High-Speed Circuit-Switched Data
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High-Speed Circuit-Switched Data (HSCSD), is a development of Circuit Switched Data, the original data transmission mechanism of the GSM mobile phone system. As with CSD, channel allocation is done in circuit switched mode. The difference comes from the ability to use different coding methods and even multiple time slots to increase data throughput.
The first innovation in HSCSD was to allow different error correction methods to be used for data transfer. The error correction used in GSM is designed to work at the limits of coverage and in the worst case that GSM will handle. This means that a large part of the GSM transmission capacity is taken up with error correction codes. HSCSD provides several levels of possible error correction which can be deployed according to the quality of the radio link. This means that in the best conditions 14.4 kbit/s can be put through a time slot which, under CSD, would normally only carry 9.6 kbit/s.
The second innovation in the HSCSD radio interface was the possibility to use multiple time slots at the same time. This allows an increase in maximum transfer rates (using four time slots) up to 57.6 kbit/s and, even in the bad radio conditions where the highest level of error correction has to be used, will still lead to a four times speed increase over CSD.
HSCSD require the time slots being used to be fully reserved for a single user. It is possible that either at the beginning of the call, or at some point during a call, it will not be possible for the user's full request to be satisfied since the network is often configured so that normal voice calls take precedence over additional time slots for HSCSD users. The user is then charged, often at a rate higher than a normal phone call, and sometimes multiplied by the number of time slots allocated, based on the period of time that the user has a connection active. This makes HSCSD relatively expensive in many GSM networks and so, packet-switched GPRS which typically has lower pricing is becoming more common than HSCSD connections.
Apart from the fact that the full allocated bandwidth of the connection is available for the HSCSD user, HSCSD also has an advantage in GSM systems in terms of lower average radio interface latency than GPRS. This is because the user of a HSCSD connection does not have to wait for permission to send a packet from the network.
HSCSD is also an option in EDGE and UMTS systems. In this case the data transmission rate is much higher. However, in the UMTS system, the advantages of HSCSD over packed data are considerably lower since the radio interface has been specifically designed to support high bandwidth, low latency packet connections. This means that the primary reason to use it in this case would be access to legacy dial up systems.