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What's a 56k-compatible Line? (Updated 17-Jun-00)
One of the requirements listed on at least one brand of 56k modems is "A 56k-compatible line" - without defining what such a line is.
Pretty slick. It's true that some telephone lines/facilities will prevent 56k from working, but there is no way for the average user to easily determine whether his line is '56k-compatible'! (If you have a 3Com/USR modem, there is an easy way of determining if your line will support 56k connections - the A/D Conversion Check.)
If you get a 56k modem, and you get 56k connections, you can safely say that you (presently) have a 56k-compatible line. However, if you don't get 56k connections, you cannot correctly assume that your line is not '56k-compatible'.
Possible reasons for not getting a 56k connect can include:
The firmware in your modem isn't working properly for your line conditions
The firmware in your modem isn't 'compatible' with the firmware in the server modem you are calling
The firmware in one of the modems isn't 'compatible' with the digital portion of the telephone network being used
Your line is not '56k-compatible'
In some of the above circumstances, you would be able to achieve a 56k connect with a different modem, or calling a different V.90 server.
56k Modem manufacturers have generally defined a '56k-compatible line' as being one that has only 1 D/A conversion (or 1 A/D conversion), and your local loop is less than 3½ miles. You may be able to get an answer - not always correct - if you ask your telco for the loop length and if your line has more than 1 A/D conversion. There is no 'requirement' or standard procedure for getting this information from a telco. But, even if you find you have only 1 A/D conversion (a '56k-compatible line'), you may still not be able to get a 56k connect with any 56k modem if your telco introduces certain types of digital impairments.
If you don't have a USR modem, you can do a somewhat less reliable test with the USR Linetest.