Serial Programming - upload.wikimedia....

Error Correction Methods
(Some people think of "streaming" as one big packet the size of the window using "stop-andwait"
protocol, divided into smaller "sub-packets").
8.2.1 Selective Repeat ARQ
w:Selective Repeat ARQ 4
A selective repeat ARQ system is a kind of streaming ARQ.
But instead of the receiver only handling 1 packet at a time, and discarding all packets
higher or lower than the one it is looking for, the receiver tries to keep a copy of all packets
it receives in a window of its own, and negotiates with the sender to try to resend *only*
the erroneous packets.
8.3 FEC
If you have only one-way communication, you are forced to use Forward Error Correction,
sometimes called EDAC (Error Detection And Correction).
You transmit the data, then (instead of a CRC) you transmit "check bits" that are calculated
from the data.
... NASA space probes ... compact disks ...
The simplest kind is "repeat the message".
If I send the same packet twice, and noise only corrupts one of them, *and* the receiver can
tell which one was corrupted, then no data was lost. If I send the same packet 3 times, and
noise corrupts any one of them, then the receiver can do "best 2 out of 3". The "check bits"
are 2 copies of the data bits. In fact, noise could corrupt a little bit of *all three* of them,
and you could still extract all the data -- align the 3 packets next to each other, and do
"best 2 out of 3" for every bit. As long as there were only a few bits of noise in each packet,
and the noise was in a different place in each packet, all the data can be recovered.
... (put picture here) ...
There are some very clever kinds of FEC (Hamming codes, Reed-Solomon codes) that can
correct all kinds of common errors better than "best 2 out of 3", and only require the same
number of "check bits" as there are data bits.
8.4 Pretend It Never Happened
A sender often streams audio and video live, in real-time.
What should a receiver do when a packet gets mangled ?
4 http://en.wikipedia.org/wiki/Selective%20Repeat%20ARQ
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