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Analyzing Connectivity for Office Communications Server 2007 R2 Audio/Video Sessions Part 2: Using the Pre Call Diagnostics Tool

Communications Server 2007 R2

This article is the second part of a two-part series. The first article is “Analyzing Connectivity for Office Communications Server 2007 R2 Audio Video Sessions Part 1. This article gives you an overview of how to use the Office Communications Server 2007 R2 Resource Kit Pre Call Diagnostic tool to analyze problems with audio/video communications. Part 1 of Analyzing Connectivity for Office Communications Server 2007 R2 Audio Video sessions provides readers with a quick look at how to analyze and troubleshoot problems with the Office Communicator client audio video sessions when using Office Communications Server.

Author: Mike Adkins

Publication date: November 2010

Product version: Microsoft Office Communications Server 2007 R2

Despite the effort that goes into the design of the applications and protocols that support audio and video communications on a network, issues with network connectivity can degrade the quality of audio and video communications between clients and peers. The Office Communications Server 2007 R2 Resource Kit Pre Call Diagnostics (PCD) tool is designed to help pinpoint a variety of network issues that can occur on the network link that supports Office Communicator 2007 R2 and Windows Live Meeting 2007. The Communications Server 2007 R2 PCD tool monitors the Windows client audio/video endpoint for criteria such as:

  • TCP/IP round trip time (RTT)
  • Packet loss due to bandwidth saturation or network errors
  • Inter-arrival jitter due to network latency issues

The results of the analysis are displayed by the Communications Server 2007 R2 PCD console as a value ranging from 1 (poor quality) to 5 (excellent quality). For more details, see Mean Opinion Score and Metrics at http://technet.microsoft.com/en-us/library/bb894481(office.12).aspx.

The following requirements must be met before using the PCD tool:

  1. The PCD tool is supported on x86 and x64 bit Windows XP SP2, Vista SP1, and Windows Server 2008.
  2. The PCD tool requires that the Communications Server 2007 R2 A/V Edge Server be part of the Communications Server 2007 R2 deployment.
  3. The PCD tool must be installed on a supported Windows client computer that has access to the Communications Server 2007 R2 A/V Edge Server that is hosted in the perimeter network.
    noteNote:
    The network connectivity just described requires that the Communications Server 2007 R2 internal firewall configuration for the perimeter network be configured as per the Communications Server 2007 R2 A/V Edge Server’s deployment documentation. For details, see Firewalls for Office Communications Server 2007 R2
  4. A valid domain user account that is enabled for Communications Server 2007 R2. The user’s SIP account will be used to provide the SIP URI for the PCD tool’s sign-in process.

The following steps explain how the PCD tool works.

  1. The default installation folder for the PCD tool is typically located at C:\Program Files\Microsoft\PreCallDiagTool.
  2. After the PCD tool is installed locally, you can open it from the Start menu of the Windows client computer.
    • The PCD tool's GUI interface will prompt the user for their SIP credentials.
    • Any subsequent domain user who signs in to the Windows client computer must open the Media Network Monitoring Tool dialog box in the PCD tool, and then select the Options menu to update their SIP URI.
  3. In the Media Network Monitoring Tool dialog box, on the Options menu, you can also choose the graph style the PCD tool will use to display the Network MOS value. The choices are Bar or Circle.
  4. In the upper right corner of the Media Network Monitoring Tool dialog box, you can click the arrow to display the run-time information for:
    • Network MOS
    • Packet loss rate
    • Inter-arrival jitter (ms)
    This graph will display up to 15 minutes of statistical information based on the quality of the network connectivity between the Windows client computer and the internal edge of the Communications Server 2007 R2 A/V Edge Sever.
  5. The PCD tool will make a SIP service credentials request to the RTCMRAUTH, or Media Relay Authentication Service (MRAS), which is hosted on the internal interface of the A/V Edge Sever. The response for this MRAS request will provide the PCD client with the listening ports of the RTCMRAUTH and RTCMEDIARELAY services and the FQDN of the internal interface of the A/V Edge Server.

When in use, the computer that has the PCD tool will establish a persistent connection to the RTCMEDIARELAY located on the internal interface of the A/V Edge Sever by using UDP port 3478 (STUN). The STUN connection will be authenticated using the pseudo user information that is generated by the PCD client. After the preliminary STUN requests and responses take place, the STUN connection between the two endpoints will remain persistent throughout the length of the PCD connectivity test. This connection is the basis for the run-time connectivity reports that are provided by the PCD tool.

Figure 1 shows an example of excellent Network MOS.

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Excellent network MOS occurs when there is no or a small amount of packet loss and inter-arrival jitter. Good network behavior is what we expect to occur between clients that share the same A\V conference.

Figure 2 shows an example of what poor Network MOS looks like on a network that is suffering from heavy packet loss.

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Packet loss typically occurs when some of the IP packets that make up the data stream are lost or dropped along the network path to the receiving client. Intermittent packet loss on a network is usually the fault of mis-configured or faulty network hardware. Inter-arrival jitter occurs when IP packets become out of sequence along the network path to their destination. Inter-arrival jitter is a symptom of latency in the delivery of IP packets. This slow delivery of IP packets on a network is usually due to network congestion caused by the saturation of bandwidth on a network link.

Figure 3 provides an example of what excellent Network MOS looks like on a network that is suffering from packet arrival latency.

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Network MOS does not include the late arrival of IP packets. This is why Figure 3 displays a large amount of inter-arrival jitter while providing a Network MOS number that represents good to excellent network connectivity. In this case, the inter-arrival jitter is caused by approximately 800 (ms) of network latency in both directions between the PCD client and the internal interface of the Communications Server 2007 R2 A/V Edge Sever. This high amount of network latency has caused a small amount of IP packet loss between the two endpoints. Because STUN UDP is a connectionless protocol at the transport layer, there is no resend algorithm that will ensure the delivery of IP packets as there would be if the TCP transport was used instead. The PCD client does not test the actual streaming Real Time Protocol (RTP) traffic between clients on your network. The PCD client is designed to test network connectivity for the TCP and UDP transports between a designated Windows client computer and the internal interface of the A/V Edge Server.

Most of the problems that clients will have with A/V communications are due to issues with the network that is hosting the Communications Server 2007 R2 Unified Communications clients. Microsoft provides Office Communications Server enthusiasts with the Communications Server 2007 R2 Resource Kit PCD tool to trouble shoot client A/V communications issues. The PCD tool can be used to isolate network issues such as slow TCP/IP round trip time (RTT), packet loss due to bandwidth saturation or network errors, and Inter-arrival jitter due to network latency issues on the network that is hosting the clients.

 
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