Why cross-over Fiber Cables?
Occasionally, there will be instances in which you need to cross over fiber optics cables. The reasons may vary, but at the end of the day, the transmit (TX) and receive (RX) will need to be crossed in order to make a proper connection.
One of these situations is when you have to make a connection, and the cable manufacturers, instead of crossing the cable from end to end, they run them straight through. In other words, when you try to plug in you take the TX from one end but the other end shall also be a TX, which will definitely not work.
That being said, you need to take the TX and plug it into the RX of the other side of the port -this is what's actually known as a cross-over. You need to make sure to identify the type of connector you are dealing with. If the fibers were reversed, all you need to do is to pull the fibers out, criss-cross them and put them back in, and then the link should be established.
One thing that should be definitely pointed out is that, whenever you are making a cross-over, you should be able to correct both ends properly. That is a good way to prevent subsequent confusions and misfortunes.
For example, you may have done a single cross-over connection within a panel that contains hundreds of them. So this may be not only inconvenient to you, but to the rest of the team in charge of managing those connections.
Another scenario you may have is to have an LC connectors on both sides. To cross over these fibers, all you need to do is to take the fiber connectors out of the holding bracket and criss-cross them manually. The way to do this is, to first pinpoint how the connectors are put together. If they're split on the bottom, for example, the fibers should come out of the bottom, so you need to figure out a way to pull the fiber out and repeat the procedure for the other one.
Then make sure to handle carefully and not bend any fiber too much so you don't break it, and then you reattach the bracket to the fiber and make sure it's sealed properly just by taking a close look at the mechanism of the connector. Sometimes you will hear a click when you lock the bracket. Also, make sure both connectors are put together at the same height so that when you plug them in you can establish a proper connection.
Another scenario that you may come across, it's the one in which you may have a different type of holding bracket on one of the ends. In this case, you need to pull the fiber out of the bracket. Just to be very careful to identify the orientation so you can keep it after you tear the bracket apart to make a proper cross-over. Once you have the right bracket, insert the fibers into it and make sure to make an appropriate link.
Source: SparkFun Tutorials
Be advised that every bracket has a particular mechanism, so look at it closely so you can make a good change. If you feel like you're forcing the fibers too much, or that the mechanism is not loosing up organically, then you may be making a wrong move and you take the risk of breaking the fiber.
Long story short, the reason fiber cables need to be crossed is rather simple: when connecting fiber from one device to another, the fiber strands need to be cross so that TX goes to RX on each end. So if you face a situation like the ones explained above, you don't need to worry. Fibers can -and must- be crossed-over. Just make sure you're making the necessary adjustments in a delicate yet competent way to properly establish the connection you need to make.
We really hope you can find this information very useful and we hope you can make a proficient connection of your system!