 need to be punctured to fit the output within the transmission time interval. It can be shown that the BER performances of the two techniques are similar. Error Control Coding Error Error Source detection correction Modulator Transmitter source encoder matcher encoder encoder Channel Error Error Source Rate de- detection correction Receiver decoder matcher modulator decoder decoder Noise Figure 6.23 A digital telecommunications bit sequence processing order It follows that the exact position of the repeated or punctured bit must be known to both encoder and decoder. A decoder generally uses the repeated bits to increase the reliability of the received symbol (more energy is received as the output is received twice). Conversely, a marker is placed by the receiver where data has been punctured. The decoder treats this bit as unknown (neither a 0 nor a 1) and proceeds to calculate the codeword Hamming distance without considering the punctured position. Data Flow Order The order of information transmission can therefore be described as in the following. First, a source encoder converts continuous or discrete source information into a structured and effi- cient bit sequence. This data sequence then undergoes a CRC process to generate a parity check sequence. The combined sequences are then FEC encoded, followed by a rate matching process to ensure the total bit sequence fits within the transmission time interval of the system. These binary bits are then transmitted using a process of digital modulation which is the sub- ject of the next chapter. On the receiver side, the received bit sequence is rate dematched and then FEC decoded. This process ensures that a highly reliable data sequence is obtained with ideally no errors. A CRC decoder then checks to determine whether any errors have occurred and if not the resulting decoded sequence is passed to a data sink. This is the process chain implemented in almost all digital telecommunications system in use today (Figure 6.23). Error-free communication of information has