Page 32 - From GMS to LTE

18 From GSM to LTE-Advanced Pro and 5G

Table 1.2 Mobile country codes.

MCC Country
234 United Kingdom
310 United States
228 Switzerland
208 France
262 Germany
604 Morocco
505 Australia

As an IMSI is internationally unique, it enables a subscriber to use their phone
abroad if a GSM network is available that has a roaming agreement with their home
operator. When the mobile device is switched on, the IMSI is retrieved from the SIM
card and sent to the MSC. There, the MCC and MNC of the IMSI are analyzed and
the MSC is able to request the subscriber’s record from the HLR of the subscriber’s
home network.
The phone number of the user, which is called the Mobile Subscriber Integrated
Services Digital Network Number (MSISDN) in the GSM standards, has a length of up
to 15 digits and consists of the following parts:

The country code is the international code of the subscriber’s home country. The
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country code has one to three digits such as +44 for the United Kingdom, +1 for the
United States, and +353 for Ireland.
The NDC usually represents the code with which the network operator can be
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reached. It is normally three digits in length. It should be noted that mobile networks
in the United States use the same NDCs as fixed‐line networks. Thus, it is not possi-
ble for a user to distinguish whether they are calling a fixed‐line or a mobile phone.
This impacts both billing and routing, as the originating network cannot deduct
which tariff to apply from the NDC.
The remainder of the MSISDN is the subscriber number, which is unique in the
●
network.
There is usually a 1:1 or 1:N relationship in the HLR between the IMSI and the
MSISDN. Furthermore, a mobile subscriber is normally assigned only a single MSISDN.
However, as the IMSI is the unique identifier of a subscriber in the mobile network, it is
also possible to assign several numbers to a single subscriber.
Another advantage of using the IMSI as the key to all subscriber information instead
of the MSISDN is that the phone number of the subscriber can be changed without
replacing the user’s SIM card or changing any information on it. To change the MSISDN,
only the HLR record of the subscriber needs to be changed. In effect, this means that
the mobile device is not aware of its own phone number. This is not necessary because
the MSC automatically adds the user’s MSISDN to the message flow for a mobile‐originated
call establishment so that it can be presented to the called party.
Many countries have introduced functionality called mobile number portability
(MNP), which allows a subscriber to retain their MSISDN even if they want to change