What is GPRS?
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:

GPRS Class      Slots   Max. data transfer speed
Class 2         3       8 - 12 kbps upload / 16 - 24 kbps download
Class 4         4       8 - 12 kbps upload / 24 - 36 kbps download
Class 6         4       24 - 36 kbps upload / 24 - 36 kbps download
Class 8         5       8 - 12 kbps upload / 32 - 40 kbps download
Class 10        5       16 - 24 kbps upload / 32 - 48 kbps download
Class 12        5       32 - 48 kbps upload / 32 - 48 kbps download
 

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) 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.