General Packet Radio Service (GPRS) is a second and a half generation (2.5G) packet-switched Mobile Data Service, developed exclusively for users of Global System for Mobile Communications (GSM) and IS-136 mobile phones, and integrated into GSM Release 97 and newer releases.
Through the use of otherwise unused time division multiple access (TDMA) channels, GPRS is able to transfer data at a moderate speed. It enables services such as email and internet access through Wireless Application Protocol (WAP), as well as Short Message Service (SMS), Multimedia Messaging Service (MMS) and Push to talk over Cellular (PoC/PTT).
Internet Protocol (IP), Point-to-Point Protocol (PPP) and X.25 connections were all supported by GPRS, although X.25 has since been removed. GPRS was standardized first by the European Telecommunications Standards Institute (ETSI), and then by the 3rd Generation Partnership Project (3GPP).
GPRS-enabled devices have three capability classes: Class A, which can be connected to GPRS service and GSM service simultaneously; Class B (the most common classification for GPRS-enabled devices), which can be connected to GPRS service and GSM service, but only one at a time, and service switching is done automatically; and Class C, which can be connected to GPRS and GSM service, but only one at a time, and service switching is done manually.
Unlike data communication through traditional circuit switching, which is billed per minute of connection time, GPRS data transfer is charged for every megabyte or kilobyte of data transmitted or received. This is advantageous for GPRS users, as users of the Circuit Switched Data (CSD) standard, found in older GSM standards, get billed per second of connection time, regardless of whether or not they are actually transferring data. This is because in a CSD network, the bandwidth is made unavailable to other users once a user establishes a connection.
Since GPRS is a packet-switched technology, total bandwidth on a transmission channel is maximized, since multiple users are able to share the same channel. They only transmit when they have data to send, thus ensuring that the bandwidth is dedicated to users who are transmitting data at any given moment. This is favorable in comparison to CSD, where a connection establishes and reserves full bandwidth of the circuit for the entire duration of the connection.
GPRS makes use of multiple access methods that are based on frequency division duplex (FDD) and frequency division multiple access (FDMA). Each user is assigned a pair of up-link and down-link frequency channels, although it is possible for several users to share the same frequency channel, enabled by time domain statistical multiplexing (also known as "packet mode communication").
A first-come first-served packet scheduling scheme is used for the down-link. A scheme based on reservation ALOHA is used for the uplink. During a contention phase, reservation inquiries make use of slotted ALOHA (S-ALOHA). Data transmission occurs with the use of dynamic TDMA, and it follows a first-come first-served scheduling scheme.