Serial Programming - upload.wikimedia....

Error Correction Methods
for the receiver to tell), the receiver discards it completely and (in most cases) pretends it
never saw it.
If it's not good, the sender sends it again.
How does the sender know it wasn't good ?
It never got the ACK. (So either the packet was corrupted, *or* the ACK was corrupted --
there's no way for the sender to know).
"Stop-and-wait ARQ"
The simplest version of ACK-NAK is "Stop-and-wait ARQ".
The sender sends a packet, then waits a little for an ACK. As soon as it gets the ACK, it
immediately sends the next packet. If the sender doesn't hear the ACK in time, it starts
over from the beginning, sending the same packet again, until it does get an ACK.
The receiver waits for a packet. If the packet passes all the error-detection tests perfectly,
the receiver transmits an ACK (acknowledgment) to the sender.
Subtleties: If the receiver receives the packet perfectly, but the ACK message is delayed too
long, then the transmitter sends another copy of the message (a "communication echo").
Imagine the packet contained the message "deduct $11,000 from Fred's account.". When the
receiver gets this second copy of the packet, what should it do? Certainly it should send an
ACK (otherwise the transmitter will keep trying to send this packet over and over). Either
or both of the following problems could occur:
• The delayed first ACK could hit the transmitter after it transmits the second copy of
the message, so it transmits the next packet. Then the second ACK hits the transmitter,
tricking the transmitter into thinking that "next packet" has been successfully received,
when it hasn't.
• When the receiver gets 2 identical consecutive packets saying "deduct $11,000 from Fred's
account", are these 2 legitimate independent transactions, and so it should deduct $22,000
from Fred's account? Or is it really just 1 transaction, with a bit of echo, and so should
deduct a total of only $11,000 from Fred's account?
Both of these problems can be solved by adding a "sequence number". The transmitter
keeps a count of how many independent packets it has transmitted to that receiver, and
puts that sequence number in the header of each packet. But when it re-transmits a packet,
it re-transmits that same identical packet with that same identical sequence number. Also,
the receiver, rather than sending a generic "ACK" message, specifies which particular packet
it is responding to by putting its sequence number in the ACK message. When there is a
communication echo, the receiver sees the same sequence number, so ACKs that sequence
number (again) but then discards and ignores the extra, redundant copy of a packet it
already received. When the transmitter is sending a new packet that merely happens to
contain the same data, the receiver sees a different sequence number, so it ACKs that new
sequence number, and takes another $11,000 out of Fred's account. Poor Fred.
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