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Questions about PDV and the QoS sensors



Hi, I have a question regarding the PDV (Packed Delay Variation) unit used in your QoS sensors.

The question is basically how you measure this. After reading up a bit on the subject it seems as if it is the difference in delay between two selected packets. This can then be used to measure the amount of maximum difference in latency between two packets, and thereby make it possible to use it as an indicator to detect "lagging" audio over a VoIP connection.

So my questions are:

1) Have I understood it correctly?

2) Why is Min PDV a negative number - how is it being calculated?

3) How do you measure/calculate the PDV Average since it is always zero?

It would be nice if I could see the latency of the packets as well, since it would show approximately how long it takes for person A to hear what person B is saying in a VoIP call.

I understand it if you're not displaying it in case the two clocks on the systems are not synchronized, thus having a chance of making the data unreliable. However if one would use the round trip sensor, wouldn't it then be the same computer comparing the times?

Is there any other sensor I could use for delay/latency? I guess I could use a ping sensor, the downside of this however is that the RTP packets transmitted during a VoIP call are often prioritized in devices such as routers etc, compared to data packets such as those carrying the ping requests.

I know it is a lot of questions, but I appreciate any answer disregarding which/how many questions you answer.

Thanks in advance.

latency pdv qos

Created on Dec 19, 2011 10:30:06 AM

1 Reply

Accepted Answer



Please refer to RFC 3393 (http://www.ietf.org/rfc/rfc3393.txt) for details on PDV and the mathematics behind it, we can't describe it any better than them.

The round trip sensor will eventually get a RTT (Round Trip Time) channel, that's on the wishlist.

[UPDATE] As of PRTG version, the values for RTT (Minimum, Maximum, Average) are included in the QoS (Quality of Service) Round Trip Sensor.

Created on Dec 19, 2011 6:58:41 PM by  Dirk Paessler [Founder Paessler AG] (11,025) 3 6

Last change on Dec 20, 2011 3:07:24 PM by  Daniel Zobel [Product Manager]

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