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High Speed Circuit Switched Data is an enhancement to the GSM mobile communications system that enables up to four 14.4 Kbps channels to be combined to provide 57.6 Kbps data transfer. Part of GSM Phase 2, HSCSD is suited for videoconferencing and multimedia transmission.

Here's from the N800 Manual:

To use a packet switched data service in a cellular network, such as GPRS, select Packet Data.

To use a circuit switched data service in a cellular network, such as HSCSD, select Data Call.

inductance

Data cannot be transferred until a circuit has been established