Lync PreCall Diagnostics
Network MOS. The network MOS ratings differed on the two days. In the January 7 capture (Figure 1), the network MOS rating steadily climbed to 3.5 during the two-minute sampling. In the January 8 capture (Figure 2), the network MOS rating was a bit better. Overall, there isn't anything alarming about the network MOS ratings, as they're pretty good. However, if you want to improve the ratings, you could potentially use a PC to Public Switched Telephone Network (PSTN) call, which leverages RTAudio Narrowband codec, or use a Lync conference call with the Siren codec.
Packet loss rate. The packet loss rate captured on both days is similar in that both captures show consistent packet loss. However, the packet loss is more significant in the January 7 capture. As you can see in Figure 1, there's a peak of 8 percent packet loss. The question that should come to mind is what is acceptable? In a voice conversation, an acceptable rate is up to 5 percent packet loss. So, the packet loss captured on January 8 is acceptable, but the packet loss captured on January 7 is unacceptable. When the packet loss is over the 5 percent threshold, it can affect the conference and audio calls made during that time.
Interarrival jitter. From what you see in Figure 1 and Figure 2, is the jitter is considerably low for both days. This is a good sign. When the jitter is low, video and conference calls should transpire with little choppiness in the feed. A high level of jitter means that packets are being discarded on the receiving device, which can cause choppiness.
When a Lync client is experiencing poor audio quality or choppy video feeds, it's usually not due to problems with the Lync client itself. Instead, these types of problems are typically due to network variables such as jitter, packet loss, or network MOS. The PreCall Diagnostics Tool can shed a lot of light into the problem, no matter whether a home network, a wireless network at a Starbucks shop, or the company LAN is being used in the Lync audio, video, and conference calls.
Source = http://windowsitpro.com/lync/lync-precall-diagnostics-tool