Calnex SNE: Jitter tool vs "Delay +/- Jitter"
- Paul Hudson
- Giles Raine (Deactivated)
There are two tools in the SNE which can be used to apply jitter to packets, and they work in different ways. It’s useful to have an understanding of the way each tool works and their relative advantages and disadvantages.
The Jitter tool (shown below) ensures that packets passing through it are not reordered, even when the maximum amount of jitter is applied to them.
Logically, if the Jitter tool prevents reordering of packets while also maintaining the original packet rate then there will be a finite amount of jitter that can actually be achieved, because there’s a finite amount of time by which each packet can be delayed before the next packet comes along. For example, if you set a Maximum Jitter of 1ms as shown above, that maximum jitter value can only be achieved at packet rates up to 1000 packets per second, because 1000 packets per second means one packet is sent every 1/1000th of a second (every 1ms). At higher packet rates some jitter will still be applied, but the actual amount tails off exponentially. The chart and graph below are based on actual jitter measurements, as measured using Spirent TestCenter while applying 1ms “max jitter” using the SNE’s Jitter tool:
The Delay tool, with the Jitter option enabled as shown in the screenshot below, does not prevent packets from being reordered (see the note at the bottom of the screenshot: “Please note desequencing (i.e. reordering) of packets will occur”)
This means that the Delay tool (when its Jitter option is used as shown above) can apply jitter to packets at much higher traffic rates than is possible using the standalone Jitter tool. What the Delay tool actually does when the Jitter option is enabled is to apply a random amount of delay to each packet, varying by up to a user-defined percentage above and below the Constant Latency value; in the screenshot above, it applies a random delay between 45ms and 55ms to each packet. If packets become out of order as a result, that is an expected consequence of using this particular tool, and is unavoidable at higher traffic rates.