Packets transmitted continuously on the network will have differing delays, even if they choose the same path. This is inherent in a packet-switched network for two key reasons. First, packets are routed individually. Second, network devices receive packets in a queue, so constant delay pacing cannot be guaranteed.
This delay inconsistency between each packet is known as jitter. It can be a considerable issue for real-time communications, including IP telephony, video conferencing, and virtual desktop infrastructure. Jitter can be caused by many factors on the network, and every network has delay-time variation.
What Effects Does Jitter Have?
Packet Loss - When packets do not arrive consistently, the receiving endpoint has to make up for it and attempt to correct. In some cases, it cannot make the proper corrections, and packets are lost. As far as the end-user experience is concerned, this can take many forms. For example, if a user is watching a video and the video becomes pixelated, this is an indication of potential jitter.
Network Congestion - Network congestion occurs on the network. Network devices are unable to send the equivalent amount of traffic they receive, so their packet buffer fills up and they start dropping packets. If there is no disturbance on the network at an endpoint, every packet arrives. However, if the endpoint buffer becomes full, packets arrive later and later, resulting in jitter. This is known as incipient congestion. By monitoring the jitter, it is possible to observe incipient congestion. Similarly, if incipient network congestion is occurring, the jitter is rapidly changing.
Congestion occurs when network devices begin to drop packets and the endpoint does not receive them. Endpoints may then request the missing packets be retransmitted, which results in congestion collapse.
With congestion, it’s important to note that the receiving endpoint does not directly cause it, and it does not drop the packets. Consider a highway with sending house A and receiving house B. Congestion is not caused by B because it does not have enough parking spaces. Congestion is caused by A, because it continues to send cars on the highway to B.