Triplett TVR10/100/1000 - Manual (Page 42)

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"Hub", "Switch" and "PC" defined: A “PC” is a device that
transmits on a pair of wires that use pins 1 and 2 (“1,2”). A “hub” and
"switch" are devices that transmit on pins 3 and 6 (“3,6”). When the
TVR1000 senses a link pulse or data packet, it lights the Data and
Link LEDs on the faceplate to indicate its source (hub or PC) based
on the above standard. "Hub" on the faceplate also means switch.
Link Pulses and Data Packets: LAN devices interact with one
another using Link pulses and Data packets. The TVR1000 detects
and displays both data and link pulses on the DATA and LINK LEDs.
The Link pulses and Data packets are different depending on whether
the devices are 10, 100 or 1000 Base-T devices.
10 Base-T Link pulses and Data packets: 10 Base-T devices
synchronize communication with one another first by each sending
link pulses (a series of single pulses) followed by any data packets
being sent. During quiet time (when there is no data being
transmitted) both devices send link pulses causing the Main Unit's
LINK LEDs to light. The speed and duplex of the link is displayed on
the Speed Verification LEDs.
100 and 1000 Base-T Link Pulses and Data packets: In the 100
and 1000 Base-T environment, a LAN device sends a burst of Link
pulses (containing setup parameters) to the other LAN device. These
bursts continue until acknowledged by the other device (this is termed
"negotiation"). Once the negotiation is complete, data packets are
sent back and forth. No more link pulses are required as long as the
link is not broken. The existence of Link Pulses and Data packets will
cause the DATA and LINK LEDs to light. During link pulse and
data transmission, the link speed and duplex will be displayed on the
Speed Verification LEDs. Notes: Some single speed 100 and 1000
Base-T devices do not send link pulses (just data packets). Also,