GPRS technology tutorial [3] - coding schemes and layers

- a summary or tutorial describing GPRS, General Packet Radio Service, giving the details of the layers and coding schemes used

In order to accommodate the packet data within GPRS it has been necessary to develop the coding schemes. Additionally the layers based on the OSI system has become more important than it was for some of the previous systems and descriptions what are contained within these layers are found below.

GPRS coding

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:

1. CS-1: - This 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. It results in a throughput of 9.05 kbps actual throughput data rate.



2. CS-2: - This error detection and coding scheme is for better channels. It effectively uses a 2/3 encoder and results in a real data throughput of 13.4 kbps which includes the RLC/MAC header etc.



3. CS-3: - This effectively uses a 3/4 coder and results in a data throughput of 15.6 kbps.



4. 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 of 21.4 kbps. If all eight slots were used then this would enable a data throughput of 171.2 kbps to be achieved.

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.

Layers

Software plays a very large part in the current cellular communications systems. To enable it to be sectioned into areas that can be addressed separately, the concept of layers has been developed. It 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.

Further pages of this GPRS tutorial
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