The MS is having several identifiers, which it uses while communicating with the network. Those identifiers are the IMSI, MSISDN, IMEI, and P-TMSI.
In case there is an MS that is having Dual SIM, every SIM is having its own IMSI, MSISDN, IMEI, and P-TMSI.
IMSI refers to 'International Mobile Subscriber Identity', and it is a globally unique identifier for every SIM card in the world. The IMSI is the main identifier for the MS.
if you changed your SIM card for any reason, e.g. the old SIM card is damaged, the IMSI of your SIM will be changed.
The structure of the IMSI is as following,
- MCC refers to 'Mobile Country Code', and it is a unique identifier for every country in the world.
- MNC refers to 'Mobile Network code', and it is a unique code for every network inside the country.
- MSIN refers to 'Mobile Subscriber Identification Number', and it is a unique code given to every SIM card inside the network.
- The PLMN address is an address for every network in the world, it contains (MCC+MNC). PLMN refers to 'Public Land Mobile Network'.
MSISDN refers to 'MS international ISDN numbers', and it is the dial number of your phone line. It is the dial number exit in your contact list , and you give it to people in order to dial you.
When you change your SIM card, your MSISDN will not change. This is because the MSISDN identify the phone line, not the SIM card.
The structure of the MSISDN is as following,
- CC refers to 'country code', which is a unique code for every country.
- NDC refers to 'National Destination Code', and it is a unique code for every network inside the country.
- SN refers to 'Subscriber Number', and it is the number of your phone line.
IMEI refers to 'International Mobile station Equipment Identity', and it is a unique code given to the Hardware of the MS.
in case the MS is having dual SIM, every 'SIM slot' will have its different IMEI. Therefore,the MS will have two IMEIs, one for each 'SIM slot'.
When you buy a new MS, you will find the IMEI number printed on the box of the MS.
There is another identifier related to the IMEI, which is the'‘IMEISV' (IMEI software version), and it represents the software version for the software exist on the MS.
P-TMSI refers to 'Packet – Temporary Mobile Subscriber Identities'. For security reasons, the MS shouldn’t send its IMSI or MSISDN every time it interacts with the network.
Therefore, as a result of the attach procedure, the SGSN assigns the MS a temporary identifier, which is the 'P-TMSI'.
Later, we will go deeper into 'Attach procedure'.
The MS stores this P-TMSI on its SIM card, and uses it in the future interactions with the network, to identify itself, instead of its IMSI.
There is an identifier related to the P-TMSI, which is the 'P-TMSI signature'.
The SGSN assigns the 'P-TMSI signature' for the MS, and the MS stores it along side the P-TMSI on the SIM card.
The MS sends the 'P-TMSI signature' with the P-TMSI when it interacts with the network. The 'P-TMSI signature' is used for authentication purposes on the SGSN.