High-Speed Circuit-Switched Data
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High-Speed Circuit-Switched Data (HSCSD), is an enhancement to 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/or multiple time slots to increase data throughput.
One innovation in HSCSD is to allow different error correction methods to be used for data transfer. The original error correction used in GSM was 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 different levels of possible error correction which can be used according to the quality of the radio link. 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, for a 50% improvement in throughput.
The other innovation in HSCSD is the ability to use multiple time slots at the same time. Using the maximum of four (4) time slots, this can provide an increase in maximum transfer rate of up to 57.6 kbit/s (4 times 14.4 kbit/s) and, even in bad radio conditions where a higher level of error correction needs to be used, can still provide a four times speed increase over CSD (38.4 kbit/s versus 9.6 kbit/s). By combining up to 8 GSM time slots the capacity can be increased to 115 kbit/s.
HSCSD requires the time slots being used to be fully reserved to 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 typically charged for HSCSD at a rate higher than a normal phone call (e.g., by the number of time slots allocated) for the total period of time that the user has a connection active. This makes HSCSD relatively expensive in many GSM networks and is one of the reasons that packet-switched 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.
Apart from the fact that the full allocated bandwidth of the connection is available to 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 from the network to send a packet.
HSCSD is also an option in EDGE and UMTS systems where packet data transmission rates are much higher. In the UMTS system the advantages of HSCSD over packet data are even lower since the UMTS radio interface has been specifically designed to support high bandwidth, low latency packet connections. This means that the primary reason to use HSCSD in this environment would be access to legacy dial up systems.
