Jitter
Jitter is defined as a variation in the arrival of received packets. On the sending side, packets
are sent in a continuous stream with the packets spaced evenly. Because of network
congestion, improper queuing, or configuration errors, this steady stream can become
uneven because the delay between each packet varies instead of remaining constant, as
displayed in Figure 2-1.
Chapter 2: Considering VoIP Design Elements 57
Time
Same Packet Stream After Congestion or Improper Queuing
Jitter in IP Networks
When a router receives an audio stream for VoIP, it must compensate for the jitter that is
encountered. The mechanism that handles this function is the play out delay buffer, or
dejitter buffer. The play out delay buffer must buffer these packets and then play them
out in a steady stream to the DSPs to be converted back to an analog audio stream. The
play out delay buffer, however, affects overall absolute delay.
When a conversation is subjected to jitter, the results can be clearly heard. If the talker
says, “Watson, come here. I want you,” the listener might hear, “Wat....s...on.......come
here, I......wa......nt........y......ou.” The variable arrival of the packets at the receiving end
causes the speech to be delayed and garbled.