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I just bought a TP-Link MC200CM GBit Ethernet (1000-T) to Fiber (1000-SX) media converter.

The device works but I'm unhappy with the performance: When connecting my computer over 1000-T (twisted pair, Cat 6, 18meters) with my server, I get a throughput of about 610MBit/s. If I replace the cable with two media converters, I'm left with about 310-315MBit/s (i.e. half the performance).

My setup is like this:

Computer <-> GBit switch <-> long cable <-> GBit switch <-> server

Computer <-> GBit switch <-> MC200CM <-> 30m fiber <-> MC200CM <-> GBit switch <-> server

Is there a way to improve the performance? Will another MC be faster? Or is that about as much as I can expect with the additional 2 converters?

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What transmition media do you use? –  MicTech Apr 11 '10 at 21:09
    
Multimode fiber. –  Aaron Digulla Apr 12 '10 at 7:36

2 Answers 2

Do the fibre converters offer any diagnostic capabilities? A slightly dirty fibre ferrule or damaged fibre might be causing drop-outs. If the problem is only marginal it might affect throughput without being immediately obvious.

What about jumbo-packet support? If they don't support jumbo packets but your computer and server expect it, you could easily see a halving in performance, if not more.

Edit: Cleaning fibre ferrules must be done with care. It is very easy to scratch the most critical parts of the fibre at the centre of the ferrule (which are only microns across, much thinner than the thickness of your hair).

When I was working on a fibre polishing machine, I would test the quality control systems by deliberately dirtying and then cleaning them. Dirtying them was easy, just some finger grease was enough for our QC system to measure significant back-scatter losses.

When it came to cleaning them, I would take a small, fresh piece of lint free cloth and wet half of it wih IPA (Isopropyl alcohol). I would then swipe the ferrule once along the we side of the cloth and then once on the dry. This was generally enough to get a perfect signal, unless the ferrule was damaged - in which case it has to go back into the machine to be polished again.

One thing that you should not do is use the same piece of cloth more than once (it is very easy for the ferrule to pick up dirt deposited on the cloth by a previous swipe), scrub the ferrule over the surface (as previous comment, but also more likely to scratch the surface) or apply too much pressure (you don't want any grit in the cloth to gouge a scratch in the ferrule).

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Unfortunately, there are no diagnostics. These devices are KISS. I'm not aware that I have enabled Jumbo packets but I'll check. –  Aaron Digulla Apr 12 '10 at 13:17
    
And I could try with a different cable but I'm pretty sure the cable is good (it's fresh out of the box, no indication of strain and with the protective caps still installed). –  Aaron Digulla Apr 12 '10 at 13:19
    
I always clean FO cables before installation. Amazing how dirty fresh out of the box ones can be. –  Brian Knoblauch Apr 12 '10 at 13:40
    
@Brian: How do you clean them? –  Aaron Digulla Apr 12 '10 at 15:26

Is there a reason to use fibre and converters (i.e. it is already installed and an extra copper link would be inconvenient) instead of just getting a longer CAT6/CAT5e cable? CAT6 and CAT5e copper links using decent cables should cope with GBit speeds for up to 100m so your 30m distance shouldn't be an issue.

Any extra devices in the link are going to add latency which could affect throughput.

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In short: yes. I need to connect more places and the space is very limited. In the volume where I can run a single CAT6, I can run 12 fibers. –  Aaron Digulla Apr 14 '10 at 7:36

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