Resources on the topic of Ethernet Auto-Negotiation.
Auto-Negotiation was first defined in 1995 as an optional feature for 10 and 100 Mbps twisted-pair Ethernet media systems in Clause 28 of the 802.3u Fast Ethernet supplement to the IEEE 802.3 standard. The twisted-pair Auto-Negotiation system defined in Clause 28 of the standard has since been extended to include all three speeds of Ethernet supported over twisted-pair cable: 10 Mbps 10BASE-T, 100 Mbps 100BASE-TX and 1000 Mbps 1000BASE-T.
The physical signaling portion of all three twisted-pair systems use the same Auto-Negotiation signaling standard. While Auto-Negotiation can be disabled on 10BASE-T and 100BASE-TX links, it is required on 1000BASE-T systems since Gigabit Ethernet systems use Auto-Negotiation to establish the master-slave signal timing control required to make the link operational.
With Auto-Negotiation in place, all three speeds of twisted-pair Ethernet can determine the common set of options supported between a pair of "link partners." Twisted-pair link partners can use Auto-Negotiation to figure out the highest speed that they each support, for example, as well as automatically setting full-duplex operation if both ends support that mode.
Fiber Optic Auto-Negotiation
The various fiber optic Ethernet standards (10 Mbps 10BASE-F, 100 Mbps 100BASE-FX and 1000 Mbps 1000BASE-X) use different wavelengths of optical signaling which made it impossible to come up with an Auto-Negotiation signaling system that would work across all three.
Instead, only the 1000BASE-X fiber optic media system has an Auto-Negotiation spec that allows the link partners on a Gigabit Ethernet fiber optic link to determine which modes of operation they support in common (e.g, full-duplex) and then configure the modes. The 1000BASE-X Auto-Negotiation standard is defined in Clause 37 of the IEEE 802.3 standard.
Auto-Negotiation Resources
Wikipedia: Autonegotiation
Auto-Negotiation tutorial. This tutorial provides some examples of 10 and 100 Mbps Auto-Negotiation, and also describes potential problems when Auto-Negotiation proceeds over a cabling system that cannot support high speed Ethernet signaling.
"Gigabit Ethernet Auto-Negotiation" This article was written by Rich Hernandez at Dell. It discusses 10, 100 and 1000 Mbps twisted-pair Auto-Negotiation.
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