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SFP Adapters VS media converters

Unanswered Question
Apr 14th, 2004
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I'm getting ready to buy a couple of 3750 switches. I have 6 fiber cables that I will be connecting to these switches. Since these switches come with 4 SFP connectors each, all I need to do is just buy the SFP Adapters.


My question is, what is the difference between buying the SFP Adapters and buying a fiber media converter such as the Allied Telesyn devices?


I mean, price-wise, I'm only spending $180 for each fiber cable VS $400 for the SFP Adapter.


Can someone give some advice?

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konigl Wed, 04/14/2004 - 19:54
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The 6 fiber cables that are to connect to the 3750 switches, are they coming from 100Base-FX Fast Ethernet interfaces at the remote ends over multimode fiber (MMF)? What did they used to connect to?


If they are Fast Ethernet, then the $180 or so per media converter sounds right for Allied Telesyn's Fast Ethernet media converters with SC connectors. (Their 1000Base-T to 1000Base-SX Gigabit Ethernet media converters will run at least $400 per device, probably a little more.)


If the MMF feeds are carrying Gigabit Ethernet using 850nm wavelength, then the SX SFPs are a more elegant solution. They don't require external power (they get their electricity from the switch they're installed in). SFPs are Gigabit Ethernet only.


Downside to the SFPs is, if you want to re-use your current MMF patch cables, you're out of luck: SFP optical transceivers use LC connectors, which are smaller than the SC ones. So you'll need new patch cables with LC connectors at one end to attach the the SFPs to your fiber, which adds to the cost.


Anyway, trying to run Fast Ethernet FX from the remotes into Gigabit Ethernet SX SFPs over MMF will NOT work. They are not directly compatible: optical wavelengths are different, physical layer signaling protocols are different.


Also beware: while the Fast Ethernet standard specifies that 100FX can go 2000 meters over MMF in full duplex mode, Gig Ethernet SX over MMF is limited to 220, 275, 500, or 550 meters depending on the fiber core size and quality.


That distance extends to 550 meters (standard) or 700 meters (field-tested) over MMF if you use the more expensive LX or LH SFPs, but then you also need specialized mode conditioning patch cords to make them work. These generally cost more than regular patch cables. (LX/LH is normally used over single mode fiber, and is good to 5km standard, or 10km per most manufacturers' specifications.)


Hope this helps.


apaxson Thu, 04/15/2004 - 04:16
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Thank you Konigl,


Very well thought-out and informative post. I believe that does help me out a great deal. Especially the LC-SC deviation. I was not aware I had to get new patch cables.


The company (prior to my arrival) were using 3Coms, and they were just spread out EVERYWHERE! like an eagle's nest. The fibers are actually coming from 3Com dual-speed hubs. They might connect at 100Mbps but only half-duplex. I plan on replacing those switches with 2950's soon, but first I have to build my core.


From the looks of things, I should buy the Allied Telesyn Media Converters. Any idea on how to figure out if the fiber is multi-mode or single mode?


Thanks!!

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