in home network; and never erase. For the timer-based erasure trigger the erasure time is preferably temporally distributed over the population of mobile stations.

The at least one predetermined criterion, to erase all networks from the forbidden PLMN list, can be when a user has manually selected a forbidden network and obtained service .

A method is disclosed to clear all or a part of the forbidden PLMN list stored in the memory accessible to the mobile station processor. The method has steps of (a) calculating a time at which to clear all or a part of a forbidden PLMN list; (b) comparing the calculated time to a current time; and (c) if not already erased, erasing all or a part of the forbidden PLMN list if the calculated time equals or is greater than the current time. The current time may be obtained from a calendar function or it may estimated. In the preferred embodiment the current time and the calculated time are expressed as a day of the month, and the calculated time is determined from the mobile station IMSI or from the mobile station IMEI . An initial step sets an indicator in the mobile station for indicating that clearing the forbidden PLMN list is one of enabled or disabled.

BRIEF DESCRIPTION OF THE DRAWINGS

The above set forth and other features of the invention are made more apparent in the ensuing Detailed Description of the Invention when read in conjunction with the attached Drawings, wherein:

Fig. 1 is block diagram of a wireless telecommunications system that is suitable for practicing this invention; and

Fig. 2 is logic flow diagram that illustrates a method in accordance with the teachings herein.

DETAILED DESCRIPTION OF THE INVENTION

Referring to Fig. 1, there is illustrated a simplified block diagram of an embodiment of an exemplary wireless telecommunications system 1 that includes a plurality of mobile stations 10. Two mobile stations (MSs) are shown in Fig. 1, with one being designated MS#1 and the other MS#2. Fig. 1 also shows an exemplary network operator 2 having, for example, a mobile switching center (MSC) 3 for connecting to a telecommunications network, such as the Public Switched Telephone Network or PSTN, at least one base station controller (BSC) 4, and a plurality of base transceiver stations (BTS) 5 that transmit in a forward or downlink direction both physical and logical channels to the mobile stations 10 in accordance with a predetermined air interface standard. It is assumed that a reverse or uplink communication path exists from the mobile station 10 to the network operator, which conveys mobile originated access requests and traffic, which may be voice and/or packet data traffic.

In the presently preferred, but not limiting, embodiment of these teachings, the air interface standard conforms to a Time Division Multiple Access (TDMA) air interface known as GSM and variants thereof, but the use of a GSM or GSM- like air interface standard is not a limitation upon the practice of this invention.

The network operator 2 can include a Message Service Center (SMSC) that receives and forwards messages for the mobile stations 10, such as Short Message Service (SMS) messages, or any wireless messaging technique including e-mail and Supplementary Data Services. Furthermore, enhancements to SMS can be used, such as one under development and known as Multimedia Messaging Service (MMS) , wherein image messages, video messages, audio messages, text messages, executables and the like, and combinations thereof, can be transferred between a network and a mobile station.

The mobile station 10 typically includes a data processor embodied in a microcontrol unit (MCU) 12 having an output coupled to an input of a display 14 and an input coupled to an output of a keyboard or keypad 16. The mobile station 10 may be considered to be a handheld radiotelephone, such as a cellular telephone or a personal communicator, and may have a microphone and a speaker (not shown) for conducting voice communications. The mobile station 10 could also be contained within a card or module that is connected during use to another device. For example, the mobile station 10 could be contained within a PCMCIA or similar type of card or module that is installed during use within a portable data processor, such as a laptop or notebook computer, or even a computer that is wearable by the user.

The MCU 12 is assumed to include or be coupled to some type of a memory 13, including a read-only memory (ROM) for storing an operating program, as well as a random access memory (RAM) for temporarily storing required data, scratchpad memory, received data packets and data packets prepared for transmission, etc. A separate, removable SIM 26 that is accessible to the MCU 12 by being bidirectionally coupled to the MCU 12 can be provided as well, the SIM storing, for example, a preferred Public Land Mobile Network (PLMN) list 26B, a forbidden PLMN list 26A, as well as other subscriber-related information. The ROM is assumed, for the purposes of this invention, to store a program enabling the MCU 12 to execute the software routines required to operate in accordance with the presently preferred forbidden PLMN list 26A management methods .

