Optimizing 100G VPXStory
March 20, 2023
At the 2018 Embedded Tech Trends conference, results were presented from early simulations that conclusively established 100 Gigabit Ethernet could be successfully implemented on VPX using existing and newly announced copper connectors. At that time, many in the industry thought that optical interconnects and fiber would be required to support 100 GbE speeds.
VPX began back in 2005 with a focus on protocols such as Serial RapidIO, which were operating around 2.5 to 3.1 Gbaud/sec per lane. As these protocols progressed with successive generations, and new protocols were considered and adopted, transmission speeds increased substantially. It wasn’t long before industry was designing Gen 3 VPX products at 8 Gbaud for PCI Express and 10 Gbaud for 40G Ethernet. Then along came higher-speed PCIe Gen 4 (16 Gbaud) and 100G Ethernet (25 Gbaud) and it became evident that the VPX transmission channel needed an upgrade. (Figure 1.)
[Figure 1 | VPX transmission channel testing at 100G]
One of the main elements of the transmission channel is the VPX connector, which is a key enabling technology for 100G systems. Although it was proven back in 2018 that 100G could be achieved using the first two generations of VPX connectors (known as RT2), the more recent VITA 46.30 Higher Data Rate VPX Standard – which supports data rates to at least 25 Gbaud per lane – enables more signal margin and supports larger systems. Currently, there are two options for VITA 46.30 connectors: the TE Connectivity RT3 and Amphenol R-VPX EVO2 connectors, both of which meet the ANSI/VITA 46.30 standard intended for 100G (4-lanes @ 25 Gbaud) systems, and potentially beyond.
One of the main tenets of VPX and OpenVPX is interoperability, and that concept extends to the signal integrity (SI) of the VPX channel. The VITA 68 (VPX Compliance Channel) series of standards addresses the SI interoperability of OpenVPX modules and backplanes. The most recent addition to VITA 68 is VITA 68.3, which is intended for 16G (PCIe Gen 4) and 100G channels. At these speeds, each of the physical blocks that comprise the parts of the VPX transmission channel needs to be modeled in 3D using specialized tools such as Ansys HFSS, and then solved in the frequency domain to create so-called “s-parameter” characteristics. It is these s-parameters that get stitched together to form the channel model in VITA 68.3.
The figure of merit for a good 100G channel is a calculation called channel operating margin (COM), a measure of the overall signal to noise ratio. The IEEE 802.3 Ethernet standard states that COM should be greater than 3 dB for a measured channel. VITA 68.3 uses simulations rather than quantitative measurements, and measured data shows that 100G VPX performance needs to be greater than 3.5 dB in VITA 68.3 to provide a reliable signal channel.
As an example of how to use VITA 68.3, Curtiss-Wright has simulated 100G VPX Fabric100 modules at either end of the VPX channel – and using two VITA 68.3 cases (i.e., long lossy and med-long lossy) – resulting in a comfortable margin of at least 0.25 dB over the 3.5 dB requirement. In this scenario, the Fabric100 simulated modules are based on several worst-case assumptions, so results will only improve in many applications.
Multiple rounds of testing prove that the products and channels will meet the COM target and are the base of the proposed 3.5 dB COM target for VITA 68.3.
For the warfighter, today’s technology is all about data-centricity, meaning they receive the relevant data they need when they need it to make superior decisions. In fact, 100G connectivity and digital convergence are enabling that kind of decision-making superiority through open standards like CMOSS [C5ISR/Electronic Warfare Modular Open Suite of Standards], SOSA [Sensor Open Systems Architecture], and VITA.
The VITA Standards Organization is already looking ahead to the next generations of VPX: Study groups are actively investigating what it will take to implement even higher speeds and bandwidths using protocols such as PCI Express 5.0, operating at 32 Gbaud, and 200G Ethernet, which complicates signal integrity with PAM4 (multibit) encoding. There is even discussion of going up to 400G Ethernet, increasing signal speeds up to 50 Gbaud.
With the Modular Open Systems Approach (MOSA) mandate now firmly entrenched in the defense-electronics industry, the VITA 68 signal-integrity standards have paved, and will continue to pave, the path to high performance and true interoperability that MOSA delivers to the warfighter.
Curtiss-Wright Defense Solutions https://www.curtisswrightds.com/