System Administration Guide: Network Services

ProcedureHow to Diagnose Modem Problems

Modems can be major problem areas for a dial-up link. The most common indicator of problems with the modem configuration is no response from the peer. However, you might have difficulties when determining if a link problem is indeed the result of modem configuration problems.

For basic modem troubleshooting suggestions, refer to Troubleshooting Terminal and Modem Problems in System Administration Guide: Advanced Administration. Modem manufacturers' documentation and web sites contain solutions for problems with their particular equipment. The following procedure helps 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 to check for faulty modem configuration.

  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    826808   0     826808   0     0      0     
    hme0  1500 13800032 0     1648464  0     0      0     
    sppp0 1500     210      0     128      0     0      0

    If interface errors increase over time, the modem configuration might have problems.


When you display the resulting /var/log/pppdebug log, the following symptoms in the output can indicate a faulty modem configuration. The local machine can hear the peer, but the peer cannot hear the local machine.