100BASE-T4 Link Integrity Test

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.

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

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