High Speed Circuit Switched Data(HSCSD)

High-speed circuit-switched data (HSCSD), is an enhancement to circuit switched data (CSD), the original data transmission mechanism of the GSM mobile phone system, four times faster than GSM, with data rates up to 38.4kbit/s. But actually on mobile the speed is less than 15kb/s. It is not 3G.

Like in circuit-switched data, 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.

HSCSD is a circuit-switched protocol for large file transfer and multimedia applications. The physical layer of HSCSD is the same as that for the Phase 2 GSM data services. The data rate of HSCSD has been increased by using multiple TDMA time slots (up to 8) instead of one time slot in the current data applications. The data rate can also be increased by data compression techniques.

The data computing session is performed at a terminal equipment (TE) such as a computer connected to the MT. The network inter-working function (IWF) supports adaption between GSM and the external networks. TAF and IWF are sensitive to the end-to-end services. The GSM entities between TAF and IWF are independent of the services. They only provide the bearer capabilities required to transport the corresponding data flow.

The radio interface is the same as that of the current GSM system except that multiple, independent time slots can be utilized to provide high speed link as previously described. The radio link protocol (RLP) has been enhanced in HSCSD to support multi-link (time slot) operation. The protocol may or may not recover the frame errors between TAF and IWF.

Network Architecture [1]

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A new functionality is introduced at the network and MS to provide the functions of combining and splitting the data into separate data streams which will then be transferred via n channels at the radio interface, where n = 1, 2, 3, ... 8.Once split, the data streams shall be carried by the n full rate traffic channels, called HSCSD channels.Logically the n full rate traffic channels at the radio interface belong to the same HSCSD configurationTherefore they shall be controlled as one radio link by the network for the purpose of cellular operations, e.g. handover.

Network Architecture [2]*

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 14.4kbit/s can be put through a single time slot that under CSD would only carry 9.6kbit/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 time slots, this can provide an increase in maximum transfer rate of up to 57.6kbit/s (i.e., 4 14.4kbit/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 (38.4kbit/s versus 9.6kbit/s). By combining up to eight GSM time slots the capacity can be increased to 115kbit/s.

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 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.

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 an HSCSD connection does not have to wait for permission from the network to send a packet

HSCSD is also an option in enhanced data rates for GSM evolution (EDGE) and universal mobile telephone system (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.

ConclusionSimple implementation relative to GPRSMinimal impact on infrastructureQuick to deploy relative to GPRSInefficient for moving bursty dataUser data rates ultimately less than GPRS

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