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GPRS employs four levels of error correction in its data encoding.
The level of error correction used depends upon a number of variables and it is defined as four levels, CS1, CS2, CS3, and CS4.
GPRS offers a number of coding schemes with different levels of error detection and correction. These are used dependent upon the radio frequency signal conditions and the requirements for the data being sent. These are given labels CS-1 to CS-4:
- CS-1: - This GPRS coding scheme applies the highest level of error detection and correction. It is used in scenarios when interference levels are high or signal levels are low. By applying high levels of detection and correction, this prevents the data having to be re-sent too often. Although it is acceptable for many types of data to be delayed, for others there is a more critical time element. This level of detection and coding results in a half code rate, i.e. for every 12 bits that enter the coder, 24 bits result.
- CS-2: - This error detection and GPRS coding scheme is for better channels. It effectively uses a 2/3 encoder and results in an improved data rate over CS-1.
- CS-3: - This GPRS coding scheme effectively uses a 3/4 coder.
- CS-4: - This scheme is used when the signal is high and interference levels are low. No correction is applied to the signal allowing for a maximum throughput.
|GPRS Coding Algorithms & Data Rates|
|Coding scheme||Max Data rate for One Slot||Max Data rate for Two Slots||Max Data rate for Eight Slots|
** Note: Data rates for the different GPRS coding schemes given in kbps.
In addition to the error detection and coding schemes, GPRS also employs interleaving techniques to ensure the effects of interference and spurious noise are reduced to a minimum. It allows the error correction techniques to be more effective as interleaving helps reduce the total corruption if a section of data is lost.
As blocks of 20 ms data are carried over four bursts, with a total of 456 bits of information, a total of either 181, 268, 312, or 428 bits of payload data are carried dependent upon the error detection and coding scheme chosen, i.e. from CS-1 to CS-4, respectively.
GPRS data rates
The maximum data rates quoted in some marketing literature may differ from the rates quoted above. There are many reasons for this:
- Protocol overhead: The maximum throughput quoted in some literature gives a maximum rate of 171 kbps for CS-4 coding with eight slots. This refers to the maximum theoretical speed of the lowest protocol layer, i.e. the raw data. With the addition of required protocols including TCP/IP this reduces to 160kbps or user data. Similar reductions are applied to the other GPRS coding schemes.
- Number of available time slots: Although maximum data rates of 160 kbps user data, or 171 kbps of raw data may be quoted as peak rates, these are very seldom achieved because the network is very unlikely to allocate all slots to one mobile. Depending upon the network capacity as well as the number of active users in the cell, the number of time slots that are allocated may vary between 1 and 4.
- Channel Interference: The level of interference and the signal level also plays a major role in the data rates that can be achieved. If interference levels are low and the signal levels are high, then the cell may select GPRS coding scheme CS-4 and this will provide a high data rate. However if the signal levels are low and the interference is high then the network will need to select coding scheme CS-1 and this will result in lower data rates being achieved.
- Number of phones sharing time slots: The data rate that can be achieved is also highly dependent upon the number of phones sharing the same time slots. As the number of users increases, so the available capacity in that slot has to be shared and the rate for each user falls.
- Direction of traffic: Most traffic occurs in the downlink - i.e. downloads to the phone. However if uploads from the phone are needed, then this data is likely to be transmitted more quickly because there are normally fewer users utilising this link, and the data being passed in this direction is less. As the capacity for GPRS is the same in both directions, there is less pressure on the uplink.
- Phone multislot class: The class of phone also plays a role in determining the data rate that can be achieved. The multi-slot class for the phone defines its capabilities and can limit the performance in any direction.
The different GPRS classes enable base stations to understand the capabilities of the handset and in this way provide the required capabilities for it. The GPRS class forms an easy method of passing the data over.
Software plays a very large part in mobile phone systems and their development. To enable it to be sectioned into areas that can be addressed separately, the concept of layers has been developed. The system that is used is the OSI data layer. The Open Systems Interconnection model, OSI model is a conceptual model that characterizes and standardises the communication functions of a telecommunication or computing system without regard to their underlying internal structure and technology. Its goal is the interoperability of diverse communication systems with standard protocols.
The OSI data model is used in GSM and other cellular systems but as they become more data-centric, the idea takes a greater prominence. Often these are referred to as layers, 1, 2, and 3.
Layer 1 concerns the physical link between the mobile and the base station. This is often subdivided into two sub-layers, namely the Physical RF layer that includes the modulation and demodulation, and the Physical link layer that manages the responses and controls required for the operation of the RF link. These include elements such as error correction, interleaving and the correct assembly of the data, power control, and the like.
Above this are the Radio Link Control (RLC) and the Medium Access Control (MAC) layers. These organise the logical links between the mobile and the base station. They control the radio link access and they organise the logical channels that route the data to and from the mobile.
There is also the Logical Link Layer (LLC) that formats the data frames and is used to link the elements of the core network to the mobile.
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