SIM Card – How to Identify a Mobile Subscriber?

A brief introduction to how sim cards work.

Posted by Umut Canbolat on November 17, 2018 · 3 mins read

SIM Card

SIM cards showed up for the first time with 2G GSM technology and stand for “Subscriber Identity Module”. It stores IMSI number, ICCID, PIN/PUK codes, security authentication and ciphering information and the list of the subscriber’s services. Let’s go for some details of these acronyms.

Subscriber Identity Module

Mini, Micro and Nano SIM

IMSI (International Mobile Subscriber Identity) IMSI is a unique number and identifies the subscriber. IMSI is sent to the network as rarely as possible to prevent the subscriber from being tracked by eavesdroppers. Instead, randomly generated TMSI is sent.



IMSI consists of MCC, MNB and MSIN.

  • MCC: Mobile Country Code
  • MNC: Mobile Network Code
  • MSIN: Mobile Subscription Identification Number

For example, 286-01-1111111111 could be the IMSI of a subscriber from Turkey (286) and belongs to Turkcell (01).

A full list of the MCC and MNC codes can be found here:

TMSI (Temporary Mobile Subscriber Identity)

TMSI is the identity that is most commonly sent between the MS and the network. It is randomly assigned by VLR to every mobile device in the area it controls. VLR attaches TMSI to a mobile when it is switched on and stores it until the MS leaves its geographical area.

TMSI is sent to the network instead of IMSI in order to avoid subscriber from being identified and tracked by eavesdroppers on the radio interference.

ICCID (Integrated Circuit Card ID)

ICCID is a unique ID which identifies SIM card. It is stored in the SIM and also printed on.


ICCID on different SIM Cards


PIN (Personal Identification Number) is a password that allows the phone of a subscriber to access the information of a SIM.

PUK (PIN Unblocking Key) is asked to subscribers to recover the PIN code when they forget.

Authentication and Ciphering Keys

A SIM card has its own authentication key and ciphering algorithms to calculate session keys in order to authenticate and MSC successfully. The subscriber authentication key (Ki) is never transmitted over the radio channel.

MSISDN (Mobile Station International Subscriber Directory Number)

MSISDN is the telephone number that we dial when we want to call someone. MSISDN is also a unique number which identifies a subscription in a mobile network.

MSRN (Mobile Subscriber Roaming Number)

MSRN is a temporary telephone number assigned by VLR. MSC’s need MSRN number to route calls. It can be assigned to a cell phone when it is switched on and when location update occurs. Or it can be assigned on a per call basis.