To measure the voice
quality of calls that are sent and received within the network, conference
phones use these statistical metrics that are based on concealment events. The
DSP plays concealment frames to mask frame loss in the voice packet stream.
-
Concealment
Ratio metrics: Show the ratio of concealment frames over total speech frames.
An interval conceal ratio is calculated every 3 seconds.
-
Concealed Second
metrics: Show the number of seconds in which the DSP plays concealment frames
due to lost frames. A severely “concealed second” is a second in which the DSP
plays more than five percent concealment frames.
-
MOS-LQK metrics:
Use a numeric score to estimate the relative voice listening quality. The
conference phone calculates the mean opinion score (MOS) for listening quality
(LQK) based audible concealment events due to frame loss in the preceding 8
seconds, and includes perceptual weighting factors such as codec type and frame
size.
MOS LQK scores are
produced by a Cisco proprietary algorithm that is an implementation of P.VTQ,
an ITU provisional standard.
Note |
Concealment ratio
and concealment seconds are primary measurements based on frame loss while MOS
LQK scores project a “human-weighted” version of the same information on a
scale from 5 (excellent) to 1 (bad) for measuring listening quality.
|
Listening quality
scores (MOS LQK) relate to the clarity or sound of the received voice signal.
Conversational quality scores (MOS CQ such as G.107) include impairment
factors, such as delay, that degrade the natural flow of conversation.
For information
about configuring voice quality metrics for conference phones, see the
Cisco Unified
Communications Manager System Guide, “Cisco Unified IP Phones” chapter,
“Phone Features” section.
You can access voice
quality metrics from the conference phone by using the Call Statistics screen
or remotely by using Streaming Statistics.
To use the metrics
for monitoring voice quality, note the typical scores under normal conditions
of zero packet loss, and use the metrics as a baseline for comparison.
It is important to
distinguish significant changes from random changes in metrics. Significant
changes are scores that change about 0.2 MOS or greater and persist in calls
that last longer than 30 seconds. Conceal Ratio changes should indicate greater
than 3 percent frame loss.
MOS LQK scores can
vary based on the codec that the conference phone uses. The following codecs
provide these maximum MOS LQK scores under normal conditions with zero frame
loss:
A Conceal Ratio of
zero indicates that the IP network is delivering frames and packets on time
with no loss.
When you observe
significant and persistent changes to metrics, use the following table for
general troubleshooting information.
Table 2. Changes to Voice
Quality Metrics
Metric Change
|
Condition
|
MOS LQK scores
decrease significantly
|
Network
impairment from packet loss or high jitter:
Cross-check
with Conceal Ratio and Conceal Seconds for evidence of packet loss and jitter.
|
MOS LQK scores
decrease significantly
|
|
Conceal Ratio
and Conceal Seconds increase significantly
|
Network
impairment from packet loss or high jitter.
|
Conceal Ratio
is near or at zero, but the voice quality is poor
|
Check packet
transmit (TxCnt) and packet receive (RxCnt) counters to verify that voice
packets are flowing.
|
Note |
Voice quality
metrics do not account for noise or distortion, only frame loss.
|