packet radio

General Packet Radio Service

General Packet Radio Service (GPRS) is a packet oriented Mobile Data Service available to users of Global System for Mobile Communications (GSM) and IS-136 mobile phones. It provides data rates from 56 up to 114 kbit/s. GPRS is a technology which is changing the present shape of mobile communication.

GPRS can be used for 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. 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).

Basics

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. 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.

GPRS originally supported (in theory) Internet Protocol (IP), Point-to-Point Protocol (PPP) and X.25 connections. The last has been 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 router to perform encapsulation or intelligence built in to the end-device/terminal e.g. UE(User Equipment). In practice, the mobile built-in browser uses IPv4. In this mode PPP is often not supported by the mobile phone operator, while IPv6 is not yet popular. But if the mobile is used as a modem to the connected computer, PPP is used to tunnel IP to the phone. This allows DHCP to assign an IP Address and then the use of IPv4 since IP addresses used by mobile equipment tend to be dynamic. 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.

GPRS is new technology in which 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

Coding scheme

 Coding 
scheme
 Speed 
(kbit/s)
CS-1 8.0
CS-2 12.0
CS-3 14.4
CS-4 20.0

Transfer speed depends also on the channel encoding used. 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.

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.

 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

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.

Services and hardware

GPRS upgrades GSM data services providing:

USB GPRS 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 add in cards (for laptop) or external USB devices which are similar in shape and size to a computer mouse.

GPRS can be used as the bearer of SMS. 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

The GPRS context 1 is set with +CGDCONT Hayes command .

Availability

In many areas, such as France, telephone operators have priced GPRS relatively cheaply (compared to older GSM data transfer, CSD and HSCSD). Some mobile phone operators offer flat rate access to the Internet, while others charge based on data transferred, usually rounded up to 100 kilobytes.

During the heyday of GPRS in the developed countries, around 2005, typical prices varied from EUR €0,24 per megabyte to over €20 per megabyte. In developing countries, prices vary widely, and change. Some operators gave free access while they decided pricing, for example in Togocel.tg in Togo, West Africa, others were over-priced, such as Tigo of Ghana at one US dollar per megabyte or Indonesia at $3 per megabyte. Mero Mobile of Nepal charges users up to a set amount and then has unlimited Internet access. AirTel of India charges $0.025 per megabyte. As of 2008, data access in Canada is still prohibitively expensive. For example, Fido charges $0.05 per kilobyte, or roughly $50 per megabyte.. In Venezuela, Digitel charges about $20 per 100 Mb or $25 for unlimited access.

Pre-Paid SIM Cards allow travelers to buy short term internet access. The mean price in developing nations is US$1 per hour .

The maximum speed of a GPRS connection offered in 2003 was similar to a modem connection in an analog wire telephone network, about 32 to 40 kbit/s, depending on the phone used. Latency is very high; a round-trip ping is typically about 600 to 700 ms and often reaches 1s. 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 must specify an 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 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.

See also

References

External links

ca;GPRS

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