HSCSD (High-Speed Circuit-Switched Data)
HSCSD (High-Speed Circuit-Switched Data) is essentially a new high speed implementation of GSM (Global System for Mobile Communication) data transfer. Four times faster than GSM, with a transfer rate of up to 57.6Kbps, it achieves this speed by allocating up to eight time slots to an individual user. This speed makes it comparable to many fixed-line telecommunications networks and will allow users to access the Internet and other datacom services via a GSM network.
HSCSD operates across a GSM network, and therefore no extra hardware is required by a mobile communications operator to offer the service, just a network software upgrade. In a GSM network single slots are allocated to each user, which has a standard data transfer rate of 9.6Kbps, although some networks are now being upgraded to 14.4Kbps, an increase of 50%. In HSCSD, users are allocated multiple slots so that the transmission speed can be drastically increased, with some service providers offering rates of up to 57.6Kbps. This enables internet access at the same speed of many dial-up modem services across fixed line networks.
THE GSM RADIO SPECTRUM
Since radio spectrum is a limited resource shared by all users, a method must be devised to divide up the bandwidth among as many users as possible. The method chosen by GSM is a combination of Time and Frequency Division Multiple Access (TDMA/FDMA). The FDMA part involves the division by frequency of the (maximum) 25MHz bandwidth into 124 carrier frequencies spaced 200kHz apart. One or more carrier frequencies are assigned to each base station.
Each of these carrier frequencies is then divided in time using a TDMA scheme. The fundamental unit of time in this TDMA scheme is called a burst period and it lasts 15/26ms (or approx. 0.577ms). Eight burst periods are grouped into a TDMA frame (120/26ms, or approx. 4.615ms), which forms the basic unit for the definition of logical channels. One physical channel is one burst period per TDMA frame.
Channels are defined by the number and position of their corresponding burst periods. All these definitions are cyclic, and the entire pattern repeats approximately every three hours. Channels can be divided into dedicated channels, which are allocated to a mobile station, and common channels, which are used by mobile stations in idle mode.
HSCSD is being rolled out across many GSM networks as a stop gap before broadband mobile services becomes more readily available. The service has a limited number of users, as when more slots are allocated to each user then network will become overloaded far more quickly than for voice users. Service providers in Europe include E-Plus (Germany), Orange (UK), Telia (Sweden) and Telenor (Norway). Cost is relatively low for operators who wish to roll out HSCSD, for example Ericsson won a contract worth $17 million from Singapore's SingTel to upgrade the software of its switches to support HSCSD and add hardware to run other intelligent services.