Auto-Negotiation Protocol
With equipment running at both 10 Mbps and 100 Mbps, a means was sought that would allow end stations and switching hubs to automatically adapt to one another. The result was the Auto-Negotiation protocol. With auto-negotiation, not only is communication speed determined, but duplex mode is determined as well.
Each device that participates in the Auto-Negotiation protocol knows its own capabilities and "advertises" these attributes to its link partner upon initial connection. When connecting two devices over a twisted-pair link, each is to advertise its available speeds and its duplex capability (half or full). A duplex connection allows communication in both directions. For full-duplex links, bi-directional communication can occur simultaneously. For half-duplex, bi-directional communication can occur but not at the same time. Full-duplex links offer higher throughput but cannot reinforce collisions and therefore can only be used with switching hubs. Half-duplex links can work with either switching or repeating hubs. For the range of devices we are discussing here, there are four possible conditions of interest:
100 Mbps, full-duplex
100 Mbps, half-duplex
10 Mbps, full-duplex
10 Mbps, half-duplex
The four possible conditions have been ranked with the highest performance at the top of the list and the lowest at the bottom. Notice that preference has been given to full-duplex links since they offer higher performance. Of course, 100 Mbps offers higher performance over a 10 Mbps link.
During negotiation, the two partners advertise their possible levels of performance and agree upon the highest common denominator for optimum communication. If one device advertised 10 Mbps, full-duplex, and the other 100 Mbps, full-duplex, they would settle on 10 Mbps, full-duplex, and begin communicating accordingly. Devices that participate in auto-negotiation are end stations and switch ports. Repeating hubs do not participate. What happens if one of the two devices was manufactured before the Auto-Negotiation protocol was invented and does not participate in the advertising phase? The Auto-Negotiation protocol would anticipate such an occurrence and invoke "parallel detection." With parallel detection, the more intelligent device will recognize either an NLP or an FLP and adjust its speed accordingly to either 10 Mbps (in the case of an NLP) or 100 Mbps (in the case of an FLP). The duplex mode will default to half. The reason that auto-negotiation works on twisted-pair links is that connectors, pin-outs, distances, and cabling remain the same so communication is possible once speed and duplex are determined. The same potential does not exist over 100BASE-FX fiber optics but it does with 100BASE-SX fiber optics. However, most fiber optic switch ports only support 100BASE-FX.
Connecting to Switch Ports
Most 100 Mbps switching hubs have the capability of operating their twisted-pair ports at either 10 Mbps or 100 Mbps. Usually, auto-negotiation is enabled but some switches offer fixed settings of duplex and speed. Fiber optic ports are another situation. A 100BASE-FX port offers the greatest fiber optic length so you tend to see this technology versus 100BASE-SX. Since auto-negotiation on 1300 nm fiber is not possible, the speed of the fiber port is usually fixed at 100 Mbps. A half-duplex, 100 Mbps fiber link is limited to 430 m — so to achieve the maximum fiber distance, the duplex must be set to full. That is the reality when dealing with 100 Mbps fiber optic ports on switches. They are usually fixed to 100 Mbps full-duplex with no auto-negotiation. For copper, you can usually assume auto-negotiation and 10/100 Mbps operation. This should be kept in mind when applying a media converter between end stations and switches or between two switches.
