Question Ethernet disconnects when power devices start

Apr 15, 2023
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My configuration is "Cisco EPC 3925 coax cable modem" + "35 m UTP CAT5e cable (plastic connectors)" + "Marvell Yukon 88E8071 ethernet card" and the connection drops when a tank-less gas water heater or an electric oven starts, but gets reconnected 1-2 s afterwards.

No connection drops on WiFi.

The heater, oven, and modem are all in separate rooms and quite apart from each other. The flat contains mainly 2 prong outlets. UTP cable runs close to the electric meter and parallel to the flat's power lines along 2 m and at a distance of 10 cm.

I checked, if it makes any difference by distancing the UTP cable more from the power lines, but without luck.
I tried disabling energy efficient ethernet (and or green energy) and wake on magic packet, but that doesn't help.
I tried replacing my ethernet card with "Lenovo P24h-2L Realtek RTL8153" and "ThinkPad universal USB-C dock 40AY0090EU", but that doesn't help.
I tried to run my laptop on battery only, but that doesn't help.
I check connection drops by following command "ping -t 8.8.8.8".

There is no connection drops with following configuration "Cisco EPC 3925 coax cable modem" + "35 m UTP CAT5e cable (plastic connectors)" + "TP-Link WR740N v2.5 router" + "5 m FTP CAT6 cable (metal connectors)" + "Marvell Yukon 88E8071 ethernet card".

What can I do to prevent connection drops without additional TP-Link router, and what magic does the TP-Link router to keep the connection alive?
 
That is interesting part number for them to reuse. A 3925 was one of cisco very popular commercial routers.

What happens if you use the tplink router as a switch. Turn off the DHCP server and then use 2 lan ports to connect the 2 ethernet cables. Not the optimum configuration but it will not affect your performance using it as a switch.

This is very bizarre. When I first read it I though both the heater and the over are high electric loads until I read again and it says your "heater" is a gas water heater. Depending on the model it might not even have any electrical connections, depends if it uses a pilot or uses electric to create a tiny spark but uses no power after that.

It almost has to be the modem you would think. You run the laptop on battery which means you are completely isolated from any power. You have tried other ethernet which means it is not likely a defective nic.

Although still unlikely it might be your ethernet cables. There are massive amounts of fake ethernet cable on the market mostly because of the price of copper metal. The most common one you see are those flat cable. The wires are much too thin to meet the standards for ethernet cable and have more issues than a official cable.

What you want is your cable to have pure copper wire (no CCA) with wire size 22-24. If you can't find that information about your cables in the sellers advertising it is highly likely they are fake cable. Reputable vendor clearly point out these specs because they know about the fake cables and want to show why their cable likely cost slightly more.
The cable you have that you say has metal connectors is likely a shielded cable but these do nothing for you in a home install and they can in theory at least increase interference when improperly grounded. You must have a special ground wire hooked to the shield but this does not exist in consumer equipment. These are used mostly in industrial or data center installs. If meets the other specs for wire size etc then you just payed more than you had to.
 
Apr 15, 2023
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Hi there and thanks for your interest.

Using the tplink router as a switch also doesn't drop my connection.

Regarding the water heater, it's "De Dietrich MCR Home 20/24 MI" and it has an electrical connection. Its rated electrical power is 84 W. It's also used for central heating, so it heats water in an automatic mode, but in that mode not every switch on causes a connection drop, so it's something that makes me wonder.

Rated power of the oven is 2 kW. On the other hand a microwave oven doesn't cause a connection drop although its rated power is at 1,1 kW.

Regarding the Ethernet cable, it's stranded Alantec 4x2x26AWG bought in rather reputable store and by an electrician. Its specification sheet says 100 % copper. It's not 22-24 as you wrote, but it works without a connection drop with the tplink router and achieves maximum speeds promised by my ISP.

The installation of 35 m UTP cable is fresh, and the cable itself is bought as new.

Regarding the shielded cable, its shield probably does no good, but I use what I have at hand. If I would have a short one unshielded, I would check if it would have any influence as well. I was amazed that the tplink router fixed my problem, and I don't understand why.

I also thought about powering 3925 through a battery, but buying even the cheapest UPS is an expensive check.
 
You could try a surge suppressor maybe that would be enough since many also filter the power. Since the 3925 uses a power block type of power supply maybe a different one might help. 1.2kw can easily cause a voltage drop which might cause issues. 84watts is too small to cause a voltage drop so it must be some kind of interfering signal.

BUT

I don't think it is a direct power issue because the modem would still have this power issue even with the tplink plugged in the path. So if for example it quickly reboots it would do that no matter what. It is more likely there is some kind of interference between the ethernet and the power cables. This is a extremely rare thing but maybe the cisco is extremely sensitive and there is some strange issue when it talks to your machine rather than the tplink.

If you are going to spend any money I would just put it toward replacing the cisco since that is a very old and outdated model. The cheapest solution is to just keep using the tplink as a switch and ignore the problem for now.
 
Apr 15, 2023
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A surge suppressor in a power strip? I read that they've limited life due to a MVO component in them. I believe its life would be shortened relatively fast in my case.
Besides doesn't such a suppressor require three prongs? The plug of my power block is only two prong.

Alas the 3925 is the property of my ISP. A replacement on my request costs $50, and I would probably get 3928S or 3928AD, because that's what they have in their offer. I don't think those cable modems would do better than the current one.

I don't necessarily want to buy something. I just want to know what can I do, should my old tplink break or maybe I could suggest something educated in the installation of the UTP cable.

What should the minimum sane distance be between the Ethernet and the power cables?

Could acute bends in the cable cause connection drops?
Could it be that the tplink is somehow active and Ethernet cards are passive, so the signal gets strengthend in the first case?
 
Although people claim interference between a power cable and ethernet I am not so sure. The twists in the ethernet cable are what protect it from outside interference.

Where I used to work we would put over 20 servers in 1 rack. Each had a dual power supply as well as 2 ethernet ports and a third ethernet port for out of band management. So in a very small area we had 40 power cables and 60 ethernet cables all crammed into the cable management channels along the sides. The ethernet and power cables were touching and running in parallel at times. There were many racks like this in the data center and I never saw any issues....and since the ethernet were in commercial switches it kept track of every packet that might get a error. Those counters were almost always zero even over years of running.

If the tplink breaks buy a cheap switch. Since your modem/router is old and only has 100mbps port you can get a 5 port switch for under $15. A gigabit one only costs about $20.
 
Apr 15, 2023
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Ok, thanks for your support.

I'm still not sure, if it would work with whatever router I would use, or is it working only with this particular tplink.