A “this is as simple as it gets” version of how the SIM interacts with the handset and network.
Every SIM is created in a secure production facility, and each SIM contains an IMSI (identifier on the network), ICC-ID (SIM serial number) and MS-ISDN (phone number). All of these are received via a secure transport key as encrypted input from the host mobile network. This information is stored within the SIM fields or “drawers” which contain both static and dynamic information.
A network authentication key (Ki) is also generated in this process. Upon production an output file is securely transported from the SIM to the host network. The host network loads the output file, making sure it matches the input file, as it will otherwise be rejected. With the output file now residing in decrypted form, the SIM is enabled to authenticate on the home network using the IMSI and the secret Ki.
The phone and the SIM negotiate the speed of communication and the voltage to use.
The mobile phone reads the service table fields from the SIM, which tells the SIM of the abilities and settings indicated in the SIM. Examples are PIN active Y/N, Pin Number, Barred dialing numbers, SMS Centre number, LP (language profile), MS-ISDN (phone number), name of the network to display and much more.
The IMSI (International Mobile Subscriber Identity) is the unique identifier within the SIM and contains the MCC (mobile country code) and MNC (mobile network code). The IMSI is installed using the output file on the mobile network AuC (authentication centre).
The mobile phone sends out a “hello world” containing the IMSI and the Ki for authentication, which is guided to the correct network and rejected by the wrong networks. To ensure the security, the IMSI and Ki pair is encrypted into a T-IMSI.
After the network recognizes the SIM as one of its own, and it matches the preloaded information on the network, the SIM is asked for authentication in a SIM calculated process, RunGSM. Answering the network with the correct answer to RunGSM allows the SIM to authenticate on the network and receive its rights. These rights given on the network can be, for example, roaming allowed, SMS allowed, calls allowed and more.
Now you are ready!
Now your phone is booted and authenticated with the network, and you are ready to use it.
During mobile/SIM usage, you will be logged into the mobile network, and your SIM will be authenticated with the network using RunGSM authentication on a variable basis.
If it fails to do so, you have been rejected from the network. A fail can, for example, happen when your subscription is cancelled and you are rejected from the network; alternatively your SIM information can be deleted from the network resulting in your SIM to be rejected on the next network refresh.
About our associate editor
Glenn Eric Tornow is our associate editor at esim.net and provides a different angle on SIM and eSIM. Glenn is a blackbelt mobile technologist and has hands-on experience from the worldwide market for mobile technology with a focus on SIM and eSIM since the mid-1990s. https://www.linkedin.com/in/glennerictornow/
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