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Whilst GSM (2G) networks are excellent for
voice calls, they are limited when it comes to sending and receiving
data. GSM phones use a technology called CSD (Circuit Switched
Data) to transfer data. CSD requires the phone to make a special
connection to the network before it can transfer data (like making
a voice call) which can take up to 30 seconds. Once connected, the
data is sent or received and the user is billed for the time spent
online. Data transfer is relatively slow: 14.4 kbps (kilobits
per second) for GSM 1800 networks (Orange
and T-Mobile) and 9.6 kbps for GSM
900 networks (Vodafone
and O2).
GPRS (General Packet Radio Service) is a
method of enhancing 2G phones to enable them to send and receive
data more rapidly. With a GPRS connection, the phone is "always
on" and can transfer data immediately, and at higher speeds:
typically 32 - 48 kbps. An additional benefit is that data
can be transferred at the same time as making a voice call. GPRS
is now available on most new phones.
GPRS
is part of a series of technologies that are designed to move 2G
networks closer to the performance of 3G networks. The key characteristic
of a 3G network is its ability to transfer large amounts of data
at high speed (up to 2 Mbps), enabling applications like video calling,
video downloads, web browsing, email, etc. By increasing the speed
of a 2G network, some of these applications become possible, e.g.
web browsing and sending or receiving emails with large attachments.
These technologies are called 2.5G and include enhancements
to the CSD technology, such as HSCSD and EDGE.
GPRS Class
Types
The class
of a GPRS phone determines the speed at which data can be transferred.
Technically the class refers to the number of timeslots available
for upload (sending data from the phone) or download (receiving data
from the network). The timeslots used for data are in addition to
the slot that is reserved for voice calls. These
timeslots are available simultaneously, so the greater the number
of slots, the faster the data transfer speed. Because GPRS transmits
data in packets, the timeslots are not in use all the time, but are
shared amongst all users of the network. That increases the overall
data capacity of the network, and it also means that you are billed
for the quantity of data transmitted, not the time that you are online.
It may mean that during busy times, data transfer rates slow down,
because the network will give priority to voice calls.
The
most common GPRS classes in use are as follows:
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GPRS
Class
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Slots
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Max.
data transfer speed
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Class 2
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3
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8 - 12 kbps upload
/ 16 - 24 kbps download
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Class 4
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4
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8 - 12 kbps upload
/ 24 - 36 kbps download
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Class 6
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4
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24 - 36 kbps upload
/ 24 - 36 kbps download
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Class 8
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5
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8 - 12 kbps upload
/ 32 - 40 kbps download
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Class 10
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5
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16 - 24 kbps upload
/ 32 - 48 kbps download
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Class 12
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5
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32 - 48 kbps upload
/ 32 - 48 kbps download
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Generally speaking, the higher the GPRS class,
the faster the data transfer rates.
HSCSD
HSCSD (High Speed Circuit Switched Data) enables data
to be transferred more rapidly than the standard GSM (Circuit Switched
Data) system by using multiple channels. The maximum number of timeslots
that can be used is four, giving a maximum data transfer rate of 57.6
kbps (or 38.4 kbps on a GSM 900 network). HSCSD
is more expensive to use than GPRS, because all four slots are used
simultaneously - it does not transmit data in packets. Because of
this, HSCSD is not as popular as GPRS and is being replaced by EDGE.
EDGE
EDGE
(Enhanced Data rates for GSM Evolution) or EGPRS provides data transfer
rates significantly faster than GPRS or HSCSD. EDGE increases the
speed of each timeslot to 48 kbps and allows the use of up to 8 timeslots,
giving a maximum data transfer rate of 384 kbps. In places
where an EDGE network is not available, GPRS will automatically be
used instead. EDGE offers the best that can be achieved with a 2.5G
network, and will eventually be replaced by 3G.
GPRS Resources
GSM
World Guide to GPRS
Nokia GPRS Guide
Nokia EDGE Guide

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