In communications, Circuit Switched Data (CSD) (also named GSM data) is the original form of data transmission developed for the time-division multiple access (TDMA)-based mobile phone systems like Global System for Mobile Communications (GSM). In later years, High Speed Circuit Switched Data (HSCSD) was developed providing increased data rates over conventional CSD. After 2010 many telecommunication carriers dropped support for CSD and HSCSD, having been superseded by GPRS and EDGE (E-GPRS).
The Global System for Mobile Communications (GSM) is a family of standards to describe the protocols for second-generation (2G) digital cellular networks, as used by mobile devices such as mobile phones and mobile broadband modems. GSM is also a trade mark owned by the GSM Association. "GSM" may also refer to the voice codec initially used in GSM.
2G networks developed as a replacement for first generation (1G) analog cellular networks. The original GSM standard, which was developed by the European Telecommunications Standards Institute (ETSI), originally described a digital, circuit-switched network optimized for full duplex voice telephony, employing time division multiple access (TDMA) between stations. This expanded over time to include data communications, first by circuit-switched transport, then by packet data transport via its upgraded standards, GPRS and then EDGE. GSM exists in various versions based on the frequency bands used.
General Packet Radio Service (GPRS), also called 2.5G, is a mobile data standard that is part of the 2G cellular communication network Global System for Mobile Communications (GSM). Networks and mobile devices with GPRS started to roll out around the year 2001; it offered, for the first time on GSM networks, seamless data transmission using packet-switched data for an "always-on" connection, eliminating the need to dial up, providing improved Internet access for web, email, Wireless Application Protocol (WAP) services, Short Message Service (SMS), Multimedia Messaging Service (MMS) and others.
Up until the rollout of GPRS, only circuit-switched data was used in cellular networks, meaning that one or more radio channels were occupied for the entire duration of a data connection. On the other hand, on GPRS networks, data is broken into small packets and transmitted through available channels. This increased efficiency also gives it theoretical data rates of 56–114 kbit/s, significantly faster than the preceding Circuit Switched Data (CSD) technology. GPRS was succeeded by EDGE ("2.75G") which provided improved performance and speeds on the 2G GSM system.
The Universal Mobile Telecommunications System (UMTS) is a 3G mobile cellular system for networks based on the GSM standard. UMTS uses wideband code-division multiple access (W-CDMA) radio access technology to offer greater spectral efficiency and bandwidth to mobile network operators compared to previous 2G systems like GPRS and CSD. The original version of UMTS provides a peak theoretical data rate of 384 kbit/s.
Developed and maintained by the 3GPP (3rd Generation Partnership Project), UMTS is a component of the International Telecommunication Union IMT-2000 standard set and compares with the CDMA2000 standard set for networks based on the competing cdmaOne technology. The technology described in UMTS is sometimes also referred to as Freedom of Mobile Multimedia Access (FOMA) or 3GSM.