GSM Mobile Network – mobile network of 2nd generation
CSD Data Transmission in GSM Network

Digital cellular networks like GSM have been primarily developed for transferring of human voice. Nevertheless, even the human voice is transmitted in digital form. This makes it relatively easy to transfer general data instead of human voice. However, there are certain limitations associated mainly with the maximum achievable bit rates.

The single GSM radio channel has a maximum transfer rate of 33.8 kbps. Nevertheless for data transfer, the maximum available transfer rate is equal only to 9.6 kbps. The reason is that 11 kbps is used as service channel capacity and it is designed to ensure the functionality of the GSM network. The remaining capacity of 13.2 kbps is utilized to ensure the reliability of transmission, treatment errors and failures. This principle of data transfer is referred as CSD.

Data transfer by CSD

After some time it has been successfully tested, that the data transfers always do not require a highly robust protection mechanisms and it is possible to increase the available transfer rate for data transmission. Specifically, the data transfer rate has been increased to 14.4 kbps but in conjunction with the condition on availability of signal with high quality.

To increase the reliability of data transfer by adding bits to the transmitted data is not the only option. It is possible to use solutions based on the feedback between the sender and the recipient. If the recipient recognizes corrupted data, the sender transmits data again on the basis of request for retransmission. This process obviously requires that both sides agree on this procedure and it is therefore necessary to define the protocol. This protocol is called RLP (Radio Link Protocol).