There are two types of fiber optic transceivers according to the number of fiber cores, one is single-mode dual fiber fiber optic transceivers, and the other is single-mode single fiber optic transceivers. The A and B ports we are talking about are only used by single-mode single fiber fiber transceivers, because single-mode single fiber transceivers transmit light through a single fiber core, so both transmitting and receiving light are transmitted simultaneously through a single fiber core. In this situation, to achieve normal communication, two wavelengths of light must be used to distinguish.

The optical module of a single-mode single fiber transceiver emits light with two wavelengths, generally 1310nm/1550nm for short distances and 1490nm/1550nm for long distances. In this way, there will be a difference between the two ends of the interconnection of a pair of transceivers, with one end transmitting 1310nm and receiving 1550nm. On the other end, it emits 1550nm and receives 1310nm. So for the convenience of users to distinguish, letters are usually used instead, resulting in A-end (1310nm/1550nm) and B-end (1550nm/1310nm). Users must use AB pairing instead of AA or BB connection.