#WHY LOOPBACK CABLE MANUAL#
Consult the equipment manual of the equipment being tested to determine if gigabit loopback is supportedįrom all my testing and research on this subject my conclusion is that unless you're dealing with exclusively ancient 100Mbps equipment, the loopback adapter concept is now obsolete due to advanced signalling in modern switches. The Gigabit Loopback Jack & Plug is intedned solely for testing systems where the Near End Crosstalk (NEXT) function can be disabled and the equipment under test can support being looped back to itself. Not all gigabit Ethernet systems support loopback operation. Gigabit switches may behave differently (but I’m not sure what the spec says or the real-world variation is)Īlso, even commercial loopback adapters come with a lengthy caveat regarding gigabit compatibility
Meaning it’s often only useful on NICs which let you disable crosstalk detection. As the CM4148 has 48 ports you need to test ports, 1-9, 10-19. The screen will then show 7 columns for ports 10 through 16 and one for Ethernet. To test ports 10 through 16 on the CM4116 you need to type loopback -e eth0 /dev/port1 0-6.
#WHY LOOPBACK CABLE SERIAL#
Gigabit NICs have crosstalk detection (detects how much signal interferes onto other wires), and will likely decide that the loopback is an extreme amount of crosstalk - any may not show link. The screen will show 9 columns for serial loopback and one for Ethernet: This will test port 1 through 9. Turns out "Gigabit loopback is a limited concept" so maybe 100Mbps works ONLY on 100Mbps switch? I could live with that if that's how that works, but what about 1Gbps loopback with 4 pairs? Why is that not lighting up?)Ĭan anyone shed some light on this? I just don't get why it's behaving the way it is. But maybe that's to be expected because that's also what happens when I plug in my other 2 pair 100Mbps RJ45 loopback. The other weird thing is that 1Gbps switch that's 100Mbps capable doesn't detect this at least as 100Mbps (I could live with it not showing up as 1Gig). I also tried several switches, and it's always the same: 100Mbps lights up, 1Gbps doesn't. I'm not new to crimping cables either, so I'm 99% sure I succeeded in crimping it well. I triple checked my pinout and I just can't get it to work. The obvious problem could be that I crimped the cable wrong, but now I'm on the 3rd one and I get exactly same frustrating results.
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