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Support for 50/125 FO on PA-A3-OC3MM

Somebody knows about support for 50/125 fiber optic on Cisco 7206VXR with PA-A3-OC3MM?


Re: Support for 50/125 FO on PA-A3-OC3MM

I don't know if Cisco has an official position on this, but I can give you my opinion from a fiber optic engineering perspective.

It will probably work, but you should assume a reduced optical power budget (around 5.5dB) relative to the one for 62.5/125 multimode (which is 9dB).

This is because the minimum transmit power figure will likely be a couple of dBm lower for the 50/125 than it is for 62.5/125 fiber. That's the way it is on Cisco's 100BASE-FX MMF interfaces, which are the closest match. Maximum transmit power figure stays the same for the OC-3MM interface, -14dBm. You can rationalize the decrease in min. TX power by picturing what's happening as the light leaves the interface: cross-sectional surface area of the 50/125 fiber core is reduced to 64% of what's available in 62.5/125, so some of the light is possibly just not making it into the launch aperture. But there's still enough going in to get a usable signal out the other end. But how far?

The reduced power at launch may be offset somewhat by the higher quality and slightly better attenuation-per-km characteristics of the 50/125 fiber at 1310nm wavelength. You may even be able to make it all the way out to 2km, I just never ran the calculations to check that; never needed to. I rarely encounter 50/125 here: it's either 62.5/125 multimode, or else single mode fiber, for most of my work.

How far were you planning on going over multimode? 2km is the supported limit for 62.5/125. You can probably do at least 1km with 50/125. If it's just the length of a long multimode patch cable in a server room, wiring closet, or distribution frame, you most likely have nothing to worry about there.

Hope that helps.

New Member

Re: Support for 50/125 FO on PA-A3-OC3MM

The distance involved is 10 meters, this will represent a problem?

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