Administration of Microsoft Lync Server 2010 : Configuring Quality of Service

Quality of Service (QoS) can be used in networks where the media traffic used by Lync Server 2010 servers and clients should have a higher priority than other traffic using the same infrastructure. This is done by having the Lync servers and endpoints tag their media traffic packets with a specific Differentiated Services Code Point (DSCP) value. Routers within the network are then configured to prioritize traffic based on the DSCP values included with packets.

DSCP marking in Lync Server 2010 is done by using the policy-based QoS first introduced with Windows Server 2008 and Windows Vista. These policies are controlled through Windows Group Policy and can mark traffic with DSCP codes based on application names and port ranges. Because these policies can be centrally managed and controlled by the organization, it should be acceptable to trust the markings sent from client endpoints. Using a separate port range for each type of traffic enables the policy-based QoS to tag the traffic appropriately. For example, audio traffic using one port range can be assigned a DSCP code with higher priority than the port range used for video or application sharing.

Server Configuration

Lync servers operate similarly to clients and tag media traffic through the use of policy-based QoS in Group Policy. Each media type has one of the following default port range assigned:

• Audio (49,152–57,500)

• Video (57,501–65,535)

• Application sharing (49,152–65,535)

These port ranges can then be matched through policy-based QoS and assigned a DSCP marking. The default port range for application sharing overlaps with both audio and video. If application sharing is tagged differently than either traffic, a separate port range must be configured. Port ranges for each media type can be configured using the Set-CsConferenceServer and Set-CsMediationServer cmdlets.

For example, to separate application sharing into its own port range, run the following command:

Set-CsConferenceServer –AppSharingPortStart 32768

This changes application sharing to use ports 32,768–49,151 while keeping the same amount of ports available.

Client Configuration

QoS tagging of the media is performed by the Lync client itself, so it must be provisioned in a way that it understands what ports to use for each type of traffic. By default, no tagging is done and all traffic uses the port range 1,024–65,535. The Lync client supports using different port ranges for the following types of traffic and recommends a minimum number of ports for each modality. Using port ranges that are below the recommended minimums increases the chance for a media request to fail.

• Audio (20 ports required)

• Video (40 ports required)

• Application sharing (4 ports required)

• File transfer (4 ports required)

First, to enable separate port ranges for each media type, run the following command:

Set-CsConferencingConfiguration –ClientMediaPortRangeEnabled $true

Next, define a unique port range for each type of traffic. As an example, the ports used on the client side can be the same as on the server. The sample numbers used here are well beyond the minimum number of ports required and are used only to show how the default port ranges can be moved. The port ranges used should be limited to the recommended sizes for ease of management and troubleshooting.

Set-CsConferencingConfiguration –ClientAudioPort 49152 –ClientAudioPortRange
8348 –ClientVideoPort 57501 –ClientVideoPortRange 8034 –ClientAppSharingPort
32768 –ClientAppSharingPortRange 16383 –ClientFileTransferPort 24733
–ClientFileTransferPortRange 8034


					  

Creating a QoS Policy

After modifying the Lync clients and servers to use specific port ranges, QoS policies must be created to add the appropriate DSCP value to traffic originating from each port range. The steps required to use policy-based QoS through Group Policy are similar, but the port ranges used for servers and clients will likely differ. Create at least two separate policies: one for servers and one for clients. To create a new policy, perform the following steps:

Repeat these steps for each type of media using a unique port range, which should be tagged differently. Figure 1 demonstrates what a policy with separate audio and video settings looks like.

Figure 1. Policy-Based QoS

Windows XP QoS

Because QoS policies can be created only on Windows Vista or later operating systems, the process for enabling QoS on Windows XP is slightly different. Windows XP QoS marking is also not as flexible as policy-based QoS and can only mark audio traffic as SERVICETYPE_GUARANTEED and video as SERVICETYPE_CONTROLLEDLOAD.

First, verify the QoS packet scheduler is installed on the client. Secondly, run the following command on a Lync server:

Set-CsMediaConfiguration –EnableQoS $true

Before the Lync client attempts to tag packets, the DSCP values should be modified using Group Policy.

Repeat these steps for the Controlled load service type to tag video media.

Lync Phone Edition QoS

The final client type that can use QoS settings is the Lync Phone Edition client, which can run on third-party partner hardware. Lync Phone Edition settings are determined through in-band signaling to the devices. In addition to a DSCP value for audio traffic, the Lync Phone Edition clients also support 802.1p. To set the Lync Phone Edition QoS settings, use the following command on a Lync server:

Set-CsUcPhoneConfiguration –VoiceDiffServTag 46 –Voice8021p 5