What Is a SIM Card?

A Subscriber Identity Module (SIM) card is a portable memory chip used mostly in cell phones that operate on the Global System for Mobile Communications (GSM) network. These cards hold the personal information of the account holder, including his or her phone number, address book, text messages, and other data. When a user wants to change phones, he or she can usually easily remove the card from one handset and insert it into another. SIM cards are convenient and popular with many users, and are a key part of developing cell phone technology.

Activating a SIM Card

Since all of a user's data is tied to the SIM card, only it needs to be activated when the person opens an account with a cell phone service provider (also called a carrier). Each card has a unique number printed on the microchip, which the carrier needs to activate it. In most cases, the phone's owner can go either to the carrier's website and enter this number in the appropriate tool or call the service provider directly from another phone to get it turned on. SIM cards are tied to a particular carrier and can only be used with a service plan from that carrier.

Swapping Handsets

One of the biggest advantages of SIM cards is that they can easily be removed from one mobile phone and used in any other compatible phone to make a call. This means that, if the user wants to buy a new handset, he or she can activate it quickly by inserting his or her old SIM card. The user's phone number and personal information is carried on the card, so there's no need to do anything else to transfer this information. Most phone applications (apps) are stored in the phone's memory or secure digital (SD) card, however, so they will not be transferred to the new handset.

   Some cell phone carriers lock their handsets; this means they will only work with SIM cards from that carrier. This is especially common in the US, where service providers sell the handsets at a discount in exchange for the consumer signing a long-term contract for service. Handsets can be unlocked with the right code, however, although the exact process varies by manufacturer and model. Any SIM card can be used with an unlocked phone. Phones without a card, or with one that is not compatible, can typically only be used for emergency calls.

Pre-Paid SIMs

Pre-paid SIM cards are also available, allowing a phone that's locked to the pre-paid carrier's network or an unlocked phone to be used without a long-term contract. This type of card is useful for people who don't want to be tied to one carrier, or who want to try out a service provider before committing. It's especially useful for international travelers, who can purchase a local SIM card to use their phone abroad. This allows the user to keep the same phone - with the apps and other settings that are stored in it - without paying international roaming charges from the carrier back home. It also gives the traveler a local phone number, making it less expensive for people to call the traveler from within the same country.

SIM Cards Sizes

SIM cards are made in three different sizes to accommodate different devices. Most phones use mini-SIM or micro-SIM cards, which are quite small - the mini is 25 mm by 15 mm (0.98 in by 0.59), and the micro is 15 mm by 12 mm (0.59 in by 0.47 in). Full-sized cards are much larger, 85.6 mm by 53.98 mm (3.37 in by 2.13 in), and are too big for most phones. All cards are only 0.76 mm (0.03 in) thick, however, and the microchip contacts are in the same arrangement. This means that, with the proper adapter, the smaller cards can be used in devices designed for larger ones.

Security

A SIM card offers security for both the user's data and his or her calls. The cards can be locked, meaning that only someone who has the correct personal identification number (PIN) can use the card. If the phone is stolen, the thief cannot use a locked SIM or get any information off of it without the PIN.

   In addition, the card has a secret authentication code and an encryption key that protect the phone's transmissions. Although it is possible to "clone" a cell phone on the GSM network and thus steal service, it's much more difficult than it is to clone a phone on the competing Code Division Multiple Access (CDMA) network. Because of the way the encryption information is transmitted to the carrier, it's usually necessary to have physical access to the SIM card in order to copy it.

Other Cell Phone Technologies

Phones that operate on the CDMA network do not use SIM cards; instead, most save the phone number and other identifying information in the handset itself. While this can be less convenient for users, CDMA is most common in the US, where handsets are usually heavily subsidized and users may have less incentive to switch phones frequently. In addition, some carriers, mostly in Asia, do use their own removable card format, called a Re-Useable Identification Module (RUIM).

   As technology changes, however, more cell phone providers are upgrading to 3GPP Long Term Evolution (LTE), which is based on GSM technology. This means that some sort of SIM card is likely to be required for devices that use this network.