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FC vs FCoE

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FC vs FCoE infrastructure, usually a common debate when designing the network infrastructure of a new Data Center or new part of a Data Center, after all the advantages of running a converged storage & IP network are hard to turn down. Many of us are probably already aware of the why FCoE is always good option vs the traditional dedicated FC design, but I wanted to point out one interesting  fact that make FCoE more efficient and that fact stems down the Ethernet transport.

When FC frames are sent over a Fibre Channel network they are placed onto the physical medium in 8b/10b encoding which has an overhead of 25% to ensure the signal is sent successfully. (and intact with no corruption, essentially for every 8-bits, 2-bits are used to ensure the signal is still intact think of this an CRC or FCS for the electrical signals)

When we consider Ethernet, it is sent over the wire with a 64b/66b encoding, about a 3% overhead as the information hits the physical medium. So while FCoE may encapsulate the already large FC Frame with additional header & trailers FCoE from physical perspective is much more efficient means transmission. (So for every 64-bits sent to the wire, 2-bits are used for this ‘error checking’ and much better ratio than 8b/10b)

The 8b/10b encoding is used for 1, 2, 4, & 8 Gb Fibre Channel, the new 10+Gb Fibre Channel technologies are relying on 64b/66b encoded, which may tip the scales. However the converged infrastructure & cabling make FCoE the better option for most environments from a cost management perspective.

Random tidbit:

This is also an aspect the differentiates 100Mbps from 1Gbps from 10Gbps

  • 100Mb Ethernet –  4b/5b encoding
  • 1Gb Ethernet – 8b/10b encoding
  • 10Gb Ethernet – 64b/66b encoding

 

Written by Stephen J. Occhiogrosso

August 17, 2014 at 11:41 AM

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