13.5 Examples of Auto-Negotiation
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.
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