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T68i
White Paper, April 2002
26
Using GPRS in the T68i
Instead of occupying an entire voice channel for
thedurationofadatasession,theT68isends/
receives data in small packets, as needed, much
like IP on the Internet. Because of this, the T68i
maintains a constant online connection, its data
transmission abilities summoned by the
application in use on an as-needed basis.
The GPRS specification includes four coding
schemes – CS1, CS2, CS3 and CS4 – that allow
data speeds of 9,050 bps, 13,400 bps, 15,600
bps and 21,400 bps respectively. The T68i works
with all four coding schemes, but data speed will
naturally vary according to network configuration.
At the moment, CS-3 and CS-4 are not supported
in any live network, i.e present speed is limited to
40,200 bps.
The GSM system limits the ability to use all eight
time slots, so the T68i uses up to three time slots
for receiving data, and one slot for transmitting.
This means the speed for receiving data is up to
64,200 bps and up to 21,400 bps for sending
data.
Information about the identity of the phone and
the characteristics of the connection are
described in the PDP (Packet Data Protocol)
context. This information is stored both in the
phoneandinthemobilenetwork,sothateach
phone is identified and “visible” to the system.
GSM
9,600/14,400
9,600/14,400
9,050/13,400/
15,600/21,400
1
14,400, 19,200 or 28,800 bps (HSCSD)
GPRS
9,050, 13,400,
15,600, 18,100,
21,400, 26,800,
27,150, 31,200
40,200, 42,800,
46,800 or 64,200 bps
2
9,600/14,400
9,600 bps
9,050/13,400/
15,600/21,400
9,050/13,400/
15,600/21,400
9,050/13,400/
15,600/21,400
Figure 5 A comparison between GSM and GPRS
A normal GSM call uses only one of eight
repeating time slots in the GSM channel, giving a
data speed of 9,600 bps. The T68i supports a
more efficient coding scheme, giving data
speeds of up to 14,400 bps (with necessary net-
work support). Furthermore, High Speed Circuit
Switched Data (HSCSD) adds the possibility of
using two time slots for receiving data, increasing
the data speed to as much as 28,800 bps (net-
work dependent).
In GPRS, data is sent in packets, with up to
threetimeslotsbeingcombinedtoprovidethe
necessary bandwidth. The T68i is prepared to
support 3+1 time slots, giving speeds of up to
64,200 bps for receiving data, depending on cod-
ing scheme.