An Explanation of the %L & %Q readings on the SupraFAXModem v.32 & v.32bis


%L function:

How to measure %L

         1. Dial and connect to another modem.
         2. Escape to COMMAND mode by issuing +++ and then wait for OK.
         3. Type AT%L and press return.  Repeat the AT%L a few times and
                average the values together to get a average value.
         4. Issue ATO to return to "ONLINE" mode, or ATH to hang-up.

What does %L measure, and what do the values mean:

The %L value is the measurement of the received carrier level in decibels (dB).
FCC specifications state that a modem will transmit at a -10dB level, and the
SupraFAXModem is able to use signals down to at least -25dB.  The modem reports
this level as a positive value, but it should be understood that the real value
is negative.  Readings of %L that are less than 10 (i.e. 0 to 9) show that the
signal is being amplified to a level beyond the original signal level.  Such
amplification will cause distortion and may have a VERY high level of noise
mixed with the original signal.



%Q function:

How to measure %Q:

        1. Dial and connect to another modem.
        2. Escape to COMMAND mode by issuing +++ and then wait for OK.
        3. Type AT%Q and press return (also note the displayed carrier rate
                on the modem.)  Repeat the AT%Q a few times and average the
                values together to get a average value.
        4. Issue ATO to return to "ONLINE" mode, or ATH to hang-up.

                                                                  
What does %Q measure, and what do the values mean:

The %Q values returned give an instantaneous reading of the received carrier
signal quality.  Values of greater than 20 are generally considered to be poor.
When poor %Q values occur, Supra suggests reducing the carrier rate one level,
and checking the value again.  This process should be repeated until a good %Q
reading can be obtained.  Values of less than 8 show the line quality is very
good at the present carrier rate, and that a higher rate may also be supported
under the present conditions.


What does the modem look at to determine the %Q value:

%Q readings on the Supra v.32 & v.32bis are based from the root-mean-square
of the ERROR VECTOR value currently generated in the modem.  (The larger the
vector the larger the %Q value reported.)

The ERROR VECTOR is the average of the vector error for all the points in the
type of carrier encoding pattern currently in use (either TCM or QAM -see
below).  The number of points in the carrier increases with the carrier rate.
The error basically measures the amount of drift between the current pattern of
the points on the carrier and the ideal location of these points.  The more the
carrier signal is damaged, the greater the drift (or total loss) of these
points will become.


Causes of damage to the carrier:

The carrier is degraded by echoes, signal loss, phase shifts, frequency shifts,
background noise, and other issues that normally plague transmission lines.
Also harmonics from out-of-band signals may prove damaging to the carrier
signal.  Basically, the better the transmission lines is able to faithfully
transfer the original signal, the lower the error level rate will be.


It should be fully understood that the %Q value is a measure of the damage to
the carrier single when compared against the ideal carrier pattern.  The
measurement is in no way a direct measurement of the telephone connection, but
the telephone connection from one modem to the other is the major source of
damage to the carrier signal.


Should you require more information on v.32 modem operation or the structure
of modem carriers, please consult with a Data Communication Engineer, or look
in an engineering level text on modem data communications.  This type of data
is beyond the scope of Supra's Technical Support department.


This document is a combination of data from Rockwell, Engineers at Supra, and
my own knowledge and education relating to data communication.  Please feel
free to post this document in its original and unedited form.

Patrick Moore
Supra Technical Support
SysOp Supra BBS.
Feb. 25, 1993



For information on telephone line conditions, please download TEL_LINE.TXT.


TCM -Trellis Coded Modulation
QAM -Quadrature Amplitude Modulation (see page 58 in the Supra v.32 Ref. Manual)