[Asterisk-Users] need help troubleshooting clipping and garbled VOIP calls

[Rich Adamson]

T. Shaw wrote:
> Hello all,
> I have a problem with call quality with my Asterisk setup. I'm doing 
> VOIP only so far, but have a zaptel TDM400P in the box not being used. 
> The problem i'm having is that when calls are placed, connected, and the 
> far-end is reporting that they are experiencing clipping, choppy, and 
> garbled voice conversations. So bad that we have to resort to using our 
> cell phones. This entire setup is still being built, but any phone 
> attached is experiencing this. Call volume is almost nil (under 20 total 
> incoming calls a day). This is a small business setup. The server is 
> used exclusively for Asterisk, so it isn't a fileserver, or anything else.
> 
> The setup is as such:
> 
> ipphone  <--->cisco 2900XL switch <----> Cisco 2621 router <---> dsl 
> modem <-->DSL <---> VOIPprovider
> 
> I've configured the switch and the router to set priority and qos to 
> prioritize voice traffic above data.
> Funny thing is, there is not data REALLY hitting the network. I have 
> setup 2 vlans, data vlan, and voice vlan. There are two work stations on 
> the network, and neither is being used to hit the internet heavily 
> (office is still being setup).
> 
> Any pointers or suggestions anyone have for me as to were to look for 
> this poor quality?
> It seems only the Far-end (called party), is hearing this and not the 
> calling party.
> 
> I haven't tried switching out the phones because we only have 1 type, 
> and any of the phones i used exhibit these problems. I will try 
> softphones to see if it is truly a "networking" issue or Phone issue.
> 
> Is anyone using a cisco 2900 switch or router and care to provide config 
> samples of their COS/QOS setup?

Highly unlikely the ipphones or switch have anything to do with the 
problem. Most likely cause is the dsl "upload" speed (which is usually 
substantially slower then download speed).

Just as an FYI... the QoS settings in any Cisco product (including the 
2900 switch) do not actually kick in "until" a switch port becomes 
congested. So if your switch ports are running at only 10 megabits, 
you'd have to be moving data (including voice) at that rate before QoS 
begins to give priority to voice packets.

If your voip provider supports a low bit-rate codec, you might consider 
using something like g729 to see if that impacts the problem. If it does 
improve it, then either your dsl connection is the problem (to slow), 
or, your dsl provider's data network is less then adequate to move the 
voice packets through in a reliable way.

There are several internet sites that you can use to test your bandwidth 
if you want to try those. One is dslreports.com, but there are others. 
Goggle should give you plenty to work with.