Keeping the data safe

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The Home Location Register (HLR) is the location in the 2G network where various subscriber items are stored. This function is included in the Home Subscriber System in 4G and part of the Unified Database Management in 5G.

These systems store (amongst other things) the International Subscriber Identity which is essentially an International version of the SIM card. It also contains the MSISDN which is the phone number of the device. In 2G there is a separate Authentication Centre that creates the authentication key used in any data transmissions. 4G and 5G also include this but it is part of the HSS or UDM.

The ISIM, gives a unique index to these databases. This is made up of a country code (234 or 235 in the UK), an operator code, then the Subscriber Identity Module (SIM) number. For the UK these are the codes :

BT	234-76 or 234-77
EE	235-02
EE (Orange)	234-33 or 234-34
EE (T-Mobile)	234-30 or 234-31 or 234-32
Three	234-20 or 234-94
O2	234-02 or 234-10 or 234-11
Vodafone	234-15 or 234-27 or 234-91

The Mobile Subscriber ISDN is the phone number that is being used by the device. In the UK you can change the phone number that is linked to your SIM and so the phone number cannot be used as the index into the database. In the UK the MSISDN starts as '447 ...' and has 12 digits.

In a 2G network there is a separate Authentication centre. The Authentication Centre (AuC) provides 2 functions to the GSM network. Firstly it stores the key ( Ki) associated with the SIM. This key is only stored in the AuC and SIM and provides the mechanism for encrypting data on the wireless network. Secondly the AuC will use the key to generate a random number used in the authentication and encryption processes on the network. This authentication is done within the HSS or UDM for a 4G / 5G network

The Subscriber Identity Module (SIM) is the small card inserted into a MS to give the user access to the network. Each SIm has a unique number, IMSI. This is made up of 3 components. The Mobile Country Code (MCC), Mobile Network Code (MNC) and a unique number.

In addition each SIM will have a home number stored in the HLR. This is the Mobile Subscriber ISDN (MSISDN) which in the UK will be of 11 digits starting with 07.

The HLR is a database used by a specific operator. In a 4G and especially in a 4G / 5G network the function of the HLr / AUC is implemented as a cloud based API.