Packet Jitter

Introduction
A crucial component of end-to-end delay is the varying queuing delays that a packet experiences in
the network s routers. Because of these varying delays, the time from when a packet is generated
at the source until it is received at the receiver can
uctuate from packet to packet (as shown in
Figure 1-lecture 2). This phenomenon is called jitter. As an example, consider two consecutive
packets in our VoIP application. The sender sends the second packet 20 msecs after sending the
rst packet. But at the receiver, the spacing between these packets can become greater than 20
msecs. To see this, suppose the rst packet arrives at a nearly empty queue at a router, but just
before the second packet arrives at the queue a large number of packets from other sources arrive
at the same queue. Because the rst packet experiences a small queuing delay and the second
packet suers a large queuing delay at this router, the rst and second packets become spaced
by more than 20 msecs. The spacing between consecutive packets can also become less than 20
msecs. To see this, again consider two consecutive packets. Suppose the rst packet joins the end
of a queue with a large number of packets, and the second packet arrives at the queue before this
rst packet is transmitted and before any packets from other sources arrive at the queue. In this
case, our two packets nd themselves one right after the other in the queue. If the time it takes to
transmit a packet on the router s outbound link is less than 20 msecs, then the spacing between
rst and second packets becomes less than 20 msecs. If the receiver ignores the presence of jitter
and plays out chunks as soon as they arrive, then the resulting audio quality can easily become
unintelligible at the receiver. Fortunately, jitter can often be removed by using sequence numbers,
timestamps, and a playout delay, as discussed below.