Need help to analyze a packet capture for VoIP traffic
We have been dealing with a VoIP call quality issue for almost 2 weeks ago. The PBX has been working for about 6 years without issue and then, 2 weeks ago, they brought in a new IT vendor and all the issues started. The IT vendor swears they made no changes. We have done some packet captures of bad calls but I don't see anything terribly out of line. Can someone help me interpret what we are seeing and tell me if it helps narrow down the source of the problem?
Here is a summary of the RTP stream analysis...
Forward
- 192.168.4.139 (IP telephone) → 192.168.4.246 (PBX)
- SSRC: 0x5afdd899
- Max Delta: 27.34 ms @ 9354 Max
- Jitter: 2.00 ms
- Mean Jitter: 0.95 ms
- Max Skew: 14.76 ms
- RTP Packets: 4873
- Expected: 4873
- Lost: 0 (0.00 %)
- Seq Errs: 0
- Start at: 17.075851 s @ 114
- Duration: 97.43 s Clock Drift: 11 ms
- Freq Drift: 8001 Hz (0.01 %)
Reverse
- 192.168.4.246 (PBX): → 192.168.4.139 (IP telephone)
- SSRC: 0xfddd139d
- Max Delta: 55.05 ms @ 9577
- Max Jitter: 5.92 ms
- Mean Jitter: 1.53 ms
- Max Skew: -44.15 ms
- RTP Packets: 4860
- Expected: 4860
- Lost: 0 (0.00 %)
- Seq Errs: 0
- Start at: 17.131184 s @ 119
- Duration: 97.18 s
- Clock Drift: -3 ms
- Freq Drift: 8000 Hz (-0.00 %)
- Forward to reverse start diff 0.055333 s @ 5
The above call was not horrible but it was not clear like it should be. We did a capture on a good call and the only major differences I can see between the two are the Max Delta and the Max Skew. On the good call, the max delta is 21.80 ms and the max skew is -2.12 ms. On this poor call, the max delta is 55.05 ms and the max skew is -44.15 ms.
Can someone help me understand what this means? While I am sure this doesn't tell us what device is causing the issue, can someone help me understand these numbers? Is there anything else that you see that could explain this problem?
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