Jitter on Point to Point circuit

Unanswered Question
Feb 14th, 2007

I'll keep this simple (or at least try to). I have a point-to-point that I have established to send only voice traffic. There is a 1700 series router on both sides of the circuit. When I run trace routes from both directions, I see that the circuit is connceting with a couple of hops averaging 4ms. Not bad. During this time, i am running a network qualifier application that simulates 32 phone calls going across the circuit from my laptop to our new voice mail server. These tests are showing jitter which will spike up to 80 ms. Wouldn't the traceroute be more accurate than a network qualifier application? Or does something need to be tweaked on the routers?

I have this problem too.
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Richard Burts Wed, 02/14/2007 - 12:39


I am a little puzzled about part of your explanation. You describe a point to point connection, but you say the traceroute shows a couple of hops. How could there be a couple hops over a point to point connection?

The bigger question is whether traceroute or your qualifier application are more likely to be more accurate. I know nothing about your qualifier application so can say nothing about it. But I can say that ping and traceroute are good tools for checking network connectivity but not good tools for accurate measurement of response time. This is partly because responding to ICMP is a low priority task for the router. When the router is not busy it will respond very quickly. But if the router is busy with some task (routing update, cache update, or some other task) then the response may be slowed down. So my guess is that your qualifier may be more accurate than your traceroute.



david.lowe Wed, 02/14/2007 - 20:05

Simulating 32 phone calls over a T1 line is taxing the circuit entirely too much. Even if you're doing G.729 over this WAN link. You're probably exhausting the buffers on both routers as well during these tests, therefore, creating additional latency.

You may even want to consider configuring LFI and LLQ for optimal voice quality on the WAN interfaces of both routers. LFI helps to prevent serialization delays on low speed WAN circuits and LLQ will better support the voice across the WAN. Just something to keep in mind.

Good luck!


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