Fiber - Single Mode/Multimode

Unanswered Question
Mar 17th, 2003

I have a couple of 6509's that have GBIC's in them. I've generally used orange fiber patch cords for the connections which I believe are multi-mode. I also have some connections coming from another building with single mode fiber in between the buildings. To connect switches between these buildings I use orange fiber patch cords which, like I said, I believe are multi-mode.

My questions are, how do I tell whether I have multi or single mode GBIC's? How do I tell if my patch cords multi or single mode? Are there consequences for mixing multi-mode and single-mode fiber?

Thanks in advance,

Mike

I have this problem too.
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matthew.scala@m... Mon, 03/24/2003 - 08:12

GBICs will more than likely have a label on them that identify the type of fiber they support. Examples that you will see:

SX - Short-wave (Multimode)

LX - Long-wave (Singlemode)

LX/LH - Long-wave/Long-haul (Singlemode/Multimode)

ZX - Extended Distances (Singlemode)

You must use the same type of GBIC for each end of your fiber connection. Not doing so can result in damage to your physical hardware.

Additionally, most fiber optic cables are labeled with a "core" specification (50/62.5/125, etc.) Most fiber optic cables that I have encountered in the field have either been orange (multimode) or yellow (singlemode).. however, there is no set standard of colors. Hope this helps..

- Matt

konigl Tue, 03/25/2003 - 05:16

Mike,

To expand on what Matt said, the LX - Long-wave should be considered (Singlemode/Multimode) as well. LX and LX/LH GBICs can both work over multimode fiber runs. Typically, they require a mode-conditioning patch cable to help "launch" the LX transmit signal from the GBIC at an offset from the center of the multimode fiber strand. (SX GBICs do this naturally, if I remember correctly.) LX or LX/LH over multimode is specified out to 550m, versus 220-275m using SX GBICs.

You must use the same type of GBIC for each end, because of wavelengths. SX GBICs run at 850nm, LX/LH GBICs run at 1310nm, and ZX GBICs run at 1550nm. If you mix GBIC types on a link, they won't "see" each other.

You can damage your physical hardware in the case of the ZX GBICs if you do not have sufficient optical attenuation between the TX port of one GBIC and the RX port of the other GBIC, and vice versa. In a "lab" environment and on single-mode fiber runs out to about 50km, you need optical attenuators on the RX ports of each ZX GBIC.

I have seen multimode fiber patch cables come in colors other than orange; for example, an Extreme Networks field engineer showed me some ugly purple ones (think "Barney") that matched their switches!. I have never seen a single-mode patch that wasn't yellow, though.

There can be consequences to mixing multi-mode and single-mode fiber on a given run. For example, if you use multimode patches with single-mode long-distance runs, and you're using LX/LH or ZX GBICs, the multimode patches may cause excessive attenuation. That having been said, I have a customer who I routinely catch using orange MMF patches on single-mode LX/LH runs (and even two ZX runs once!), and they still worked. I got them changed out anyway.

Hope this helps.

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Posted March 17, 2003 at 6:19 AM
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