jitter:    1. Deviation in timing that a bit stream encounters as it traverses a physical medium. 2. The slight movement of a transmission signal in time or phase that can introduce errors and loss of synchronization 3. A variation of connection latency.

See also: 8b/10b encoding, data dependent jitter, eye, eye diagram, eye opening, jitter tolerance pattern, jitter timing reference, zero crossing

Concepts

Most communications clocking at the receive side of a circuit is derived from the signal itself. Within the mechanics of transmission, we need to assume that all signals are in some way altered either by their source or during transmission. This alteration could prevent the derivation of clock. A practical approach suggests that if we try to for perfect synchronization with the ideal reference clock, nothing would be transmitted successfully. So signal jitter, signal strength, phase shift, attenuation and other signal characteristics all require compensation or agreement to a series of acceptable limits. The known limits enable clock derivation.

Applied to Storage Networking

In this graphic, you need to imagine each green line as being painted on an oscilloscope screen independently as data traverses the medium. Because 8b/10b encoding balances the number of 0's and 1's when the scope was properly adjusted, you would see the balance. Yet, there remains a deviation in the signal of time or phase. Amplitude is often a function of distance, a pure function of medium. The other characteristics are a combination of medium (particularly when using wire), processing, codec's and other circuitry. For example, phase shift may be the result of variations in the source crystal, and they might be constant, random or cyclic.

Applied to Storage Subsystems

Taking a different approach, both jitter and isochronous transmission are significant factors in audio and video, which are often acquired from the storage infrastructure. The storage system, under load, may create jitter, perhaps by delay or inability to accurately seek a specific sector, processing variations when decompressing or decrypting the stream. In a video application, you might see this as distortion of a signal, flickering, and horizontal lines. You can see that if your client is doing a video editing from multiple sources, this is very important and could get in the way of the creative process. It is not enough that one throws bandwidth at the problem, it requires understanding the I/O infrastructure from end-to-end.

Knowledge Statements (things useful in an exam)

 

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