Weird ethernet cable issue (probably)

Greg66

Active Member
I hope this is the right forum for this Q.

Is it possible for an ethernet cable to "die" (fail) and if so how does one test for this?

Background:

1. My set up is router -> 24 port switch -> wired connections around the house plus wireless connections off various APs.

2. One of the wired connections off the switch runs to an outbuilding. Max 50m cable run (I ran the original cable run from a 50m roll and had spare).

3. The first half or so of this cable run, which is newer, is exterior use grade cat 5 cable. That's been there for maybe six years. The second half is internal use grade cable, run inside trunking. That's been there for 8 years.

4. In the outbuilding is a wifi AP, with wired connections to back up NAS drives, Sonos, Apple TV etc. The backup NAS drives make this an important connection.

5. Recently the outbuilding has been suffering from slightly slower connections and increasing Sonos drop outs. There's been no physical activity around any part of the cable during this time. Now though - as of yesterday - the outbuilding is to all intents and purposes dead. By which I mean (a) the AP shows up as an attached device on the router's attached devices page, with its designated IP address, and broadcasts a wi-fi signal. But it carries no data - you can't access the web via its wifi signal, and if you ping it from the house at least one, usually 2 and sometimes three of the four packets of data sent are lost. (b) I've tried two different APs to rule out AP failure. No difference. (c) I've tried a different port on the switch to rule out a switch issue. No difference. (d) I powered down everything in the house and did a complete cold restart. First up was the router, then the switch, then the outbuilding AP, and nothing else. Even then the AP was doing nothing, so it's not a traffic issue.

7. The only thing left I can see as the source of the issue is the cable. Next step will be to open up the junction box. If that is dry, that suggests either a break in the cable somewhere, or perhaps a failed plug / socket somewhere in the outbuilding.

Anyone have any ideas on what might be a cause here? I am running out of options.

TIA.
 

Bryn1

Active Member
How are the two differing cables connected? sounds like potentially moisture has got into the junction box/joint
 

lencuff

Active Member
First thing I would try is to get or borrow another 'long' cable and replace the original. That way if all works again then you know the problem lies in the original cable. That's the way I sorted a problem with some cameras being a bit intermittent.
 

MaryWhitehouse

Well-known Member
If the switch is managed it can probably tell you if there’s a cable fault and possibly where.
 

mickevh

Distinguished Member

Just to sound a note of caution about the linked ad, I note the description cites "Tests Cat 6, Cat 5e, Cat 3 & Flat." Beware that cheap 10ish GBP continuity testers do not test the "cat" of anything - they do not run the battery of frequency sweeps, crosstalk assessment, delay skew, etc. etc. needed to prove a cable lobe meets a "cat" standard. (Devices that do that cost hundreds/thousands.) Most don't even do basic wire mapping to check for split/crossed pairs.

The cheap testers are just doing a basic electrical continuity test, you could do the same with a light bulb and a battery. Better than nothing, but not much.

Devices are not in any meaningful way "attached" to routers. Routers operate "stateless" and there's no dialogue between a router and any end points. These "maps of attached devices" that SOHO routers present are just for human amusement and almost for the sake of pretty pictures. Thusly, in this case, it may be that the router "saw" the AP once and "knows" about it and which port it can be reached on, but the router is not constantly monitoring that device is still there. Thus, AP (or anything else) could be in the map, but not actually active until whatever logic in the router decides it "hasn't seen it for a while" and thence removing it. Possible router doesn't even do that and it'll show it forever.

Point being, don't rely on your router to tell you whether anything is "on the network" or not - "ping" the target instead.

Similarly, an AP will (usually) continue to advertise it's SSID irrespective of whether it has a backhaul to the rest of the network or not. Thusly, that we can see an SSID being advertised on the Wi-Fi airwaves tells us nothing about whether the backhaul link is working or not.
 

Greg66

Active Member
How are the two differing cables connected? sounds like potentially moisture has got into the junction box/joint

First reply - correct answer!

A continuity tester would be where I would start

This device has been a Godsend.

I used the tester to test the full cable run. A half/dim light on ch 1 at the switch end and no light on ch 1 at the receiving end.

The guys that fitted the exterior cable seemed to know what they were doing (eg using exterior cable in the first place, and TBF to them every other connection has been fine) so I assumed the junction box they’d used was weatherproof. As it happens it wasn’t and rainwater had been getting in.

From there it was a fairly straightforward job, save that I was working 12’ up a ladder desperately trying not to drop anything into the 12’ high bush underneath me never to see it again; I had to redo the conduit a bit to make space for a proper IP66 junction box; I had to not only make plugs for the two ends of Ethernet cable in situ to join them via a couple but also break into and rejoin two sat cables and a coax cable so that everything sat inside the junction box with no chance of water ingress.

And now it works again! Using the continuity tester on my new plugs and seeing a full 8 lights was a bit of a punch the air moment, as was getting net access back up and running in the outbuilding.

Thanks to all for the tips and pointers.

PS the switch is unmanaged so no help from that.
 

spile

Active Member
Just to sound a note of caution about the linked ad, I note the description cites "Tests Cat 6, Cat 5e, Cat 3 & Flat." Beware that cheap 10ish GBP continuity testers do not test the "cat" of anything - they do not run the battery of frequency sweeps, crosstalk assessment, delay skew, etc. etc. needed to prove a cable lobe meets a "cat" standard. (Devices that do that cost hundreds/thousands.) Most don't even do basic wire mapping to check for split/crossed pairs.

The cheap testers are just doing a basic electrical continuity test, you could do the same with a light bulb and a battery. Better than nothing, but not much.

The first caveat I think is fair.
I have issues with the second paragraph. The mechanics of these simple testers make it far, far easier than using a continuity tester for the home user. Overstated claims aside, I stand by my comment that these simple (and cheap) devices do what they do and do it well.
 

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