IDENTITY NUMBERS
GSM identities
The GSM network is complex and consists of the Switching System (SS) and the Base Station System (BSS). The switching system, which consists of HLR, MSC, VLR, AUC and EIR, interfaces both the Base Station System and also other networks like PSTN/ISDN, data networks or other PLMNs.
In order to switch a call to a mobile subscriber, the right entities need to be involved. It is therefore important to address them correctly. The numbers used to identify the identities in a GSM/PLMN network is described in this chapter. See also Figure 56.
Numbering plans are used to identify different networks. For a telephone number in the PSTN/ISDN network, numbering plans E.164 is used.
Mobile Station ISDN Number (MSISDN)
The MSISDN is a number which uniquely identifies a mobile telephone subscription in the public switched telephone network numbering plan. According to the CCITT recommendations, the mobile telephone number or catalogue number to be dialled is composed in the following way:
MSISDN = CC + NDC + SN
CC = Country Code
NDC = National Destination Code
SN = Subscriber Number
A National Destination Code is allocated to each GSM PLMN. In some countries, more than one NDC may be required for each GSM PLMN. The international MSISDN number may be of variable length. The maximum length shall be 15 digits, prefixes not included.
Each subscription is connected to one Home Location Register (HLR).
The length of the MSISDN depends on the structure and numbering plan of each operator, as an application of CCITT recommendation E.164.
The following is an example of dialling a GSM subscriber.
Example:
A Swedish PSTN subscriber calls a Swiss GSM PLMN subscriber.
The digits 89 define the 2 or 3 digits which identify the GSM PLMN area code. The digits 902147 define the 6 digits which identify the mobile subscriber.
International Mobile Subscriber Identity (IMSI)
The IMSI is the information which uniquely identifies a subscriber in a GSM/PLMN.
For a correct identification over the radio path and through the GSM PLMN network, a specific identity is allocated to each subscriber. This identity is called the International Mobile Subscriber Identity (IMSI) and is used for all signalling in the PLMN. It will be stored in the Subscriber Identity Module (SIM), as well as in the Home Location Register (HLR) and in the serving Visitor Location Register (VLR).
The IMSI consists of three different parts:
IMSI = MCC + MNC + MSIN
MCC = Mobile Country Code (3 digits)
MNC = Mobile Network Code (2 digits)
MSIN = Mobile Subscriber Identification Number (max 10 digits)
According to the GSM recommendations, the IMSI will have a length of maximum 15 digits.
All network–related subscriber information is connected to the IMSI. See also Figure 56.
Mobile Station Roaming Number (MSRN)
HLR knows in what MSC/VLR Service Area the subscriber is located. In order to provide a temporary number to be used for routing, the HLR requests the current MSC/VLR to allocate and return a Mobile Station Roaming Number (MSRN) for the called subscriber, see Figure 56.
At reception of the MSRN, HLR sends it to the GMSC, which can now route the call to the MSC/VLR exchange where the called subscriber is currently registered.
The interrogation call routing function (request for an MSRN) is part of the Mobile Application Part (MAP). All data exchanged between the GMSC - HLR - MSC/VLR for the purpose of interrogation is sent over the No. 7 signalling network.
The Mobile Station Roaming Number (MSRN), according to the GSM recommendations, consists of three parts:
MSRN = CC + NDC + SN
CC = Country Code
NDC = National Destination Code
SN = Subscriber Number
Note: In this case, SN is the address to the serving MSC.
Temporary Mobile Subscriber Identity (TMSI)
The TMSI is a temporary number used instead of the IMSI to identify an MS. It raises the subscriber’s confidentiality and is known within the serving MSC/VLR-area and changed at certain events or time intervals. The structure of the TMSI may be chosen by each administration but should have a maximum length of four octets (8 digits).
International Mobile
The IMEI is used for equipment identification. An IMEI uniquely identifies a mobile station as a piece or assembly of equipment. (See IMEI, chapter 5.)
IMEI = TAC + FAC + SNR + sp
TAC = Type Approval Code (6 digits), determined by a central GSM body
FAC = Final Assembly Code (2 digits), identifies the manufacturer
SNR = Serial Number (6 digits), an individual serial number of six digits uniquely identifying all equipment within each TAC and FAC
sp = spare for future use (1 digit)
According to the GSM specification, IMEI has the length of 15 digits.
Location Area Identity (LAI)
LAI is used for location updating of mobile subscribers.
LAI = MCC + MNC + LAC
MCC = Mobile Country Code (3 digits), identifies the country. It follows the same numbering plan as MCC in IMSI.
MNC = Mobile Network Code (2 digits), identifies the GSM/PLMN in that country and follows the same numbering plan as the MNC in IMSI.
LAC = Location Area Code, identifies a location area within a GSM PLMN network. The maximum length of LAC is 16 bits, enabling 65 536 different location areas to be defined in one GSM PLMN.
Cell Global Identity (CGI)
CGI is used for cell identification within the GSM network. This is done by adding a Cell Identity (CI) to the location area identity.
CGI = MCC + MNC + LAC + CI
CI = Cell Identity, identifies a cell within a location area, maximum 16 bits
Base Station Identity Code (BSIC)
BSIC allows a mobile station to distinguish between different neighboring base stations.
BSIC = NCC + BCC
NCC = Network Colour Code (3 bits), identifies the GSM PLMN.
Note that it does not uniquely identify the operator. NCC is primarily used to distinguish between operators on each side of border.
BCC = Base Station Colour Code (3 bits), identifies the Base Station to help distinguish between BTS using the same BCCH frequencies
Location Number (LN)
Location Number is a number related to a certain geographical area, as specified by the network operator by ”tying” the location numbers to cells, location areas, or MSC/VLR service areas.
The Location Number is used to implement features like Regional /Local subscription and Geographical differentiated charging.