A 3G SIM card has data that allows a 3G cell phone access to a 3G network. If the cell phone is incapable of accessing a 3G network, the 3G SIM card still functions as a regular SIM card. The difference between a 3G SIM card and a regular SIM card is data and a special label.
Subscriber Identity Module
A subscriber identity module (SIM) card is used to store customer-specific information in a cell phone. Security codes, contact lists and network information are stored in the SIM and can be moved from one cell phone to another. The cards come in three sizes. The standard size is 86 by 54mm, mini is 25 by 15mm and micro is 15 by 12mm. Each SIM has a unique ID number that ties it to the owner.
3G Cellular Standard
The International Telecommunication Union (ITU) established the cellular standard for the third generation of mobile communications in the year 2000 under the brand IMT-2000. A 3G (third generation) capable device accesses the radio spectrum between 400 MHz and 3 GHz and has data transmission speeds of 2 Mbits per second at walking pace and 348 kbits per second in moving vehicles. The 3G standard is compatible with multiple radio access protocols used by cell phone service carriers.
The 3G standard functions with Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA) protocols. FDMA is an analog system where each user is assigned a frequency channel that is closed to other calls until the connection is closed. TDMA splits frequencies into time slots to allow multiple users to share the same channel with a base station switching users at intervals. CDMA allows users access to all channels at once with unique codes to tell other users apart.
IMT-2000 Radio Interfaces
IMT-2000 defines five radio interfaces that can be used by CDMA, TDMA and FDMA-based networks to ease the adoption of the 3G standard. Direct Spread (IMT-DS) and Multicarrier (IMT-MS) interfaces are designed for CDMA networks. Time-Code (IMT-TC) can be used in CDMA and TDMA networks. The Single Carrier (IMT-SC) interface is for TDMA-only networks. The fifth interface, Frequency Time (IMT-FT), is shared by TDMA and FDMA-based networks.
Switching Out SIM Cards
A SIM card from one cell phone can be used in another cell phone if both are from the same carrier. Carriers lock their cell phone with data stored on the SIM card, so a SIM card from one carrier will not function in a cell phone from another carrier. 3G SIM cards can be inserted into non-3G cell phone and function as a standard SIM card, but a SIM card originally from a non-3G cell phone will not properly function in a 3G cell phone.