Modal Bandwidth Discussion

[1ceHacka]

A co-worker and I got into a discussion about fiber this morning. We are planning on using 10gig links from our closets to our core. Currently we have both MMF and SMF going from the closets to the core.

We got into a discussion about the distances that the MMF can sustain certain bandwidths before hitting the 3dB loss (i.e. half power loss). This lead to a discussion on Modal Bandwidth, and we realized we didn't have a clear understanding of what this measurement actual means to us. Anyone have a decent explanation of how this measurement comes into play in determining the freq. used vs. the distance of the fiber link? Thanks in advance.

 

[cmetz]

In modern fiber systems, dispersion is a limit long before loss. With MMF, you get a lot more dispersion in the fiber, plus you're dealing with cheaper optics on both ends. So your bit rate (often but incorrectly called "bandwidth") gets limited by your ability to tell the bits apart reliably.

 

[1ceHacka]

I just read an article by Corning. They are stating that their 50nm fiber rates 10Gb/s with a 2000 EMB (Effective Modal Bandwidth) up to 300m. This tells me that to attain the 10Gbps, new cable would be needed seeing as we currently have 62.5nm fiber which is rated at 10Gbps up to 33-26m.

So is dispersion linked to Modal Bandwidth? Still not clear on where MB plays its part in all this...

 

[spidey07]

62.5 fiber is effectively dead (because of low modal bandwidth, the larger core means more dispersion "scattering".). You could use your single mode. Modal bandwidth and dispersion are kind of the same thing, at higher data rates the dispersion becomes more problematic to be able to recover the bits. Think of dispersion as noise and modal bandwidth as a noise floor, the farther you go the greater the noise/dispersion.

For anybody else reading any new MM fiber should always be premium high modal bandwidth 50 micron fiber.