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More than 1 A/D Conversion? (Updated 27-Apr-04)
If you have a 3Com/USR 56k modem, there is a fairly reliable method to determine if you have more than 1 A/D conversion, or other serious impairment on your line. 56k (x2, Flex, V.90 and V.92) will not work if you have more than 1 A/D conversion! [Note: USR PCI Softmodems based on Conexant chipset do not support A/D conversion detection.]
To determine if you have this kind of impairment, call any 56k server with a terminal program. After you get a CONNECT, wait 15 seconds or so and disconnect the call. Then, give the modem the ATY11 command. The modem will respond with a list of frequencies and the receive level of each. What is important is the difference between the value reported for 3750 and 3300 hz. If this difference is 25 or more, it is an indication of more than 1 A/D conversion or other serious impairment. If the number is close to, but less than, 25, you may or may not get a 56k connection, but if you do, it will likely be a very poor 56k connection. A 'good' value for this difference is lower than 18.
Additionally, if the level reported for 3750 is above 50-55, it may indicate a poor local loop which may prevent or result in poor 56k performance.
Here's a sample of the ATY11 output on a connection that does not have more than 1 A/D conversion:at y11 Freq Level 150 16 300 15 450 14 600 14 750 14 900 14 1050 14 1200 15 1350 15 1500 15 1650 16 1800 16 1950 16 2100 16 2250 17 2400 17 2550 17 2700 17 2850 18 3000 18 3150 19 3300 21 <==== Subtract the 3300 value from the 3750 value 3450 24 3600 29 3750 35 <==== ( 35 - 21 = 14. This indicates only 1 A/D conversion)
Also see - Jeff Lieberman's page that includes graphs from 7 different lines.
Note: If you do not have a 3Com/USR modem, you can call the USR line test number which uses the same process, but from the perspective of the server modem to 'guess' whether you have more than 1 A/D conversion. This procedure is not as accurate as doing it from your perspective to a local number. The USR line test number is listed on the Access Numbers page. The USR line test number often is unavailable.
Why don't other modem vendors include this diagnostic data? Here's what a 3Com engineer (not speaking for 3Com) posted in the usr-tc discussion group:
The only diag data I have found is AT I11 in Lucent's, and AT&V1 in Rockwell's. It is very scant, and often shows blanks or erroneous zero's (particularly on V.90 calls). In no case have I found freq probe data (Y11) on anything except 3Com/USR modems. It would be trivial for them to do it (the data exists in the modem, they just need to parse and display it).
The information on this page is based upon the 3Com/USR Troubleshooting Guide for ISPs. (Acrobat .pdf format document; page 7) [Thanks to Gage Robinson for the updated link to the Guide!]