This happens for the same reason that 33.6 modems tend to connect at 28.8
initially but then renegotiate up to 31.2 or higher immediately after
connecting.
You have to remember that the modem isn't sending any data while it's
handshaking. It's simply probing the line and making some educated guesses
about the quality of the circuit it's on. Once it gets a carrier, it *is*
sending data, and it can see how reliably the data is getting to the other
side. At a certain point, the retransmit delay because of bad data
surpasses the speed increase gained by being at the higher rate. At this
point, the modem (and rightfully so) drops to a lower rate to avoid as many
errors as possible.
The opposite is possible as well... the modem may detect a bad line while
probing and negotiation but see no errors once it's connected to the other
side. It will then jump up a rate to see if it can squeeze some more speed
out of the line.
Chad Scott
Network Operations
Internet Direct, Incorporated
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