Unusual cable test results

[Steffan]

Hi there,

I hope someone can clarify some things with me, we have a number of base stations with netonix switches which are great but recently we went to site to try to work out some bandwidth issues and discovered that our mimosa B5 had abnormal termination on its cable tests, we changed cable and eventually changed the kit to a B5c after realising that the cable we discovered the original cable was testing fine.

We then tested some of the others and some test OK on all 4 pairs and some test with "abnormal termination" so we dragged the B5 back to our workshop and tested it using our test netonix and a pre-terminated cable and are still having the same results, there's no errors on the port and no errors on the B5 either, mimosa looked at it remotely and they dont think there's a problem with the unit.

I can understand having one faulty unit possibly and possibly one faulty cable or even a faulty switch or a faulty port on one switch, or even a compatibility issue that the netonix doesn't like with the B5 but some test fine.....

Firmware is on the latest on both mimosa and netonix btw..

Kind regards

Steff

[sirhc]

There is a search feature where you can find a lot of information.

Read this posts:
viewtopic.php?f=6&t=1178#p8809

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Question: Why do I see abnormal termination when I do a cable diagnostics?

Answer: Because every manufacturer does not follow the proper design specs so when the device is unpowered it is actually doing the cable test into the device electronics and every device will report differently. They "should" report OK, OK , OK , OK once the device is powered up however some Ethernet surge protectors will always report abnormal termination.

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Question: Why do I see short on pairs 3 and 4 on a of devices especially UBNT airMAX 10/100 gear.

Answer: Because every manufacturer does not follow the proper design specs and use a 4 channel Ethernet Transformer designed for POE with a center tab for each pair. Since pair 3 and 4 is not needed to carry data with 10/100 only links they use them for power only which allows them to use a cheaper 2 channel transformer without a center tap for pairs 1 and 2. They then connect the 2 wires from each pair to the board skipping the need for a transformer with a center tap so the 2 wires from pair 3 are connected directly to the board shorted together for positive and the 2 wires from pair 4 are shorted together directly on the board for negative.

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[Julian]

Mimosa uses gas discharge tubes, or GDTs, as surge suppression on their ethernet ports.

https://www.mouser.com/pdfdocs/bourns_g ... _paper.pdf describes the operation of a GDT - Of note here is the graph labeled figure 1, detailing the 'glow region' of the device - Any change in voltage across the conductors (like the pulse sent down the line during cable test) will start to cause the gas inside to ionize, which can "confuse" cable diags - this issue gets worse due to component choices on Mimosa's end. Also an interesting architectural note with the Mimosa radios is that since there is either a cable or a short PCB between the glanded RJ45 jack and the "worky bits", even atmospheric humidity and ambient temperature can have an effect.

So, in short, there's probably not anything wrong with the switch, or the radio, they just do that sometimes. I wish I had the time to give you a better picture of the nuts and bolts of it.

Cable diags are meant (primarily) to catch cross-pair shorts, they are not meant to replace a TDR or other cable tester.

[sirhc]

Once you power it up you should see a differnt test result of ( OK, OK, OK, OK ) across all 4 pair.

It is only when the unit is unpowered you get abnormal termination.

To achieve a 1G connection all 4 pair MUST be working properly.