signal to noise ratio check?

signal to noise ratio check?

Post by Dirk Peppe » Fri, 24 Mar 1995 13:23:19



Is there a way to check signal to noise ratio without going out and
buying some kind of meter? ie a software diagnostic application that can
do this? Thanks! :-)
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signal to noise ratio check?

Post by bu.. » Fri, 24 Mar 1995 23:12:24



> Is there a way to check signal to noise ratio without going out and
> buying some kind of meter? ie a software diagnostic application that can
> do this? Thanks! :-)

Indeed there is.  Pick up one of the new Multi-Tech MT2834 series modems with
ver 2.07 or later firmware (2.07A in the MT2834ZDX)

Then send the command AT%DP1&W to the modem, then dial some V.34 host.
At the end of the call view your results with the commands ATL8 L9 L10 and L11.
The L10 command will give you the SNR graph.  Pretty impressive, IMO

std. disclaimer. I do not represent my employer in this forum

 
 
 

signal to noise ratio check?

Post by Dave Houst » Fri, 24 Mar 1995 23:57:49




>>Is there a way to check signal to noise ratio without going out and
>>buying some kind of meter? ie a software diagnostic application that can
>>do this? Thanks! :-)
>I think most modern modems can report a number called the 'eye quality', which
>represents the quality of the connection. Trouble is, it's not always
>documented. (i.e, hidden command, for example).

Rockwell chipset modems have a command to measure 'received signal level' or
signal plus noise (%L) and another that lets you measure EQM which gives a
rough indication of line quality (%Q).

To use them you must be connected to another modem and then:

1. Issue +++ to enter command mode. Wait for OK.
2. Issue AT%L<CR>
The modem will return an OK (I think) followed by a number between 009 and
043 which is the received signal (+ noise) level in -dBm.
3. Issue ATO to return to online mode or ATH to hang up.

or

1. Issue +++ to enter command mode. Wait for OK.
2. Issue AT%Q<CR> (repeat a few times - good lines are fairly stable)
The modem will respond with OK and then with a number between 0 and 255
which is related in some mysterious way to line quality. Just how it is
related is hard to say as all Rockwell manuals that I have found give no
information on how to interpret the values.
3. Issue ATO to return to online mode or ATH to hang up.

CAUTION: If the remote modem has not been set to ignore +++, it too will be
in command mode.

I sent Tony Zuccarino (of Rockwell marketing) email asking for clarification
of %Q but he merely responded with RTFM which I had done prior to asking.

Absent any definitive explication of EQM measurement and interpretation, the
best that you can do is to establish a baseline using a line that has good
quality and then use the baseline readings for comparison with lines where
the quality is suspect. But be aware that you must compare apples with
apples; only readings made at the same rate and modulation are comparable.

In response to my earlier plea in this forum for an explication of
Rockwell's EQM measurement, someone with a modem manufacturer that uses
Rockwell chipsets posted the following:

-----begin quote-----

The EQM value returned by AT%Q has different meanings based on
carrier modulation. Most of the time it reports the square of the error
vector in the eye pattern. In TCM modes (VFC, V.34, etc.) it reports the
minimum trellis path length. From a practical standpoint it is a
measure of line quality, small numbers are good, large numbers are bad.

"Small" and "large" are relative to the actual carrier modulation being
used and the current carrier rate. The EQM value for rate renegotiation
downward ranges from about 30 to 55, depending on carrier. The EQM value to
force a retrain is from 2 to 10 higher than the rate renegotiation EQM. The
EQM values for a rate renegotiation upward varies from 12 to 20.

The EQM values reported on TCM carriers (VFC, V.34, etc.) are larger
than the values reported for other modes. A EQM of 20 on a V.32 connection
indicates a fairly bad connection. An EQM of 20 on a VFC connection means a
poor connection but not poor enough to require a rate renogotiation or
retrain.

The modem just reports the EQM of the signal it is receiving. The
remote modem may have a high EQM while the local modem has a very low
EQM. A bad local phone line may not increase the EQM values seen on the
local modem, but it may increase the EQM values on the remote modem.  

-----end quote-----

The following EQM readings were taken at the indicated connect rates on
a BBS with which I always connect at 28800. I set s37 to limit the maximum
connect rate, connected and then measured the EQM at that rate and
modulation. And, since I was calling a USR Courier v.everything modem with
my V.FC modem, I didn't have to worry about retrains or fallbacks <g>.






The high readings in parentheses were made on other lines where I do not get
always get 28.8 connects. In every case where I do not get 28.8, I have
measured EQMs that are higher than my baseline. On all lines where I do get
28.8, EQMs are about the same as the baseline.

YMMV




^^ ~~ ^^                                                ^^ ~~ ^^

 
 
 

signal to noise ratio check?

Post by Greg Sulliv » Sat, 25 Mar 1995 09:49:42



>Is there a way to check signal to noise ratio without going out and
>buying some kind of meter? ie a software diagnostic application that can
>do this? Thanks! :-)

I think most modern modems can report a number called the 'eye quality', which
represents the quality of the connection. Trouble is, it's not always
documented. (i.e, hidden command, for example).

Greg.

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1. Signal to noise ratio?

Below is the data from calling USR's modem test number. From what they said
I must have a SNR better than 45 to get better connect speeds.

What is it?
What is the minium number the phone company requires for a good line?
Is there a site to find out more about phone lines & modems?

3Com USRobotics Courier V.Everything Link Diagnostics...

Modulation               V.34+

Carrier Freq    ( Hz )   1600/1800

Symbol Rate              2400/2400

Trellis Code             64S-4D/64S-4D

Nonlinear Encoding       ON/ON

Precoding                OFF/OFF

Shaping                  ON/OFF

Preemphasis Index        0/10

Recv/Xmit Level (-dBm)   25.5/11.3

SNR             ( dB )   37.3

Near Echo Loss  ( dB )   11.5

Far Echo Loss   ( dB )   39.2

Roundtrip Delay (msec)   30

Timing Offset   ( ppm)   -160

Carrier Offset  ( ppm)   47

RX Upshifts              0

RX Downshifts            0

TX Speedshifts           0

V.90 Status              0007

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