How to Diagnose Modem Problems

The following steps help determine whether a faulty modem configuration causes link problems.

1. Call the peer with debugging turned on, as explained in How to Turn on PPP Debugging.

2. Display the resulting /var/log/pppdebug log.

   Either of the following symptoms in the output can indicate a faulty modem configuration:

   • No “recvd” messages have come from the peer.

   • The output contains LCP messages from the peer, but the link fails with too many LCP Configure Requests messages sent by the local machine.

     These messages indicate that the local machine can hear the peer, but the peer cannot hear the local machine.

   • The link terminates with a SIGHUP signal.

3. Use ping to send packets of various sizes over the link.

   For complete details about ping, refer to the ping(1M) man page.

   If small packets are received, but larger packets are dropped, modem problems are indicated.

4. Check for errors on interface sppp0:

   % netstat -ni Name Mtu Net/Dest Address Ipkts Ierrs Opkts Oerrs Collis Queue lo0 8232 127.0.0.0 127.0.0.1 826808 0 826808 0 0 0 hme0 1500 172.21.0.0 172.21.3.228 13800032 0 1648464 0 0 0 sppp0 1500 10.0.0.2 10.0.0.1 210 0 128 0 0 0

   If interface errors increase over time, this can indicate problems with the modem configuration.