[Asterisk-Users] SPA-841 "Decode Latency"?

[Matias G.]

From: "alan" <alan at pair.com>
To: <asterisk-users at lists.digium.com>
Sent: Monday, October 03, 2005 2:56 PM
Subject: Re: [Asterisk-Users] SPA-841 "Decode Latency"?


>> Subject: Re: [Asterisk-Users] SPA-841 "Decode Latency"?
>
> Luki <lugosoft at gmail.com> wrote:
>
>> > Does anyone have any familiarity with "decode latency," specifically
>> > with Sipura devices? Why would it take 200+ms to decode a 20ms RTP
>> > packet? G.711u has existed for over 30 years, how hard could it be?
>>
>> Although I have never seem the decode latency to go above 30 ms on a
>> LAN, it does go up to 80 ms if the Sipura device (phone or ATA) is
>> connected via an Internet link which has jitter. So I don't know for
>> sure, but my understanding is that it's the delay from the arrival of
>> the packet until it's played; this is not due to the actual decoding
>> but probably mostly due to the jitter buffer in the device, which is
>> adjusted dynamically depending on the traffic conditions. More jitter
>> = larger buffer to try to compensate for late packets rather than
>> considering them lost. Anyone correct me if I'm wrong here.
>>
>> Having said that, I don't notice the delay or distorted voice even if
>> the decode latency is as high as 80 ms. Not sure about 200+ ms, but it
>> seems rather high and would imply to me that you have a connectivity
>> issue somewhere on your LAN.
>
> The explanation of jitter adding to decode latency sounds reasonable.
> However, as I said before, I have never seen jitter go above 5ms even
> when our "decode latency" spirals out of control.
>
> Our latency is under 1ms, generally. It's 100 base T fully switched, and
> not highly utilized, with 2 switches between the phone and the PBX.
>
> Our current working theory, which we will test "soon", is that this may
> be caused by periodic high levels of ARP broadcast traffic. I'm not
> familiar with the hardware of these phones, and for most ethernet
> devices they should ignore ARP with no performance effects. But if the
> SPA-841 is set up in such a way that it eats CPU for the phone to
> discard ARP packets, then this could be a problem for us.
>
> I'll keep you posted on what we find. If anyone has any insight into the
> networking hardware the SPA-841 uses, I'd be interested in that.

Alan,
pls keep us informed on wether you find what is going wrong with this 
issue... thanks a lot.

M.