• In the  smaller bit errors, a single bit within a character may have been corrupted by noise and changed to its opposite.
• There is no way that the receiving part of the system can detect this error. The character count agrees with the number of characters actually received.
• But a single bit error can have a severe impact when a character is changed or a number is corrupted.
• So a techniques of detecting and even correcting errors have been developed.
• And here is the advantage of digital system over analog system.
• In analog system, it cannot be confirmed that voltage measured 1.65 V  was actually 1.2 V ( signal corrupted by 0.45v of noise).
• In Digital system, it is possible to single-bit errors, multiple-bit errors and it can be corrected this errors from this corrupted bit without re-transmissions.
• There are two methods of error detection, Parity and Checksums.
• The error detection and correction bits are transmitted along with the actual data bit as the part of the final system format.
• If the bits as received agree with the bits are recalculated by the receiver, it is assumed that there is no error.

CHECK SUMS

• Another approach is used to overcome the weakness of parity bits called Check Sums
• Instead of a single bit, a group of bits is developed by checksum
• The receiver develops its summary check sum using the same rules as the receiver uses and then compares results.
• A single checksum of just 16 bits length can act as the summary of several hundred or thousands data bits.