This describes an instrument to check allsorts of different cables. This is
an ethernet cable being checked,
Although the RJ11 ethernet plug has got eight conductors only four are needed for basic network communications two receive and two transmit pairs wired 1 to 3 and 2 to 6 both ways as in this testing Rig. Conventional ethernet cable can be up to 600 feet long so you would simply flag the yellow terminator in at one end and the blue box at the other to test the continuity of DC voltage. It does not do much checking for resistance between the leads as might happen if water got in to the conductors as it does not really measure resistance very well but it would for example show you if you had a defective plug if you wiggled it in the connector…
The display shows the simplest cable having wires 1,2,3 & 6 all connected but the other four conductors not. This will not operate at gigabit speeds– that will need all 8 connected!
some remote Wi-Fi equipment for example can be powered through the ethernet cable using a technology called POE. Note that it is not as simple as just pushing some voltage up the wires because the specification makes allowance for line resistance and proper correction for voltage drops with special terminating gear..
the DC voltage ‘rides under’ the high frequency communications on some of the same wires.
This is especially important for those Wi-Fi antennas and powered relays that are way up high in trees for example