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'Circuit Switched Data' ('CSD') 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). CSD uses a single radio time slot to deliver 9.6 kbit/s data transmission to the GSM Network and Switching Subsystem where it could be connected through the equivalent of a normal modem to the Public Switched Telephone Network (PSTN) allowing direct calls to any dial-up service.
Prior to CSD, data transmission over mobile phone systems was done by using a modem, either built into the phone or attached to it. Such systems were limited by the quality of the audio signal to 2.4 kbit/s or less. With the introduction of digital transmission in TDMA-based systems like GSM, CSD provided almost direct access to the underlying digital signal, allowing for higher speeds. At the same time, the speech oriented audio compression used in GSM actually meant that data rates using a traditional modem connected to the phone would have been even lower than with older analogue systems.
A CSD ''call'' functions in a very similar way to a normal voice call in a GSM network. A single dedicated radio time slot is allocated between the phone and the base station. A dedicated "sub-time slot" (16 kbit/s) is allocated from the base station to the transcoder, and finally another time slot (64 kbit/s) is allocated from the transcoder to the Mobile Switching Centre (MSC).
At the MSC, it is possible to use a modem to convert to an "analog" signal, though this will typically actually be encoded as a digital pulse-code modulation (PCM) signal when sent from the MSC. It is also possible to directly use the digital signal as an Integrated Services Digital Network (ISDN) data signal and feed it into the equivalent of a remote access server.
GSM data transmission has advanced since the introduction of CSD:

★ High-Speed Circuit-Switched Data (HSCSD) is a system based on ''CSD'' but designed to provide higher data rates by means of more efficient channel coding and/or multiple (up to 4) time slots.

★ General Packet Radio Service (GPRS) provides more efficient packet-based data transmission directly from the mobile phone at speeds similar to HSCSD.

★ Enhanced Data Rates for GSM Evolution (EDGE) (E-GPRS) and Universal Mobile Telecommunications System (UMTS) provide improved radio interfaces with higher data rates, while still being backward compatible with the GSM core network.

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In the third quarter 2006, CSD was cheaper than GPRS and other data services. The only charges are as for voice calls, out of the subscriber's "bucket of minutes." It can even be free at certain times, depending on the plan. It is ideal for simple POP3 email checking (one does need a dial-up ISP), but Web viewing is increasingly difficult due to modern web page clutter.
The T-Mobile system is area/city specific. It works in Denver, Colorado and Savannah, Georgia, and (2003) Las Vegas into SoCal roaming onto Cingular. There is no service in Sarasota, Florida or anywhere in Texas. These were the experiences using T-Mobile on a Nokia 9290 Communicator in 2006. Customer service had essentially no knowledge of their CSD services; always ask to speak to a data specialist, and then one that has CSD knowledge. Fax services required a second number for an additional $10/mo.
When auto-roaming onto Cingular, CSD will work if it works for Cingular. The other national auto-roam partners (Dobson, et al.) apparently do not support CSD. Cingular states that it stopped providing CSD for new customers as of December 2005. But, customers have apparently gotten CSD service for $3.99/mo, either enrolling on line or stating that they need it for faxes.