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
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