2G Technology

2G (or 2-G) is short for second-generation wireless telephone technology.

Second generation 2G cellular telecoms networks were commercially launched on the GSM standard in Finland by Radiolinja (now part of Elisa) in 1991. 2G networks were fully digital while previous 1G networks were analog. Three primary benefits of 2G networks over their predecessors were that phone conversations were digitally encrypted, while on analog systems it was possible for third parties to eaves-drop on calls; 2G systems were significantly more efficient on the spectrum allowing for far greater mobile phone penetration levels; and 2G introduced data services for mobile, starting with SMS text messages.

After 2G was launched, the previous mobile telephone systems were retrospectively dubbed 1G. While radio signals on 1G networks are analog, and on 2G networks are digital, both systems use digital signaling to connect the radio towers (which listen to the handsets) to the rest of the telephone system.

2G technologies :
2G technologies can be divided into TDMA-based and CDMA-based standards depending on the type of multiplexing used. The main 2G standards are:

GSM (TDMA-based), originally from Europe but used in almost all countries on all six inhabited continents (Time Division Multiple Access). Today accounts for over 80% of all subscribers around the world.
IS-95 aka cdmaOne, (CDMA-based, commonly referred as simply CDMA in the US), used in the Americas and parts of Asia. Today accounts for about 17% of all subscribers globally. Over a dozen CDMA operators have migrated to GSM including operators in Mexico, India, Australia and South Korea.
PDC (TDMA-based), used exclusively in Japan
iDEN (TDMA-based), proprietary network used by Nextel in the United States and Telus Mobility in Canada
IS-136 aka D-AMPS, (TDMA-based, commonly referred as simply TDMA in the US), was once prevalent in the Americas but most have migrated to GSM.
2G services are frequently referred as Personal Communications Service, or PCS, in the United States.

2.5G services enable high-speed data transfer over upgraded existing 2G networks. Beyond 2G, there's 3G, with higher data speeds, and even evolutions beyond 3G, often called 3.5G. Currently there is no commercial 4G but that is being standardized globally, with expected commercial 4G launches starting around 2012-2015.

Capacity :
Using digital signals between the handsets and the towers increases system capacity in two key ways:

Digital voice data can be compressed and multiplexed much more effectively than analog voice encodings through the use of various CODECs, allowing more calls to be packed into the same amount of radio bandwidth.
The digital systems were designed to emit less radio power from the handsets. This meant that cells could be smaller, so more cells could be placed in the same amount of space. This was also made possible by cell towers and related equipment getting less expensive.

Advantages :
Digital systems were embraced by consumers for several reasons.

The lower powered radio signals require less battery power, so phones last much longer between charges, and batteries can be smaller.
The digital voice encoding allowed digital error checking which could increase sound quality by reducing dynamic and lowering the noise floor.
The lower power emissions helped address health concerns.
Going all-digital allowed for the introduction of digital data services, such as SMS and email.
Greatly reduced fraud. With analog systems it was possible to have two or more "cloned" handsets that had the same phone number.
Enhanced privacy. A key digital advantage not often mentioned is that digital cellular calls are much harder to eavesdrop on by use of radio scanners. While the security algorithms used have proved not to be as secure as initially advertised, 2G phones are immensely more private than 1G phones, which have no protection against eavesdropping.

Disadvantages :
The downsides of 2G systems, not often well publicized, are:

In less populous areas, the weaker digital signal may not be sufficient to reach a cell tower. This tends to be a particular problem on 2G systems deployed on higher frequencies, but is mostly not a problem on 2G systems deployed on lower frequencies. National regulations differ greatly among countries which dictate where 2G can be deployed.
Analog has a smooth decay curve, digital a jagged steppy one. This can be both an advantage and a disadvantage. Under good conditions, digital will sound better. Under slightly worse conditions, analog will experience static, while digital has occasional dropouts. As conditions worsen, though, digital will start to completely fail, by dropping calls or being unintelligible, while analog slowly gets worse, generally holding a call longer and allowing at least a few words to get through.
While digital calls tend to be free of static and background noise, the lossy compression used by the CODECs takes a toll; the range of sound that they convey is reduced. You'll hear less of the tonality of someone's voice talking on a digital cellphone, but you will hear it more clearly.

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