CSD: Circuit Switched Data

What is CSD?

Circuit Switched Data, often referred to as CSD, is the original form of data transmission developed for TDMA-based mobile phone systems like 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 PSTN allowing direct calls to any dial up service.

Advantages

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.

Mechanics

CSD call is similar 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 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 a remote access server.

GSM data transmission has advanced since the introduction of CSD:

• 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.
• GPRS provides more efficient packet-based data transmission directly from the mobile phone at speeds similar to HSCSD.
• Finally EDGE (E-GPRS) and UMTS provide improved radio interfaces with higher data rates, while still being backward compatible with the GSM core network.