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13.5 Examples of Auto-Negotiation

Operating at the Highest Performance Mode

If the 10BASE-T hub in the previous example is later replaced with a 100BASE-T repeater hub, then the dual-speed interface will receive FLPs when the hub is turned on, and the Auto-Negotiation protocol will result in the interface and hub port both operating at 100-Mbps. The switch from 10-Mbps to 100-Mbps will occur with no manual intervention.

The Auto-Negotiation protocol provides mechanisms for hub ports and connected interfaces to negotiate the highest common denominator of performance over a given link. However, the protocol does not specify the behavior of a repeater hub with multiple ports that may be connected to interfaces with differing capabilities. Since a repeater hub is used to create a shared signal channel for all devices attached to the repeater ports, that shared signal channel must operate at the same speed for all ports. If an Auto-Negotiation repeater hub has one of its ports attached to a device that only supports 10BASE-T and the rest of the ports are attached to 100BASE-T devices, then the hub may take several approaches to resolve the speed mis-match.

The designer of the repeater hub may choose to negotiate a lower speed on all ports, to accommodate the 10-Mbps interface. This drops the link speed for all other stations connected to ports on the repeater hub to 10-Mbps. Rather than reduce the link speed for the other stations, the designer of a 100BASE-T repeater hub may decide not to allow a fixed-speed 10-Mbps interface to make a connection, and instead the hub could send a message to its management interface reporting the failed connection attempt.

Yet another approach suitable for a higher cost hub is to design a hub with multiple repeater backplanes and use Auto-Negotiation to dynamically connect the ports operating at 10-Mbps to a 10-Mbps repeater backplane in the hub, and to connect the 100-Mbps ports to a separate 100-Mbps backplane. The two repeater backplanes in the hub could communicate through an Ethernet switch. Ethernet switching is described in Chapter 15.

Unlike a repeater hub, in which all ports must operate at the same speed, a switching hub provides ports that operate independently. A hub with switched ports can simultaneously support 10-Mbps operation on one port and 100-Mbps operation on another port of the same hub.

If there is no common technology detected at either end of the link, then the Auto-Negotiation protocol will not make a connection, and the port will be left in the off condition. For example, if a 100BASE-T4 device is connected to a port on a 100BASE-TX switch, no connection will be established on that link.

Quick Reference Guide to Auto-Negotiation - 25 MAY 96
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