Digital data communication techniques require synchronization between transmitters and receivers. There are two main solutions: 1) Asynchronous transmission synchronizes on a per-character basis, resynchronizing with each character. 2) Synchronous transmission synchronizes transmitters and receivers at the bit level or block level, preventing timing drift through synchronized clocks. 3) Error detection and correction techniques like parity checks and cyclic redundancy checks are used to detect errors, while forward error correction allows detection and correction of errors.

1. Digital Data Communications Techniques Key point

4. Asynchronous (diagram)

9. Synchronous (diagram)

13. CRC

16. Error correction k = message, FCS = frame check sequence,FEC=forward error correction n = total bit to be transmit (k+FCS or k + FEC)) On transmission end , each k bit block of data is mapped into an n-bit block (n> k ) called codeword, using FEC encoder, the codeword is then transmitted. Due to the impairments, which may produce bit errors in the signal, the block of data is passed through an FEC decoder with one of four possible outcome DIAGRAM

18. Block Code Principles HAMMING DISTANCE = d( V1, V2 ) which v1 disagree v2 Rule = select lowest d(v1,v2) as original codeword example v1 = 011011 v2= 110001 , so d (v1,v2)= 3 Example For k =2 and n = 5 Data block codeword received codeword = 00100 00 00000 d(v1,v2) = 1 01 00111 d(v1,v2) = 2 10 11001 d(v1,v2)=3 11 11110 d(v1,v2)=4

19. Block code principles

20. ERROR CORRECTION PROCESS