What is Jitter?

The Computer Corner

By Charles Miller

When clients call because they are experiencing problems connecting to the Internet, the source of the problem is often not apparent to someone untrained in network engineering. One of these invisible gremlins is known as “jitter.”

The technical dictionaries define jitter as “the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications.” English translation: jitter is a connection that is getting through okay, but the order is all scrambled up. Imagine you are sending the following message via your Internet connection: “Now is the time for all good men to come to the aid of their country.” Now imagine that somewhere along the line your connection introduces some jitter so that the order of the words in your message gets a bit scrambled to read: “Is now the time country men come for all good to the aid of their to.” All the words in the message got through, but they did not arrive in the exact order in which they were sent.

This is not necessarily a big problem for computers; in fact it is often not even noticeable. If part of an email arrives out of order, your computer will simply wait a few milliseconds for all the pieces to arrive, then it will assemble the email correctly, and you never see the problem. Similarly, you might pull up a web page that because of jitter might load with the elements out of order, perhaps the sound starting before the video. This is the type of thing the user very rarely ever notices so long as the jitter is not excessive and your computer can compensate.

Buffers can be used to counter jitter in packet-switched networks to improve audio or video streams. Some software is intelligent enough to use adaptive de-jittering to buffer or delay streams a bit, as much as a few seconds, and if that is enough time for all the pieces to arrive, the program can reassemble your stream in the correct order. The jitter can thus be mitigated, and you might never notice your stream was slightly delayed. Trivia question: Who hears the music first, the person in the back row of an auditorium in New York or the person listening the same music live on the radio in Mexico? Answer: Because signals travel through the Internet faster than the speed of sound, it is usually the listener in Mexico unless there is jitter.

While surfing the web or using email, nobody will ever notice jitter in their Internet connection. The place where users will notice degradation resulting from jitter is while using any Internet application dependent on reliable real-time connection. Streaming video as well as Voice Over Internet Protocol applications, such as Skype, can tolerate very little jitter, and buffering only makes matters worse.

Recently I attempted to explain to a client that her Vonage voice terminal was probably okay, and that the reason she could not make phone calls was jitter and the quality of her Internet connection. She responded: “You’re wrong! I never have trouble with email so my Internet is fine.” I found it difficult to explain to her a problem that could not be seen, and she was frustrated when the new Vonage box she ordered did not work any better than the old one.

Charles Miller is a freelance computer consultant, a frequent visitor to San Miguel since 1981, and now practically a full-time resident. He may be contacted at 415 101-8528 or email FAQ8 (at) SMAguru.com.


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