A SIM card, which stands for Subscriber Identity Module, is a small removable chip that allows devices like mobile phones to connect to a cellular network. SIM cards are most commonly associated with cellular phones and mobile devices, but can also be used in other devices like some satellite phones, computers, cameras and other Internet of Things (IoT) devices.
When a SIM card is inserted into a compatible device, it enables the device to connect to the wireless network of the SIM card’s issuing carrier. The SIM card stores data that authenticates the device to the carrier’s network and provides account details so the device can access the account associated with the SIM card.
Some of the key data stored on a SIM card includes:
- International Mobile Subscriber Identity (IMSI) – A unique number that identifies the subscriber
- Authentication Key (Ki) – A secret key used to authenticate the SIM card to the mobile network
- Local Area Identity (LAI) – The current location of the SIM card
- Operator-Specific Emergency Number – Emergency service numbers for the network
- SMSC (Short Message Service Center) number – The number used to send and receive SMS messages
- MSISDN (Mobile Station Integrated Services Digital Network) number – The subscriber’s phone number
- PIN (Personal Identification Number) and PUK (PIN Unlock Key) – Security codes
- Contacts list – Phone book entries
- Text messages
In addition to these standard data sets, a SIM may also contain additional carrier-specific applications like value-added services. The memory on a SIM card is limited, so the amount of data that can be stored varies by card.
IMSI
The International Mobile Subscriber Identity or IMSI is one of the most crucial pieces of data stored on a SIM card. The IMSI is a unique 15-digit number that identifies the subscriber and confirms their eligibility to access the carrier’s network.
It serves a similar purpose to an account number and allows the network to identify the specific SIM card being used. The IMSI is tied directly to the contract associated with the subscriber. It remains the same even if the subscriber changes their device or phone number. The IMSI is stored on the SIM card and is sent to the network when the SIM-enabled device connects.
The IMSI is used by the carrier to authenticate subscribers and grant them access to the network services under their subscription plan. No two SIM cards will ever have the same IMSI number. It begins with a 3-digit Mobile Country Code (MCC) that identifies the country, followed by the Mobile Network Code (MNC) which identifies the mobile network. The remaining digits are the Mobile Station Identification Number (MSIN) which identifies the individual subscriber.
Authentication Key (Ki)
The Authentication Key or Ki is a 128-bit secret key that is shared between the SIM card and the mobile network. It is used to mutually authenticate the network and the subscriber’s SIM card each time the device connects to the network. This authentication process confirms that the SIM card is valid and should be granted access.
The Ki key is stored in a protected memory location on the SIM card that cannot be accessed from the device directly. When the device connects, the carrier’s authentication center (AuC) will generate a random number and perform an authentication algorithm using the Ki. The SIM card has an identical copy of the Ki and performs the same calculation. If the results match, the SIM card is authenticated.
The Ki is randomly generated when the SIM card is manufactured and loaded by the mobile network operator. It is a cryptographic key that prevents fraud by making it extremely difficult to spoof a connection using an unauthorized SIM card. If the mobile device provides an incorrect result for the algorithm calculation using the Ki, authentication fails and network access is denied.
Local Area Identity (LAI)
The Local Area Identity or LAI represents the specific location that the SIM card is connecting from. It identifies the country, mobile network and location area. When a device connects, it provides the LAI to identify its approximate geographical location.
The LAI is composed of the MCC (Mobile Country Code), MNC (Mobile Network Code), and Location Area Code (LAC). The LAC specifies a geographical local area within the operator’s network. For example, a city or borough. Combining this with the country and network codes narrows down the location.
This allows the carrier to route calls and messages to the device based on its current location. As the device moves between different LACs, the SIM card updates the network with its new location. The granularity of the LAC depends on the concentration of cell towers in a given region. Rural areas may have coarser LACs than urban areas.
The mobile network uses the LAI to efficiently route communications directly to the cell where the device is currently located, without having to broadcast across all cells. If an incoming call or message arrives, the LAI ensures it is sent right to the device’s current cell.
Operator-Specific Emergency Number
Most SIM cards contain an emergency services number assigned by the issuing operator. This allows users to call emergency services even without an activated mobile subscription. Legislations like the Wireless Communications and Public Safety Act in the United States mandates that users must be able to call 112 (Europe), 911 (North America) or 000 (Australia) to reach emergency services.
The SIM stores a list of emergency number(s) specific to the carrier and will allow calls to these numbers even when outgoing calls are restricted or service has lapsed. For example, an AT&T SIM card would contain 911, while a SIM from Deutsche Telekom in Germany stores 112. This provides access to emergency services independent of account status.
In some cases, the emergency numbers are hard-coded into the device firmware. But storing them on the SIM as well ensures users can call emergency services regardless of device or SIM card changes. The SIM may also contain additional emergency numbers beyond the mandated 112/911/000 standard.
