HSCSD and GPRS

Another advantage of GSM is its support for several extension technologies that achieve
higher rates for data applications. Two such technologies are High Speed Circuit Switched
Data (HSCSD) and General Packet Radio Service (GPRS). HSCSD is a very simple upgrade
to GSM. Contrary to GSM, it gives more than one time slot per frame to a user; hence the
increased data rates. HSCD allows a phone to use two, three or four slots per frame to achieve
rates of 57.6, 43.2 and 28.8 kbps, respectively. Support for asymmetric links is also provided,
meaning that the downlink rate can be different than that of the uplink. A problem of HSCSD
is the fact that it decreases battery life, due to the fact that increased slot use makes terminals
spend more time in transmission and reception modes. However, due to the fact that reception
requires significantly less consumption than transmission, HSCSD can be efficient for web
browsing, which entails much more downloading than uploading.
GPRS operation is based on the same principle as that of HSCSD: allocation of more slots
within a frame. However, the difference is that GPRS is packet-switched, whereas GSM and
HSCSD are circuit-switched. This means that a GSM or HSCSD terminal that browses the
Internet at 14.4 kbps occupies a 14.4 kbps GSM/HSCSD circuit for the entire duration of the
connection, despite the fact that most of the time is spent reading (thus downloading) Web
pages rather than sending (thus uploading) information. Therefore, significant system capacity
is lost. GPRS uses bandwidth on demand (in the case of the above example, only when
the user downloads a new page). In GPRS, a single 14.4 kbps link can be shared by more than
one user, provided of course that users do not simultaneously try to use the link at this speed;
rather, each user is assigned a very low rate connection which can for short periods use
additional capacity to deliver web pages. GPRS terminals support a variety of rates, ranging
from 14.4 to 115.2 kbps, both in symmetric and asymmetric configurations.