The mobile station 10 also contains a wireless section that includes a digital signal processor (DSP) 18, or equivalent high speed processor, as well as a wireless transceiver comprised of a transmitter 20 and a receiver 22, both of which are coupled to an antenna 24 for communication with the network operator 1.

The numerous innovative teachings of the present invention will now be described with particular reference to the presently preferred embodiment. However, it should be understood that this class of embodiments provides only a few examples of the many advantageous uses of the innovative teachings herein. In general, statements made herein do not necessarily delimit any of the various embodiments. Moreover, some statements may apply to some inventive features but not to others.

This invention provides for the automatic erasure of the forbidden PLMN list by the mobile station 10, and to the definition of rules as to whether (and when) to make the erasure, and whether to erase the entire forbidden PLMN list 26A or only a portion thereof.

One location to configure whether the forbidden PLMN list erasure feature is enabled or disabled is in the SIM 26. This can be done by defining a new datafield in the SIM 26 and, if the datafield exists in the SIM 26, then the feature is enabled. The datafield may also store additional information related to the forbidden PLMN erasure method, such as whether to erase the entire list of forbidden networks or only certain networks, such as outside of the home country. The datafield may also store an erasure interval which functions as a trigger to make the automatic erasure, such as once a month, or when there are only forbidden networks available when outside of home country, or when there are only forbidden networks available, as described in further detail below.

In addition, an EASY ERASURE trigger may be made part of the applicable standards. For example, if the user manually selects a network which is in the forbidden PLMN list 26A, and obtains service there, according to current GSM standards only this manually selected network is removed from the forbidden PLMN list 26A. However, the inventor has realized that in actuality the ability to obtain service from a formally forbidden network is an indication of the fact that the operator has made at least one new roaming agreement, and thus it may be desirable to trigger the erasure of the entire forbidden PLMN list 26A (i.e., the list may be out of date) . This EASY ERASURE may also be configurable .

An example of one possible configuration of this SIM 26 datafield is as follows: Byte Contents

0 USE THIS FIELD: 0= feature disabled, 1 = feature enabled

1 ERASURE TYPE: 0 = entire forbidden PLMN list 26A is erased, 1 = erase only networks that are from different country than the home network, 2 = erase only networks from the country the mobile station is currently visiting (country the mobile station 10 is currently roaming in) , 3 = erase only networks from countries other than where the mobile station 10 is visiting roaming

2 ERASURE TRIGGER: 0 = timer-based (e.g., once per month) , where the mobile station manufacturer is responsible for even distribution over the population of mobile stations, 1 = when, after switch- on, only forbidden networks are found and the mobile station 10 is roaming in a country other than the home country, 2= when after switch-on, only forbidden networks are found regardless of the country, 3 = when returning to home network, after roaming, 4 = when roaming after being in home network, 5 = never (another way to disable the feature, apart from that the fact that EASY ERASURE may be supported)

4 EASY ERASURE: when the user has manually selected a forbidden network and obtained service, instead of removing just that particular network from the forbidden PLMN list 26A, remove all forbidden networks. 0 = feature enabled, 1= feature disabled.

It should be evident that there are numerous ways and numerous criterion with which to configure the forbidden PLMN list erasure feature of this invention, and that different erasure types and triggers may be specified than those specifically listed above. Note that several erasure TYPES and TRIGGERS can be made a function of the roaming status of the mobile station 10 (e.g., roaming or not roaming, location where mobile station is roaming, roaming history, etc.) Also note that the timer-based method may be made configurable, so that the timer interval is made programmable and stored in the SIM 26, e.g., 1-255 days, where 0 specifies basically the same operation as ERASURE TRIGGER 1 or 2.

