Thursday, January 5, 2012

Attacking VoIP Call-Quality Issues

Call-quality issues are a bit trickier with VoIP than with traditional telephony.

One of the main problems with VoIP are the negative effects of the delays in transmission of packets.  The latency is always inherent in converting a call to and from VoIP.  But no matter how quickly that conversion occurs, if there are several, the cumulative effective latency can have noticeable degradation in quality.  An end-to-end cumulative latency of great than 200 milliseconds (ms) causes human perceptible delay and results in conversations with participants that repeatedly interrupt each other.

Fluctuations in latency are also common on VoIP networks.  This is commonly referred to as jitter.  Jitter is in most cases caused by a leg in the call path that is sharing traffic with other applications or calls.  The most common  symptom of jitter is called "clipping."

A normal conversation sounds like this:
"Hi dear, shall I stop at the store to pick up some milk?"
but clipping produces:
"I ear, all op de ore ick om ilk"

In future posts we'll explore how to isolate latency and jitter to one or many legs of a VoIP call.