Auto-Negotiation

With the proliferation of Fast Ethernet and the similarity of the cabling components to conventional Ethernet, a means was proposed in IEEE 802.3u to automatically configure Fast Ethernet ports to work with either legacy Ethernet ports or other Fast Ethernet ports. This configuration protocol was based upon National Semiconductor’s NWay standard. There is a way for twisted-pair links to automatically configure compatible formats in order for links to begin communicating. This scheme is only suitable for twisted-pair links and not coaxial or fiber optic links. Coaxial cable is a legacy 10 Mbps standard that is not in the plans for evolving Ethernet. Fiber optics is a different story. Although fiber optics is very much in the plans for evolving Ethernet, there is just no simple way for two fiber optic devices to auto-negotiate data rates since a 10BASE-FL device operates at 850 nm while a 100BASE-FX device operates at 1300 nm. These devices will not interoperate. The Auto-Negotiation protocol is only intended for twisted-pair links where there are only two devices on a segment. Notice that twisted-pair bus segments, which are favored in industrial automation, are nowhere supported by Ethernet standards. Industrial customers must be content wiring in a star topology and using either repeating hubs or switching hubs.
The benefit of auto-negotiation is to provide hands-free configuration of the two devices attached to the link segment. At connection time, each of the two devices will advertise all their technical abilities. These abilities have been ranked by the standard as shown in Table 4. The lowest possible ranking is 10BASE-T which assumes half-duplex or shared Ethernet operation. The very next ranking is 10BASE-T full-duplex indicating that full-duplex has higher performance than half-duplex. Notice that there is no ranking for 100BASE-T4 full-duplex. This is because this technology is not capable of full-duplex operation. Finally, the highest ranking is 1000BASE-T full-duplex. This ranking scheme has been provided for completeness. It is not assumed that a particular adapter can handle all technologies. In fact, some of these technologies may not have been commercialized. However, they are all listed consistent with the IEEE 802.3 standard.

100BASE-T full-duplex
100BASE-T
100BASE-T2 full-duplex
100BASE-TX full-duplex
100BASE-T2
100BASE-T4
100BASE-TX
10BASE-T full-duplex
10BASE-T


Table 4 — Auto-negotiation assumes a ranking of priorities.
10BASE-T is at the bottom of this ranking.


Each device examines each other’s technical abilities and determines the lowest common denominator. For example, if an Ethernet adapter can only handle 10BASE-T while a switch port can handle either 10BASE-T or 100BASE-TX, 10BASE-T will be chosen by both. If two Ethernet adapters, one only advertising 10BASE-T and the other only advertising 100BASE-TX connect, there will be no subsequent communication since no compatibility exists.

FLP and NLP

With legacy 10BASE-T systems, a Normal Link Pulse (NLP) is sent between two devices in order to demonstrate that the link is functional in both directions. With Fast Ethernet, the NLP is replaced with a Fast Link Pulse (FLP) that carries the Auto-Negotiation information. The FLP is interpreted by legacy 10BASE-T devices as a NLP so as to provide backward compatibility. If a connection is broken, as evidenced by a lack of link pulses, a series of FLPs will be observed once the link is reestablished. After auto-negotiation is completed, link pulses or link activity are monitored as with legacy 10BASE-T networks.

CONCLUSION

Fast Ethernet provides the path to higher performance Ethernet systems with the ability to interoperate with legacy 10 Mbps systems. Added features such as full-duplex and auto-negotiation also make this migration path attractive.
References

Ethernet—The Definitive Guide, Charles E. Spurgeon, 2000, O'Reilly & Associates, Inc.

Switched and Fast Ethernet, Second Edition, Robert Breyer and Sean Riley, 1996, Macmillan Computer Publishing USA

International Standard ISO/IEC 8802-3 ANSI/IEEE Std 802.3, 2000, The Institute of Electrical and Electronic Engineers, Inc.