I've been trying to upgrade our small fiber network from 100BASE-FX to 1000BASE-SX. The cables are 62.5/125 multimode fibers, all terminated in patch panels with ST plugs (the round ones).

As I understand whatever sources I could find, this combination should work fine ... but whenever I test part of the network, it never manages to establish a connection.

I've tried hooking up the fiber converters directly with a 3m cable, and it works fine. But when I hook it up to even our shortest fiber run (approx 5m (!)), it doesn't work again...

Our hardware:

  • We rely primarily on using fiber-converters, as they are cheaper to replace when they fail.
  • The 1000BASE-SX'es I've been trying with are TrendNet TFC-1000MSC's (the title currently says 1000BASE-FX, but the specs (and chassis) says SX).
  • The buildings are part of Danish heritage, so we're not allowed to change so much. They installed the network by putting it in the same places as the power mainline - hence the fiber...

I have a few theories about why this is:

  • I've missed something (RTFM &c.)
  • Our plugs doesn't support 1000BASE-SX.
  • I bought the wrong hardware.

The TFC-1000MSC has SC (square) connectors, so if you're using ST (round) termination, you must be using some sort of dongle or coupler between the ST jack on the patch panel and the SC fiber jack on the fiber converter, right? Is the 3-meter test fiber terminated SC-SC, or are you using an SC-ST dongle or coupler for your test? Are your old 100base-FX fiber converters SC or ST?

In general, fiber is fiber. We use both 62.5/125 and 50/125 for 1000base-SX connections have no problems with either (using LC or SC termination). However, this page notes that if you have to couple multiple fibers together, it's important that all of them be of the same type, or you can get light loss.

If you're using all 62.5/125, then it may be that your ST connectors are not designed to pass the wavelength of light used by 1000base-SX. If you look at the Wikipedia pages you referenced, you'll see that 100base-FX operates at a wavelegth of 1300nm, while 1000base-SX typically uses 850nm. I don't know if connectors even have such limitations or not, but I do know that ST is not commonly used anymore, and I've never seen it used for GigE.

Also, keep in mind that 100base-FX and 1000base-SX are not compatible. If you're only testing one end of a run at a time, you definitely will not get a link. Aside from the fact that the use different wavelengths, neither of them is 10/100 or 10/100/1000 auto-sensing. In my experience, this is not universally understood. :)

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  • IIRC, the diameter of the fiber (62.5 vs 50) mainly affects distance limits. – sysadmin1138 Aug 16 '09 at 19:52
  • I've been trying to upgrade both ends at a time, but to no avail. The patch cables are ST-SC, where needed. I'll have to check their widths, though. – Morten Siebuhr Aug 16 '09 at 22:08
  • There apparently is indeed nothing intrinsic about ST connectors that would prohibit them from being used for GigE. (I truly didn't know, I just threw it out as a possibility.) At least one company I found sells 1000base-SX NICs with ST connectors (aei-it.com). – James Sneeringer Aug 17 '09 at 2:51

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