There are two basic types of fiber optic cabling: one is multi-mode and the other single mode. In the case of multi-mode fiber, these diagrams, the white part in the middle of the blue is supposed to be representing the portion of the glass that carries the light. And you can see with multi-mode the portion of the glass that carries light is much larger than in single mode. What the red lines here represent is the way the light travels inside of that glass. With multi-mode fiber if you looked at the end of a piece of multi-mode fiber you would see the cladding, that's the blue part on the outside, and then you'd see in a smaller portion of the glass that carries the light. And unlike this diagram kind of tends to imply the light does not bounce up and down down that piece of fiber. It enters the the glass and it goes in a circular fashion but if it enters the glass in the very outside very near the cladding, it will stay near the cladding and if it near enters right in the middle, it'll stay in the middle. So the reason it's called multi-mode is the light follows multiple paths. Single mode, the light carrying component, is so small that there's only one path for the light. Single mode because it's a single path. Let's take a look at multi-mode fiber. So there's some very legacy stuff. It's 62.5 microns. This is what we installed on my campus in the 1986 kind of time frame. And then there are a number of newer styles of multi-mode fiber that are all 50 micron And I've listed the standards here that you should be aware of. OM1 is the legacy 62.5 micron. OM2 is a 50 micron, still legacy. OM3 and OM4 are optimized for laser and then OM5 is optimized for some wave division multiplexing functions. Single mode fiber on the other hand, all of single mode fibers have cores of between 8 and 10 microns and there's a whole bunch of different standards. So I've listed the same standards, the OSI standards, that we were talking about for a multi-mode fiber. So there's OS1, it's kind of a legacy standard and it's for indoor fiber. And OS2 is a newer standard and it'll cover both indoor and outdoor fiber. There are lots and lots of single mode fiber types that are designated by the International Telecommunication Union or ITU. These are telephone folks because fiber for many, many years and decades has supported telephone communications. A common type is G.652 and in fact G.652d and OS2 are exactly the same. There are other standards associated with long haul and wave division multiplexing. There's fiber that's, you know, single mode fiber is what is used in undersea fiber cables. None of these are relevant to campus installations. You really want OS2 or G.652d. Let's just take a minute and talk a little bit. This is going on the interface between the network equipment whether it's a router or a switch and the optical fiber or plant itself. Typically the way this works is you have things called SFPs. Those are the two guys, they're kind of in the middle and they plug into empty holes. You can see on the Cisco 3750g on the left-hand side a multi-mode fiber cable plugged into an SFP that's placed in the SFP port and then on the right that's actually a QSFP and the QSFP plugs into a wider port and you can see that on that switch on the right. And the QSFPs will support 25 gigabits and above. So the SFP form factors, the smaller form factors, will support 1 gig and 10 gig and then QSFP will support above 10 gig.
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