[NTLUG:Discuss] question about modems

[Steve Baker]

Collin wrote:
> 
> This isn't about linux - but rather a general question about modems 
> Once the modem handshake has been accomplished, and the modems agree on a
> frequency - how do they stay in sync once data transfer has finished?

Dead reckoning.  It's just RS-232 - so the transmitter is required to sent
at a speed which is within some small percentage of the nominal frequency
and the reciever is required to accept data at a speed that is within a
slightly larger percentage of nominal.

The protocol's allowed error rate is designed such that the maximum amount
of error is always less than the width of a single bit.

When you see the beginning of a 'start bit', you start reading the data
at your clock rate.  If the source data is coming anywhere from 5% too fast
to 5% too slow, you'll still be in sync after as many as 10 bits.   Since there
is a delay between bytes and a new 'start bit' before each new byte, everything
is inherently resynced ready for the next byte.

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