It has been said that you can’t manage what you can’t measure. This especially holds true for VoIP quality. In some cases administrators may simply implement VoIP without understanding the bandwidth demands on their networks from existing applications. Given the importance of call quality for ensuring a positive user experience, constant monitoring of VoIP networks is critical. It requires hard numbers beyond subjective user assessments of quality (although these can play an important role in troubleshooting specific issues).

Call Quality Scoring
Call quality scoring uses industry standard methodologies for assigning a grade to a call. Measuring call quality provides objective and subjective scores to evaluate existing call conditions and compare them with historical conditions. Two well-established metrics for measuring call quality and overall VoIP network health are the R-factor and the Mean Opinion Score (MOS).

The R-factor uses a scale of 1 through 100 to measure call quality. It takes into account how various impairments, including jitter, delay and packet loss, affect the average user’s perception of call quality. Typically the maximum value of a call would be 93.2 after standard degradation. If the R-factor falls below 70, call quality will noticeably degrade.

MOS measures user satisfaction with a call. It takes into account a variety of factors impacting call quality, such as handset quality, ambient noise, and network performance. MOS measures quality on a scale of one through five, where 4.0 or higher indicates a satisfied user. If the average MOS falls below 3.5, it’s likely that there are several dissatisfied VoIP users. If you notice these statistics trending downward, it’s essential to perform more detailed analysis to determine and resolve any performance issues.

It’s important to keep in mind that call quality will be perceived differently depending upon the environment. For example, a user making a call from a factory floor may tolerate lower quality than a user making a call from a conference room.

[Stephen Brown and Charles Thompson of Network Instruments]

[tags]voip,delay,site survey,benchmark,network performance,voip metric,cdr,call detail record,packet loss[/tags]