-=> Quoting Danny Keogh to Meng-Shi Lim <=-

 -=> Quoting Meng-Shi Lim to All <=-
 
 ML> Another question: on a 28.8k modem, if the connection is made at less
 ML> than 28.8k (eg 21600), can the modem pick up and renegotiate a higher
 ML> speed from this point onwards if the quality of the line improves?
 ML> (Personally I don't think so but you'll never know). I know that the
 ML> modem can retrain up and down from the connection speed (eg 21600 ->
 ML> 19200 ->21600) but I'm not sure about the above.
 DK> 
 DK> The connect speed is the highest speed the modem will go too.  It
 DK> can move anywhere down from this speed.   If you want a higher
 DK> speed, you have to hang up and ring again.

As someone else has pointed out and from what I've experienced, this is
NOT correct. A well designed modem with good v34 implementation can fall
both backwards and forwards. eg 21600 -> 19200 -> 21600 -> 24000 etc.
This has happened to me a few times (confirmed firstly with the CONNECT
speed message after handshake and ATI6 after hanging up). I have been
experimenting and what I have noticed is this:

1) If you let the modem negotiate the connection as per factory setting,
you don't see fall forward much. For example, the modem starts
negotiating from 28.8k and if the quality of the line isn't all that
good connects at 21600. I don't think the modem can fall forward beyond
21600. I think the reason is that during the initial negotiation, the
v34 has already determined the noise tolerance range of the line and set
a limit. However,

2) If you force a v34 connection eg 16800 v34, the likelihood of the
speed falling forward is good because there is a good chance that the
line quality is good enough to support higher speeds and that during the
v34 negotiation phase, your forced connection speed is well within the
toleranced range of the line.

Care to try out my theory?

mslim{at}ne.com.au


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