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2.2.2 Further Reading
For more information on WECA and on the Wi-Fi standard, check the Wi-Fi™ web site at
http://www.wi-fi.com/. The web site also contains a list of Wi-Fi certified products.
2.3 GSM
GPRS and HSCSD operate in GSM networks at the 900 MHz and 1800 MHz bands. The
maximum transmit power is 2 W for GSM 900 and 1 W for GSM 1800.
The data services in the first commercial GSM systems in 1991-1992 were based on circuit
switched technology. The connection over the air interface used one time slot per user
offering a maximum user data rate of 9.6 kbit/s. Since then, mainly due to the increased
Internet usage and ever increasing amount of information in the Internet, there has been a
constantly increasing user demand for higher data rates. The fixed telephone modems have
responded to these requirements with increased data rates – modem data rates have in fact
increased over time from 1.2 kbit/s (V.23) to current 56 kbit/s (V.90). In the light of this, it has
been very clear that, in order to remain competitive and to meet the customer requirements,
the GSM system has to offer higher bit rates. Therefore, the work on GSM High Speed
Circuit Switched Data (HSCSD) was started in 1994.
HSCSD is an enhancement of the original circuit switched data service to enable higher user
data rates. The increased data rates are achieved by using multiple time slots per a user
connection. In theory, one to eight time slots can be combined dynamically giving maximum
speed of 76.8kbit/s (9.6 kbit/s * 8) for one user. It should be noted that GSM circuit switched
data shares the capacity with voice services, i.e. a data user using one time slot decreases
the maximum amount possible for voice calls in that particular cell by one. Furthermore, if the
data user uses several time slots for the connection, the maximum amount of voice calls is
decreased by the amount of the used time slots.
In 1997, as a parallel activity to HSCSD, the GSM user data rate per single time slot was
increased from 9.6 kbit/s to 14.4 kbit/s by applying different error coding mechanisms in the
standard. The motivation for this activity was: 1) to better meet the data rates in the fixed
telephone domain (that developed as multiples of 14.4 kbit/s) and 2) to further increase the
maximum offered bit rates. The GSM 14.4 kbit/s data service gives a theoretical maximum
user data rate of 115.2 kbit/s with eight time slots.
Fax is also a part of GSM data services. Fax uses the above mentioned data channels.
Because the transmission over GSM is digital, there is no need for a modem in a mobile
station. Instead, during the communication over PSTN, a modem located at the GSM core
network is used.
For HSCSD, the corresponding modem rates at the fixed telephone network side are for
example V.90 and V.110. For example, with a V.90 modem the maximum bit rate of 43.2
kbit/s can be achieved, which matches with a GSM HSCSD mobile using 3 time slots.
An important feature in HSCSD is that it supports asymmetric connections: data rates (the
amount of time slots) can be different in uplink (to the network) and in downlink (to the
mobile). This is important for example in Internet browsing, where the nature of the traffic is
asymmetric (more data comes to the terminal than leaves the terminal). In HSCSD, it is
possible to reserve more time slots for downlink traffic than for uplink. Also streaming is
possible over HSCSD since it offers guaranteed bit rate service.
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