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Need help to analyze a packet capture for VoIP traffic

asked 2021-06-28 21:47:07 +0000

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...


  • (IP telephone) → (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 %)


  • (PBX): → (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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answered 2021-06-29 08:01:01 +0000

BigFatCat gravatar image

updated 2021-06-29 09:55:20 +0000

The max delta time is the time between packets. Max delta of 55.05ms can be an issue if the receiver is expecting the subsequent RTP packet sooner. It depends on the ptime, which I have seen at 20ms or 30ms. I suspect it is 20ms for your system because you have a good call with the max delta of 21.80ms, which it normal. If the receiver is expecting the subsequent RTP packet around 20ms, but it gets there a 55.05 ms then this could affect the audio quality if it happens frequently on the call.
I would check the QOS markings? It should be realtime COS1/EF, and be trusted. If not, then there could be large delays between RTP packets when the network is busy.

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Thanks so much for the suggestion and the explanation. We will have the IT company check that QoS setup and see if there is any issues there.

TPischieri gravatar imageTPischieri ( 2021-06-29 23:18:08 +0000 )edit

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Asked: 2021-06-28 21:47:07 +0000

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Last updated: Jun 29 '21