Set up QoS policy for VoIP in Active Directory

This information is primarily for administrators and/or people who manage Zylinc solutions

You can make sure that network packets for voice over IP (VoIP) calls made with Zylinc internal softphone get the correct priority—and therefore won’t run the risk of becoming delayed or lost, because of interference from other network packets of a lower priority—if you set up a quality of service (QoS) policy for VoIP in your Active Directory.

If you set up a QoS policy for VoIP, callers are likely to notice a much better audio quality.

Without a QoS policy, audio quality can in severe cases on very busy networks become so distorted that VoIP calls can’t be completed.

How quality of service works

DSCP (Differentiated Services Code Point) is a protocol for packet classification and management of network traffic. With DSCP markings in the headers of network packets, you can determine which network traffic gets a high priority, high bandwidth, etc.

That can help you provide a specific quality of service, and it can also help you measure and document that you provide the required quality of service.

The quality of service policy uses the differentiated services field (DS field) in the IP header to indicate when a network packet is related to VoIP, and therefore must take priority over other network packets, for example, packets for HTTP or FTP.

According to industry best practices, you should set SIP traffic to a DSCP value of 24 (also known as Class Selector 3 or CS3), and you should set RTP traffic to a DSCP value of 46 (also known as Expedited Forwarding or EF).

The following scenario is based on Windows Server 2012. The procedure is roughly similar on other Windows Server versions.

First, set up a QoS policy for RTP:

  1. On a domain controller, in Group Policy Management, right-click your domain, and select Create a GPO in this domain, and Link it here…
  2. In Name, enter a name for the GPO (Group Policy Object), for example, VoIP RTP, and click OK.
  3. Right-click the new GPO that you created in the previous step, and select Edit…
  4. In the new window, Group Policy Management Editor, expand Computer Configuration > Policies > Windows Settings
  5. Right-click Policy-based QoS, and select Create new policy…
  6. In Policy name, enter a name for the policy, for example, VoIP RTP
  7. In Specify DSCP Value, enter 46
  8. Click Next
  9. Select Only applications with this executable name, and in the box, enter %programfiles(x86)%\Zylinc\ZyDesk\ZyDesk.exe
  10. Click Next
  11. Make sure that both Any source IP address and Any destination IP address are selected, and click Next
  12. In Select the protocol this QoS policy applies to, select UDP
  13. Select From this source port number or range, and in the box, enter 4000:4063
  14. Select To any destination port
  15. Click Finish

To set up QoS policy for SIP, repeat the previous steps, but with the following changes:

  • In Name, enter a different name, for example, VoIP SIP
  • In Specify DSCP Value, enter 24
  • In Select the protocol this QoS policy applies to, select TCP and UDP
  • Select From any source port
  • Select To this destination port number or range, and in the box, enter 5060

The QoS policy will take effect the next time computers in the domain refresh their group policy settings, typically when the computers restart, or if you open a command prompt and type the following command: gpupdate /force

To test if the policy is active on a client computer:

  1. In a command prompt, type gpresult /Scope Computer /v

  2. In Applied Group Policy Objects, the names that you entered for your RTP and SIP policies should be visible, like in this example:

    ...

    Applied Group Policy Objects
    -----------------------------
    Default Domain Controllers Policy
    Default Domain Policy
    VoIP RTP
    VoIP SIP

    ...