Autonegotiation - Overview

Overview

In 1995, a standard was released to allow connected network adapters to negotiate the best possible shared mode of operation. The initial autonegotiation standard contained a mechanism for detecting the speed but not the duplex setting of Ethernet peers that did not use autonegotiation.

Autonegotiation can be used by devices that are capable of different transmission rates, different duplex modes (half duplex and full duplex), and/or different standards at the same speed (though in practice only one standard at each speed is widely supported). Each device declares its technology abilities, that is, its possible modes of operation, and the best mode is chosen from those shared by them, with higher speed preferred over lower, and full duplex preferred over half duplex at the same speed.

Parallel detection is used when a device that is capable of autonegotiation is connected to one that is not. This happens if the other device does not support autonegotiation or autonegotiation is administratively disabled. In this condition, the device that is capable of autonegotiation can determine and match speed with the other device. This procedure cannot determine the presence of full duplex, so half duplex is always assumed.

The standards for 1000BASE-T and 1000BASE-TX require autonegotiation to be always present and enabled. Other than speed and duplex mode, autonegotiation is used to communicate the port type (single port or multiport) and the master-slave parameters (whether it is manually configured or not, whether the device is master or slave if this is the case, and the master-slave seed bit otherwise).