A further enhancement to the existing GSM network is the new High Speed Circuit Switched Data (HSCSD) service. HSCSD introduces two new technologies to the existing GSM network. First, it increases the maximum user data rate that can be transported per GSM time slot from 9.6 kbit/s to 14.4 kbit/s by employing a new coding scheme with less error protection capabilities. Thus providing more space for user data per time slot. Second, with HSCSD up to four time slots can be allocated for one data call.
The introduction of HSCSD within existing GSM networks is also very simple as the update can be achieved by software upgrades. HSCSD can be used in conjunction with both 9.6 kbit/s and 14.4 kbit/s bearers, enabling a maximum data transfer speed of up to 38.4 kbit/s (9.6 kbit/s bearer) or up to 57.6 kbit/s (14.4 kbit/s bearer). However, HSCSD with the 14.4 kbit/s bearer service cannot be implemented within 900 MHz GSM networks, due to the resulting requirement for superior radio channel conditions. Because of the new coding scheme with less error protection capabilities, the 14.4 kbit/s bearer service can only be implemented within the 1800 MHz GSM networks. The biggest disadvantage of HSCSD is that it is very expensive for the user. Four channels means you have to pay four times.
HSCSD offers both transparent and non-transparent types of service so that all sorts of circuit switched applications can be used. For non-transparent (NT) HSCSD calls, the number of timeslots can be changed during the call, and can be allocated asymmetrically with more bandwidth in the downlink than in the uplink. With NT HSCSD, there are no hand-over problems, because the time-slots will be allocated according to availability. For transparent HSCSD calls, the number of timeslots cannot be changed during a call. With transparent bit-rates, HSCSD offers constant bit-rates and transmission delay, very useful for video applications requiring a constant rate of transmission.
Copyright © 2001-2003 by Rainer Hillebrand and Thomas Wierlemann