GSM frequency bands
GSM frequency bands or frequency ranges are the cellular frequencies designated by the ITU for the operation of GSM mobile phones and other mobile devices. GSM frequency usage around the world. A dual-band 900/1800 device is required to be compatible with most networks apart from deployments in ITU Region 2. GSM-900, EGSM/EGSM-900 and GSM-1800. GSM-900 and GSM-1800 are used in most parts of the world (ITU-Regions 1 and 3): Africa, Europe, Middle East, Asia (apart from Japan and South Korea where GSM has never been introduced) and Oceania. In common GSM-900 is most widely  used. Fewer operators use GSM-1800. Mobile Communication Services on Aircraft (MCA) uses GSM-1800. In some countries GSM-1800 is also referred to as "Digital Cellular System" (DCS). GSM-850 and GSM-1900. GSM-1900 and GSM-850 are used in most of North, South and Central America (ITU-Region 2). In North America, GSM operates on the primary mobile communication bands 850 MHz and 1900 MHz. In Canada, GSM-1900 is the primary band used in urban areas with 850 as a backup, and GSM-850 being the primary rural band. In the United States, regulatory requirements determine which area can use which band. The term "Cellular  is sometimes used to describe GSM services in the 850 MHz band, because the original analog cellular mobile communication system was allocated in this spectrum. Further GSM-850 is also sometimes called GSM-800" because this frequency range was known as the "800 MHz band" (for simplification) when it was first allocated for AMPS in the United States in 1983. In North America GSM-1900 is also referred to as Personal Communications Service (PCS) like any other cellular system operating on the "1900 MHz band". Frequency mixing between GSM 900/1800 and GSM 850/1900. Some countries in Central and South America have allocated spectrum  in the 900 MHz and 1800 MHz bands for GSM in addition to the common GSM deployments at 850 MHz and 1900 MHz for ITU-Region 2 (Americas). The result therefore is a mixture of usage in the Americas that requires travelers to confirm that the devices they have are compatible with the bands of the network at their destination. Frequency compatibility problems can be avoided through the use of multi-band (tri-band or, especially, quad-band) device. The following countries are mixing GSM 900/1800 and GSM 850/1900 bands: GSM-450. Another less common GSM version is GSM-450. It uses  the same band as, and can co-exist with, old analog NMT systems. NMT is a first generation (1G) mobile system which was primarily used in Nordic countries, Benelux, Alpine Countries, Eastern Europe and Russia prior to the introduction of GSM. The GSM Association claims one of its around 680 operator-members has a license to operate a GSM 450 network in Tanzania. However, currently all active public operators in Tanzania use GSM 900/1800 MHz. There are no publicly advertised handsets for GSM-450 available. Very few NMT-450 networks remain in operation. Overall, where the 450 MHz NMT band has  been licensed, the original analogue network has been closed, and sometimes replaced by CDMA. Some of the CDMA networks have since upgraded from CDMA to LTE (LTE band 31). Multi-band and multi-mode phones. Today, most telephones support multiple bands as used in different countries to facilitate roaming. These are typically referred to as multi-band phones. Dual-band phones can cover GSM networks in pairs such as 900 and 1800 MHz frequencies (Europe, Asia, Australia and Brazil) or 850 and 1900 (North America and Brazil). European tri-band phones typically cover the 900, 1800 and 1900 bands giving  good coverage in Europe and allowing limited use in North America, while North American tri-band phones utilize 850, 1800 and 1900 for widespread North American service but limited worldwide use. A new addition has been the quad-band phone, also known as a World Phone, supporting at least all four major GSM bands, allowing for global use (excluding non-GSM countries such as Japan, South Korea and as well countries where 2G system was shut down to release frequencies and spectrum for LTE networks like Australia (since 2017), Singapore and Taiwan (since 2018). There are also multi-mode  phones which can operate on GSM as well as on other mobile phone systems using other technical standards or proprietary technologies. Often these phones use multiple frequency bands as well. For example, one version of the Nokia 6340i GAIT phone sold in North America can operate on GSM-1900, GSM-850 and legacy TDMA-1900, TDMA-800, and AMPS-800, making it both multi-mode and multi-band. As a more recent example the Apple iPhone 5 and iPhone 4S support quad-band GSM at 850/900/1800/1900 MHz, quad-band UMTS/HSDPA/HSUPA at 850/900/1900/2100 MHz, and dual-band CDMA EV-DO Rev. An at 800/1900  MHz, for a total of 'six' different frequencies (though at most four in a single mode). This allows the same handset to be sold for AT&T Mobility, Verizon, and Sprint in the U.S. as well as a broad range of GSM carriers worldwide such as Vodafone, Orange and T-Mobile (Excluding-US), many of whom offer official unlocking.