Ethernet Auto Negotiation
The 802.3 Ethernet standard was modified to include 802.3u 100 Mbps Fast Ethernet in 1995. Along with Fast Ethernet, the IEEE incorporated a feature called full-duplex mode of operation. Previously, all Ethernet communication was in a half-duplex CSMA-CD mode of operation. This meant that Ethernet stations would first listen to the wire and only transmit if they did NOT hear another device already transmitting. If by chance two end stations tried to transmit at the same time, a COLLISION would occur and each end-station would back off for a random period of time before re-transmitting. This is true even when there are ONLY two devices on the network.
Full-duplex operation eliminates the CSMA-CD protocol and implies a point-to-point Ethernet network connection between two devices. With full-duplex operation each point-to-point connection is a separate 10/100/1000 Mbps Ethernet segment. In a full-duplex network there are no collisions and end-stations at opposite ends of a full-duplex Ethernet link may transmit simultaneously.
Along with the full-duplex mode of operation, IEEE implemented another Ethernet feature called auto-negotiation. Auto-negotiation is a mechanism that enables devices to negotiate the SPEED and MODE (duplex or half-duplex) of an Ethernet Link. As an Ethernet standard this mechanism has to accommodate ANY Ethernet compliant device. As with any new technology, there are legacy issues that must be dealt with as users migrate to the latest improvements.
Many early implementations of the auto-negotiation feature do not comply with the final standard. Some may be fixed with driver updates, while others require new hardware. Many Ethernet products older than mid-year 1997 do not support auto-negotiation. These issues have created a situation where the new standard compliant products appear to be creating a problem, when in fact it is the older non-compliant hardware that cannot take advantage of this new valuable feature.
Fast Ethernet devices that support auto-negotiation send fast link pulses (FLP) to the adjacent link station to negotiate the SPEED and MODE of the link. The FLP signals carry information necessary to negotiate the highest level of service available for the link. The priority for services includes 100BaseTX full-duplex as the highest priority, then 100BaseT4, 100BaseTX, 10BaseT Full-Duplex, and finally 10BaseT (which has the lowest priority).
All Brocade products support the standard compliant Ethernet auto-negotiation protocol.