How to detect if QoS is in effect

Unanswered Question
Nov 24th, 2008


Not sure if this is too broad a question but assuming I am an end user, how would I be able to detect if QoS or some sort of traffic shaping is in effect on the network?


I have this problem too.
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Collin Clark Mon, 11/24/2008 - 11:44

Voice calls should not drop or be degraded at anytime. IMO. the end users they should never notice anything.

ece344609_2 Mon, 11/24/2008 - 11:57


Thanks for the quick reply. I think I should rephrase.

I want to figure out a way to detect whether QoS or traffic shaping is in effect on the network from an end user perspective. I know ideally it should be invisible to the user.

But is there a way for me to tell if QoS is in effect on the network from an end user perspective.

Thanks again.

Collin Clark Mon, 11/24/2008 - 12:01

From the end-user experience Joe's explaination is accurate. When we first implemented QOS management wanted to know when QoS was "on". I used SNMP to monitor the TX queue of key routers. When they saw the line jump, QoS was "on".

Joseph W. Doherty Mon, 11/24/2008 - 11:55

From an end user perspective, if QoS is implemented, it might be easy to tell, or not.

If at different times of the day, you ping a host with different DSCP markings and see notable differences when only the DSCP marking changes, DSCP sensitive QoS is likely active. However, many QoS policies don't trust hosts to mark as they desire, so your different DSCP markings could be ignored even though QoS is active in the network. Or, the QoS isn't just looking at DSCP markings.

Since shaping tries to emulate a certain bandwidth, it's difficult to determine whether you're really dealing with physical bandwidth or a shaper. However, shapers work with time averages, so if you obtain a high bandwidth within just a few small packets but it slows over more time with more packets, the rate might be controlled by a shaper. Physical bandwidth shouldn't vary (except for load) over short vs. long intervals.

Also note, QoS usually requires some amount of load for it to "kick in". If you're traffic doesn't exceed the QoS threshold, QoS might be implemented, but effectively inactive and you wouldn't be able to detect it.

Other clues to the QoS might be you're using a VoIP phone and transferring a large data file at the same time. If VoIP quality suffers only after the data transfers starts, good chance there's no QoS. The converse, doesn't mean there is QoS.

Hope the foregoing shows you might get some indications of QoS as an end user in some situations, but there are lots of variables.


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