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Simon Hodgson
Asked: 2009-06-02 07:20:32 +0800 CST

Prioritise VOIP traffic over a VPN link

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We have recently opened a branch office for developers to work out of. The guys there all have Aastra VoIP 'hard' phones on their desks.

The main office and branch offices are linked together via a VPN (using two Sonicwall Firewalls) the VoIP PBX we have is a Trixbox installation, interfacing to 4 PSTN lines.

Most of the time this setup works great, but if someone opens a file on a server at the 'other' end of the link the call quality drops off.

Is there some easy (and cheap) way we can make the VoIP traffic a higher priority?

vpn sonicwall voip

4 Answers

  1. Best Answer

    SonicWall devices have QoS configuration options in the Enhanced OS version. We use this internally to prioritize VoIP and Citrix traffic on our VPN tunnels. I would think this would be your best option since it utilizes existing hardware.

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  2. It may depend on which model of SonicWALL Firewall and its software you have but you should be able to define a 'Quality of Service' (QoS) for your VoIP traffic. This can be complex and implementing this may require some professional help.

    It PAINS me to point you here but this question appears to have been asked on experts-exchange.com before, perhaps you could look there too.

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  3. Unless you are dealing with a point-to-point circuit or MPLS connection between your branch offices, simply adding QoS to your SonicWall or other VPN/Firewall device won't be enough. If your SonicWall's are using public internet connections, they will have no control over the inbound data received for the WAN and outbound QoS will be stripped off once it hits your ISP's router.

    In fact, added any QoS policies to inbound traffic can cause more network conjestion. Consider a SMB packet which has traversed your bandwidth limited download link and reaches the SonicWall. The SonicWall sees that this is Windows file sharing, which is low priority, and drops or delays the packet due to higher priority VoIP that is also being received. Well, the sending client gets the TCP retransmission and has to send that SMB packet again which increases the overall bandwidth usage on your link than if the packet had been allowed in the first place.

    I guess my suggestion to increase VoIP quality is:

    1. Utilize MPLS to connect to your branch offices which will honor your QoS policies.
    2. When using the public Internet for VPN's, use links which are dedicated to VPN traffic only (so when Bob in accounting downloads 100MB of family photos from Flickr it won't affect VPN traffic).
    3. If you combine internet and VPN traffic on one connection do not set QoS on inbound/ingress traffic.
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    1. Install ubuntu 9.04 server and two network interface cards on a surplus PC (mine is 800MHZ P3).

    2. Set up a firewall as described here: http://ubuntuforums.org/showthread.php?t=926001

    3. Install wondershaper: "apt-get install wondershaper". I used the improved version of wondershaper from here: http://serendipity.ruwenzori.net/index.php/2008/06/01/modified-wondershaper-for-better-voip-qos.

    You need to do some measurements on your system, so that you can give upload and download values to wondershaper when you start it (see /usr/share/doc/wondershaper/README.Debian).

    I can get good quality VOIP calls while filling my DSL line with a torrent download.

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