IMSI conversion method

Details IMSI conversion method IMSI conversion method

2001-04-03

2004-11-16

Tran, Cong Van (Department: 2683)

Telecommunications

Radiotelephone system

Security or fraud prevention

C455S551000, C455S552100, C455S414400, C370S342000

Reexamination Certificate

active

06819918

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the processing of International Mobile Station Identity (IMSI) information used in mobile telephone communications systems.
DISCUSSION OF THE RELATED ART
The use of mobile station identifiers in wireless communication is widespread for registration, authentication, SMS (short message service), and other call processing and billing purposes. With the proliferation of mobile communications devices throughout the world, efforts have been made to standardize the mobile station identifiers so that a mobile station can be used in many different countries, provided it is capable of interconnecting with the type of service (e.g., TDMA, CDMA) offered. To this end, ITU-T E.212 43 (hereinafter E.212) recommends an international identification plan for land mobile stations and offers a recommendation to establish principles for allocating an International Mobile Station Identity (IMSI) to mobile stations. The idea is to enable mobile stations to roam among public land mobile networks located in different countries by adherence to an international identification plan with a unique international identification for each mobile station.
The E.212 recommendation is directed to the IMSI numbering plan only, and the numbering plan may be implemented on an over-the air interface or through a signaling interface in a wireless network in a variety of ways. The E.212 numbering plan recommendation identifies and uses the following terms:
(1) Mobile Country Code (MCC) which is the part of the mobile station identifier which uniquely identifies the country of domicile of the mobile station; it is the first part of an IMSI designation and is 3 digits long;
(2) Mobile Network Code (MNC), which uniquely identifies the home network of the mobile station; it is the second part of an IMSI designation and follows the MCC and is 1 to 3 digits long;
(3) Mobile Station Identification Number (MSIN), which uniquely identifies the mobile station within a network; it is the third part of an IMSI designation and follows the MNC;
(4) National Mobile Station Identity (NMSI), which uniquely identifies the mobile station nationally; and is formed of the MNC and MSIN;
(6) International Mobile Station Identity (IMSI) which uniquely identifies the mobile station internationally and is formed of the MCC, MNC and MSIN.
The relationship of the MCC, MNC, MSIN, NMSI and IMSI as set out in E.212 is illustrated in FIG.
1
.
The recommendation continues by specifying that only numerical characters (0-9) shall be used and that the IMSI shall be variable in length, never to exceed 15 digits. The MCC shall always consist of 3 digits. The national mobile station identity is to be assigned by each network administration.
The assigned IMSI for a mobile may be stored in the mobile station, transmitted in an over-the-air interface and/or through signaling interfaces, and stored and processed within the network in a variety of ways. It is important to note that because of the flexibility given to various network administrations, once concatenated to form E.212 IMSI, it is impossible to derive algorithmically the various components which go into forming an assigned IMSI. Thus, the manner in which IMSI is formatted, transmitted and processed varies on various interfaces and in different types of networks.
For example, for an IS-95 CDMA system, the air interface specification, as set out in Section 6.3.1 at IS-95, partitions the IMSI into smaller code entities for efficiency.
The partitioning of the IMSI into smaller entities allows the interface to define methods in which only some of the digits are transmitted for a given call. The remaining digits are known to the base station and broadcast to mobiles in an overhead channel. For CDMA mobiles the IMSI, as defined in E.212, is partitioned into the following components: MCC, IMSI_S (which is sub-divided into IMSI_SI and IMSI_S
2
) and finally IMSI_
11
_
12
. Under the CDMA IS-95 standard, an IMSI that is 15 digits in length is called a class 0 IMSI. An IMSI that is less that 15 digits is called a class 1 IMSI. The first 3 digits of the IMSI are the MCC. These digits are encoded into 10 bits in the manner defined in the IS-95 standard. The IMSI_S is a 10 digit (34 bit) number derived from the IMSI. When the IMSI has 10 or more digits, IMSI_S is equal to the last 10 digits. When the IMSI has fewer than 10 digits, the least significant digits of IMSI_S are equal to the IMSI and zeros are added to the most significant side to obtain a total of 10 digits. The 10 digit IMSI_S consists of 3 and 7 digit parts, called IMSI_S
2
and IMSI_S
1
respectively. The IMSI_S
2
is stored in 10 bits and the IMSI_SI in 24 bits. The IMSI_
11
_
12
is the 11
th
and 12
th
digits of the IMSI. A class 1 IMSI is padded with leading zeroes if needed when computing the IMSI_
11
_
12
. The IMSI_
11
_
12
is encoded in 7 bits. As noted, per E.212, the NMSI is defined as the digits of the IMSI after the MCC.
The encoding scheme for the various CDMA IMSI components entails representing digits in binary coded decimal and then modulating the values to minimize the number of bits required for storage. For example, if the first digit is D
1
and the second D
2
, the binary representation is found by computing 10×D
1
+D
2
−11.
When a class 1 IMSI is transmitted over-the-air, an additional parameter, IMSI_ADDR_NUM is sent which is the NMSI length minus four. The IS-95 CDMA IMSI air interface format is shown in FIG.
2
.
For example, in a CDMA system the IMSI 310001234567890, which is a class 0 IMSI, is represented by encoding the following, as shown in FIG.
3
:
MCC=310
IMSI_
11
_
12
=00
IMSI_S
2
=123
IMSI_S
1
=4567890
FIG. 4
shows an example of a class 1 IMSI having the digit identifier 67898234567:
MCC=678
NMSI=98234567 (per E.212)
IMSI_
11
_
12
=06
IMSI_S
2
=789
IMSI_S
1
=8234567
IMSI_ADDR_NUM=4 (NMSI Length−4) NMSI length=8 in this example.
It should be noted that the CDMA air interface allows transmission of an E.212 IMSI which begins in a leading zero. This is accomplished by use of the IMSI_ADDR_NUM length parameter which is sent for any class
1
IMSI.
The manner in which the IMSI is handled in an IS-136 TDMA system is quite different. As spelled out in Section 8.1.1.2, IS-136, the IMSI is always encoded as a 50-bit Mobile Station Identification (MSID). Any IMSI less than 15 decimal digits in length is first padded with leading zero digits (i.e., d
15
, d
14
. . . ) as necessary to produce a 15 decimal digit string. The 15 decimal digits are then divided in 5 groups of 3 digits each. Each 3 digit group is translated into its 10-bit binary equivalent using a normal decimal to binary conversion (e.g., 271=0100001111). The resulting 10-bit groups are then concatenated to form a 50-bit MSID for transmission in the over-the-air interface. At the receiving end, the actual IMSI is recovered by removing all leading zero digits that may result when translating the 50-bit MSID back into 15 decimal digits. The TDMA encoded IMSI format is shown in FIG.
5
. It should be noted that leading digit zero values are considered as fill and not considered part of the IMSI.
The foregoing are but two examples of how IMSI is formatted and transmitted over the air differently in a CDMA and TDMA system. The IMSI may also be formatted and transmitted differently in an over-the-air or signaling interface in other types of wireless systems which use the IMSI.
Accordingly, because of the different way the IMSI is formatted and transmitted in the over-the air interface and through a signaling interface between various wireless and network elements as specified in the appropriate standards documents, different IMSI processing software is required on a network element for each affected interface. This creates a burden on the network element operator as different software is required to handle each IMSI format on each interface (air, signaling) for each functional area (call processing, bil

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