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OpenVPN phones have intermittent audio loss - Printable Version

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OpenVPN phones have intermittent audio loss - amartin - 09-10-2015 02:00 AM


I have successfully deployed several SIP-T32G, SIP-T38G, and SIP-T42G Yealink phones at remote locations. These phones use the following OpenVPN config to connect to an OpenVPN server running on my Internet gateway and then to an Asterisk server on my LAN. Some of the time, this works very well - calls are clear and both parties can hear each other. However, at other times the call quality will significantly degrade, so that there are intermittent gaps in the audio from one party or both. I have not experienced these same problems with other phones on the LAN, so I suspect the issue lies in the gateway, remote network, or OpenVPN config. Since this is occurring at several different remote locations, I feel that I can probably rule out the remote network as the culprit, which leaves just the gateway or the OpenVPN config on the phones. I am not showing congestion on the gateway, as it is mostly just used for this purpose. I do not see anything obvious in the syslog of one of the phones.

I have done a Wireshark packet capture and and am seeing some jitter (around 10ms average) and delta (around 100ms average) on the packets, but it is hard to exactly correlate this intermittent loss with the audio problems.

OpenVPN client config:
dev tun
cipher DES-CFB
auth SHA1
resolv-retry infinite
remote <gateway public ip> 1194 udp
ca /phone/config/openvpn/keys/ca.crt
cert /phone/config/openvpn/keys/client1.crt
key /phone/config/openvpn/keys/client1.key
ns-cert-type server

Is wrapping the SIP/RTP traffic inside of an OpenVPN tunnel causing some jitter or latency issues? What else can I do to debugt his prboelm?T

RE: OpenVPN phones have intermittent audio loss - James_Yealink - 09-10-2015 07:09 AM


There is no issue to the client config.
So the quality issue only happens when you enable OpenVPN?

You mentioned that "at other times the call quality will significantly degrade", the other times means some random call or a period of time?

If it happens in some specific call, please try hold and unhold multiple time to check whether the issue will be solved. Or just hang up and make a call as soon as possible to check the quality in next call.

If it happens to multiple phones in a period of time, it seems more like a gateway issue.


RE: OpenVPN phones have intermittent audio loss - amartin - 09-10-2015 11:03 PM


Thanks for the fast response! The call quality will degrade significantly at random points during a call. Some calls will work completely fine with no problems, other calls may start out fine and then degrade and then may recover again later if the call is long enough.

I will try holding the hold button the next time this happens.

Are there any settings on my gateway that I should attempt to adjust? I just tried lowering the MTU from the default 1500 to 1200 using "mssfix 1200" in my OpenVPN server config on the gateway yesterday.

Note that I have tried changing the Jitter Buffer settings from their defaults to the following, with no improvement:
Type: Fixed
Minimum Delay: 120
Maximum Delay: 120
Normal Delay: 120

Note that I am experiencing it on the following phones with the following firmware versions:

RE: OpenVPN phones have intermittent audio loss - amartin - 09-11-2015 04:43 AM


Also note that every time someone reports bad quality on a call, I have been able to correlate the timestamp with an entry like this in the OpenVPN server log:

Could this renegotiation be disrupting the VPN connection during an active call, causing the problems?

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RE: OpenVPN phones have intermittent audio loss - shanondink - 12-03-2015 12:53 AM

Hey Martin,

I am experiencing the exact same thing.

Did you ever figure it out?


RE: OpenVPN phones have intermittent audio loss - amartin - 01-19-2016 06:04 AM

Hi Shanon,

Unfortunately no, I never did isolate the source of this problem. It is still intermittently happening, with no resolution. Have you found any insights into how to resolve this problem?

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