SMSC Number
The SIM card contains the Short Message Service Center (SMSC) number which allows the device to send and receive text messages over the network. The SMSC number identifies the network’s message center which handles routing and storing SMS messages when devices are unavailable.
When a device sends a new text message, it transmits the SMSC number from the SIM card to identify where to send the message. Similarly, incoming SMS messages are routed to the device’s SIM using the SMSC number. If the recipient is powered off, the SMSC will hold the message until it can be delivered.
The SMSC number is assigned by the mobile operator and varies between different carriers and countries. With the SMSC, any changes to the subscriber’s phone number or device do not impact text messaging. Messages are routed using the persistent SIM card’s SMSC address instead.
MSISDN
The MSISDN or Mobile Station Integrated Services Digital Network number is simply the cell phone number tied to the SIM card and subscriber account. It is the number that others can use to call and text the device.
The MSISDN is stored on the SIM card so that the subscriber can be identified by their number when connecting to the network. However, unlike the IMSI, the MSISDN can easily be changed without replacing the SIM card, through a process called porting. The IMSI remains constant while MSISDN portability allows keeping the same number when switching carriers.
When a call or text is placed to a device, the network identifies the recipient based on the MSISDN. The IMSI then maps the MSISDN to the specific SIM card and subscriber account to route the communication. The MSISDN also determines the phone number displayed on the recipient’s caller ID.
PIN and PUK
The PIN or Personal Identification Number is a 4-8 digit passcode that protects access to the SIM card. It is typically requested each time the mobile device is powered on or the SIM is inserted. Entering the correct PIN will unlock the SIM card and allow normal usage of services.
The PIN prevents unauthorized access if the SIM card is removed and inserted into another device. The user can set and modify their PIN code through device settings. If the incorrect PIN is entered too many times, the SIM will become blocked until the correct PUK code is entered.
The PUK or PIN Unblocking Key is an 8-digit master code assigned by the carrier to unblock a blocked PIN. If the incorrect PIN has been entered three times in a row, the SIM will require the PUK before allowing any further PIN attempts. The PUK can reset the PIN and unblock the SIM.
Both the PIN and PUK serve as security credentials protecting access to the SIM card. Since all subscriber details and service access are controlled through the SIM, this prevents fraudulent use of a lost or stolen SIM card. The PIN and PUK are unique codes only known to the subscriber.
Contacts List
In addition to subscriber and network data, SIM cards can also store a contacts list containing names and phone numbers. Storage space on the card is limited but can typically hold 100-250 contacts depending on the SIM size.
Storing contacts on the SIM makes them easily portable across different devices. The user can access the same list of contacts after swapping the SIM into a new phone. Contacts are stored using .vcf files on a DF_TELECOM ISO directory on the SIM card.
New SIM cards start with empty contact lists that get populated as users add more entries. Users can manage the contacts through their device address book settings which interface with the storage on the SIM. Any changes or additions are synced to the SIM card.
Text Messages
The SIM card can also store text messages, though the number depends on available storage capacity. SIM cards have space allocated for storing SMS messages, usually 20-50 messages. New SIMs start empty until text exchanges occur.
Incoming and sent text messages get copied and saved to the message storage on the SIM card. Users can view the stored history and the messages remain available even when swapping SIMs across devices. As with contacts, SMS storage enables maintaining message history across phones.
Once the allocated space fills up, older messages start getting overwritten by newly received ones in chronological order. The SIM card helps provide a unified text messaging experience across changing devices while the network’s SMSC handles the actual routing.
Other Data
In addition to core network access data, contacts and messages, SIM cards may contain supplemental software applications. This includes value-added services provided by the mobile operator that run using the SIM card memory and processor.
Examples include news/entertainment apps, games, and network configuration tools. These optional apps can access subscriber details and account information stored on the SIM to provide customized experiences. Carriers can develop and deploy their own apps tailored to their offerings.
Because apps take up additional limited space, many carriers opt to make minimal use of SIM-based apps outside of core network functions. With internal space ranging from 16KB up to 128KB, most SIM cards focus on the essential subscriber data needed for network registration and authentication.
Summary Table of Data Stored on a SIM Card
| Data | Description |
|---|---|
| IMSI | Unique identification number for the subscriber |
| Ki | Authentication key used to validate SIM to network |
| LAI | Current geographical location of SIM card |
| Emergency Number | Carrier-assigned emergency call number |
| SMSC | Address of SMS message center for texts |
| MSISDN | Subscriber phone number |
| PIN | Personal Identification Number security code |
| PUK | PIN Unlock Key to unblock blocked PIN |
| Contacts | Stored phone book entries |
| Text Messages | Received and sent text messages |
| Applications | Optional carrier apps and services |
Conclusion
The SIM card plays a crucial role in authenticating devices to mobile networks and providing access to subscriber accounts and phone numbers. Though SIM cards have limited storage, they contain a range of important data like the IMSI, Ki, contacts, text messages, and other carrier-specific applications. Standardized SIM data allows seamless connectivity and portability across different mobile providers and devices.