GSM specifies different mechanisms for data transmission for data transmission for the original GSM allowed for data rates up 9600 bit/s for non-voice services. Bearer services allow for both transparent and non-transparent, synchronous or asynchronous data transmission.
Transparent bearer services use only the functions of the physical layer (layer 1 ISO/OSI reference model) to transmit data; data transmission consequently has a constant delay and throughput, that is if no errors occur. The only mechanism of any use to try and increase the quality of the transmission is forward error correction (FEC). This mechanism codes redundancy into the data-stream and Depending on the FEC, data rate of 2.4, 4.8, or 9.6 kbit/s are possible.
Non-Transparent bearer services use protocols of the layers two and three to implement error correction and flow control. Non-transparent bearer services use the transparent bearer services, while adding a radio link protocol (RLP). This protocol uses mechanisms of high-level data link control (HDLC) (Halsall, 1996), and special selective-reject mechanisms to trigger retransmission of erroneous data. The achieved bit error rate is less than 10-7, but now throughput may vary, this depending on the transmission quality.