When connecting devices via a pre-built system of patch panels, the optical patch cables must be used with a certain criterion. These notes, valid in general, refer in particular to a distribution system for dual-link connections with connectors of the LC type.

Patch Cables

LC-LC duplex patch cables are usually assembled to directly connect two SFP tranceivers.
These patch cables are called "A to B" given that they optically connect the "B" transmitter ferrule of an SFP to the "A" receiver ferrule of another SFP.

Patch Panels and long range connections

Patch panels with LC duplex connectors are usually assembled as to establish a 1-to-1 correspondence between the ferrules of their ports.
In this case, the "A" ferrule of a given port of a panel is connected to the "A" ferrule of the corresponding port on the remote panel.

When connecting two devices via a pre-built system of patch panels, it must be taken into account the number of permanent links (segments of multi fiber cable, with patch panels at both ends) employed to connect the devices.
If the number of permanent link is even (2,4,...), then you can employ standard "A to B" patch cables all the way (see fig. below). Otherwise, if the number of permanent link is odd (1,3,...), then you have to use one "A to A" patch cable along the path to match transmitter and receiver. Even if it is not important where this cable is used, it is good to always adopt the same method, for example by placing the "A to A" cable on the first connection (see fig. below). An "A-A" patch cable can be obtained from a standard "A-B" cable by carefully disassembling one of the two LC connectors and swapping the position of its ferrules.

In these cases, it is adviced to insert the PATCH CABLE A-A at the beginning of the connection that is between the central switch and the first patch panel (or switch) encountered.