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8 Error Correction Methods
8.1 Introduction
There are 3 main types of handling errors:
• acknowledge or retry (ACK-NAK).
• "Forward Error Correction" (FEC)
• Pretend It Never Happened
8.2 ACK-NAK
Each packet is checked by the receiver to make sure it is "good".
If it *is* good, the receiver (eventually) tells the sender that it came through OK -- it
acknowledges (ACK) the packet.
All versions of ACK-NAK absolutely require Two Way Communication 1 .
How does the receiver know it's good ?
The sender calculates a checksum or CRC for the entire packet (except for the footer), then
appends it to the end of the packet (in the footer/trailer).
The typical CRC is 32 bits, often a Fletcher-32 2 checksum 3 .
Aside: Note that the checksum or CRC are forms of hashing, ie, irreversibly shrinking
data. Checksums and CRCs are weaker algorithms than "cryptographically strong" message
authentication code algorithms such as MD5 or SHA variants. Cryptographically strong
algorithms can detect errors better than checksums or CRCs, but they take more time to
calculate.
Whenever the receiver receives a packet, the receiver calculates exactly the same checksum
or CRC, then compares it to the one in the footer/trailer. If they match, the entire packet
is (almost certainly) good, so the receiver sends an ACK.
When there's even the slightest question that the packet has any sort of error (which could
be *either* in the actual data *or* in the header *or* in the checksum bits -- there's no way
1 http://en.wikibooks.org/wiki/Serial_Programming%3ABi-directional_Communication
2 http://en.wikipedia.org/wiki/%20Fletcher%27s%20checksum
3 http://en.wikipedia.org/wiki/%20checksum%20
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