Fig. 2 depicts a logic flow chart of a presently preferred embodiment of the teachings of this invention.

A) Fixed interval: A determination is made at Step 102 if the mobile station 10 contains a calendar function. If Yes, at Step 104 the method calculates the erasure date based on the IMSI (international mobile subscriber identity) that is stored in the mobile station 10, such as by using the 2-5 last digits of the IMSI divided by the number 28 (or 23, which is a prime number) . There will be a remainder (0-26, or 0-22) , and the erasure date is thus defined to be the remainder + 1. The erasure date could be calculated instead based on the IMEI (international mobile equipment identity) that is also stored in the mobile station 10. For example, assume that the IMSI last five digits are 56742. Then 56742/28 = 2026 with remainder equal to 14. The erasure date for this particular mobile station 10 would thus be the (14+1) or 15th of each month. If the current day of the month is equal to the calculated date (or past the calculated date) then it may be time to clear the forbidden PLMN list 26A (Step 106) .

At Step 108 the mobile station 10 needs to "remember" whether it has already cleared the forbidden PLMN list 26A in the current day (and month) . If it is time to clear the forbidden PLMN list 26A list, and if it has not yet been cleared, then the list is cleared at Step 110.

The current date may also be obtained, in some embodiments, from the network operator 2 , or from some other source external to the mobile station 10 (e.g., the mobile station 10 includes a GPS reception capability) .

The date calculation techniques described above are exemplary, and those skilled in the art may derive other suitable techniques to determine a particular time for the mobile station 10 to automatically clear the PLMN list 26A. In all such cases it is desirable and preferred that not all mobile stations 10 (e.g., MS#1 and MS#2) clear their respective forbidden PLMN lists 26A on the same day. It is desirable to temporally distribute the clearing of the forbidden PLMN lists 26A for different mobile stations to minimize the risk that on some particular date many network operators do not experience a large number of failed registration attempts.

Returning to Step 102, if the mobile station 10 does not have a built-in calendar function, control passes to Step 112 where, for example, a switch-on counter (typically a memory 13 location) is tested to determine if the mobile station 10 has been turned on some predetermined number (e.g., 100) times. If Yes, control passes to Step 110 to clear the forbidden PLMN list 26A. Also, a mobile station 10 timer function can be used, such that after some number of hours (e.g., 12) the switch-on counter is increased by one. In this case, if the mobile station 10 is left on continuously the forbidden PLMN list 26A will be cleared every 50 hours. The interval may be programmable and may be obtained from a SIM datafield defined for this purpose.

In some embodiments of this invention the operator may define the interval to be zero in order to disable the automatic forbidden PLMN list 26A erasure function. This may be a default value, i.e., information is stored in the SIM 26 in the form of a datafield to enable periodic forbidden PLMN list 26A clearing, and if this data is not present the periodic forbidden PLMN list clearing is disabled.

A description of this invention in the context of roaming- based forbidden PLMN list 26A erasure will now be given.

Solution 1: Clear when registered network changes

Every time the mobile station 10 changes the network from the last registered network, the entire forbidden PLMN list 26A or portions of it may be cleared when the mobile station 10 is switched off (those portions corresponding to networks from other countries or networks from the visited country.

Solution 2: Clear when no attempts for registration are made

Alternatively, if there are only forbidden networks available when the mobile station 10 was switched on and then off again, then the list is cleared (i.e., in the case where the mobile station 10 did not access any network) .

However, the roaming-based approach has more drawbacks than the other methods described above, as it is more likely to cause unnecessary registration attempts for forbidden networks .

There are a number of possible techniques to implement the teachings of this invention, but basically the mobile station 10 is operated to delete either the entire forbidden PLMN list 26A or a selected portion of the forbidden PLMN list 26A based on the assumption that a forbidden network ceases to be a forbidden network as operators make new roaming agreements.

