T68i
White Paper, April 2002
26
Using GPRS in the T68i
Insteadof occupying an entire voice channel for
thedurationofadatasession,theT68isends/
receivesdata in small packets, as needed, much
like IP on the Internet. Because of this, the T68i
maintainsa constant online connection, its data
transmissionabilities summoned by the
applicationin use on an as-needed basis.
The GPRS specificationincludes 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 codingschemes, but data speed will
naturallyvary according to network configuration.
Atthe moment, CS-3 and CS-4 are not supported
in any livenetwork, 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 characteristicsof the connection are
described in the PDP (PacketData Protocol)
context.This information is stored both in the
phoneandinthemobilenetwork,sothateach
phoneis identifiedand “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 comparisonbetween GSM and GPRS
A normal GSM callu sesonly one of eight
repeating time slots in the GSM channel, giving a
data speedof 9,600 bps. The T68i supports a
more efficientcoding scheme, giving data
speeds of up to 14,400 bps (with necessary net-
work support).Furthermore, High Speed Circuit
Switched Data (HSCSD) adds the possibilityof
usingtwo time slots for receiving data, increasing
the datas peedto as much as 28,800 bps (net-
work dependent).
In GPRS, data is sent in packets,with u pto
threetimeslotsbeingcombinedtoprovidethe
necessary bandwidth. The T68i is prepared to
support3+1 time slots, giving speeds of up to
64,200bps for receiving data, depending on cod-
ing scheme.