– Jurrie van den Breekel,
director of cloud solutions at Spirent Communications (www.spirent.com), says:
Forwarding performance, low-latency and flow scalability are critical metrics for data center switch vendors but accurately measuring all these metrics together is no easy task. Resolution, precision, and accuracy are three words that get thrown around a lot, often interchangeably. However, these three words actually mean three different things when testing devices and networks for latency, particularly in switch fabrics designed for data centers and cloud computing.
Resolution and precision both refer to the smallest unit measurable by an instrument; the difference is on how reliable and repeatable a measurement is. Neither refers to whether the measurement is correct. That’s what accuracy is all about—correctness.
· Resolution: The smallest unit to which an instrument can be read
· Precision: The smallest unit to which an instrument can be read reliably and repeatedly
· Accuracy: The condition or quality of being correct
A common illustration of the difference between resolution and precision compares an analog stopwatch to a digital stopwatch. The analog stopwatch may have ten tick marks between seconds, indicating a resolution of one tenth of a second, whereas the digital stopwatch may have two digits after the decimal, indicating a resolution of one hundredth of a second. However, human response isn’t that fast. It takes about a tenth of a second from a stimulus to a button press. So despite the fact that the digital stopwatch has a resolution of one hundredth of a second, its precision (repeatability) when operated by a human is the same as the analog watch, one tenth of a second.
However, neither resolution nor precision will tell you whether the stopwatch is accurate, meaning that when the stopwatch marks off ten seconds, whether exactly ten seconds has passed. Accuracy in an instrument is usually assured by calibrating it to a known value. In electronic instruments that measure sub-second values, accuracy depends on the resonant frequency of crystals, which can measure millions or billions of changes per second.
Because of the importance of sub-microsecond latency measurements in high-speed environments, Spirent engineers designed their testing platform to measure time intervals with the highest resolution, precision, and accuracy in the industry. The test modules have a resolution of 10 ns at 10 Gbps and 2.5 ns at 40 Gbps and 100 Gbps. To yield the best results when measuring latency between interfaces of different speeds, the test signature has a resolution of 2.5 ns on all modules.
The shortest frame insertion time is 51.2 ns at 10 Gbps and 20.48 ns at 40 Gbps, so a timestamp resolution of 10 ns is adequate. However, the shortest frame insertion time is 5.12 ns at 100 Gbps, so the 2.5 ns resolution of Spirent TestCenter 100 Gbps test modules is required to accurately measure latency at that speed.
Scalability tests for data centers require high port counts, which involve multiple test chassis. To achieve the accuracy required for high-speed ports at such granular precision, Spirent calibrates for synchronization cable length between chassis, port location with each chassis, and heat, as temperature affects the resonant frequency of a crystal. Spirent’s ability to display negative latency allows its users to validate calibration and optionally adjust to near zero nanoseconds when using Direct Attached Copper cables vs. fibers with optics.
Recently Gnodal, known for its ultra-low-latency switches, looked to Spirent to showcase the performance of the new GS-Series switches, including its latest 40 GbE switch. Gnodal conducted low-latency performance testing of the GS4008 and GS0072 High Speed Ethernet switches individually and also as part of a two-tier data center fabric.
The test utilized the 32-port Spirent HyperMetrics dX, generating traffic to Gnodal top of rack switches, interconnected with the new GS-Series 40G Ethernet switch. Spirent and Gnodal successfully demonstrated 40GbE performance with sub 285 nanosecond latency within the GS0072 and also the scalability of the Gnodal Fabric vital in delivering data center applications.
As illustrated by the Gnodal test, modern switch and router fabrics span multiple chassis and typically expanded to build systems supporting hundreds or thousands of ports. These systems must be tested to assess system performance and verify reliability when deployed in large mission-critical environments supporting millions of user application flows. The density and mesh-pattern traffic-generation capabilities of Spirent can not only fully-load modern, multi-terabit backplanes and fabrics, but can measure port-to-port and end-to-end latency across a full-mesh network with industry-leading accuracy and precision.
Jurrie van den Breekel is director of product marketing at Spirent Communications, a global leader in test & measurement for networks, services and devices. www.spirent.com
