Installed wiring segments have to be installed with a crossover in each fiber pair, whereby the A side of one connector pair matches to the B side of the other connector pair in any patch cord or permanent link.
As shown in figure 1 , the optical fibers connects optical transceivers (S1,S2) of the active element 1 with the transceivers (E1,E2) of the active element 2.
This is also prescribed in the cabling standard ISO/IEC 11801 and CENELEC EN 50173.

In reality fiber optic transmission channels are not just consisting of an optical cable, but mostly it is the combination of fix installed optical fiber cables between a patch panel and a connection socket at work place whereby a patch cord is on every end. ( see details in figure 2).

The FOC transmission channel is made of three crossed cable parts. Two crossings cancel out each other, therefore the transmission channel can be seen as one crossing. That principle works only if the number of cable parts within the transmission channel is odd, whereby installed optical fiber in the cable part is crossed.

If the installed cable is made of two single-side assembled optical fibres, which is spliced together in the middle, the outcome is still the same. While splicing, it is important to connect just fibers with the same colour, in order to avoid additional crossings. If the number of parts in transmitter channel is even it is important to use right number of crossed and not crossed cable parts to guarantee the right polarity