Wednesday, September 15, 2010

More on 10G Cabling in the Data Center – 10GBASE-T versus 10GBASE-CR

By Lisa Huff

Even though Augmented Category 6 cabling has been around for more than four years and has actually been installed in many data centers, it has really just started to be used for 10G applications. 10GBASE-T ports have supposedly been available from both server and switch manufactures for over a year, but trying to actually get your hands on them has been difficult. And, when you look at the power dissipation and latency specs, data center operators are choosing 10GBASE-CR (SFP+ direct-attach copper) instead. So what are the tradeoffs?

SFP+ direct-attach copper is designed to be used for short interconnects – 15m or less – which makes it a perfect solution for ToR switches. It’s also available with 30AWG cable, which makes it much more manageable than the 24AWG (or sometimes 23AWG) CAT6A – especially if you’re using a shielded CAT6A solution. Because it plugs into a SFP+ port, if you purchase a passive cable (no DSP chip to clean up the signal), there is essentially no added power consumption. If you add an optical SFP+ module, it’s still less than 1W. The 10GBASE-T port still burns about 3W. Latency of a 10GBASE-T solution could be as much as 2.6 µsec, while SFP+ is 0.3 µsec max. For some firms, like trading companies, this could mean millions of dollars made or lost.

With SFP+ DAC, you could buy a ToR solution today that will accommodate both short connections within the rack and up to 40km outside the rack. If you choose a 10GBASE-T product, it will currently only cover up to 100m within one switch – there are no products available so far that have both 10GBASE-T and SFP+ ports. But, I’m sure these are being developed. And, once they are, it will most likely be more cost effective to use 10GBASE-T within the rack with LOMF SFP+ (or maybe 40G QSFP+) uplinks outside the rack. Just like what we have today with Gigabit – 10/100/1000BASE-T within the rack and SFP uplinks.

Both SFP+ DAC and 10GBASE-T products will be needed in the long term – 10GBASE-T for inexpensive connections and 10GBASE-CR (SFP+ DAC) for lower latency and lower power consumption.

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