To summarize, the method for the timer-based solution may be a value 1-28 (30 or 31) that is calculated based on the IMEI or the IMSI. In this case the forbidden PLMN list 26A erasure date is dependent on the unique identity of the mobile station 10. The mobile station 10 stores the month in which it erased the forbidden PLMN list 26A (for instance May) , and when June arrives it checks when the date is equal to or greater than the calculated date. If the date is equal to or greater than the erasure date, and the SIM 26 requires the timer-based erasure, the erasure is performed (by using any appropriate erasure method) . The mobile station 10 also stores June (e.g., it stores a 6 in a month datafield in the SIM 26) as the month when the erasure was done. In this manner the mobile station 10 will not again erase the forbidden PLMN list 26A during June.

In those mobile stations without a calendar function, a long timer (e.g., 12 hours) can be used, either alone or in combination with the switched-on timer, for estimating the time. Naturally this approach is not as accurate, as the mobile station 10 can be switched off for several days. Another method may employ a datafield in the SIM 26 which is decreased (or increased) by one every day (24 hours) . Based on this day counter the erasure of the forbidden PLMN list 26A is made. In this case the SIM 26 may perform the erasure automatically.

Alternatively, the SIM 26 datafield may just indicate the month when the erasure was made. In this case the mobile station 10 performs the next erasure if the date is one month (or more past) the date or, in the case where the interval is configurable, the mobile station 10 may calculate whether the erasure should be made at this time. In the case where the SIM 26 datafield includes the month when the erasure was made, the algorithm is basically the same as the fixed interval method.

For example, a value 1-28 (30 or 31) is calculated based on the IMEI or on the IMSI . In this case the forbidden PLMN list 26A erasure date is dependent on the identity of the mobile station 10. This value could also be given by the SIM 26 so that the mobile station 10 can calculate it from the IMSI or given directly in some datafield. The SIM 26 stores the month it has erased the list (for instance May) and when it June arrives the mobile station 10 again checks if the date is equal or greater than the calculated date. If it is, and if the SIM 26 requires timer-based erasure of the forbidden PLMN list 26A, the erasure is performed using an appropriate erasure method. The mobile station 10 also stores June in the SIM 26 as the month when the erasure was performed to SIM.

It should be noted that by placing the time counter in the SIM 26, and allowing the mobile station 10 to update the time counter (e.g., once a day) , the SIM 26 is enabled itself to clear the forbidden PLMN list 26A or portions thereof .

It should further be noted that by periodically sending the date to the SIM 26, the SIM 26 is enabled itself to clear the forbidden PLMN list 26A or portions thereof at appropriate times.

While the invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that changes in form and details may be made therein without departing from the scope and spirit of the invention.

Claims

CLAIMSWhat is claimed is :

1. A method to clear all or a part of a forbidden PLMN list stored in a memory accessible to a mobile station processor, comprising steps of:

calculating a time at which to clear all or a part of a forbidden PLMN list;

comparing the calculated time to a current time; and

erasing all or a part of the forbidden PLMN list if the calculated time equals or is greater than the current time.

2. A method as in claim 1, wherein the current time is obtained from a calendar function.

3. A method as in claim 1, wherein the current time is obtained from a source external to the mobile station.

4. A method as in claim 1, wherein the current time and the calculated time are expressed as a day of the month.

5. A method as in claim 1, wherein the step of calculating uses the mobile station IMSI.

6. A method as in claim 1, wherein the step of calculating uses the mobile station IMEI.

7. A method as in claim 1, and comprising an initial step of setting an indicator in the mobile station for indicating that clearing the forbidden PLMN list is one of enabled or disabled.

8. A method to erase a forbidden PLMN list stored in a memory accessible to a mobile station processor, comprising mobile station executed steps of:

determining whether at least one predetermined criterion is met; and

if the at least one predetermined criterion is met, erasing the forbidden PLMN list.

