Author Topic: Strange results testing link speed of cables  (Read 370 times)

inworksit

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Strange results testing link speed of cables
« on: October 01, 2018, 04:16:04 PM »
Hi,
we've been testing some cables between two of our buildings to see if there are broken wires etc.
During testing we've received improbable results. One of the cables is knowingly bad, PE says it's a 100 MBit FD/MDIX cable with no errors.
The cables should all be 1 GBit, PE reports them as 100 MBit FD/MDIX or 100 MBit FD/MDI, however.

We've then connected the loopback adapter directly to the PE via the included cable. Since PE reports this cable as also being 100 MBit, we assume a hardware issue with the PE device.

Regards, Jürgen

ShadowLord

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Re: Strange results testing link speed of cables
« Reply #1 on: October 03, 2018, 09:33:33 AM »
Hi,
we've been testing some cables between two of our buildings to see if there are broken wires etc.
During testing we've received improbable results. One of the cables is knowingly bad, PE says it's a 100 MBit FD/MDIX cable with no errors.
This can be a valid result, depending on how you test the cable and what the failure is. If you use the wiremap tool (with the terminator) it should show you which wires are damaged.
If you use link detection with the wire connected to a gigabit switch and only one or 2 of the wire pairs are damaged 100MBit FD/MDI(X) can be a valid result as due to auto negotiation the still working pairs can be selected for use.

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The cables should all be 1 GBit, PE reports them as 100 MBit FD/MDIX or 100 MBit FD/MDI, however.
If there are no issues with the wire and the switch they are hooked up to is is normal working condition, this may be a bug. I have once seen this behavior as well and I think in my case it was caused by the switch not liking the quick network speed switching the PE does. In my case testing the link a second time resolved the issue.

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We've then connected the loopback adapter directly to the PE via the included cable. Since PE reports this cable as also being 100 MBit, we assume a hardware issue with the PE device.
This is expected behavior as the gigabit negotiation does not work with a passive loopback connector, thus link testing will never ever show gigabit with the loopback. If you want to check the wire is suited for gigabit use, perform a BER test with the loopback connector instead (at gigabit speed).

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Regards, Jürgen