Did you try a "show interfaces status" command? Your WS-G5484 SX GBIC should show up on Gi0/1 or Gi0/2 as Type 1000BaseSX.
The SX GBIC should be supported in all of the firmware versions for the 2950G, it's a basic building block of any Gigabit Ethernet network. I don't think they would have dared leave that out, even in the initial release.
You didn't say what you were trying to connect it to. Cisco switch to Cisco switch or router generally works fine. If you're connecting it to some other manufacturer's switch, the Cisco may need a couple parameters set on the Gig interface so that it gets along with the other switch. (Other posts here in the forums have dealt with this topic on Dell and 3Com switches, to name a few.)
Could it be a physical media problem? SX GBIC works multimode fiber cable only; make sure you're not using single mode fiber patches to make the connection.
Also, some patch cables come with their SC connectors tied together such that you can't swap the position of the SC connector in the GBIC's TX port over to the RX port and vice versa at one end of the link. Some of these (e.g. Siecor) are easy to separate and reverse; others (e.g. Amp) can be a pain, you may need a sharp knife to cut the plastic that holds them together. The fixed connectors may be forcing you to run straight-through from TX on the GBIC to TX on the other GBIC (and RX to RX) which won't get you a link light.
Remember: for a two-strand fiber link to work, TX needs to go to RX, and RX to TX. Trace your strands to check this.
You could also have two patches that cross-over, but when used in series (near-end patch---horizontal or riser fiber run---then far-end patch) they cancel each other out, leaving you with a straight-through connection. Again, trace your strands to check this.
A simple, quick-and-dirty test of an SX or LX/LH GBIC to verify that the GBIC's not in a bad slot in the switch is to take a known-good single strand of fiber with SC connectors at each end, plug one end into the TX port, then the other end of the same strand into the RX port. (You can use one strand of a duplex patch cable for this, too, as long as the connectors aren't tied together as discussed previously.) As long as the port's not administratively shutdown, you should see the link LED light up. When it does, disconnect that patch cable because you have created an Ethernet loop. If no link LED, you could have a bad GBIC.
Hope this helps.
UPDATED to add:
How far is the fiber run? Multimode works for Ethernet at half or full duplex out to 2000 meters; Fast Ethernet at half duplex to 412m, full duplex to 2000m. But Gigabit Ethernet with SX GBICs is generally limited to 220m or 275m, full duplex, depending on the modal bandwidth of the fiber. If you're working with multimode that's been around for a while or used previously with a slower-speed technology (FDDI or ATM OC-3c even), the run could be too long. LX/LH GBICs and mode conditioning patch cords can push Gigabit Ethernet to 550m standard, and even 700m at the extreme. Any farther than that, you need single-mode fiber or some of that new performance-enhanced 50/125-micron multimode.
I'm connecting it across approximately 150m of *what I'm told is* mmf to a brandnew fibre to Gigabit Ethernet media converter, then on to a another brandnew 3550 I'll clearly need to check all of this out for veracity. Sadly, the site is 300 miles away, so a quick check is out!
Thanks very much for that reply: its been very illuminating.
I don't know of any converters that will work with the cisco gbic. They don't even negotiate speed. It's 1000 or nothing at all. I would get another gbic for the far end or do a ethernet to fiber converter for the span and convert it back to ethernet on the other side.
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