Autonegotiation and Autosensing

Autonegotiation is a mechanism defined in the IEEE 802.3u specification whereby devices sharing a link segment can exchange information and automatically configure themselves to operate at the highest capability mode shared between them.

Autonegotiation is like a rotary switch that automatically switches to the correct technology such as 10Base-T or 100Base-TX or between half- and full-duplex modes. Once the highest performance common mode is determined, auto-negotiation passes control of the link to the appropriate technology, sets the appropriate duplex mode, and then becomes transparent until the link is broken.

Following is the IEEE 802.3u-defined hierarchy for resolving multiple common abilities for a 10/100Base-TX card. The PCI 10/100Base-TX/9000 product provides the means for interfacing various types of HP 9000 workstations to either a 10Base-T or 100Base-TX network. 100Base-TX is a subset of 100Base-T networking defined by the IEEE 802.3u-1995 standard. 100Base-TX provides 100 Mbit/s data transmission over category 5 unshielded twisted-par (UTP) cable. Two pairs of wires in the cable are used—one wire pair is for receiving data, and one wire pair is for transmitting data. The same card port that supports 100Base-TX operation can also support 10Base-T operation.

For example, if both devices on the link support 10Base-T (half-duplex) and 100Base-TX (half-duplex), autonegotiation at both ends will connect the 100Base-TX (half-duplex) instead of the 10Base-T (half-duplex).

Most Fast Ethernet devices on the market today such as hubs and switches do not support autonegotiation. Either the speed and duplex mode of the device are fixed (as is usually the case with hubs), or they are often manually configured at the desired speed and duplex (as is often the case for switches). However, switches that support autonegotiation are starting to be offered.

If the PCI 10/100Base-TX/9000 card is connected to a device, such as a switch, that is autonegotiating, the PCI card will autonegotiate with the device to mutually determine the highest possible speed and duplex settings between them.

	NOTE: If the PCI 10/100Base-TX/9000 card is connected to a device that does not support autonegotiation or a device that has autonegotiation disabled, the PCI card will autosense the speed of the link and set itself accordingly. The duplex mode of the card will be set to half-duplex in this case. If you want the card to operate in full-duplex mode, you have to set it using the method described in Manual Speed and Duplex Mode Configuration.

The PCI 10/100Base-TX card will sense when the connection between itself and a hub or switch on the other end of a link has been broken. If a connection is made to another (or the same) device, the autonegotiation and autosensing process will be done again automatically. Autonegotiation and autosensing are also done whenever the interface is reset.