Ask the Expert

GBICs and fibre cable

There are short wave and long wave GBICs. What is the maximum distance you can run the fibre cable with each? And, why would someone want to use copper GBICs and cables?

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This is discussed in pages 346 to 350 in Building Storage Networks, 2nd edition.

Despite the way this information is normally presented, you can run cabling to all kinds of distances, however, the FC spec calls for certain max cable lengths that will support the max error limits.(10 x-12). Also don't expect vendors to support problems if your cable lengths exceed the spec.

The limiting factor is not the GBIC, it is the internal diameter (ID) of the cable - and the laser technology used. In general larger IDs, disperse the signal faster - resulting in shorter supported cable lengths. Here are a few specs for 1 GB FC (yes, 2GB FC is different and shorter):

  • 9 micron (used w/ long wavelength lasers) 10,000 meters
  • 50 micron (used w/short wavelength lasers) 500 meters
  • 62.5 micron (used w/short wavelength lasers) 300 meters

There are also special long distance lasers that exceed the supported distances by a large amount.

If you are thinking of implementing 2 GB in the future, check into the current specs for 2GB support. You do not want to implement cable lengths today for 1GB FC that cannot be upgraded for use with 2GB equipment because the cable distances are too long. In general, the 2GB distances are approximately half the distance of 1GB distances. Pay particular attention to 9 micron, long wavelength runs, the supported 2GB distances might be less than a 25% of the supported 1GB distance.

As to copper, both certain varieties of coax and shielded twisted pair cables are supported. The ends are definitely not RJ-45. I don't think 2GB FC supports copper and severely doubt that higher speeds will support copper. So, why copper in the first place? Because it's cheaper than fiber optic cabling and it works. 1GB distances are supported to 25 meters. In general it looks like copper is falling by the wayside. It never did have a chance for FC, given that the technology is called FIBRE Channel.

Regards,
Marc Farley

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This was first published in November 2001

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