2.5.2.1 Error Control Techniques

2.5.2.1 Error Control Techniques:

1-Automatic Repeat request (ARQ):

            The transmitter sends a packet of the encoded message and the receiver checks it. If no error detected an acknowledgement(ACK) is sent by the receiver to sent another package. But if an error detected no acknowledgement (NACK) sent by the receiver and the transmitter will retransmit the packet again after a certain period of time.

2-Forward Error Correction(FEC):

            The transmitter encodes the data with an error correcting code (ECC).The receiver here doesn’t send any acknowledgement to the transmitter. But It decodes the received message to the most likely data.

3-Hybrid ARQ (H-ARQ) (ARQ+FEC):

            It is a combination of both the previous techniques. The minor errors are corrected by the ECC without any retransmission request and major errors are requested to be retransmitted again.

2.5.2.2 Error detection schemes:

1-Repetition codes:

            A repetition code is a coding scheme that repeats the bits across a channel to achieve error-free communication. Given a stream of data to be transmitted, the data is divided into blocks of bits. Each block is transmitted some predetermined number of times. If one of the blocks was different  the decoder will detect that an error occurred.

2-Parity bits:

            A parity bit is a bit that is added to a group of source bits to ensure that the number of set bits (i.e., bits with value 1) in the outcome is even or odd. It is a very simple scheme that can be used to detect single or any other odd number of errors in the output. But an even number of flipped bits will make the parity bit appear correct even though the data is erroneous.

3-Checksums:

            A checksum of a message is a modular arithmetic sum of message code words of a fixed word length (e.g., byte values). The sum may be negated by means of a ones'-complement operation prior to transmission to detect errors resulting in all-zero messages.

4-Cyclic redundancy checks (CRCs):

            A cyclic redundancy check (CRC) is a single-burst-error-detecting cyclic code and non-secure hash function designed to detect accidental changes to digital data in computer networks.

            It is characterized by specification of a so-called generator polynomial, which is used as the divisor in a polynomial long division over a finite field, taking the input data as the dividend, and where the remainder becomes the result.