A Primer on Lync Audio Quality Metrics

The topic of audio quality can be vast and complex. The purpose of this post is to provide some practical and easy to read guidance on the key metrics used in several Lync Server monitoring reports. This will enable IT professionals to quickly gain insight into the audio quality of a particular Lync call or conference and troubleshoot poor audio quality if need be.

Note: this post is focused on audio metrics for Lync Audio – not enterprise voice audio – although some of the same metrics apply.

See my previous blog post Key Tips to Get Started with Lync Monitoring & Reporting to get started with the Lync Monitoring role and reports.  Microsoft has a set of 4 documents to get you started and interpret the reports: Microsoft Lync Server 2010: Work Smart Guide for Monitoring Server Reports. Lastly, fellow Lync MVP Stale Hansen recently posted a good article regarding “Lync Server 2010 Monitoring reports and why you always should deploy them” which covers how and why you should deploy the Lync monitoring reports, and some of the metrics covered in this post.

Key Audio Quality Metrics Used in the Lync Monitoring Reports

Here are the 7 audio quality metrics I have found most effective for analyzing Lync audio quality and what numbers represent an audio quality issue.

Quality Metric Key Notes Thresholds
Avg. listening MOS Mean Opinion Score (MOS) is the gold standard measurement to gauge the perceived audio quality (an algorithm calculates how a typical user would rate the voice quality).

It is is an integer rating from 0 to 5.

  • 5 – excellent
  • 4 – good
  • 3 – fair
  • 2 – poor
  • 1 – bad
Avg. round trip Network Round Trip Time (RTT) is the most common measure of latency and is measured in ms.

This measure is the average round trip time for RTP packets between endpoints.

When latency is high, users will likely hear the words, but there will be delays in sentences and words.

For RTP packets as reported in the monitoring reports:

  • < 200 ms is good
  • > 200 ms is poor
  • > 500 ms is bad

For basic UDP packets used by the ping command line:

  • < 100 ms is considered decent
  • > 150 ms is considered problematic
Avg. jitter Jitter (ms) measures the variability of packet delay and results in a distorted or choppy audio experience.

Jitter can increase latency on networks.

  • < 20 ms is good
  • > 30 ms is not good (but can be ‘ok’)
  • > 45 ms is considered very bad
Avg. packet loss rate Packet Loss (%) represents the % of packets that did not make it to their destination.

Packet loss will cause the audio to be distorted or missing (on the receiver end).

  • < 3% is considered good
  • an average pack loss rate of > 5% will impact audio
  • > 7% is not good (some consider +7% packet loss “huge”)
  • > 10% is bad
  • > 50% packet loss … no chance!
Avg. network MOS degradation Average network MOS degradation is an integer represents the amount of the MOS value lost to network affects.
  • > 1 is not good.
  • < 0.5 represents acceptable degradation.
Avg. concealed samples ratio Concealing audio samples is a technique used to deal with dropped network packets.

Average concealed samples Ratio (%) is the % of packets that were concealed.

  • < 2% is good
  • > 3% is not good
  • > 7% is bad
Bandwidth estimates (Kbps) This is available bandwidth estimated on the client-side.
  • Absolute thresholds are not that helpful, but when the client detects bandwidth is low (< 100 Kbps) audio quality can easily be impacted by other applications or network congestion.

There are other metrics such as Burst Duration and Burst Density which can help gain insight into audio quality, however the above seven metrics will usually provide a good indication of audio quality.

Feel free to comment on this blog post with your own experiences so we can learn from each other.

Key Lync Monitoring Audio Quality Reports

The above quality metrics are reported in several Lync Monitoring Reports. In my experience, the 3 reports below give you the best insights (these are available from the “/ReportServer/Pages/ReportViewer.aspx?/LyncServerReports/Reports%20Home%20Page”>Monitoring Server Reports” reports on the Lync Monitoring report server).

Tip #1: some of the reports contain many different sections. The audio quality metrics are available in the Media Quality section or various reports; not to be confused with the Diagnostic Reports section.

Tip #2: in the Media Quality reports, if a reported metric value is colored

  1. Yellow : it has reached a threshold that could cause poor quality
  2. Red : it has reached a threshold that would classify the call as ‘poor’

1] The User Activity Report

This is available under the “Call Diagnostic Reports (per-user)” section, and provides Lync audio quality metrics for peer-to-peer and conferencing activities (for a user and all the participants in a conference).

To get the audio quality metrics for a A/V conference:

  1. Either filter by Activity Type (Conference), or scroll down to conferences.
  2. Select the Conference URI link and the Media Quality link for the Audio or A/V conferencing modality for a conference participant to see the quality metrics for the media stream.

Here is a sample of the quality metrics for a participant in an audio/video conference:

image

2] The Media Quality Summary Report

This is available under the “Media Quality Diagnostic Reports” section and provides a summary of quality data across the different endpoints deployed in your environment. It can be most useful to identify groups of specific endpoints having poor audio quality experiences.  Here is a sample:

image

3] The Location Report

Many times audio quality issues are caused by the network. Network problems typically affect groups of users in a particular location. The Location Report provides a list or network locations – identified by the caller subnet – ranked by highest percentage of poor call quality.

Other reports such as the Server Performance Report and Device Report give audio quality metrics from the standpoint of servers and devices respectively, and can be useful if a particular Lync server or device is causing audio quality issues. From my experience it is *usually* more related to the network.


Miscellaneous Notes

The audio codec used by the Lync 2010 client can adapt to low bandwidth conditions. The codec used will affect the bandwidth consumed by that client and the audio quality. See my previous blog entry on The Lync 2010 Client Audio Codec Selection in Conferences for information on how the Lync 2010 client adapts to changing network conditions.

Lync audio quality over a WiFi connection can be troublesome given the latency, jitter, and packet loss on WiFi networks. Lync audio over WiFi should be viewed as a ‘best-effort’ experience.

This post won’t go into the vast area of how to improve audio quality, but it can be improved through several ways. Microsoft Lync supports Quality of Service (QoS) through networks that use DiffServ, and there are also Call Admission Control features which can be used to improve audio quality.


Very Useful Microsoft References

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