GSM SIM Card

GSM technology has live with us since 20 years ago. I bet, only a small part of GSM (or mobile communication) user understand how could GSM SIM card works and make us able to talk with other people at different places everywhere, send short message, browsing the internet, or even show your account balance from your cellphone display. But, don't you know, there are several important process before SIM card could help you to do some cellular activity (like making call or sending SMS)? It was called starting process, how SIM card connected with the tower/BTS (Base Transceiver Station).

Now let's talk about GSM SIM card. In that small chip, Subscriber Identity Module (SIM) loaded with data. This data is used to authenticate and identify subscribers on the network. In simple word, 'network' refers to an area or zone which covered by mobile operator's signal.

So, how does this thing work? At first, when you turn your cellphone on, it obtains IMSI from the SIM card. By mobile operator, IMSI (International Mobile Subscriber Identity) is used for identifying SIM card. BTS (Base Transceiver Station) which throw mobile operator's signal use this IMSI to connect or communicate with cellphone.

Okay, back to the process. Once the cell phone get IMSI, it will be passed to the mobile operator for requesting access and authentication. The mobile phone may have to pass a PIN to the SIM card before the SIM card will reveal this information. Then, operator search its database for the incoming IMSI and SIM card's Ki. Ki (Authentication Key) is used in authenticating the SIM. Each SIM holds a unique Ki assigned by mobile operator during personalization process. Mobile operator also store Ki in their database.

After finding IMSI and it's associated Ki in database, operator then generates a Random Number (RAND) and signs it with Ki and IMSI, and then computing another number known as Signed Response (SRES_1). Mobile operator then sends this RAND to the cellphone, which passes it to the SIM card. In the next process, SIM card signs RAND with it's Ki and producing SRES_2. SIM gives this SRES_2 to mobile phone along with encryption key. Cellphone passes SRES_2 on to the mobile phone operator.

Operator then compares computed SRES_1 with computed SRES_2 from mobile phone. If this two number match, the SIM is authenticated. From this point, mobile phone granted access to the operator's network. Cellphone is allowed to catch signal from the operator.