General Packet Radio Service
General Packet Radio Service (GPRS) is a Mobile Data Service available to users of GSM and IS-136 mobile phones. GPRS data transfer is typically charged per megabyte of transferred data, while data communication via traditional circuit switching is billed per minute of connection time, independent of whether the user has actually transferred data or he has been in an idle state. GPRS can be utilized for services such as WAP access, SMS and MMS, but also for Internet communication services such as email and web access. In the future, it is expected that low cost voice over IP will be made available in cell phones.
2G cellular systems combined with GPRS 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 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 standards releases starting with Release 97 and onwards. First it was standardized by ETSI but now that effort has been handed onto the 3GPP.
GPRS basics
GPRS is different from the older Circuit Switched Data (or CSD) connection included in GSM standards. In CSD, 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 are 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 are being transferred, the bandwidth is unavailable to other potential users.
The multiple access methods used in GSM with GPRS are based on frequency division duplex (FDD) and FDMA. During a session, a user is assigned to one pair of uplink and downlink 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. In the downlink, first-come first-served packet scheduling is used. In the uplink, a scheme that is very similar to reservation ALOHA is used. This means that slotted Aloha (S-ALOHA) is used for reservation inquiries during a contention phase, and then the actual data is transferred using first-come first-served scheduling.
GPRS originally supported (in theory) IP, PPP and X.25 connections. The last 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. In practice, mainly IPv4 is used. PPP is often not supported by the operator, while IPv6 is not yet popular.
The GPRS capability 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. See List of Class A GPRS Phones.
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 to be pseudo Class A. Some networks are expected to support DTM in 2007.
GPRS multislot classes
GPRS speed is a direct function of the number of TDMA time slots assigned, which is the lesser of (a) what the particular cell supports and (b) the maximum capability of the mobile device expressed as a GPRS Multislot Class.
Multislot Class |
Downlink Slots |
Uplink Slots |
Active Slots |
1 |
1 |
1 |
2 |
2 |
2 |
1 |
3 |
3 |
2 |
2 |
3 |
4 |
3 |
1 |
4 |
5 |
2 |
2 |
4 |
6 |
3 |
2 |
4 |
7 |
3 |
3 |
4 |
8 |
4 |
1 |
5 |
9 |
3 |
2 |
5 |
10 |
4 |
2 |
5 |
11 |
4 |
3 |
5 |
12 |
4 |
4 |
5 |
32 |
5 |
3 |
6 |
The most common GPRS multislot classes are:
Class 2
Minimal GPRS implementation
Class 4
Modest GPRS implementation, 50% faster download than Class 2
Class 6
Modest implementation, but with better uploading than Class 4
Class 8
Better implementation, 33% faster download than Classes 4 & 6
Class 10
Better implementation, and with better uploading than Class 8, seen in better cell phones and PC Cards
Class 12
Best implementation, with maximum upload performance, typically seen only in high-end PC Cards
GPRS coding scheme
Transfer speed depends also on the channel encoding used. The least robust (but fastest) coding scheme (CS-4) is available near the Base Transceiver Station (BTS) while the most robust coding 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 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.
Coding
scheme |
Speed
(kbit/s) |
CS-1 |
8.0 |
CS-2 |
12.0 |
CS-3 |
14.4 |
CS-4 |
20.0 |
Technology |
Download (kbit/s) |
Upload (kbit/s) |
Configuration |
CSD |
9.6 |
9.6 |
1+1 |
HSCSD |
28.8 |
14.4 |
2+1 |
HSCSD |
43.2 |
14.4 |
3+1 |
GPRS |
80.0 |
20.0 (Class 8 & 10 and CS-4) |
4+1 |
GPRS |
60.0 |
40.0 (Class 10 and CS-4) |
3+2 |
EGPRS (EDGE) |
236.8 |
59.2 (Class 8, 10 and MCS-9) |
4+1 |
EGPRS (EDGE) |
177.6 |
118.4 (Class 10 and MCS-9) |
3+2 |
Note: Like CSD, HSCSD establishes a circuit and is usually billed per minute. For an application such as downloading, HSCSD may be preferred, since circuit-switched data are usually given priority over packet-switched data on a mobile network, and there are relatively few seconds when no data are being transferred.
GPRS 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 and hardware
GPRS upgrades GSM data services providing:
USB GPRS modem
USB GPRS modems use a terminal like interface USB 2.0 and upper, data formats V.42bis, and RFC 1144 and external antennas.
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. Some mobile phone operators offer flat rate access to the Internet and some other mobile phone operators base their tariffs on data transferred, usually rounded off per 100 kilobyte.
During its heyday, the mid 2000s, typical rates for GPRS service varied widely, ranging from EUR €0,24 per megabyte to over €20 per megabyte.
The maximum speed of a GPRS connection (as offered in 2003) is 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; a round-trip ping being typically about 600–700 ms and often reaching one second round trip time. GPRS is typically prioritized lower than speech, and thus the quality of connection varies greatly.
In order to set up a GPRS connection for a wireless modem, a user needs to specify Access Point Name (APN), optionally a user name and password, and very rarely an IP address, all provided by the network operator.
Devices with latency /RTT improvements (via e.g. the extended UL TBF mode feature) are rather widely available. Also network upgrades the feature(s) are available within certain operators. With these enhancements the active RTT can be reduced, resulting in significant increase in application-level throuhput speeds.