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

[alan]

> 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.

Thanks,

Alan Ferrency
pair Networks, Inc.
alan at pair.com