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12.2 100BASE-T4 Components

100BASE-T4 Link Integrity Test

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The Fast Ethernet transceiver circuits (PHY) continually monitor the receive data path for activity as a means of checking that the link is working correctly. When the network is idle, 100BASE-T4 transceivers send link pulses over the segment to verify link integrity. These pulses are called Fast Link Pulses, and they are also used in the Auto-Negotiation mechanism which allows a multi-speed hub to detect the speed of operation of an Ethernet device that is connected to it, and to adjust the speed of the hub ports accordingly. Auto-Negotiation is described in Chapter 13

It's important that both link lights be lit, one at each end. That indicates that there is a correctly wired signal path between both devices. However, the presence of the link lights merely means that the 100BASE-T4 segment is correctly wired. Since the link pulse operates more slowly than actual Ethernet signals, the link lights are not a guarantee that Ethernet signals will work over the segment. Odds are good that a correctly wired segment will work, but if the signal crosstalk on the segment is too high, then it may not work despite the presence of the link lights.

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Quick Reference Guide to 100BASE-T4 - 09 SEP 95

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