Technology behind a SIM card

Mobile SIM card

Today, the phone is the most important part of our life. It’s how we chat with our friends and make fun. It’s the reason we remember every single person’s anniversary. It’s also a portable camera. These days it can even be a handy replacement for a printed-out ticket or debit card. But, have you ever think that it’s actually the SIM card hiding inside that makes all the good stuff possible? That tiny chip, under the hood, is a total game-changer. Do you know, what actually is a SIM card or how does it work? In this article, we will solve all these questions that can help you know better of your phone and SIM card. So, let's dive into the deep of SIM card -

What is a SIM card?

A SIM card, also known as a subscriber identity module, is a smart card that stores data for GSM cellular telephone subscribers. Such data includes user identity, location and phone number, network authorization data, personal security keys, contact lists and stored text messages. Security features include authentication and encryption to protect data and prevent eavesdropping. The first SIM card was made in 1991 by Giesecke and Deviant of Sagem communications in France.

A SIM card is internationally identified by its Integrated circuit card identifier (ICCID), which is engraved on the body of the card. The data stored in the SIM card includes  International Mobile Subscriber Identity or IMSI, Security Authentication information, temporary information about the network, a Personal Identification Number or PIN and a Personal Unblocking code or PUK for unlocking. SIM card contains its internal memory in which stores the data, personal and financial information, identity for GSM/CDMA. Modern SIM cards allow the storage of application data that communicate with the handset or server using the SIM application toolkit. The SIM card stores network specific information to authenticate the identity of the subscriber in the network. Out of the many keys, the most important keys are ICCID, IMSI, Authentication key or Ki, Local Area Identification or LAI, and an operator specific emergency number.

The Micro sim has been invented for the latest mobile phones. The SIM also contains other data like Short Message Service Centre number or SMSC, Service Provider Name or SPN, Service Dialing Number or SDN, Value Added Service or VAS etc. The SIM comes in various data capacities ranging from 32KB to 128K and can store 250 contacts.

Type of SIM cards

There are two types of SIM cards -

GSM

GSM technology stands for Global System for Mobiles and its foundation can be credited to Bell Laboratories in 1970.  It basically uses circuit switched system and divides each 200 kHz signal into 8 25 kHz time slots and operates in 900 MHz, 800 MHz, and 1.8GHz bands. It uses a narrow band transmission technique- basically Time Division Access Multiplexing. The data transfer rates vary from 64kbps to 120kbps.

CDMA

CDMA means code division multiple access which explains about the communication channel principle that employs spread-spectrum technology and a special coding scheme which are time division multiplexing scheme and frequency division multiplexing scheme.

Formats of the SIM card

Full-size SIM

The full-size SIM (or 1FF, 1st form factor) was the first form factor to appear. It has the size of a credit card (85.60 mm × 53.98 mm × 0.76 mm). Later smaller SIMs are often supplied embedded in a full-size card that they can be pushed out of.

Mini-SIM

The mini-SIM (or 2FF) card has the same contact arrangement as the full-size SIM card and is normally supplied within a full-size card carrier, attached by a number of linking pieces. This arrangement (defined in ISO/IEC 7810 as ID-1/000) lets such a card be used in a device that requires a full-size card - or in a device that requires a mini-SIM card, after breaking the linking pieces. As the full-size SIM is no longer used, some suppliers refer to the mini-SIM as a "standard SIM" or "regular SIM".

Micro-SIM

The micro-SIM (or 3FF) card has the same thickness and contact arrangements, but reduced length and width as shown in the table above. The micro-SIM was introduced by the European Telecommunications Standards Institute (ETSI) along with SCP, 3GPP (UTRAN/GERAN), 3GPP2 (CDMA2000), ARIB, GSM Association (GSMA SCaG and GSMNA), GlobalPlatform, Liberty Alliance, and the Open Mobile Alliance (OMA) for the purpose of fitting into devices too small for a mini-SIM card. Micro-SIM cards were introduced by various mobile service providers for the launch of the original iPad, and later for smartphones, from April 2010. The iPhone 4 was the first smartphone to use a micro-SIM card in June 2010, followed by many others.

Nano-SIM

The nano-SIM (or 4FF) card was introduced on 11 October 2012, when mobile service providers in various countries started to supply it for phones that supported the format. The nano-SIM measures 12.3 × 8.8 × 0.67 mm and reduces the previous format to the contact area while maintaining the existing contact arrangements. A small rim of isolating material is left around the contact area to avoid short circuits with the socket. The nano-SIM is 0.67 mm thick, compared to the 0.76 mm of its predecessor. 4FF can be put into adapters for use with devices designed for 2FF or 3FF SIMs and is made thinner for that purpose, but many phone companies do not recommend this.

Embedded-SIM/embedded universal integrated circuit card (eUICC)

An upcoming new type of SIM is called e-SIM or eSIM (embedded SIM). It is a non-replaceable embedded chip in the SON-8 package that is soldered directly onto a circuit board. It has M2M and remote SIM provisioning capabilities.

How do SIM cards work?

  • The most important bits of data include the IMSI (International Mobile Subscriber Identity) and the authentication key that validates the IMSI. This authentication key is provided by the carrier. SIM authentication works as -

  • On startup, the phone obtains the IMSI from the SIM card and relays it to the network. Think of this as the “request for access.”

  • The network takes the IMSI and looks in its internal database for that IMSI’s known authentication key.

  • The network generates a random number, A, and signs it with the authentication key to create a new number, B. This is the response it would expect if the SIM card is legitimate.

  • The phone receives A from the network and forwards it to the SIM card, which signs it with its own authentication key to create a new number, C. This number is relayed back to the network.

  • If the network’s number A matches the SIM card’s number C, then the SIM card is declared legitimate and access is granted.

Type of data in SIM

Integrated Circuit Card Identifier (ICCID)

It is the Primary account number that has 19 digits long. The number has sections like Issuer Identification Number (IIN), Individual Account Identification, Check digit etc.

International Mobile Subscriber Identity (IMSI)

It is used to identify the individual operator network. Normally it has 109 digits. Its first 3 digits represent Mobile Country Code or MCC, the next 2 to 3 digits represents the Mobile Network Code or MNC, The next digits represent the Mobile Subscriber Identification Number or MSIN.

MSISDN

The Mobile Station International Subscriber Directory Number is intended to convey the telephone number assigned to the subscriber for receiving calls on the phone.

Abbreviated dialing numbers (ADN)

Any number and name dialed by the subscriber are saved by the ADN EF. The type of number and numbering plan identification is also maintained under this. This function works on the subscriber’s commonly dialed numbers. The ADN cannot be changed by the service provider and they can be attributed to the user of the phone. Most SIMs provide 100 slots for ADN entries.

Fixed dialing numbers (FDN)

The FDN EF works similar to the ADN because it involves contact numbers and names. With this function, The user doesn’t have to dial numbers; by pressing any number pad of the phone, he can access the contact number.

The last number dialed (LND)

The LND EF contains the number most recently dialed by the subscriber. The number and name associated with that number are stored in this entry. Depending upon the phone, it is also conceivable that the information may be stored in the handset and not on the SIM. Any numbers that may be present can provide valuable information to an investigator.

Messaging Information

Messaging is a communication medium by which text is entered on one cell phone and delivered via the mobile phone network. The short message service contains texts and associated parameters for the message. SMS entries contain other information besides the text itself, such as the time an incoming message was sent, as recorded by the mobile phone network, the sender’s phone number, the SMS center address, and the status of the entry.

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