Link establishment and full and half duplex

Link establishment and full and half duplex
6.2.10 This page will explain how links are established through Auto-Negotiation and introduce the two duplex modes.


Link partners are allowed to skip offering configurations of which they are capable. This allows the network administrator to force ports to a selected speed and duplex setting, without disabling Auto-Negotiation.

Auto-Negotiation is optional for most Ethernet implementations. Gigabit Ethernet requires its implementation, though the user may disable it. Auto-Negotiation was originally defined for UTP implementations of Ethernet and has been extended to work with other fiber optic implementations.

When an Auto-Negotiating station first attempts to link it is supposed to enable 100BASE-TX to attempt to immediately establish a link. If 100BASE-TX signaling is present, and the station supports 100BASE-TX, it will attempt to establish a link without negotiating. If either signaling produces a link or FLP bursts are received, the station will proceed with that technology. If a link partner does not offer an FLP burst, but instead offers NLPs, then that device is automatically assumed to be a 10BASE-T station. During this initial interval of testing for other technologies, the transmit path is sending FLP bursts. The standard does not permit parallel detection of any other technologies.

If a link is established through parallel detection, it is required to be half duplex. There are only two methods of achieving a full-duplex link. One method is through a completed cycle of Auto-Negotiation, and the other is to administratively force both link partners to full duplex. If one link partner is forced to full duplex, but the other partner attempts to Auto-Negotiate, then there is certain to be a duplex mismatch. This will result in collisions and errors on that link. Additionally if one end is forced to full duplex the other must also be forced. The exception to this is 10-Gigabit Ethernet, which does not support half duplex.

Many vendors implement hardware in such a way that it cycles through the various possible states. It transmits FLP bursts to Auto-Negotiate for a while, then it configures for Fast Ethernet, attempts to link for a while, and then just listens. Some vendors do not offer any transmitted attempt to link until the interface first hears an FLP burst or some other signaling scheme.

There are two duplex modes, half and full. For shared media, the half-duplex mode is mandatory. All coaxial implementations are half duplex in nature and cannot operate in full duplex. UTP and fiber implementations may be operated in half duplex. 10-Gbps implementations are specified for full duplex only.

In half duplex only one station may transmit at a time. For the coaxial implementations a second station transmitting will cause the signals to overlap and become corrupted. Since UTP and fiber generally transmit on separate pairs the signals have no opportunity to overlap and become corrupted. Ethernet has established arbitration rules for resolving conflicts arising from instances when more than one station attempts to transmit at the same time. Both stations in a point-to-point full-duplex link are permitted to transmit at any time, regardless of whether the other station is transmitting.

Auto-Negotiation avoids most situations where one station in a point-to-point link is transmitting under half-duplex rules and the other under full-duplex rules.

In the event that link partners are capable of sharing more than one common technology, refer to the list in Figure . This list is used to determine which technology should be chosen from the offered configurations.

Fiber-optic Ethernet implementations are not included in this priority resolution list because the interface electronics and optics do not permit easy reconfiguration between implementations. It is assumed that the interface configuration is fixed. If the two interfaces are able to Auto-Negotiate then they are already using the same Ethernet implementation. However, there remain a number of configuration choices such as the duplex setting, or which station will act as the Master for clocking purposes, that must be determined.

This page concludes this lesson. The next page will summarize the main points from the module.