2G - Second Generation networks
Skip content2G - Second Generation mobile telephone networks were the logical next stage in the development of wireless systems after 1G, and they introduced for the first time a mobile phone system that used purely digital technology. The demands placed on the networks, particularly in the densely populated areas within cities, meant that increasingly sophisticated methods had to be employed to handle the large number of calls, and so avoid the risks of interference and dropped calls at handoffs. Although many of the principles involved in a 1G system also apply to 2G - they both use the same cell structure - there are also differences in the way that the signals are handled, and the 1G networks are not capable of providing the more advanced features of the 2G systems, such as caller identity and text messaging.
One of the successful 2G digital systems is GSM, a mobile phone standard that was developed during the 1980s, and is currently under the control of the ETSI. With the benefit of a well-specified system, commercial GSM services were able to start operating in Europe in mid-1991. The GSM system is able to utilise any of the three frequency bands at 900, 1800 and 1900 MHz, and many GSM phones can operate as dual-band or tri-band phones, whereby they are adaptable to the local frequency system in the region the user travels through.
In GSM 900, for example, two frequency bands of 25 MHz bandwidth are used. The band 890-915 MHz is dedicated to uplink communications from the mobile station to the base station, and the band 935-960 MHz is used for the downlink communications from the base station to the mobile station. Each band is divided into 124 carrier frequencies, spaced 200 kHz apart, in a similar fashion to the FDMA method used in 1G systems. Then, each carrier frequency is further divided using TDMA into eight 577 uS long "time slots", every one of which represents one communication channel - the total number of possible channels available is therefore 124 x 8, producing a theoretical maximum of 992 simultaneous conversations. In the USA, a different form of TDMA is used in the system known as IS-136 D-AMPS, and there is another US system called IS-95 (CDMAone), which is a spread spectrum code division multiple access (CDMA) system. CDMA is the technique used in 3G systems.
