18

Erasure Codes

There are four recently developed codes, namely the Tornado, Fountain, Luby Transform (LT), and Raptor codes. The first two are discussed in this chapter, and the last two in the subsequent two chapters, respectively.

18.1  Erasure Codes

Erasures were discussed in detail in §13.6. A more interesting situation arises in the following case. Consider a wide-area file system in which large files are usually partitioned into n blocks that are replicated among the servers. Different users (clients) download most of the n blocks, where replication is also permitted, although it is wasteful in terms of both space and performance. Suppose clients cannot access certain blocks on the server. That is where the erasure codes come in, to calculate ...

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