Tactical Software Tech Note


All Tech Notes

Troubleshooting a "NO CARRIER" Condition

When viewing a failed dialout attempt in our Trace Window, a modem's NO CARRIER reply to the ATDT dial string is the most common issue.  A NO CARRIER reply back from the modem is a sign that the modem was unable to establish a connection to the remote endpoint.  This can happen if the modem is told to dial a malformed number, a telco problem is present or a modem negotiation issue causes the connection to fail.

In summary:

  • A malformed number is when the format of the number being dialed is incorrect.  Possible causes include a '9' is being dialed but not needed to acquire an outside line, an area code and/or leading '1' is missing, the dialed number is simply incorrect, or there are characters in the number that the modem can not process.
  • A telco issue essentially means that the telephone service lines (T1 or POTS) connected to the modem server are not capable out making outbound calls or the lines don't have a long distance carrier. Both this and the above "malformed number" condition usually show the NO CARRIER response being returned within a couple of seconds of the dial command.
  • A modem negotiation problem occurs when the two modems can not agree on the communication protocols to be used.  To fully diagnose this type of problem, it may be necessary to see the CONNECT string from a successful connection to that endpoint with a local modem (i.e., not a modem server) to determine what baud rate, error correction and data compression are required by the destination modem.