SIM

SIM is an acronym for Subscriber Identity Module. It is a card used in Mobile telephones. The first SIM card was made in 1991 by Munich smart card maker Giesecke and Devrient who sold the first 300 SIM cards to the Finnish wireless network operator Radiolinja.

SIM is an integrated circuit that safely stores the International Mobile Subscriber Identity (IMSI) and other forms of related identification to authenticate subscribers on mobile telephony devices such as; Computer and mobile phone. When this circuit is embedded into a removable plastic card it is called SIM card.

SIM cards also vary in the amount of data they can hold. It varies between 16KB, 32KB, 64KB and 128K. but it is enough to hold your: cell phone number, text messages, PIN lock address or contact book, service features and preferences, rate plan and LAI (Location Area Identity). There are three operating voltage for SIM cards: 5V, 3V and 1.8V. The SIM is also in different sizes: Full-size SIM (1FF); Mini-SIM (2FF); Micro-SIM (3FF); Nano-SIM (4FF).

The mobile phone can also be locked to a specific SIM card by some networks, in this case no other SIM card can work in the phone except when it has been unlocked and this happens mostly in countries where phones are subsidized (such countries include United states, Canada, UK, Poland etc.). But in countries where phones are not subsidized, phones are not locked to its SIM card, so the users can switch SIM at their convenient time (such countries include Nigeria, India, etc.).