2G networks are the second generation of cellphone networks that relied on a digital signal instead of a radio signal. This technology, launched in Finland in 1991, is based primarily on a packet-switching protocol for transferring voice digitally. Phones that ran on a 2G network were smaller and had better battery life because they were not required to emit as strong of a radio signal. The voice quality on 2G phone networks was generally better and more secure because of digital encryption. Because 2G technology relied on a digital signal instead of analog signal, it was able to transmit more than voice, such as text messages and email.
2G technology was good for the carriers because it was more efficient on the spectrum. It allowed carriers to push a higher volume of calls through their network, but it relied more on proximity to a cellphone tower; when a caller moved out of range, calls were dropped entirely instead of progressively degrading. Technologies that are directly related to 2G are code division multiple access (CDMA), time division multiplex access (TDMA), and Global System for Mobile Communications (GSM).
CDMA
Used in North and South America as well as Asia, this subset of 2G technology still accounts for 17% of subscribers in the world. In code division multiple access (CDMA), the system relies on each phone being assigned a specific code, which allows multiple users to be put on the same transmission channel.
TDMA
Most 2G networks relied on time division multiplex access (TDMA) to transmit digital signals that were divided into different time slots instead of codes, as in CDMA. The signals are sent in rapid succession, all while sharing one digital channel. The timing requirements for this type of technology frequently made it unreliable as a mobile phone transmission technology because when callers moved closer to or farther away from a tower, they would misalign the timing requirements of the system and disrupt the transmission.
GSM
Global System for Mobile Communications (GSM) was developed to address some of the shortfalls of TDMA technology. It was originally created in Finland in 1991 and is now used around the world. It requires timing advance commands to be sent to the base station, which sends signals to the mobile phone, telling it whether it should transmit the signal earlier and, if so, by how much. It accounts for 80% of the subscribers around the world. GSM is the most ubiquitous set of standards for mobile phones. Because of its success, many other 2G technologies, including CDMA and TDMA, eventually transferred to GSM. GSM is so widespread that international roaming is now much more simple, because phones can almost always access a signal that they can use.
