Tornado code: Difference between revisions

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In [[coding theory]], '''Tornado codes''' are a class of [[erasure code]]s that support [[error correction]]. Tornado codes require a constant C more redundant blocks than the more data-efficient [[Reed–Solomon erasure code]]s, but are much faster to generate and can fix erasures faster. Software-based implementations of tornado codes are about 100 times faster on small lengths and about 10,000 times faster on larger lengths than Reed–Solomon erasure codes.{{sfn|Byers|Luby|Mitzenmacher|Rege|1998}} Since the introduction of Tornado codes, many other similar erasure codes have emerged, most notably [[Online codes]], [[LT codes]] and [[Raptor codes]].

Tornado codes use a layered approach. All layers except the last use an [[Low-density parity-check code|LDPC]] error correction code, which is fast but has a chance of failure. The final layer uses a Reed–Solomon correction code, which is slower but is optimal in terms of failure recovery. Tornado codes dictates how many levels, how many recovery blocks in each level, and the distribution used to generate blocks for the non-final layers.

== Overview ==

The input data is divided into blocks. Blocks are sequences of bits that are all the same size. Recovery data uses the same block size as the input data. The erasure of a block (input or recovery) is detected by some other means. (For example, a block from disk does not pass a CRC check or a network packet with a given sequence number never arrived.)

The number of recovery blocks is given by the user. Then the number of levels is determined along with the number of blocks in each level. The number in each level is determined by a factor B which is less than one. If there are N input blocks, the first recovery level has B*N blocks, the second has B*B*NB×B×N, the third has B*B*B*NB×B×B×N, and so on.

All levels of recovery except the final one use an LDPC, which works by xor (exclusive-or). Xor operates on binary values, 1s and 0s. A xor B is 1 if A and B have different values and 0 if A and B have the same values. If you are given result of (A xor B) and A, you can determine the value for B. (A xor B xor A = B) Similarly, if you are given result of (A xor B) and B, you can determine the value for A. This extends to multiple values, so given result of (A xor B xor C xor D) and any 3 of the values, the missing value can be recovered.