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Obsolete Releases => SME VoIP (Asterisk, SAIL etc) => Topic started by: phead5 on December 23, 2008, 03:14:16 PM

Title: latency
Post by: phead5 on December 23, 2008, 03:14:16 PM
Hi all,

I am having an issue with my latency from my extensions.  When I look at the latency for my trunk it is usually 14ms.  However, when I look at the latency in the extensions page, I am getting real inconsistent readings.  They vary between 40 and 110ms and spike higher some times.  The user complain of call quality at times and I am guessing it is this up and down latency.  Anybody have any idea why this might be happening?

Title: Re: latency
Post by: SARK devs on December 30, 2008, 05:38:40 PM
Anybody have any idea why this might be happening?

Nope; but you might want to verify it first by running a series of ping tests.  Latency in Asterisk is a combination of time on the wire and processing time at the phone/carrier.  This is why, for example, you will always see better latency from a Cisco 7960 than you will from a grandstream.  The Cisco is using less processing time, either due to better programming, a faster CPU or some mix of the two.

As you look to have already figured out, latency itself is not that big a deal.  We regularly run calls to the far east with latencies of 4-500Ms, however its not a problem because it is pretty contant so it just results in a slight delay on the line.  Where it hurts is when the latency is varying widely in real time.  This can happen a lot if you have a poor or heavily used internet connection or if you have something hammering your LAN or choking it.  I've seen this with badly set up network switches and QOS devices (usually Cisco - not because the Cisco's are bad but because they are flexible enough to be set up badly by people who should know better).  More recently we had a problem at a local customer which, after much frustrated searching, turned out to be a very sick HP network printer which was flooding the LAN with error messages.  So it can be lots of different things.

Wireshark and a good old fashioned hub (we keep a 24 port unit in the office) can help a lot in diagnosing local problems of this nature.  You can pick up a hub from ebay for just a few bucks and they are great for helping to diagnose local network issues.

Kind Regards