What is GPRS!!

General packet radio service (GPRS) is a packet oriented mobile data service available to users of the 2G cellular communication systems global system for mobile communications (GSM), as well as in the 3G systems. In 2G systems, GPRS provides data rates of 56-114 kbit/s.
GPRS data transfer is typically charged per megabyte of traffic transferred, while data communication via traditional circuit switching is billed per minute of connection time, independent of whether the user actually is using the capacity or is in an idle state. GPRS is a best-effort packet switched service, as opposed to circuit switching, where a certain quality of service (QoS) is guaranteed during the connection for non-mobile users.
2G cellular systems combined with GPRS are often described as 2.5G, that is, a technology between the second (2G) and third (3G) generations of mobile telephony. It provides moderate speed data transfer, by using unused time division multiple access (TDMA) channels in, for example, the GSM system. Originally there was some thought to extend GPRS to cover other standards, but instead those networks are being converted to use the GSM standard, so that GSM is the only kind of network where GPRS is in use. GPRS is integrated into GSM Release 97 and newer releases. It was originally standardized by European Telecommunications Standards Institute (ETSI), but now by the 3rd Generation Partnership Project (3GPP).
GPRS was developed as a GSM response to the earlier CDPD and i-mode packet switched cellular technologies.

[edit] Technical overview
[edit] Services offered
GPRS extends the GSM circuit switched data capabilities and makes the following services possible:
"Always on" internet access
Multimedia messaging service (MMS)
Push to talk over cellular (PoC/PTT)
Instant messaging and presence—wireless village
Internet applications for smart devices through wireless application protocol (WAP)
Point-to-point (P2P) service: inter-networking with the Internet (IP)
If SMS over GPRS is used, an SMS transmission speed of about 30 SMS messages per minute may be achieved. This is much faster than using the ordinary SMS over GSM, whose SMS transmission speed is about 6 to 10 SMS messages per minute
[edit] Protocols supported
GPRS supports the following protocols:
internet protocol (IP). In practice, the mobile built-in browser uses IPv4 since IPv6 is not yet popular.
point-to-point protocol (PPP). In this mode PPP is often not supported by the mobile phone operator but if the mobile is used as a modem to the connected computer, PPP is used to tunnel IP to the phone. This allows an IP address to be assigned dynamically to the mobile equipment.
X.25 connections. This is typically used for applications like wireless payment terminals, although it has been removed from the standard. X.25 can still be supported over PPP, or even over IP, but doing this requires either a network based router to perform encapsulation or intelligence built in to the end-device/terminal; e.g., user equipment (UE).
When TCP/IP is used, each phone can have one or more IP addresses allocated. GPRS will store and forward the IP packets to the phone during cell handover (when you move from one cell to another). TCP handles any packet loss (e.g. due to a radio noise induced pause) resulting in a temporary throttling in transmission speed.
[edit] Hardware
Devices supporting GPRS are divided into three classes:
Class A
Can be connected to GPRS service and GSM service (voice, SMS), using both at the same time. Such devices are known to be available today.
Class B
Can be connected to GPRS service and GSM service (voice, SMS), but using only one or the other at a given time. During GSM service (voice call or SMS), GPRS service is suspended, and then resumed automatically after the GSM service (voice call or SMS) has concluded. Most GPRS mobile devices are Class B.
Class C
Are connected to either GPRS service or GSM service (voice, SMS). Must be switched manually between one or the other service.
A true Class A device may be required to transmit on two different frequencies at the same time, and thus will need two radios. To get around this expensive requirement, a GPRS mobile may implement the dual transfer mode (DTM) feature. A DTM-capable mobile may use simultaneous voice and packet data, with the network coordinating to ensure that it is not required to transmit on two different frequencies at the same time. Such mobiles are considered pseudo-Class A, sometimes referred to as "simple class A". Some networks are expected to support DTM in 2007.

Huawei E220 Modem
USB GPRS modems use a terminal-like interface USB 2.0 and later, data formats V.42bis, and RFC 1144 and external antennas. Modems can be added as cards (for laptops) or external USB devices which are similar in shape and size to a computer mouse.
[edit] Coding schemes and speeds
The upload and download speeds that can be achieved in GPRS depend on a number of factors such as:
the number of BTS TDMA time slots assigned by the operator
the maximum capability of the mobile device expressed as a GPRS multislot class
the channel encoding used summarised in the following table.

The least robust, but fastest, coding scheme (CS-4) is available near a base transceiver station (BTS), while the most robust coding scheme (CS-1) is used when the mobile station (MS) is further away from a BTS.
Using the CS-4 it is possible to achieve a user speed of 20.0 kbit/s per time slot. However, using this scheme the cell coverage is 25% of normal. CS-1 can achieve a user speed of only 8.0 kbit/s per time slot, but has 98% of normal coverage. Newer network equipment can adapt the transfer speed automatically depending on the mobile location.
In addition to GPRS, there are two other GSM technologies which deliver data services: circuit-switched data (CSD) and high-speed circuit-switched data (HSCSD). In contrast to the shared nature of GPRS, these instead establish a dedicated circuit (usually billed per minute). Some applications such as video calling may prefer HSCSD, especially when there is a continuous flow of data between the endpoints.
The following table summarises some possible configurations of GPRS and circuit switched data services.
[edit] Multiple access schemes
The multiple access methods used in GSM with GPRS are based on frequency division duplex (FDD) and TDMA. During a session, a user is assigned to one pair of up-link and down-link frequency channels. This is combined with time domain statistical multiplexing; i.e., packet mode communication, which makes it possible for several users to share the same frequency channel. The packets have constant length, corresponding to a GSM time slot. The down-link uses first-come first-served packet scheduling, while the up-link uses a scheme very similar to reservation ALOHA (R-ALOHA). This means that slotted ALOHA (S-ALOHA) is used for reservation inquiries during a contention phase, and then the actual data is transferred using dynamic TDMA with first-come first-served scheduling.
[edit] Addressing
A GPRS connection is established by reference to its access point name (APN). The APN defines the services such as wireless application protocol (WAP) access, short message service (SMS), multimedia messaging service (MMS), and for Internet communication services such as email and World Wide Web access.
In order to set up a GPRS connection for a wireless modem, a user must specify an APN, optionally a user name and password, and very rarely an IP address, all provided by the network operator
[edit] Usability
The maximum speed of a GPRS connection offered in 2003 was similar to a modem connection in an analog wire telephone network, about 32-40 kbit/s, depending on the phone used. Latency is very high; round-trip time (RTT) is typically about 600-700 ms and often reaches 1 s. GPRS is typically prioritized lower than speech, and thus the quality of connection varies greatly.
Devices with latency/RTT improvements (via, for example, the extended UL TBF mode feature) are generally available. Also, network upgrades of features are available with certain operators. With these enhancements the active round-trip time can be reduced, resulting in significant increase in application-level throughput speeds.