GPRS - General Packet Radio Service

General Packet Radio Service (GPRS) is a mobile data service available to users of GSM mobile phones. It is 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 TDMA channels in the GSM network. 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 is the only kind of network where GPRS is in use. GPRS is integrated into GSM standards releases starting with Release 97 and onwards, first it was standardised by ETSI but now that effort has been handed onto the 3GPP.

GPRS service
GPRS is different from the older Circuit Switched Data (or CSD) connection included in GSM standards releases before Release 97 (from 1997, the year the standard was feature frozen) using a GSM phone, in that in the older system, a data connection establishes a circuit, and reserves the full bandwidth of that circuit during the lifetime of the connection. GPRS is packet-switched which means that multiple users share the same transmission channel, only transmitting when they have data to send. This means that the total available bandwidth can be immediately dedicated to those users who are actually sending at any given moment, providing higher utilisation where users only send or receive data intermittently. Web browsing, receiving e-mails as they arrive and instant messaging are examples of uses that require intermittent data transfers, which benefit from sharing the available bandwidth.

Usually, GPRS data is billed per kilobytes of information transceived while circuit-switched data connections are billed per second. The latter is to reflect the fact that even during times when no data is being transferred, the bandwidth is unavailable to other potential users.

GPRS originally supported (in theory) IP, PPP and X.25 connections. The latter has been typically used for applications like wireless payment terminals although it has been removed as a requirement from the standard. X.25 can still be supported over PPP, or even over IP, but doing this requires either a router to do encapsulation or intelligence built into the end terminal.


GPRS speeds and profiles
Packet-switched data under GPRS is achieved by allocating unused cell bandwidth to transmit data. As dedicated voice (or data) channels are setup by phones, the bandwidth available for packet switched data shrinks. A consequence of this is that packet switched data has a poor bit rate in busy cells. The theoretical limit for packet switched data is approx. 170 kbit/s. A realistic bit rate is 30-70 kbit/s. A change to the radio part of GPRS called EDGE allows higher bit rates of between 20 and 200 kbit/s. The maximum data rates are achieved only by allocation of more than one time slot in the TDMA frame. Also, the higher the data rate, the lower the error correction capability. Generally, the connection speed drops logarithmically with distance from the base station. This is not an issue in heavily populated areas with high cell density, but may become an issue in sparsely populated/rural areas.

GPRS class 8 is also known as 4+1. This means that 4 slots are allocated to downloading and 1 slot to uploading. This profile is appropriate for applications where data is mostly downloaded, such as web browsing. If the user reads more e-mail than he or she sends, this is also an appropriate profile. Class 8 is usually selected by default on mobile devices that support GPRS.

GPRS class 10 is also known as 4+2. This means that 4 slots are allocated to downloading and 2 slots to uploading, but no more than 5 slots may be used at the same time. This profile is appropriate for applications where data is sent back-and-forth in roughly equal amount, such as instant messaging.

Other classes exists, including GPRS class 6 (3+2) and GPRS class 4 (3+1) used in older devices. Some rare devices can do as much as 4+4 (up to 4 slots in both upload and upload, but maximum 5 total). Those are for industrial use only, though, as more than 2 upload slots are considered a health hazard for nearby user.

The transfer speed depends also on the channel encoding used. The best encoding scheme (CS-4) is available near the Base Transceiver Station (BTS) while the worst encoding scheme (CS-1) is used when the Mobile Station (MS) is further away from the BTS.

Using the CS-4 it is possible to achieve a speed of 21.4 kbit/s per time slot. However by using this scheme the cell coverage is 25% from the normal. CS-1 can achieve a speed of 9.05 kbit/s per time slot and has 98% of the normal coverage.

Each slot can reach a maximum of 14.4 kilobit per second.

                 download               upload
GPRS       4+1 57.6 kbit/s      14.4 kbit/s (class 8 & 10)
GPRS       3+2 43.2 kbit/s      28.8 kbit/s (class 10)
CSD         9.6 kbit/s                9.6 kbit/s
HSCSD    28.8 kbit/s             14.4 kbit/s (2+1)
HSCSD    43.2 kbit/s             14.4 kbit/s (3+1)

Note; Like CSD, HSCSD establishes a circuit and is usually billed per second. For an application such as downloading, HSCSD may be preferred, since circuit-switched data is usually given priority over packet-switched data on a mobile network, and there are few seconds when no data is being transferred.

GPRS packet switched data is packet based. 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). A radio noise induced pause can be interpreted by TCP as packet loss, and cause a temporary throttling in transmission speed.


GPRS Services
GPRS upgrades GSM data services providing:

Point-to-point (PTP) service: internetworking with the Internet (IP protocols) and X.25 networks.
Point-to-multipoint (PT2MP) service: point-to-multipoint multicast and point-to-multipoint group calls.
Short Message Service (SMS): bearer for SMS.
Anonymous service: anonymous access to predefined services.
Future enhancements: flexible to add new functions, such as more capacity, more users, new accesses, new protocols, new radio networks.

GPRS in practice
Telephone operators have priced GPRS relatively cheaply (compared to older GSM data transfer, CSD and HSCSD) in many areas, such as Finland. Most mobile phone operators don't offer flat rate access to the Internet (with the notable exception of T-Mobile in the United States), instead basing their tariffs on data transferred, usually rounded off per 100 kilobyte.

Typical rates vary wildly, ranging from EUR €1 per megabyte to over €20 per megabyte. In the US, T-Mobile offers US$20 per month unlimited GPRS. Other carriers such as AT&T Wireless also offer flat-rate plans. Orange (UK) offers a 1 Gigabyte package for £88 a month.

GPRS Data on pre-paid packages is usually expensive, and limited to WAP and MMS.

The maximum speed of a GPRS connection (as offered in 2003) is the same as modem connection in an analog wire telephone network, about 4-5 kB/s (depending on the phone used). Latency is very high; a round-trip ping being typically about 600-700 ms and often reaching 1 second round trip time. GPRS is typically prioritised lower than speech, and thus the quality of connection varies greatly.

External Links

• Free online tutorial (http://www.comsoc.org/livepubs/surveys/public/3q99issue/bettstetter.html).

• GSM World - Association Responsible for GSM and GPRS (http://www.gsmworld.com/technology/gprs/intro.shtml).

• GPRS Attach and PDP Context Activation Sequence Diagram .