9. A method as in claim 8, wherein an ERASURE TYPE is stored in the memory for specifying at least one of : erase the entire forbidden PLMN list; erase only networks that are from different country than a home network; erase only networks from the country the mobile station is visiting roaming in; and erase only networks from countries other than where the mobile station is currently roaming.

10. A method as in claim 8, wherein an ERASURE TRIGGER is stored in the memory for specifying at least one of : a timer-based erasure trigger; erasure when, after switch on, only forbidden networks are found and the mobile station is visiting another country than its home country; erasure when, after switch on, only forbidden networks are found regardless of the country; erasure when returning to home network, after roaming; erase when roaming after being in home network; and never erase.

11. A method as in claim 10, wherein for the timer- based erasure trigger the erasure time is temporally distributed over the population of mobile stations.

12. A method as in claim 8, wherein the at least one predetermined criterion, to cause an erasure of the entire forbidden PLMN list, is when a user has manually selected a forbidden network and obtained service.

13. A mobile station comprising a forbidden PLMN list stored in a memory accessible to a mobile station processor, said mobile station processor determining whether at least one predetermined criterion is met and, if the at least one predetermined criterion is determined to be met, for erasing the forbidden PLMN list.

14. A mobile station as in claim 13 , wherein said memory is a SIM having an ERASURE TYPE stored within for specifying at least one of: erase the entire forbidden PLMN list; erase only networks that are from different country than a home network; erase only networks from the country the mobile station is visiting roaming in; and erase only networks from countries other than where the mobile station is currently roaming.

15. A mobile station as in claim 13, wherein said memory is a SIM having an ERASURE TRIGGER stored within for specifying at least one of: a timer-based erasure trigger; erasure when, after switch on, only forbidden networks are found and the mobile station is visiting another country than its home country; erasure when, after switch on, only forbidden networks are found regardless of the country; erasure when returning to home network, after roaming; erase when roaming after being in home network; and never erase .

16. A mobile station as in claim 13, wherein for the timer-based erasure trigger the erasure time is temporally distributed over the population of mobile stations.

17. A mobile station as in claim 13, wherein the at least one predetermined criterion, to cause an erasure of the entire forbidden PLMN list, is when a user has manually selected a forbidden network and obtained service.

18. A method to erase a forbidden PLMN list stored in a SIM, comprising steps of: determining whether at least one predetermined criterion is met; and

if the at least one predetermined criterion is met, erasing the entire forbidden PLMN list, wherein the at least one predetermined criterion comprises an occurrence of a user obtaining service from a network that is manually selected, the network being one stored in the forbidden PLMN list.

19. A method to erase a forbidden PLMN list stored in a SIM, comprising steps of:

determining whether at least one predetermined criterion is met; and

if the at least one predetermined criterion is met, erasing all or a part of the forbidden PLMN list, wherein the at least one predetermined criterion comprises an occurrence of a day of the month, the day being a function of a value of a unique mobile station identifier .

20. A method to erase a forbidden PLMN list stored in a SIM, comprising steps of:

determining whether at least one predetermined criterion is met; and

if the at least one predetermined criterion is met, erasing all or a part of the forbidden PLMN list, wherein the at least one predetermined criterion comprises a number of times that the mobile station has been powered on.

21. A method to erase a forbidden PLMN list stored in a SIM, comprising steps of: determining whether at least one predetermined criterion is met; and

if the at least one predetermined criterion is met, erasing all or a part of the forbidden PLMN list, wherein the at least one predetermined criterion comprises a length of time that the mobile station has been powered on.

22. A method to erase a forbidden PLMN list stored in a SIM, comprising steps of:

determining whether at least one predetermined criterion is met; and

if the at least one predetermined criterion is met, erasing all or a part of the forbidden PLMN list, wherein the at least one predetermined criterion comprises a roaming status of the mobile station.