HSCSD

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 slot 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.

GSM

Global System for Mobile communications (GSM: originally from Groupe Spécial Mobile) is the most popular standard for mobile phones in the world. Its promoter, the GSM association, estimates that 82% of the global mobile market uses the standard. GSM is used by over 2 billion people across more than 212 countries and territories. Its ubiquity makes international roaming very common between mobile phone operator, enabling subscribers to use their phones in many parts of the world. GSM differs from its predecessors in that both signaling and speech channels are digital call quality, and so is considered a second generation (2G) mobile phone system. This has also meant that data communication were built into the system using the 3^rd partnership project (3GPP).

The key advantage of GSM systems to consumers has been better voice quality and low-cost alternatives to making calls, such as the short messaging service (SMS, also called "text messaging"). The advantage for network operators has been the ease of deploying equipment from any vendors that implement the standard. Like other cellular standards, GSM allows network operators to offer roaming services so that subscribers can use their phones on GSM networks all over the world.

Newer versions of the standard were backward-compatible with the original GSM phones. For example, release '97 of the standard added packet data capabilities, by means of general packet radio service (GPRS). Release '99 introduced higher speed data transmission using (EDGE).