A SIM card contains unique information that identifies it to the mobile network, allowing the subscriber to use the communication features of a mobile device. Without the SIM card inserted, a GSM cell phone cannot make calls, send SMS messages or connect to mobile Internet services (3G or 4G, etc.)
GSM Cell Phones
GSM, which stands for Global System for Mobile Communications, is the type of cell phone network used in a large percentage of the world outside of the U.S. In GSM cell phones, the SIM Card is comprised of the Subscriber Identity Module (SIM) and a shaped plastic surround. The important part is a small integrated chip which is able to be read by the mobile device it is inserted into, and contains a unique identification number, the phone number and other data specific to the user it is registered to.
The SIM also contains a small amount of memory which can store up to 250 contacts, some SMS messages and other information used by the carrier who supplied the card. A SIM card can usually be removed from one handset and inserted into any other compatible handset, allowing the user to switch devices without losing their contacts data, having to change their phone number or start a new mobile contract. This isn't generally possible if the cell phones originated in different countries, as cellular network frequencies vary between countries.
In many countries, SIM cards and handsets are locked to the carrier they are purchased from. This means that although a SIM card from a carrier will work in any handset sold by that same carrier, it will not work in a handset sold by a different carrier. It is usually possible to unlock a cell phone with help from the carrier.
CDMA Cell Phones
Originally, cell phones which used the CDMA network rather than the GSM network did not use a removable SIM Card. Instead, the handset itself would contain the identifying numbers and other information. This meant that CDMA handsets could not easily be switched from one carrier network to another, and could not be used in many countries outside the US. More recently, CDMA phones have begun to feature a Removable User Identity Module (R-UIM). This card looks almost identical to a SIM card and will work in most GSM handsets.
SIM Card Sizes
The SIM card size that most people use is called the Mini SIM, but Micro SIM cards are increasingly being used in mobile devices, including some models of the iPhone and the Samsung Galaxy S3. The first SIM cards were roughly the size of a credit card, and many Mini SIM's are still supplied with this large, Full SIM surround. Both Mini and Micro SIM cards feature a cut off corner to help prevent incorrect insertion into the phone or tablet. You can see the dimensions of the different types of SIM card below.
- Full SIM - 85mm x 53mm
- Mini SIM - 25mm x 15mm
- Micro SIM - 15mm x 12mm
Despite differences in size, all SIM cards contain the same types of identifying numbers and information on the small chip. Different cards do contain different amounts of memory space, but this has nothing to do with the physical size of the card. A mini SIM can actually be trimmed down using scissors to turn it into a micro SIM, as long as it is only the plastic surround which is cut.