Troubleshooting guide

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Cisco Broadband Local Integrated Services Solution Troubleshooting Guide
OL-5169-01
Chapter 4 Troubleshooting with Call Flows
Voice Quality Problems
If the LostSyncs value is high (such as once an hour), there is something physically wrong with the
connection. Send a technician to recable the MTA.
If T3Timeouts or T4Timeouts values are high, it is most likely an upstream problem.
• Look at signal-to-noise ratio, corrected errors, and uncorrectable errors. If the signal-to-noise ratio
is higher than normal, you will probably also notice an increasing proportion of corrected and
uncorrectable errors. Check for ingress noise, impulse noise, or common path distortion.
Starting at the Trunking Gateway
If you are starting to troubleshoot a VoIP over cable problem at the trunking gateway, here are some
guidelines:
• Look at values for the following objects in the Voice Over IP Call Active Table of
CISCO-VOICE-DIAL-CONTROL-MIB.my:
– cvVoipCallActiveGapFillWithSilence—duration of the voice signal replaced with signal played
out during silence, due to voice data not received on time (or lost) from the voice gateway on
this call.
– cvVoipCallActiveGapFillWithPrediction—duration of the voice signal played out with signal
synthesized from parameters or samples of data preceding in time, due to voice data not received
on time (or lost) from the voice gateway for this call. An example of such playout is
frame-erasure or frame-concealment strategies in G.729 and G.723.1 compression algorithms.
Excessive values could indicate codec problems.
• In the same MIB, look at values for these objects:
– cvVoipCallActiveHighWaterPlayOutDelay—provides the high-water mark Voice Playout FIFO
(first-in-first-out) Delay during the voice call.
– cvVoipCallActiveLowWaterPlayOutDelay—provides the low-water mark Voice Playout FIFO
Delay during the voice call.
Excessive values indicate a jitter problem.
• To determine where jitter is occurring, do the following:
– Use a sniffer to see where the jitter is occurring. The normal arrival time for packets is about 20
ms; a variation of 5 ms indicates jitter. Also look at the sequence of RTP packets to see where
packets are lost.
– View values for the following objects in the Voice Over IP Call Active Table of
CISCO-VOICE-DIAL-CONTROL-MIB.my:
• cvVoIPCallActiveLostPackets—number of lost voice packets during the call.
• cvVoIPCallActiveLate Packets—number of received voice packets that arrived too early to
store in the jitter buffer during the call.
• cvVoIPCallActiveEarlyPackets—contains the number of received voice packets that
arrived too late to playout with CODEC during the call.
Abnormal numbers here can indicate a network problem.
– Use the show call history voice brief command to view round-trip delay
(last/minimum/maximum) of packets in milliseconds.
• A high degree of processing may be the cause of the problem. Use the show processes cpu
command to view a graph of the maximum and average trunk gateway CPU to locate what process
is interfering with voice transmission quality.