Moving to optical connections at light speed

By: ra

For the past twenty years, we have been only three years away from optical backplanes and architectures taking over the computing industry. And for those twenty years, it never happened. Like the perpetuation of Moore’s Law, the signal engineers made Megahertz and Gigahertz signals run effectively on copper wires and copper connectors. But, as Moore’s Law nears the end of its ability to double the number on transistors in a given space on a die, copper may be nearing the end of its ability to reliably pass signals at higher frequencies.

It’s finally time we dropped copper connections and moved to optics. Why? Well, it just gets too hard to make copper work reliably at 10G and above, and it’s too expensive too. How do I know this? Because, the first two generations of PCI Express used 8b/10b signaling (at 2.5 GHz and 5 GHz respectively). In Gen-3 silicon, they moved to PAM (Pulse Amplitude Modulation) and 8 GHz so they could move the same amount of data as a 10G connection that used 8b/10b, but not have to deal with the 10G signal integrity problems. If you look back at the high-speed connection work over the years, you will see that every time the frequency of the signal doubled, the distance that signal could reliably travel was cut in half. The parallel PCI interface enhancements over time prove this point. So, all the advancement in backplane and cable signal frequencies over the years came at the cost of shorter and shorter runs. “Other than that, Mrs. Lincoln, how did you like the play?”

Another reason we have refrained from optical connections has been the limitations of the optical architectures. Optics do not like to be multidropped (like a bus). They do not like to be switched (like the old POTS telephone network). Optics like to run from one point to another, without any interference. Market acceptance of the high-speed serial fabrics (and the dying-off of the bus-thinker engineers) have removed that impediment. A number of the fabric-based machines are built with Meshes, which are point-to-point serial connections to and from each board in the rack. Pretty soon, the copper-thinker engineers will die-off, and we can make a wholesale move to optical.

Speed and signal integrity are not the only reasons to move to optical connections. Noise immunity is a “raison primaire” for going optical. Data can be moved in any electrically-noisy environment at GHz speeds without any concern for the ever-shrinking signal-to-noise ratio. In the Military applications, optical connections are immune to EMP (Electro Magnetic Pulse) from nuclear explosions or “pulse bombs”. But in those situations, you must use glass fiber: plastic fiber will undergo a crystalline change, turn opaque, and no longer pass light when exposed to EMP. Optical connections are lighter and more resilient to shock and vibration than copper connections. That is why we are seeing them used frequently in UGV (Unmanned Ground Vehicles) and avionics platforms (including Unmanned Aerial Vehicles and fighter planes). Anywhere weight is a factor, you find optical connections.

Many of VITA’s members have been marketing some optical connections over the years, under product names like “reflective memory”. The Holy Grail for optical connections, until now, was 10 Gigahertz Ethernet. But, Apple and Intel just announced a new optical concept, called “Light Peak”. LP also runs at 10 GHz, and is targeted to replace all the different cables now found in PCs, servers, and other computing devices (techresearch.intel.com/articles/None/1813.htm).

In the VITA Standards Organization we already have two efforts (VITA-66 and VITA-67) that are adding optical connections on VPX backplanes and XMC mezzanine cards with the existing copper connectors. Obviously, we need to sort-out the connector schemes and architectural implications for pure-optical VPX backplane systems. I think it is time we started moving to optical connections rapidly, especially in MIL/Aero and ground vehicle applications that use VPX.

We can’t wait for telecom to drive the optical revolution: telecoms are having trouble controlling their own bowel movements these days, and cannot drive anything except their journey to bankruptcy. So, it’s up to the Military applications segment to initiate this move away from copper and to a new generation of pure-optical architectures and technologies. And it’s up to VITA and its members to lead the way.