Editing Circuit Switched Data (section)

== Technical ==
CSD uses a single radio [[Time-division multiplexing|time slot]] to deliver 9.6 [[kilobits per second|kbit/s]] data transmission to the GSM [[network 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. For backwards compatibility, the [[IS-95]] standard also supports CDMA Circuit Switched Data. However, unlike TDMA, there are no time slots, and all [[CDMA]] radios can be active all the time to deliver up to 14.4&nbsp;kbit/s data transmission speeds. With the evolution of CDMA to [[CDMA2000]] and [[1xRTT]], the use of [[IS-95]] CDMA Circuit Switched Data declined in favour of the faster data transmission speeds available with the newer technologies.

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&nbsp;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 data compression|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 [[analog signal|analog]] 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&nbsp;kbit/s) is allocated from the base station to the [[transcoder]], and finally, another time slot (64&nbsp;kbit/s) is allocated from the transcoder to the [[Mobile switching centre server|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|remote access server]].