How to Diagnose Modem Problems

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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.

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.

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

Display the resulting /var/log/pppdebug log to check for faulty modem configuration.

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.

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 192.0.2.0     192.0.2.32    13800032 0     1648464  0     0      0     
sppp0 1500 198.51.100.2  198.51.100.1  210      0     128      0     0      0

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

Troubleshooting

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.

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 that are sent by the local machine.
The link terminates with a SIGHUP signal.