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Each measurement point is instrumented with a verifier, which, depending on the application and deployment location, may either be a purpose-built hardware appliance or a lightweight software agent designed specifically to perform VoIP testing. Verifiers measure VoIP transactions and communicate raw transaction performance data to a centralized administration portal that provides data analysis, aggregation, and real-time reporting. This pervasive approach is the only way to precisely pinpoint the causes of poor voice and call quality.
Continuous Measurement
Measuring the network intermittently will not tell you much about VoIP quality. That's because IP networks -- unlike more static and predictable circuit-switched networks -- are by their very nature dynamic. With asymmetric routing, dynamic routing modifications, ever-changing traffic loads, and bandwidth availability, network performance can vary from one moment to the next. Continuous measurement is, therefore, essential.
When we say "continuous" we mean both around the clock and throughout the VoIP application lifecycle, including pre-deployment testing right through the deployment and production phases. Measuring application and network performance before deployment is critical to avoid unpleasant surprises when the application is launched. Many VoIP quality problems -- and unhappy users -- could have been avoided had they been identified pre-launch. Incremental testing before and during deployment also makes the task of validating the network more manageable, accelerating time-to-launch, and avoiding costly and time-consuming analysis and redesign after deployment.
The need for monitoring doesn't end once the VoIP application is successfully rolled out. In fact, continuous monitoring is most important once the service goes live and is subjected to real world, highly changeable traffic loads and network conditions. Around-the-clock testing and monitoring of network performance enables administrators to pinpoint trouble areas rapidly to minimize or avoid impact on service quality. Continuous testing also provides the data to perform meaningful trend analyses that enable better capacity planning and network utilization decisions.
Comprehensive Measurement
To provide that meaningful data, network measurement must be comprehensive, employing both passive monitoring and active testing.
Active testing should simulate actual end-user usage patterns. This means executing the same high-level transactions, such as a call, using the same types of protocols, like SIP, H.323, MGCP, or SCCP, and involving the same call components, such as gateways or proxy servers, that users do. This testing should be flexible enough to allow precise control over every aspect of configuration, scheduling, and measurement. And it should provide the flexibility to test all aspects of the service from call initiation, media transmission, and delivery through call completion as a single transaction or by isolating specific portions. Most importantly, because active testing can be done independently of users, administrators can proactively uncover potential voice quality problems before users do.
However, it is also important to have the option to passively monitor the quality of a particular call to identify user, location, or time-specific complaints. For example, for a poorly performing on-net call between a user in the New York-based headquarters and a colleague in the Paris sales office, the network administrator must be able to identify whether the impairments can be attributed to performance issues in the WAN provided by the service provider, the LAN in New York, or the LAN in Paris in order to fix the problem. By measuring a call's performance, passive testing provides insight on the user experience -- as it happens.
A PER-CALL QUALITY METRIC
While measurement of packet loss and latency is required to diagnose performance problems, the bottom line for network administrators is whether they're providing toll-quality service to their users. How do you achieve this single measure of user satisfaction or a MOS (Mean Opinion Score) without relying on the subjective opinions of human testers? Today's most sophisticated measurement suites deliver an objective measure of voice quality based on the ITU-T G.107 (E-Model) standard.
PUTTING THE QUALITY IN QoS
As cost pressures continue to mount on enterprises, and legacy equipment continues to age, more and more organizations will embrace the advantages of VoIP. As they do so, however, enterprise IT managers must recognize the importance of adopting a VoIP measurement strategy that is pervasive, continuous, and comprehensive. Not doing so leaves mission-critical voice applications at risk for crippling service degradations and outages -- and carries with it its own steep price tag.
Planned and implemented correctly, VoIP network measurement can:
. Provide a precise picture of call quality for each and every call, using the same criteria that determines user satisfaction;
. Pinpoint trouble areas, in time to prevent service interruption;
. Enable enterprises to evaluate service provider responsibility for performance issues; and
. Provide the information needed to plan network and service enhancements effectively.
Using a sophisticated and flexible measurement approach gives enterprises what they really need: insight into the myriad network factors that impact voice quality, and finally giving them control over the user experience.
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