What is a SIM card – a Technical Breakdown

What is a SIM card – a Technical Breakdown

The first SIM card was used in 1991 by Deviant and Giescke who worked at Sagem communications in France. Currently, a SIM is a very crucial tool that is used in mobile phones. It is used to activate connections and make links with servers as well as in many electronic and electrical projects. The Subscriber Identity Module contains an integrated circuit that stores the International Mobile Subscriber Identity (IMSI) and keys used to classify the subscriber on the communications systems. The SIM is implanted in a removable card that can be inserted into different phones. 

A SIM card has an internal memory that stores data, financial and personal details, and identity for GSM/CDMA. The modern SIM cards can store application data that connects with the server or phone using the SIM toolkit. 

The encrypted information stored in a SIM includes: 

• A serial number called ICCID 
• Security authentication data 
• Temporary network information 
• International Mobile Subscriber Identity (IMSI) 
• Personal Identification Number (PIN) 
• PIN Unblocking Key (PUK) 

SIM cards store network related information to verify the character of the subscriber within the network. Among the many data keys, the most crucial keys are IMS, ICCID, Authentication key, Local Area Identification (LAI), and a network specific emergency calls number. The SIM also stores other information like Short Message Service (SMS) Centre number, Service Provider Name (SPN), Service Dialling Number (SDN), Value Added Service (VAS), etc. The SIM cards have various data sizes ranging from 32KB to 128K and storage capacity of up to 250 contacts. 

Functions of SIM card: 

A SIM card performs the following roles: 

1. It is used to identify the subscriber: The IMSI pre-set on the SIM card is used to identify a subscriber. A mobile number represents each IMSI and used on the HLR to enable a subscriber to be recognised. 

2. Authenticating a subscriber: This is the process that uses the authentication algorithm on a SIM card to provide a unique response to each subscriber, based on the IMSI in the SIM and RAND supplied by the network. When this response matches with values calculated on the network a subscriber is permitted to access the network and to utilise the services provided by the mobile service provider. 

3) Storage: The SIM can be used to store phone numbers and messages. 

Types of SIM Card: 

There are two main types of SIM cards. These include: 


The GSM technology is the abbreviation for Global System for Mobiles whose invention goes all the way back to Bell Laboratories in 1970. This technology uses circuit switched system and splits every 200 kHz signal into 8 25 kHz slots and operates between 900 MHz to 1.8GHz bands. It is based on a narrow band transmission system; Time Division Access Multiplexing. The rate of data transfer ranges from 64 to 120kbps. 


CDMA is the abbreviation for Code Division Multiple Access. This technology capitalises on communication channel principle which uses spread-spectrum technology and a unique coding system.


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