Circuit Switched Data
Circuit Switched Data, often known as CSD, is the original form of data transmission developed for the GSM mobile phone system. CSD used a single radio time slot to deliver 9.6Kb/S data transmission to the GSM Network Sub System where it could be connected through the equivalent of a normal modem to the 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 2.4Kb/S or less. With the introduction of digital transmission in GSM, CSD provided direct almost direct access to the underlying digital signal allowing for higher speeds. At the same time, the speech oriented compression used in GSM actually meant that data rates using a traditional modem connected to the phone would have been even lower than with traditional 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" (16Kb/S) is allocated from base station to transcoder and finally a time slot (64Kb/S) is allocated from transcoder to MSC.
At the MSC, it is possible to use a modem to convert to an "analogue" signal, though this will typically actually be encoded as a digital PCM signal when sent from the MSC. It is also possible to directly use the digital signal as an ISDN data signal and feed it into the equivalent of an access server.
GSM data transmission has advanced since the introduction of CSD. HSCSD is a system which was based on, and quite similar to CSD but designed to provide higher data rates. GPRS on the other hand provides a packet based data transmission directly from the mobile phone. Finally EDGE and UMTS provide improved radio interfaces with higher data rates, but still, in large part, compatible with the same GSM core network.