Auto-Negotiation is a mandatory function for 1000BASE-X interfaces. These include the 1000BASE-SX and 1000BASE-LX fiber optic interfaces and the 1000BASE-CX copper interface.
It is physically impossible for incompatible types (for example, 1000BASE-SX and 1000BASE-LX, or 1000BASE-LX and 1000BASE-CX) to communicate across a link, so the technology type is not negotiable. The capabilities that are negotiated include
Full–duplex or half-duplex capabilities.
Flow control capability (for a full–duplex link). As was the case for twisted-pair cabling, 2 bits are used to describe whether the system can send and/or receive PAUSE messages.
These Auto-Negotiation messages optionally also can describe remote fault ...