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First generation refers to the analog �brick phones� and �bag phones� as they were first introduced for mobile cellular technology. Cell phones began with 1G and signify first generation wireless analog technology standards that originated in the 1980s. 1G was replaced by 2G wireless digital standards.

  

2G signifies second generation wireless digital technology. Fully digital 2G networks have replaced analog 1G, which originated in the 1980s. 2G networks first commercially began on the Global System for Mobil Communications, or GSM, standard. 2G on GSM standards was first used in commercial practice in 1991 by Radiolinja, a Finnish GSM operator that was founded on September 19, 1988. Radiolinja is now part of Elisa, which was known in the 1990s as the Helsinki Telephone Company.

In addition to the GSM protocol, 2G also utilizes various other digital protocols, including CDMA, TDMA, iDEN and PDC. GSM is based on TDMA.

 

2.5G wireless technology is a stepping stone that bridged 2G to 3G wireless technology and is sometimes used to describe those evolved technologies that were first considered as being 2G. While 2G and 3G have been officially defined as wireless standards by the International Telecommunication Union (ITU), 2.5G has not been defined and was created only for the purposes of marketing.

As an interim step up from 2G, 2.5G has seen some of the advances inherent in 3G networks (including packet-switched systems). The evolution from 2G to 3G has ushered in faster and higher-capacity data transmission. Several technologies that have been considered as the evolutionary step to 3G include EDGE (part of the GSM family) andCDMA 2000 1X; at times these technologies are called 3G as they both meet some of the ITU requirements for 3G standards.

  

3G is the third generation of mobile phone standards and technology. 3G supersedes 2G technology and precedes 4G technology. Current 3G systems have been established through ITU�s project on International Mobile Telecommunications 2000 (IMT-2000).

 

3G technologies have enabled faster data transmission speeds, greater network capacity and more advanced network services. In May 2001, NTT DoCoMo (Japan) launched the first pre-commercial 3G network � branded as FOMA. Following the first pre-commercial launch, NTT DoCoMo again made history on October 1, 2001, with the first commercial launch of 3G in Japan.

 

UMTS-HSPA is the world�s leading 3G technology. By 2015, UMTS-HSPA and LTE 3G technologies are expected to account for 3.9 billion global subscriptions, compared to 569 million CDMA EV-DO subscriptions and 59 million WiMAX subscriptions.

  

Similar to the 2.5G acronym, the reference to 3.5G is not an officially recognized standard by the ITU. It is an interim or evolutionary step to the next generation of cellular technology that will be known as IMT-Advanced according to definitions by the ITU. IMT-Advanced will comprise the fourth generation of cell phone technology. The acronym 3.5G is also known as �beyond 3G.� 4G Americas does not use the terms 3.5G (or 2.5G) in respect of the official definitions provided by the ITU. The technologies within the GSM family that are considered as beyond 3G include HSPA+ and LTE. These 3.5G technologies are often called pre-4G as well.

  

4G is the term used to refer to the fourth generation of mobile wireless services that has been defined by the ITU and its Radiocommunication Sector (ITU-R) and established as an agreed upon and globally accepted definition in IMT-Advanced.

 

The ITU has developed requirements for a technology to be considered IMT-Advanced, which is the next-generation wireless technology. An IMT-Advanced cellular system must fulfill the following requirements. 

In Release 10, 3rd Generation Partnership Project (3GPP) addressed the IMT-Advanced requirements in a version of LTE, called LTE-Advanced, for which specifications became available in 2011. WiMAX addressed the IMT-Advanced requirements in a version called Mobile WiMAX 2.0, specified in IEEE 802.16m. Such a 4G family, in adherence to the principles defined for acceptance into this ITU process, is globally recognized to be one that can grow to include all aspects of a marketplace arriving beyond 2010, thus complementing and building upon an expanding and maturing 3G ecosystem.

 

In September 2009, the technology proposals were submitted to the International Telecommunication Union (ITU) as 4G candidates. Basically all proposals were based on two technologies: 

Current LTE and WiMAX implementations are often cited in the marketplace as 4G, although they dont fully comply with the planned requirements of 1 Gbps for stationary reception and 100 Mbps for mobile. The common argument for branding LTE and WiMAX systems as 4G is that they use different frequency bands to 3G technologies; that they are based on a new radio-interface paradigm; and that the standards are not backwards compatible with 3G, while some of the standards are expected to be forwards compatible with "real" 4G technologies. 

 

Thus in December 2010, the ITU deferred discussion on 4G technologies by stating in apress release:

 

Following a detailed evaluation against stringent technical and operational criteria, ITU has determined that �LTE-Advanced� and �WirelessMAN-Advanced� should be accorded the official designation of IMT-Advanced. As the most advanced technologies currently defined for global wireless mobile broadband communications, IMT-Advanced is considered as �4G�, although it is recognized that this term, while undefined, may also be applied to the forerunners of these technologies, LTE and WiMAX, and to other evolved 3G technologies providing a substantial level of improvement in performance and capabilities with respect to the initial third generation systems now deployed. The detailed specifications of the IMT-Advanced technologies will be provided in a new ITU-R Recommendation expected in early 2012.