User's Manual Part 1

6 Product Overview — Introduction to GSM and GPRS
Revision 1.8 P/N 9002363
Data Rates
Four major factors affect actual GPRS data rates:
•Multi-slot Class
Each timeslot can handle a given amount of data, established through
negotiation between the device and the network.
• Channel Coding Scheme
Four CS schemes are defined, each with a different level of error correction
(basically “overhead”, similar to CDPD, which takes the 19.2 down to a real
data rate somewhere between 10 and 12 kbps). As the amount of error
correction is reduced, the quantity of data in the packets increases, but the
probability that packets of data will need to be retransmitted increases.
Conversely, as the amount of error correction increases, less data can be sent
in each packet or timeslot, but the chance that re-transmission of packets is
required decreases. With no error correction, a maximum of 21.4 kbps per
channel can be supported - which, at the maximum 8 timeslots, equals 171.2
(for CS-4) kbps. At CS-1, there is 9.05 kbps per timeslot; this increases to
13.4 kbps per timeslot at CS-2 and 15.6 kbps per time slot at CS-3.
• Network Capacity
The device and the network will negotiate the appropriate combination of MS
class and CS scheme to determine the actual data, based on capabilities of
the device, and how busy the network is at a given point in time.
Because GPRS is an extension of the digital voice GSM network, voice calls
will always take priority over data calls (real time voice is more important than
data); therefore real data rates will, in many cases, be lower than the device
can support.
• Quality of Service (QoS)
There is the ability to assign a Quality of Service to a specific account. The
QoS is used to guarantee a certain data rate as well as capacity.
Most devices will be limited to MS-10, which, at CS-2, is a maximum of 53.6
kbps (4 * 13.4 = 53.6) Rx and 26.8 kbps (4 * 13.4) Rx, or nominally 56 and 28.
SIM
GPRS requires a unique SIM (Subscriber Identity Module) card for each device. The SIM
Card identifies individual users to the network for billing and other purposes, ensures a
common set of SIM-based features, and maintains security with other GSM devices.