Available GSM Frequency Bands

• GSM networks around the world operate on four different bands. These bands include 850Mhz, 900Mhz, 1,800Mhz and 1,900Mhz networks. In the United States the 850Mhz band is operated by AT&T Wireless, while the 1,900Mhz band is used by T-Mobile.
  
  

GSM Sim Card Compatibility

• SIM cards are small rectangular cards that are carrier-specific. When a card is inserted into a network-compatible phone the user's phone number becomes associated with that phone. SIM cards are swappable with various phones on GSM networks. To use a SIM card in a GSM phone, that phone must be a carrier-provided phone or unlocked to work on that carrier's phone frequency (850Mhz phones will not work on 1,900Mhz networks).
  
  

Types of GSM Phones

• A single band GSM phone offers only one GSM frequency, such as 850Mhz, a dual-band phone offers two frequencies such as 850mhz and 1,900mhz. A tri-band phone offers three frequencies (850, 1,800, 1,900mhz) and a quad-band phone brings all four available frequencies. Single-band phones are rare in most part of the worlds, while quad-band phones are often referred to as "world phones" thanks to their ability to work on any worldwide GSM carrier when unlocked.
  
  

GSM Data Networks

• GSM phones first used GPRS as their main mode of data communication. GPRS is the slowest of the data networks and provides speeds of 150-170kbps. The next fastest speed is known as the "Edge" network, this network provides speed averages four to five times faster than the GPRS network. The last data network known as 3G offers speeds in the U.S. up to 7.2Mbps and is considered a "high speed" mobile data network. The most popular version of 3G use is the iPhone 3GS.
  
  

Popular GSM Networks: Worldwide

• In the United States, the AT&T Wireless and T-Mobile networks dominate the cellular market. In Canada, those networks are replaced by Fido and Rogers service. The UK, one of the biggest GSM networks, is operated by various providers including Orange, Vodafone, O2 Wireless and numerous others. China Mobile is popular in China, while Vodafone once again shows up in the Australian market. There are other carriers found in other parts of the world. In all, over 210 countries feature GSM networks and their own carriers.