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Voice Quality of Service (QoS)

Communications Server 2007 R2

Topic Last Modified: 2009-05-22

The quality of the service associated with synchronous traffic like audio or video can be impacted by delay, jitter, and packet loss in the IP network. Although Office Communications Server 2007 R2 has been designed to work without any Quality of Service (QoS) framework, it can be deployed in IP networks with QoS implemented using Differentiated Services (DiffServ). To support the QoS environment, endpoints are configured to mark the IP traffic conveying the real-time audio and video IP traffic according to well-established classes of services designed to protect the real-time communication traffic from other asynchronous traffic in the IP network, including instant messaging (IM), application sharing data, and file downloads. These markings can be changed to map to different classes of services as desired by an enterprise.

A network enabled for Differentiated Services (DiffServ) provides class-level prioritization based on Differentiated Services Code Point (DSCP) marking of the IP packets. Each DSCP value corresponds to a class of service for forwarding packets from the sender or intermediary router to the next router or receiver in the network. The forwarding behaviors can be implemented by using a variety of techniques, including priority queuing, weighted fair queuing, or conventional leaky bucket-based techniques. Relevant classes for the delivery of audio and video media streams are the Expedited Forwarding (EF) and Assured Forwarding (AF) classes, respectively. For a description of the 6-bit DSCP field values in the Type of Service byte of any IP packet, see IETF RFC 2474.

In Office Communications Server 2007 R2, DSCP marking can be enabled to configure the media stack to mark the IP traffic conveying the real-time audio and video IP traffic according to well-established classes of services. By default, DSCP marking is not enabled. If enabled, the marking of the IP packets is done by the QoS Packet Scheduler service. The resulting marked packets can subsequently be recognized by network entities (end systems and routers) to manage the media traffic according to the QoS priorities. The QoS marking is applied to all media ports and regardless of whether the audio/video traffic is delivered over Real-Time Protocol (RTP; see IETF RFC 3350) or Secure Real-Time Protocol (SRTP; see IETF RFC 3711). Because QoS policies are often tied to UDP or TCP ports, Office Communications Server 2007 R2 also includes a Group Policy registry setting (on client platforms) or a WMI setting (on server platforms) to specify the port range for the UDP and TCP ports used in delivering media streams.

Before enabling QoS for Office Communications Server 2007 R2, you must provision the network correctly. Relevant classes for the delivery of audio and video media streams in Office Communications Server 2007 R2 are the following:

  • For audio, the Expedited Forwarding (EF) class. Audio streams affected by this marking include DTMF, Comfort Noise, and Audio with Forward Error Correction (FEC) streams.
  • For video, the Assured Forwarding (AF) class. Video streams affected by this marking include Video streams with FEC packets.

After ensuring that the network is correctly configured, Office Communications Server 2007 R2 can be configured to support a QoS environment by enabling DSCP marking, which includes doing the following:

  • Enabling QoS on the appropriate servers and clients
  • Running QoS on computers
  • Ensuring that Group Policy settings are correct on servers and client computers

The procedures in Enabling DSCP Marking describe how to configure Office Communications Server components to support a QoS environment.

Dd572295.note(en-us,office.13).gifNote:
Generally, a QoS environment is set up before Office Communications Server is deployed, and the procedures in this documentation are implemented after the Communications Server components are deployed. If you add Differentiated Services capability to the Enterprise network after you deploy Office Communications Server 2007 R2, use the information in Implementing Support for a QoS Environment in the Administering Office Communications Server 2007 R2 documentation to enable and configure Office Communications Server media traffic to take advantage of this new capability.
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