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13.5 Examples of Auto-Negotiation

Auto-Negotiation and Cable Type

The Auto-Negotiation system is designed so that a link will not become operational until matching capabilities exist at each end. However, the Auto-Negotiation protocol is not able to test the cable used on the link. Therefore, it is up to the installer to make sure that either the correct cable type is in place, or that the mode of operation on the link is set correctly.

Consider a link with a hub at one end, a station at the other end, and with Auto-Negotiation in operation in both devices. If the cable used in this link is only rated at Category 3 then operation using 100BASE-TX on this link can be a problem. When power is applied, the hub and station will use Auto-Negotiation to determine the capabilities at each end.

Let's assume that the hub and station are each capable of 10BASE-T and 100BASE-TX operation and will choose to operate at the highest performance mode they have in common, which is 100BASE-TX. The Auto-Negotiation link pulses are simply bursts of the same link pulses used in 10BASE-T, so the pulses will travel over Category 3 wire without any problems and the negotiation process will work OK. However, 100BASE-TX operation requires the use of Category 5 cable, which means that this link will either operate marginally with a high rate of errors, or not at all.

While Auto-Negotiation is a handy feature that allows the highest performance mode to be automatically selected on a given link, it still requires that the correct cable type be in place for the highest speed mode that may be selected. Auto-Negotiation devices also provide management capabilities that allow a network manager to manually set a mode for a given link, etc. By using the management interface you can make sure that a link does not negotiate a mode of operation that exceeds the capabilities of the cabling for that link.

Quick Reference Guide to Auto-Negotiation - 25 MAY 96
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