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General Packet Radio Service (GPRS) is a digital mobile phone technology. It is considered as 2.5G, between the second and third generation. It provides moderate speed data transfer, by using unused TDMA channels 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 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.
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 4r1t. 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 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 3r2t. This means that 3 slots are allocated to downloading and 2 slots to uploading. This profile is appropriate for applications where data is sent back-and-forth in roughly equal amount, such as instant messaging.
The transfer speed depends also on the channel encoding used. The best encoding scheme (CS-4) is available near the BTS while the worst encoding scheme (CS-1) is used when the 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.
Note; Like CSD, HSCDS establishes a circuit and is usually billed per second. For an application such as downloading, HSCDS 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 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's inability to differ between radio noise induced pauses and network congestion makes the protocol unsuitable for GPRS (or any radio based IP traffic). A radio noise induced pause will make TCP (unnecessarily) throttle back its transmission speed.
GPRS upgrades GSM data services providing:
Telephone operators have priced GPRS relatively cheaply (compared to older GSM data transfer, CSD and HSCSD) in many areas, such as Finland. Most telco operators don't offer flat rate access to the Internet, instead basing their tariffs on data transferred, usually rounded of per 100 kilobyte. Typical rates vary wildly, ranging from EUR 1 per megabyte to over EUR 20 per megabyte.
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 bad, a ping being about 600-700 ms and often about 1 second round trip time. GPRS is typically prioritised lower than speech, and thus the quality of connection varies greatly. Most of the limitations are not technical, as GPRS could be made to work a lot faster.
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