When network managers want to measure voice quality they typically think in terms of packet loss, jitter and delay numbers and almost always in terms of their averages. This is generally off the mark and often misleading.
First, the most important number to focus on is the packet-loss number,
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which must not only include a count of packets lost in the network but also packets
discarded by the jitter buffer. If only the packets lost in the network are counted, as is
typically done by traditional SNMP-based management methods such as MIB2 and RMON statistics,
you've probably missed the biggest source of lost packets -- jitter-buffer discards.
Why is it possible to focus on loss and discards instead of delay and jitter as well as loss and discards? That's simple. To compensate for jitter (aka packet-delay variation) and to maintain a smooth frame play-out rate, VoIP endpoints implement a jitter (aka dejitter) buffer. The jitter buffer adds a small amount of delay to each packet and plays them out at a constant rate. If a packet is too late, that is, if it arrives beyond the jitter buffer's additional delay budget, the jitter buffer will drop it. Hence, the jitter buffer turns jitter into additional delay and ultimately into packet loss (discard) if the delay variation is too large. Obviously, a similar process occurs with simple delay. If the packet arrives too late it is discarded.
Very few packets are actually lost in the network, which is why so many service providers can offer 1% or less packet loss guarantees in their customer SLAs. So, focus on the packet-loss count, but be sure it includes the number of discards. If you don't have a combined loss and discard number, use the discard number as your first guide. Unfortunately, your service provider has no way to track packets discarded by your VOIP endpoints' jitter buffers, as this occurs on your hardware, not his. But he can provide some metrics related to jitter.
Bob Massad is a VP at Telchemy, Inc, a provider of voice quality-management solutions.
This was first published in January 2003