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High Speed Circuit Switched Data (HSCSD) is an enhancement in the data rate of circuit switched data in a GSM network. HSCSD uses two techniques to increase the data rate. First, HSCSD makes it possible to use more than one time slot. GSM uses time division multiple access (TDMA). Each radio channel is divided in eight time slots. Each time slot is allocated to a different user. This makes it possible to serve eight customers on one radio channel. HSCSD makes it possible to allocate more than one time slot to a user.

The second technique used by HSCSD is that the error correction can be adapted to the quality of the radio channel. A standard slot can carry 9.6 kbps. HSCSD makes it possible to increase this to 14.4 kbps. the quality of the rado channel must be good enough to do so.

The maximum data rate of a HSCSD configuration with 14.4-kbps channel coding is 115.2 kbps, if all eight time slots are allocated to the same user. In practice, is the number of time slots allocated to a user limited to three, limiting the data rate to 43.2 kbps. Another point is that the core network is based on circuit switched data with data channels of 64 kbps.

The main benefit of HSCSD compared to other data enhancements introduced later is that it is an inexpensive way to implement higher data rates in GSM networks. There modifications to be made are relatively small. Users need to have a new terminal that supports HSCSD.

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