To check and correct errors during the transmission, redundancy data and the information calculated from the source data are added to the stream so as to increase the bit rate. For the voice, the length of these codes is 456 bits every 20ms.
The bit rate of code stream output from the voice coder is 13Kbit/s, which is divided into many 20ms continuous segments with each segment containing 260 bits. They can be classified as:
50 very important bits;
132 important bits;
78 ordinary bits,
Redundancy processing is conducted, as shown in the above diagram.
The block coder is applied with 3 bits of redundancy code; while the excited coder applies with 2 times  redundancy and then adds  4 tail bits into the data stream.
There are three channel coding modes in the GSM system: convolution coding, block coding and parity coding.