## Multiplebit error correction hamming code

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In coding theory multiplebit error correction hamming code, Hamming 7,4 is a linear error-correcting code that encodes four bits of data into seven bits by adding three parity bits. It is a member of a larger family of Hamming codesbut the term Hamming code often refers to this specific code that Richard W.

Hamming introduced in At the time, Hamming worked at Bell Telephone Laboratories and was frustrated with the error-prone punched card reader, which is why he started working on error-correcting codes. The Hamming code adds three additional check bits to every four data bits of the message.

Hamming's 7,4 algorithm can correct any single-bit error, or detect all single-bit and two-bit errors. In other words, the minimal Hamming distance between any two correct codewords is 3, and received words can be correctly decoded if they are at a multiplebit error correction hamming code of at most one from the codeword that was transmitted by the sender. This means that for transmission medium situations where burst errors do not occur, Hamming's 7,4 code is effective as the medium would have to be extremely noisy for two out of seven bits to be flipped.

The goal of the Hamming codes is to create a set of parity bits that overlap such that a single-bit error the bit is logically flipped in value in a data bit or a parity bit can be detected and corrected.

While multiple overlaps can be created, the general method is presented in Hamming codes. This table describes which parity bits cover which transmitted bits in the encoded word. For example, p 2 provides an even parity for bits 2, 3, 6, and 7.

It also details which transmitted bit is covered by which parity bit by multiplebit error correction hamming code the column. For example, d 1 is covered by p 1 and p 2 but not p 3 This table will have a striking resemblance to the parity-check matrix H in the next section.

So, by picking the parity bit coverage correctly, all errors with a Hamming distance of 1 can be detected and corrected, which is the point of using a Multiplebit error correction hamming code code. Hamming codes can be computed in linear algebra terms through multiplebit error correction hamming code because Hamming codes are linear codes.

For the purposes of Hamming codes, two Hamming matrices can be defined: As mentioned above, rows 1, 2, and 4 of G should look familiar as they map the data bits to their parity bits:.

The remaining rows 3, 5, 6, 7 map the data to their position in encoded form and there is only 1 in that row so it is an identical copy. In fact, these four rows multiplebit error correction hamming code linearly independent and form the identity matrix by design, not coincidence. Also as mentioned above, the three rows multiplebit error correction hamming code H should be familiar.

These rows are used to compute the syndrome vector at the receiving end and if multiplebit error correction hamming code syndrome vector is multiplebit error correction hamming code null vector all zeros then the received word is error-free; if non-zero then the multiplebit error correction hamming code indicates which bit has been flipped. The four data bits — assembled as a vector p — is pre-multiplied by G i. The original 4 data bits are converted to seven bits hence the name "Hamming 7,4 " with three parity bits added to ensure even parity using the above data bit coverages.

The first table above shows the mapping between each data and parity bit into its final bit position 1 through 7 but this can also be presented in a Venn diagram. The first diagram in this article shows three circles one for each parity bit and encloses data bits that each parity bit covers.

The second diagram shown to the right is identical but, instead, the bit positions are marked. For the remainder of this section, the following 4 bits shown as a column vector will be used as a running example:. Suppose we want to transmit this data over a noisy communications channel. Specifically, a binary symmetric channel meaning that error corruption does not favor either zero or one it is symmetric in causing errors. Furthermore, all source vectors are assumed to be equiprobable.

We take the product of G and pwith entries modulo 2, to determine the transmitted codeword x:. This means that would be transmitted instead of transmitting Programmers concerned about multiplication should observe that each row of the result is the least significant bit of the Population Count of set bits resulting from the row and column being Bitwise ANDed together rather than multiplied.

In the adjacent diagram, the seven bits of the encoded word are inserted into their respective locations; from inspection it is clear that the multiplebit error correction hamming code of the red, green, and blue circles are even:. What will be shown shortly is that if, during transmission, a bit is flipped then the parity of two or all three circles will be incorrect and the errored bit can be determined even if one of the parity bits by knowing that the parity of all three of these circles should be even.

If no error occurs during transmission, then the received codeword r is identical to the transmitted codeword x:. The receiver multiplies H and r to obtain the syndrome vector zwhich indicates whether an error has occurred, and if so, for which codeword bit. Performing this multiplication again, entries modulo Since the syndrome z is the null vector multiplebit error correction hamming code, the receiver can conclude multiplebit error correction hamming code no error has occurred.

This conclusion is based on the observation that when the data vector is multiplied by Ga change of basis occurs into a vector subspace that is the kernel of H. As long as nothing happens during transmission, r will remain in the kernel of H and the multiplication will yield the null vector. Since x is the transmitted data, it is without error, and as a result, the product of H and x is zero.

The diagram to the right shows the bit error shown in blue text and the bad parity created shown in red text in the red and green circles. The bit error can be detected by computing the parity of the red, green, and blue circles. If a bad parity is detected then the data bit that overlaps only the bad parity circles is the bit with the error. In the above example, the red and green circles have bad parity so the bit corresponding multiplebit error correction hamming code the intersection of red and green but not blue indicates the errored bit.

Furthermore, the general algorithm used see Hamming code General algorithm was intentional in its construction so that the syndrome of corresponds to the binary value of 5, which indicates the fifth bit was corrupted.

Thus, an error has been detected in bit 5, and can be corrected simply flip or negate its value:. Once the received vector has been determined to be error-free or corrected if an error occurred assuming only zero or one bit errors are possible then the received data needs to be decoded back into the original four bits.

Then the received value, p ris equal to Rr. Using the running example from above. It is not difficult to show that only single bit errors can be corrected using this scheme.

Alternatively, Hamming codes can be used to detect single and double bit errors, multiplebit error correction hamming code merely noting that the product of H is nonzero whenever errors have occurred. In the adjacent diagram, bits 4 and 5 were flipped. This yields only one circle green with an invalid parity but the errors are not recoverable. However, the Hamming 7,4 and similar Hamming codes cannot distinguish between single-bit errors and multiplebit error correction hamming code errors.

That is, two-bit errors appear the same as one-bit errors. If error correction is performed on a two-bit error the result will be incorrect. Similarly, Hamming codes cannot detect or recover from an arbitrary three-bit error; Consider the diagram: Since the source is only 4 bits then there are only 16 possible transmitted words. Included is the eight-bit value if an extra parity bit is used see Hamming 7,4 code with an additional parity bit. The data bits are shown in blue; the parity bits are shown in red; and the extra parity bit shown in green.

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