Currently, there are several types of networks in the world using the GSM standard, but at different frequencies.
- The GSM-900 is the most common in Europe and the rest of the world. Its extension is E-GSM.
- The DCS-1800 operates in the 1,800-MHz band and is used mainly in Europe, usually to cover urban areas. It was also introduced to avoid saturation problems with the GSM-900.
- The PCS-1900 is used primarily in North America.
- The GSM-850 is under development in America.
- The GSM-400 is intended for deployment in Scandinavian countries in the band previously used for the analog Nordic Mobile Telephony (NMT) system.
The system is based on frequency-division duplex (FDD), which means that the uplink (radio link from the mobile to the network-that is, mobile transmit, base receive), and downlink (from the network to the mobile-that is, base transmit, mobile receive) are transmitted on different frequency bands. For instance, in the 900-MHz E-GSM band, the block 880-915 MHz is used for transmission from mobiles to network, and the block 925-960 MHz is used for the transmission from network to mobiles.
Operators may implement networks that operate on a combination of the frequency bands listed above to support multiband mobile terminals. There are different ways of sharing the physical resource among all the users in a radio system, and this is called the multiple-access method. The multiple-access scheme defines how simultaneous communications share the GSM radio spectrum. The various multiple-access techniques in use in radio systems are frequency-division multiple access (FDMA), TDMA, and code-division multiple access (CDMA). GSM is based on both FDMA and TDMA techniques.
FDMA consists in dividing the frequency band of the system into several channels. In GSM, each RF channel has a bandwidth of 200 kHz, which is used to convey radio modulated signals, or carriers. Each pair of uplink/ downlink channels is called an absolute radio frequency channel (ARFC) and is assigned an ARFC number (ARFCN). The mapping of each ARFCN on the corresponding carrier frequency is given in.
TDMA is the division of the time into intervals: within a frequency channel, the time is divided into time slots. This division allows several users (eight) to be multiplexed on the same carrier frequency, each user being assigned a single time slot. A packet of data information, called a burst, is transmitted during a time slot. The succession of eight time slots is called a TDMA frame, and each time slot belonging to a TDMA frame is identified by a time slot number (TN), from 0 to 7.
