With IPHost Network Monitor you can run simple snmp requests against a Nortel device in your network.
frMuxFramerUnderruns
Frame Relay Mux Framer Underruns
1.3.6.1.4.1.1570.1.2.13.1.9
This attribute is the total number of frames transmitted to the link for which underruns occurred.
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IPHost Network monitor allows you to monitor frMuxFramerUnderruns on Allied Telesis device via the SNMP protocol. Download IPHost Network Monitor (500 monitors for 30 days, 50 monitors free forever) to start monitoring Nortel modems right now.