An Explanation of the %L & %Q readings on the SupraFAXModem series.

      %L_&_%Q.TXT version 1.1 (phm4/22/94) from Supra BBS (503-967-2444)

NOTE:   %L and %Q values ONLY look at the RECEIVED signal carrier.  For full
        evalution of a given full-duplex connection, you must have the values
        at BOTH ends of the connection.  It should also be understood that
        other vendor's products may also support these commands, however they
        may be weighted differently.

%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
(dBm).  FCC specifications state that a modem will transmit at no higher than
a -9dBm level, and the SupraFAXModem is able to use signals down to at least
-25dBm (often, but not always, much weaker signals may also be usable). The
modem reports this level as a positive value, but it should be understood that
the real value is negative. (So remember that the higher the %L value is, the
weaker the signal being received by is.) 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 amplified connections may have high levels of
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 (on V.32/V.32bis connections) 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 (in all protocols) 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 SupraFAXModem 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 high-speed modem operation or the
structure of modem carriers, please consult with a Data Communication Engineer,
or look in a modern 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 - Senior Staff
System Operator of Supra BBS.
Original: Feb. 25, 1993
Last rev: Apr. 22, 1994



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


dBm -decibel level (in Bell standard measure).
TCM -Trellis Coded Modulation
QAM -Quadrature Amplitude Modulation (a pattern using multiple symbols per
        point, where the point exist in a strick matrix of phase angles
        and amplitudes in four distint quadrants, used to modulate the
        modem's carrier signal)