Identifiers in the GSM Network

GSM uses several identifiers for the routing of calls, identifying subscribers (e.g. for charging),
locating the HLR, identifying equipment, etc. Some of these identifiers play an important role for
CAMEL.
1.4.1 International Mobile Subscriber Identity
The international mobile subscriber identity (IMSI) is embedded on the SIM card and is used to
identify a subscriber. The IMSI is also contained in the subscription data in the HLR. The IMSI is
used for identifying a subscriber for various processes in the GSM network. Some of these are:

location update – when attaching to a network, the MS reports the IMSI to the MSC, which uses
the IMSI to derive the global title (GT) of the HLR associated with the subscriber;
• terminating call – when the GSM network handles a call to a GSM subscriber, the HLR uses
the IMSI to identify the subscriber in the MSC/VLR, to start a process for delivering the call to
that subscriber in that MSC/VLR.
• roaming charging – a VPLMN uses the IMSI to send billing records to the HPLMN of
a subscriber.
Figure 1.4 shows the format of the IMSI.
• mobile country code (MCC) – the MCC identifies the country for mobile networks. The MCC is
not used for call establishment. The usage of MCC is defined in ITU-T E.212 [129]. The MCC
values are allocated and published by the ITU-T.
• mobile network code (MNC) – the MNC identifies the mobile network within a mobile country
(as identified by MCC). MCC and MNC together identify a PLMN. Refer to ITU-T E.212 [129]
for MNC usage. The MNC may be two or three digits in length. Common practice is that, within
a country (as identified by MCC), all MNCs are either two or three digits.
• mobile subscriber identification number (MSIN) – the MSIN is the subscriber identifier within
a PLMN.
The IMSI is reported to the SCP during CAMEL service invocation. The IMSI may be needed,
for example, when identifying a country; countries in North America have equal country code
(country code = 1), but different MCC (e.g. Canada = 303; Mexico = 334).
1.4.2 Mobile Station Integrated Services Digital Network Number (MSISDN Number)
The MSISDN is used to identify the subscriber when, among other things, establishing a call to that
subscriber or sending an SMS to that subscriber. Hence, the MSISDN is used for routing purposes.

Figure 1.5 shows the structure of the MSISDN.
• country code (CC) – the CC identifies the country or group of countries of the subscriber;
• national destination code (NDC) – each PLMN in a country has one or more NDCs allocated to
it; the NDC may be used to route a call to the appropriate network;
• subscriber number (SN) – the SN identifies the subscriber within the number plan of a PLMN.

The MSISDN is not stored on the subscriber’s SIM card and is normally not available in the
MS.2 The MSISDN is provisioned in the HLR, as part of the subscriber’s profile, and is sent to
MSC during registration. The MSISDN is also reported to SCP when a CAMEL service is invoked.
One subscriber may have multiple MSISDNs. These MSISDNs are provisioned in the HLR. At
any one moment, only a single MSISDN is available in the MSC/VLR for the subscriber.
1.4.3 International Mobile Equipment Identifier

The international mobile equipment identifier (IMEI) is used to identify the ME [or user equipment
(UE) in UMTS network]. Each ME has a unique IMEI. The IMEI is hard-coded in the ME and
cannot be modified. Figure 1.6 shows the structure of the IMEI. The IMEI is not used for routing
or subscriber identification.

Refer to GSM TS 03.03 [27] for the type approval code (TAC), final assembly code (FAC)
and serial number (SNR). The software version (SV) may be included in the IMEI (‘IMEISV’) to
indicate the version of software embedded in the ME. The IMEI is always encoded as an eight-octet
string. As from CAMEL Phase 4, the IMEI(SV) may be reported to the SCP.

1.4.4 Mobile Station Roaming Number
The mobile station roaming number (MSRN) is used in the GSM network for routing a call to a
MS. The need for the MSRN stems from the fact that the MSISDN identifies a subscriber, but not
the current location of that subscriber in a telecommunications network. The MSRN is allocated to
a subscriber during MT call handling and is released when the call to that subscriber is established.
Each MSC in a PLMN has a (limited) range of MSRNs allocated to it. An MSRN may be allocated
to any subscriber registered in that MSC. The MSRN has the form of an E.164 number and can
be used by the GMSC for establishing a call to a GSM subscriber. An MSRN is part of a GSM
operator’s number plan. The MSRN indicates the GSM network a subscriber is registered in, but
not the GSM network the subscriber belongs to. Figure 1.7 shows how the MSRN is used for call
routing. The MSRN is not meant for call initiation. GSM operators may configure their MSC such
that subscribers cannot dial numbers that fall within the MSRN range of that operator.