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Home Troubleshooting A/D Conversion Check

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!]


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