Buddy, Can You Spare a Nano-Second?
Customers in the finance sector often ask the question – “What is latency across an optical tap?”.
In the world of low latency trading, where servers are co located with Exchange matching engines, latencies do add up and sometimes nanoseconds matter, so the latency across an optical tap can be important. So what causes latency (or as its sometimes called – propagation delay) in an optical passive tap?
The propagation delay across a tap is primarily determined by the length of the optics used within the tap. Once the length is known, the latency can be determined by multiplying the length by the speed of light and then multiplying the result by the refractive index of the fiber (usually just under 1.5). The ‘rule of thumb’ is 5 nSec per meter.
Typical optical taps provided by vendors have optical paths of around 12-24 inches, so for 24 inches the latency will be around 3 nSec.
However, there are vendors who have very long light path lengths. Recently we examined a competitive tap from a leading vendor and this had 46 inches of fiber cable spiraled inside its housing! This introduced an additional 6 nSec of delay.
For customers who are super sensitive about latency, they should consider vendors such as Ixia who are aware of such issues. Ixia’s lowest latency optical taps are its Patch Taps. These offer very short length optics of less than 9 inches. This gives latencies of just over 1nSec. The length of 9 inches also allows the taps to be installed in patch panels or even directly in fiber paths without the need for rack mountable taps. Using such taps, high frequency traders can save up to 2 nSecs on their light length paths.
So it may not be a case of ”Buddy, Can you spare a nano-second," but can you spare two?