-=> Quoting MIKE FARRILL to RICHARD TOWN <=-
 MF> If this is all true then, what are the benefits of changing the
 MF> Carrier level?
Modems have their own AGC processes.  On Zooms during negotiation,
the line conditions are calculated into a Q factor (known as
the EyeQ Monitor)
V.FC and V34 may have fairly high EQM values (10 to 30) on a
good connection.  V.32bis typically has EQM values below 15 on a good
connection.
The EQM thresholds are the ones used by the firmware to decide when a
rate renegotiation up or down should be intiated.  Lines with EQM values
below the good EQM threshold for 60 seconds (expected to be reduced to
20 seconds in forthcoming ROM) will rate renegotiate upward.
Lines with EQM values above the bad EQM threshold for about one second
will rate renegotiate downward.
The good and bad EQM thresholds are dependant on the current data rate.
Good thresholds range from 16 to 25, bad thresholds range from
60 to 70.  (These values are for V.34 and V.FC connections, V.32 and
V.32bis connections use different thresholds)
EQM thresholds used during a retrain are different than those used
during rate renegotiation.  They are associated with a different
constellation than the data phase constellation, so the magnitude of the
EQM measured is very different.
Since EQM values are related to calculations of signal to noise ratio
of the link that the negotiating modems make as well as bandwidth
available and the efficacy of the modems' echo cancellation (and
phase shift, etc), setting a line level of -12dbm (the threshold where
you may avoid local exchanges' AGC amps' operation) allows modems to
fully use their own AGC actions between them.
The negative feedback circuits of line amplifiers can also introduce
phase distortion too, undetectable on POTs' voice, awkward for data.
Whether setting s91=12 is available on your modem, or only can be
enabled on a per call basis, or achieves any advantage for you, only
a test to your regular remote will tell.
Me?  I have AGC switched off at the local exchange and use s91=12 in
my dial string for a Zoom WorldClass.  I don't know why type approvals
nominate such a tight lower limit for line levels, especially when
telco line levels are at -12dbm anyways.
To me, fixing at -10 +/-1dbm
means telco's AGC amps will always operate to some degree.  Unless
they expect 2dbm loss between subscriber and exchange.  Dunno, anyone
here tell?  I'm not a line techie.  Prefer RF :-)
rgdZ
Richard
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