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A case of the jitters

Testing for specific problems affecting VoIP.

Network and service managers need accurate, detailed and reliable tools for VoIP call- quality analysis and diagnostics. Some make use of RMON probes, managed CSU/DSUs or network analyzers to measure network or service performance for data applications. But how will these devices help to resolve critical service problems quickly?

Unfortunately, the answer is "poorly". The biggest problem is that these devices were developed to support data applications. They don't measure many of the impairments that affect critical Voice applications and hence can't recognize Voice over IP quality problems. This approach is generally fine for data applications as there is little difference between what is seen on the wire and what is delivered to the end user. IP Phones and VoIP Gateways have fairly sophisticated algorithms that are applied to the received packet stream in order to improve call quality, this makes it difficult for a simple network analyzer to understand what impact packet impairments are having on the subjective quality of calls.

IP Phones and Gateways use a jitter buffer to reduce the negative effects of packet delay variation or jitter. Uncompensated jitter causes an apparent "personality change" in the decoded speech. Variations in packet arrival time occur because of congestion, timing drift, route changes and contention for access to LANs. The jitter buffer intentionally delays arriving packets in order to build in some tolerance to variations, and discards packets that arrive too late to be played out.

For a network analyzer to estimate the effects of network impairments on VoIP call quality accurately it must therefore understand the mechanics of jitter buffers, packet loss concealment and other end-system algorithms. A "jitter buffer emulator" allows the network analyzer to model the behavior of the "real" jitter buffer in the end system, and hence estimate which packets would be discarded and how much delay is being added. In modern IP networks packet discard due to jitter is often more prevalent than packet loss. Therefore a network analyzer cannot provide accurate diagnostic information about Voice over IP traffic unless it has a jitter buffer emulator.

In selecting management tools to measure VoIP call quality be sure that the tool understands the real time dynamics of Voice over IP systems – including jitter buffer behavior. If you need to accurately estimate call quality and to diagnose key problems be sure that your tools include jitter buffer emulation.

Bob Massad is a VP at Telchemy, Inc., a provider of voice quality management solutions.

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