Circuit Switched Data

CSD uses a single radio time slot to deliver 9.6 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.

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 analog systems.

A single dedicated radio time slot is allocated between the phone and the base station.

This means that a large part of the GSM transmission capacity is taken up with error correction codes.

This means that in the best conditions 14.4 kbit/s can be put through a single time slot that under CSD would only carry 9.6 kbit/s, i.e. a 50% improvement in throughput.

This makes HSCSD relatively expensive in many GSM networks and is one of the reasons that packet-switched General Packet Radio Service (GPRS), which typically has lower pricing (based on amount of data transferred rather than the duration of the connection), has become more common than HSCSD.

This means that the primary reason to use HSCSD in this environment would be access to legacy dial up systems.