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CAT Fight

Intel’s X-Lab: Tomorrow’s Network Happens Here
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CAT Fight

CAT5e cabling is very common. I have it running through my own walls for gigabit structured wiring. The cabling specs out at 125 MHz and simply uses four pairs of loosely twisted copper wiring. CAT6 steps up to 250 MHz, features tighter twisting, and adds in a dielectric conduit that separates the four twisted pairs and helps prevent energy from one pair bleeding into its neighbors.  Most 10GBASE-T specifically targets CAT6a cabling, which includes even more stringent control over pair twist, as well as manufacturer-specific design features for improving immunity to alien crosstalk noise.

Pete set 10GBASE-T cabling in historical perspective to illustrate some of the increasing problems networking engineers face.

“10BASE-T has been around for a long time, and it’s even been demonstrated to work over barbed wire. There’s that much signal-to-noise ratio margin, even on a very lossy channel like barbed wire. It’s 6 V peak to peak, best-case, and the pulses are very wide. It takes a lot of bad things happening in the channel for a receiver not to see it.

100BASE-TX requires you to do some funky things to the signal. Instead of the two-level signal in 10BASE-T, there’s a three-level signal. If you look at the signal energy, it’s a 2 V peak to peak system, so there’s less power, but all of this scrambling and pulse shaping gets it to work. In 1998, that all wasn’t very straightforward, but with modern signal processing, it’s pretty easy to do 100BASE-TX.

Now, for gigabit, you start to get into some magic. There is more noise power than signal power, meaning we have a negative signal-to-noise ratio. That means if there’s a lot of background noise—like in this room now—and if we get that hammering noise so loud that you can’t hear me, that’s like a gigabit Ethernet noise environment. In 2000, 2001, there were some signal processing techniques applied, some special encoding and decoding that, at a high level, means you’re taking a best guess. You know what you’re sending out, and the receiver knows that there are certain expected combinations that will be coming back. So the system takes a best guess, to put it crudely, at what that data is. Better than 1 in 10-10 times, it makes the right guess.

But gigabit sucked up a lot of power and required, for the time, a lot of gates. At the gigabit inflection point, you started to have more gates than analog circuitry because we’re sending highly-encoded analog signals—those wiggly things with an amplitude and everything else. To encode and decode that properly requires a lot of logic gates. For 10GBASE-T, you just carry that concept to the next level. If gigabit is a whisper in a rock concert, 10GBASE-T would be like a whisper in a nuclear blast. It’s that much more noise power compared to the signal power. But with today’s digital signal processing techniques, you can make a signal have more apparent power. That’s one way to think of it. Again, the ratio of analog content to gates in 10GBASE-T is—wow. It’s very significant, with much, much more digital than analog content. This is good because it suddenly becomes very Moore’s Law-friendly, plus you get the advantage of power savings as you go to each new process node. In the lab here, we have 90, 60, and 40 nm technologies. The power savings associated with each generation has been key for our 10 Gb NIC products and the broader 10GBASE-T deployment in servers and switches.”

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