How fiber splitters work
Posted by creining | Filed under Security
I ran into a problem with tapping fiber on a firewall leg using one of my fibertaps. The problem was that after connecting one set of fiber from the network, one set to the firewall, and one set to the NIDS it would kill link. If one of the Rx or Tx cables was removed from the TAP port link would be restored. After troubleshooting for a while it was determined that it probably had to do with how the TAP was functioning or it had something to do with the LC to SC cable in use (the TAP has SC connectors). After a quick chat with the vendor, they suggested re-keying the cables, or flipping the Rx/Tx pairs on the input and output ports on the TAP. That fixed it, and here’s why: a fiber splitter is made by taking 2 fibers which are twisted together until the desired split ratio (50/50 in my case) is obtained and then they fuse them together and chop off the fourth leg. If light does not enter the splitter in the proper direction an inadequate amount of light will be transmitted out the TAP port. The expected light is about -12dbm and an inadequate amount is about -38dbm. In simpler terms, I had the fiber with light traveling in the opposite direction through the TAP that it should have been.
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