There is a mantra amongst network equipment vendors: never let the customer unscrew the chassis from the rack. Replace every component with upgraded equipment, but never upgrade the chassis itself. Doing so often triggers the customer to send the whole thing out for bid to multiple vendors, and you might not win. Somebody else's chassis might get slotted into the hole you left.
So vendors work very, very hard to keep the old chassis competitive. Line cards and supervisor modules will be updated repeatedly over the lifetime of the system. Though the design of the backplane might also be upgraded in new systems, the new cards have compatible modes of operation for even the oldest deployments.
This has a couple impacts:
- It favors passive backplanes. Active electronics can't be upgraded, future generations of cards find it a hinderance.
- It means backplanes are overdesigned, and therefore expensive. Even if you plan to clock traces for 10 Gbps operation, you make the PCB handle 25 or 40 Gbps to support future upgrades. You run as many traces as will fit, even if they won't be used by the first gen cards.
If you ever see a backplane PCB removed from the sheet metal, notice how thick it is. That isn't just for stiffness over the long span of board, backplane PCBs have many signal layers. PCB fabrication gets expensive at 20 layers, but backplanes often have 26 or more. Some of those layers end up using more expensive PCB material, needing better electrical characteristics.
Over the weekend Greg Ferro wrote about a prototype optical backplane from HP on display at Interop, which manages 120 Gbps per optical tap. I think that results in 120 Gbps per slot, though I'm not sure. Optical backplanes have been in development for a long time (consider this announcement from 2005), and even the HP design is 3-5 years away from showing up in a product.
Nonetheless the potential is clear: this is a 10 year backplane. Though the waveguides are plastic not glass, over the short distance of a chassis they should have a vastly larger useable spectrum than copper traces. Subsequent generations of cards can use faster lasers and/or more colors to get the bandwidth they need. Optics (and especially multiple wavelengths) over the backplane adds cost, but the future proofing is worth it.
footnote1: Greg Ferro also hosts the Packet Pushers podcast, which I highly recommend.