[asterisk-users] Audio problems on cable modem link

[Kevin Keane]

I've had to rip out VoIP in two cable-modem situations because the call quality was too poor.

Bandwidth isn't the main characteristic you are looking for; most Internet connections have plenty of that. Latency and jitter matter far more. Latency describes how long each packet travels from your Asterisk system to the other end of the IAX connection. The easiest way to measure it is with ping. Jitter describes how consistent the data transfer speed is. Cable modems are to some extent designed for burst data. This will obviously kill call quality.

You can go to http://myvoipspeed.visualware.com/ to get an idea. Run the same test multiple times, both with no traffic on the cable modem, and with a VoIP call going on. Then compare the jitter numbers. What I found was that on my connection (also cable modem) the average jitter was supposedly acceptable, but it was highly variable - in three tests with minimal other traffic all in the middle of the night, the jitter was 1.6, 3.5 and 4.6 milliseconds. 4.6 is in the borderline quality area. And if there is more traffic, quality may well go down further, into the poor-quality zone.

-----Original Message-----
From: asterisk-users-bounces at lists.digium.com [mailto:asterisk-users-bounces at lists.digium.com] On Behalf Of Michelle Dupuis
Sent: Friday, October 15, 2010 7:53 AM
To: Asterisk Users List
Subject: [asterisk-users] Audio problems on cable modem link

We have a small office installation running over a cable modem.  (8M down, 500k up confirmed with numerous speed test sites)

When a single call is up, call quality is fine.  When a second call is up, outbound audio is immediately choppy.  We're using ulaw, and confirmed that traffic with 2 calls is <175kbps in/out.  (IAX connection out)

Asterisk doesn't report any dropped frames, the internet connection looks fine, etc.   We have a linux router in place running wondershaper that seems to be running fine (same as our other installations).

Can someone suggest where to look?  Could this be the ITSP?  
--
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