Definition of 2.5G
2.5G is a stepping stone between 2G and 3G cellular wireless technologies. The term "second and a half generation" is used to describe 2G-systems that have implemented a packet switched domain in addition to the circuit switched domain. It does not necessarily provide faster services because bundling of timeslots is used for circuit switched data services (HSCSD) as well.
While the terms "2G" and "3G" are officially defined, "2.5G" is not. It was invented for marketing purposes only.
2.5G provides some of the benefits of 3G (e.g. it is packet-switched) and can use some of the existing 2G infrastructure in GSM and CDMA networks. The commonly known 2.5G technique is GPRS. Some protocols, such as EDGE for GSM and CDMA2000 1x-RTT for CDMA, officially qualify as "3G" services (because they have a data rate of above 144 kbit/s), but are considered by most to be 2.5G services (or 2.75G which sounds even more sophisticated) because they are several times slower than "true" 3G services.
2G is the current generation of full digital mobile phone systems. It transmits primarily voice but is used for circuit-switched data service and SMS as well.
3G is now the third generation of mobile phone systems. They provide both a packet-switched and a circuit-switched domain from the beginning. It requires a new access network, different from that already available in 2G systems. Due to cost and complexity, rollout of 3G has been somewhat slower than anticipated. (Source: wikipedia.org)
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