On Sun, 8 Apr 2007, Curt Shaffer wrote:
I am experiencing a situation where I am getting intermittent choppy audio.
Here is the network layout:
Termination provider -> IAX2 over the Internet -> 20Mb fiber connection ->
router -> Asterisk
My ATA connection goes into the router between the fiber and the Asterisk
server on another interface here is the layout from me to Asterisk:
Sipura ATA (SPA1001 running 3.1.19(SE) firmware), also tested with X-lite
softest -> PIX 506 (although I have tried multiple routers and direct
connection to the radio try to fix the problem) -> 1 mile 802.11b link to AP
-> 15 mile 802.11b link Backhaul -> router -> Asterisk
I'm, er, impressed.
Some years ago I was involved with community broadband networks delivered
via Wi-Fi, and the results were dissapointing to say the least. I was
using smartbridges kit - good kit at the time and designed for the great
outdoors. Relatively expensive though.
One thing I found was putting 2 Wi-Fi links back to back - ie. with just a
switch in-between, would seriously degrade bandwidth through the link. And
15 miles! That's really stretching it, but I guess you have the kit - big
parabolics or dishes, line of sight, no fresnel zone intrusions, etc. We
tried daisy-chaining 4 links together and struggled to get 1Mb/sec through
it.
What "kills" Wi-Fi is full duplex. It's only half-duplex kit, so there is
a turn-around on the link to simulate full duplex. When your packets are
short and coming in both directions, the radio turn-around time can exceed
the packet time. Put 2 links back to back through a switch or hub and it's
worse. Put short packets through it at regular intervals and it's worse
yet. Try to run short packets both ways at the same time (which you have
to for VoIP) and it's even worser.
But you'll get good ping times and downloading data will appear "just
fine" which is what'll make it all the more frustrating!
So make sure there is nothing else on the Wi-Fi links, especially no
uploads.
My Asterisk version is Asterisk 1.2.12.1, Zaptel 1.2.9.1. Ping times are
~10ms, jitter is under 10 with an average of 5. QoS is enabled in the router
for SIP, RTP and IAX2 traffic going to and from the Asterisk box.
QoS won't help you at the end of 2 radio hops, because by the time the
packet gets to the router, it's gone over 2 Wi-Fi hops and it's too late
for the router to do anything with it, so uploads are to be avoided.
That's what used to kill my networks (I had 3 community networks with a
few 100 people in total, connected togther with a fibre backhaul) One
kiddie using p2p software would kill his entire segment of the network as
the uploads would cripple any other downloads, and because I only had one
router per community, he'd effectively cripple everyone in that community
until I cut him off and/or employed some really harsh traffic shaping
which wan't that effective anyway, but helped. I lusted after a router at
each AP, but that was never going to happen as we never had the money...
When I experience the choppiness the ATA reports packet loss on the web
interface (Call 1 Packets Lost: ). I can run something such as ping plotter
from the same leg of the network that the Asterisk box is on while this is
happening and there is not even a small glitch of lost packets on the
network but the ATA displays otherwise. The only thing I have come up with
thus far is possible retransmissions on the wireless connection (and due to
the type of gear, I'm not able to see this data). We are way out in the
country with no other real providers even close so I'm doubting interference
although I suppose it is a possibility keeping an open mind. My question is
can anyone point me to any possible reasons this would be happening? Also
can anyone tell me other reasons other than real lost packets that the ATA
would show this? My only guess on that was packets that never got an ACK due
to server congestion or some other reason other than actual loss.
As far as I'm aware, there's no "ACK" in the RTP stream - it's just a UDP
stream with the bare minimal of overhead to help sequence and time
packets.
You may find that the Wi-Fi gear is dropping UDP packets if it gets
overloaded. Can you increase the sample time - from (eg) 20ms to 40ms? So
the packets are bigger and the radio turn-arounds are less frequent? Have
you tried a lighter codec? (eg GSM - smaller packets, but less often?)
Other than that, all I can suggest is Good Luck ...
Gordon
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