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Wednesday, 16 May 2012

Mobile Wallpapers

Mobile Wallpapers Biography
Nowadays, mobile phone is one of the most essential things of people. It takes much convince for people. The technology of mobile phone is increase rapidly everyday. Form 1^st generation up to now, people want to build up a mobile
1G (First Generation) is the name given to the first generation of mobile telephone networks. These systems used analogue circuit-switched technology, with FDMA (Frequency Division Multiple Access), and worked mainly in the 800-900 MHz frequency bands. The networks had a low traffic capacity, unreliable handover, poor voice quality, and poor security.

2G - 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.

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.

2.5G (Second Generation Enhanced) is a generic term used to refer to a standard of wireless mobile telephone networks that lies somewhere between 2G and 3G. The development of 2.5G has been viewed as a stepping-stone towards 3G, which was prompted by the demand for better data services and access to the Internet. In the evolution of mobile communications, each generation provides a higher data rate and additional capabilities, and 2.5G is no exception as it is provides faster services than 2G, but not as fast or as advanced as the newer 3G systems.

Some observers have seen 2.5G as an alternative route to 3G, but this appears to be short-sighted as 2.5G is several times slower than the full 3G service. In technical terms 2.5G extends the capabilities of 2G systems by providing additional features, such as a packet-switched connection (GPRS) in the TDMA-based GSM system, and enhanced data rates (HSCSD and EDGE).

These enhancements in 2.5G systems permit data speeds of 64-144 kbps, which enables these phones to feature web browsing, the use of navigation and navigational maps, voice mail, fax, and the sending and receiving of large email messages.

Mobile Wallpapers