[asterisk-users] SIP devices with packet loss tolerance

[Gordon Henderson]

On Mon, 23 Apr 2007, Chris Bagnall wrote:

> Greetings list,
>
> Hoping someone might have experience with poorly-performing net 
> connections and which devices work best over them.
>
> One of our clients has a number of employees that work from home, and 
> are given a SIP phone to take with them and hook up to their broadband. 
> For the most part, this works fine, but there are an increasing number 
> where sound quality is poor ("chops" in and out, generally only 
> noticeable to the listener at the other end, not the employee). Logic 
> suggests it's an upstream bandwidth issue, so we asked them to try when 
> all other devices were turned off (to cut out the "kids using 
> bitTorrent" issues), but even with the phone the only device, call 
> quality was still poor.
>
> Since the connections aren't paid for by the client, we aren't in a 
> position to mandate particular providers or speeds, but in each case, 
> the minimum was a 1mb/256k up ADSL. We asked the employees to run some 
> speed tests to determine real-world speeds, and in each case upstream 
> was around 220-235k (a little off the "official speed" but not bad). 
> Certainly way more than the ~35kbps necessary for a g729 call, even with 
> packet overheads.
>
> We've also tested the connections with a constant ping, and latency for 
> nearly all of them is sub-35ms.
>
> So, that leads me towards packet loss as the only thing left. Generally 
> speaking, these connections are giving between 1 and 4% packet loss.

For (what I'm assuming is a UK ADSL connection), that packet loss is very 
high. Is it loss to their head-office where the SIP server is, or are they 
using some external hosted SIP service?

I don't see any packet loss from my home ADSL line, so something is 
"fishy"...

> Therefore, 3 questions: 1) is this level of packet loss likely to have 
> the effect we're seeing?

Generally speaking with one packet every 20ms, 1% loss is a dropped packet 
every 2 seconds. You'd barely notice it unless it was regular. 4% loss is 
a packet every 0.5 seconds. 1% would be an annoying click every now & 
then, 4% will sound a bit ropey.

> 2) If so, are there any phones people have tried with particularly good 
> jitter buffering? If not, any ideas what else might be causing the 
> issue.
>
> 3) are some codecs naturally more "tolerant" of jitter than others? i.e. 
> would there be an advantage to using something apart from g729, and if 
> so, what would you recommend?

Changing ISP.

Maybe not an acceptable solution, but on a quiet line, I'd find it hard to 
justify a constant 1-4% packet loss, however I could belive that a dodgy 
el-cheapo ISP for the masses would have issues - espeically with high 
levels of small packets....

You might also want to check the router at head-office, if it's an 
in-house hosted service. Make sure they have a good router that can handle 
the increased packet load..

Gordon