Problem with jitter over an ADSL / VPN solution

[mell0ns23]

We have 12 * 7960 handsets connected to a 3550 switch which in turn is linked to an ADSL router which is tunnelling to our corporate HQ via a GRE VPN over a 2mb link. We are currently experiencing jitter even if only one call is in progress. QOS has been implemented on the 3550 and I have tried to limit the amount of bandwidth available on Call Manager to this location.

[mgaysek]

What is the latency between the sites through the tunnel and outside the tunnel? Are you sure you are getting the full 2mb?

[mell0ns23]

please see attached file for latency results. I can't guarantee that we are getting the full 2mb.

The firmware on the handsets is P00305000300 and is out of sync with the device defaults which is P00307020400, do you think this could be causing the jitter?

[craig-allen]

Here in the UK ADSL is limited on the upload bandwidth so even though the download is 2Mb the upload is only 256K. This might be where you are having problems. What other traffic is on the line? Get something like MRTG or Cati (both free) to graph the ADSL line usage so at least you know what the current traffic usage is at.

What codec is the VoIP phones using? This will determine the bandwidth required for each individual call.

The other issue that you might be having is MTU problems as a result of using VPN/GRE tunnel.

[mgaysek]

craig brought up some good points. I see your latency hit over 1000ms, anything over 175ms ( I think, may be slightly more) will begin to cause jitter. How erratic is your latency?

Can you look and see how much outbound bandwidth you are using before and during a call?