Technical data

www.westermo.com Theoretical and general applications 91
GPRS
GPRS is an extension to the GSM network where packet switching data traffic is sup-
ported. This is different to the circuit switching data traffic that is supported in GSM.
With GPRS each channel that is not busy with call traffic is available for packet switch-
ing data traffic. Packets from several different users can be mixed within the same
channel, which results in efficient sharing of available network resources.
GPRS permits even higher transfer rates as it uses several time slots for the transfer. In
theory rates of up to 115.2 kbit/s can be achieved, however, transfer rates of between
20 – 50 kbit/s are more common (compared to HSCSD, High Speed Circuit Switched
Data, which offers rates from 9.6 – 43.2 kbit/s which some operators also offer for cir-
cuit switched GSM traffic). Transfer rate is however dependent on several factors such
as: operator, terminal, number of users on the same cell, distance to the base radio
station (retransmissions), whether the device is on the move, (hand over between
base radio stations lowers the transfer rate) etc.
The transmission rate is also dependent on how many time slots are being used as
well as which Coding Scheme the communication link is using. There are 4 Coding
Schemes (CS) in GPRS where CS1 is the most secure and the most reliable, but also
the one that has the lowest transfer rate (9.05 kbit/s) while CS4 does not have such
stringent error correction and retransmissions and thus reaches speeds of 21.4 kbit/s.
The speeds as set out above depend on the number of time slots and CS, which
means that 4 time slots on CS4 gives 4 x 21.4 = 85.6 kbit/s. It is also worth mentioning
that the GSM standard specifies 4 CS yet only the two first CS1 and CS2 (13.4 kbit/s/
time slot) are currently implemented on active GPRS networks.
The difference between circuit switching and packet switching networks can in short
be described as:
In the circuit switching network the connection works with a
physical connection between the two parties. This is con-
stantly open, and is not closed until one of the parties decides
to do so, just like a telephone call. This has both advantages
and disadvantages. The communicating units have a constant
connection with each other, they detect the available capacity
and know that this will not be used by another. On the other hand, it is a waste
of resources when the parties are not exchanging data as the line is engaged and
no one else can use it. Accordingly, the parties must hang up the connection when
it will no longer be used.
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