What is a Sim Card

What are the SIM Cards?

Mobile phone terminology and technology can be  really confusing for those who are not extremely well-versed in electronic  applications and devices. The vast majority of consumers and mobile phone  owners ignore the way they work in general, and are satisfied with the fact  that their handsets can perform a few things that they need in their everyday  life. Researches show that most people ignore completely what the SIM card  does; all of us know the term and we have all installed a SIM card at least once, but for most of us their usage remains a  bit of a mystery.

SIM cards are actually the most important  identifying factor of the mobile phone as they carry an array of vital  information for the mobile phone. All Networks and cellular providers use the  SIM cards so as to identify the phone and provide the needed information for  its good function. SIM stands for Subscriber Identity Module and stores the so  called service subscriber key, also known as IMSI. This is what carriers use to  identify the particular subscriber of a mobile device and consists of fifteen  digits.

The format of the IMSI goes as follows: The  three first digits represent the Mobile Country Code (MCC), the two next ones  the Mobile Network Code and the final ten digits the mobile station  identification number.
SIM cards are actually very smart cards, which  means that they have a unique ICCID number based on the ISO/IEC 7812. Although  the maximum length of this number consists of 20 digits, most carriers follow  the 19 digits pattern; The ICCID is composed of these subparts: An Issuer  Identification number, which can't be more  than 7 digits, the major Industry Identifier (MII), 2 digits, the unique  country code, 1-3 digits and an issuer identifier which is mostly variable.

SIM cards can be used in all types of phones:  actually when you switch cards between different handsets all your calls and  messages will be received by the phone that contains the SIM card, even if it's not the one that you used when buying the SIM  card. SIM cards contain many useful for the user information as well, as they  can store phone numbers and messages and keep track of the latest received  calls or the called numbers.

Until recently most mobile phones could carry  only one SIM card, but the last couple of years there are many handsets that  are Dual SIM ready, which means that allow the usage of two SIM cards at the  same time; this is something really useful for professionals who want to  combine the business and personal numbers in one handset.