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Noise to signal? Isn’t it supposed to be signal to noise ratio?  When you’re talking 10 Gigabit BASE-T, you’re talking noise to signal.  10 Gigabit BASE-T has been described to me as “It’s not like looking for a needle in a haystack, it’s like looking for the right snowflake in a snow storm.  At night.” Where does all that noise come from? It comes from the cable next to the cable your data is trying to travel across, it comes from wireless signals, and it comes from outer space.  It even comes from the other wires in the same cable.  With all this noise, how does all that analog chaos get transformed into digital clarity of 1s and 0s?

 

Lots and lots of processing. In our current generation of 10 Gigabit BASE-T products we use PHYs that have 5 channels of DSPs, and tons of specific analog processing silicon to turn that snow storm of waveforms into nice clear digital Ethernet frames.  Far End Cross Talk (FEXT), disturbers, Near End Cross Talk (NEXT), 8/10 encoding, it all gets to be a jumble. FEXT is signal coupled from one channel to a neighboring channel, measured at the far end. NEXT is the same as FEXT, except that it is a measure of how much of that coupled signal is reflected back onto a neighboring channel on the same end. Disturbers are other noise sources outside of the cable.  The 8/10 signaling is how the data is encoded to help make sure there is a roughly even mix of 1’s and 0’s in the data stream. It also helps to ensure the quality of the data by periodically transmitting a checksum that the link partner can use to validate the data that it has received.   If your data stream is highly biased, like all 1’s or all 0’s, the signal processing can get “tone deaf”, limiting what it thinks is a 1, making all the data a 0.  Unless the average voltage is centered you can get signal droop which makes it hard to tell a weak 1 from a strong 0.  The extra bits of data do take up some space on the wire, but things like clock recovery from the extra bits make it worth the overhead.  Just means that to get to your advertised speed, your baud rate must be higher. For example, a 64/66 10G serial signal has a clock rate of 10.3125 G/sec, which transmits data at 10G/sec.  You can’t transfer 10 Gigabits of data. There will always be space on the wire taken up by other things (TCP/IP headers is a big one).   But those extra pieces help keep 10G copper stable.

 

Time to dispel a couple of 10 Gigabit BASE-T rumors I’ve heard over the years.

 

First up, bending the cable. I was at a trade show and a gentleman comes up and says to me “That 10 Gigabit stuff is for the birds, you can’t even bend the cables without dropping link!”  I was covering for another employee that had taken sick, so I wasn’t warned of the rowdiness of this show.  “I think you’re mistaken, sir.”  I said.  He smiles and reaches behind my demo and grabs the cable.  He bends it over on itself, making the straight run into an O shape.  He pushed with his thumb and made into more of an I shape.  He let it go and walked over to the console screen, expecting error message after error message of disconnected link.  To his shock, (and my amusement) nothing happened.  “Wow.”  He mumbled. He took my card and left the show floor, all the while being heckled by his travel buddies.  Unless you damage the cable, I’ve never seen a bent cable effect link.  And I’m not gentle to my cables.

 

Second, cell phones. I have heard concerns about answering your cell phone in a datacenter and taking your whole network offline.  There is some kernel of truth to this one since the signal bands of cell phones and 10G cables do get dangerously close to one another.  But it’s just another disturber source, albeit a powerful one. Modern cell phones and modern 10 Gigabit BASE-T are both designed to use as little power as needed to reach the end station so you would have to be pretty close with a very powerful cell phone to put a ton of noise onto the wire.  And we do test cell phone interference, but with good cables you shouldn’t see any issues.  Use bad cables and things could get ugly.  Good cables include shielding to protect signal wires from disturbers. Those DSPs can only filter out so much; an investment in quality cables is an investment in the quality of your data.


I bet you a least half of you will now go to your 10 Gigabit BASE-T installs and make a call from right next to your servers to see if it drops link.  If you have Intel® Ethernet and good cables, I think you’ll have a phone call and link all at the same time.

 

Thanks for using Intel® Ethernet, and special thanks to Sam J for his help double checking my technical details.

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