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Desktop PC Ethernet halves 5GHz WiFi throughput...

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JemTheWire

Senior Member
Not sure where is the correct place to post this so forgive me.

I have a weird issue with my 5GHz WiFi.

My desktop PC is connected to my network by Ethernet and I also have a 5GHz WiFi network which I use for my iPhone and iPad and a laptop.

When my PC is switched on, my 5GHz throughput as tested by Speedtest.net more than halves from 210MB/s to 50/60MB/s.

If I turn my PC off WiFi speed goes back up. However, if I unplug the Ethernet cable from the PC the WiFi speed stays at 210MB/s regardless whether the PC is powered on.

I initially though my PC’s PSU was failing causing some RF interference but I have changed it out with a brand new one.

Running a AX88U (Router mode - Merlin 384.10 alpha 2) and AC88U (Access point - Merlin 384.10 alpha 2)

I am only running 384.10 alpha’s trying to see if the firmware is the issue. It was the same as 384.9 and 384.8.2.
 
Do you get the same speed drop when connected to the access point as well as the main router?

Using a WiFi analyser program like Acrylic on your laptop, can you see a significant change in the 5GHz RSSI when you turn off your PC?

If it is some sort of induced RF travelling down the PC's Ethernet cable it might be from the CPU or GPU. GPU's are particularly noisy in my experience.
 
If I disable all wireless (2.4 & 5GHz) on the ROUTER and test again, result is still the same.

I have already tried removing the GPU Asus GTX970 and just used the mobo built in graphics. Again, still slow.
 
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What about the effect on the RSSI level?

Is the maximum download speed from your ISP 210Mbps (that's bits not bytes)?

When your PC is on and connected are you sure that it is not uploading or downloading stuff from the internet, like Windows updates or torrents?

Does your PC also have a WiFi connection or is it Ethernet only? If it has WiFi make sure it is disabled.


For further experimentation:

Try different Ethernet cables. Some (even cheap ones) might behave better than others. I'm assuming it's a direct connection from PC to router.

How close to the router is the PC? Try physically moving the PC as far away from the router as possible.

Try putting the router's power adaptor into a completely different mains socket, preferably on the opposite side of the room (via an extension cable).
 
If I turn my PC off WiFi speed goes back up. However, if I unplug the Ethernet cable from the PC the WiFi speed stays at 210MB/s regardless whether the PC is powered on.
This is the key observation.

I have to assume you are measuring WiFi from devices other than PC. I am thinking that your PC may be acting as a DHCP server. One way to prove is configure another device with a static IP address and a router IP address effectively forcing it to use the router only. Now try the speed test.

Has this issue always existed or just after some software changes to PC?
 
Sorry for delay. Have family emergency, dad fell.

My Router is set to be the DHCP server, how can I check if the PC has magically become one?

Sorry for being dumb, heads all over the place.
 
My Router is set to be the DHCP server, how can I check if the PC has magically become one?
Depends on what operating system is on PC. Before we go into that, configure one of your devices with a static IP address and have it point directly to router address. Choose a device that can measure connection speed change.
 
Windows 10 Pro.

All my devices have static address given out from the router. Free DCHP range from 168.1.0.201>254 available for ‘visitors’
 
Windows 10 Pro.

All my devices have static address given out from the router. Free DCHP range from 168.1.0.201>254 available for ‘visitors’
Those are assigned addresses delivered by DHCP. Not the same thing. A second DHCP server on network would still cause problem for those devices with assigned addresses.
 
Those are assigned addresses delivered by DHCP. Not the same thing. A second DHCP server on network would still cause problem for those devices with assigned addresses.
I can't see why you would think having a rouge DHCP server on the PC would create the symptoms he's describing (of course that's not to say it wouldn't create other problems). Unless you're assuming that as a consequence the PC is advertising itself as the internet gateway?
 
Unless you're assuming that as a consequence the PC is advertising itself as the internet gateway?
@ColinTaylor Exactly.

A PC with two or more connections is fine assuming the PC OS assigns different costs to each. Multiple connections with same cost may cause issues. However, multiple DHCP servers could cause similar symptoms. Also, if the laptop and PC have a routing protocol enabled and the second connection on the PC is enabled problems may occur though I am not sure of symptoms.

I am keying off the reported symptom occurring only when PC has two connections enabled.
 
I am keying off the reported symptom occurring only when PC has two connections enabled.
I don't think it was ever said that the PC has two network connections. I asked that question in post #4.

@JemTheWire Can you confirm whether or not your PC has just the one network interface (Ethernet) or multiple interfaces (Ethernet+Ethernet or Ethernet+WiFi)?
 
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It has one active Ethernet connection. Connected CAT5e directly to the router sat on a shelf 3 feet above the PC.

The Ethernet port is built onto the mobo.

I remembered that I have an old Intel Gigabit NIC in the cupboard,I will fit that tonight and disable the built in NIC.

I am not at home at the moment.

I appreciate all your advice. Thanks for taking the time to reply.
 
I don't think it was every said that the PC has two network connections. I asked that question in post #4.
@ColinTaylor You are correct, I misinterpreted. PC doesn’t have WiFi, only a single Ethernet. I led us down a rabbit hole. Sorry.

However, I go back to this:
“If I turn my PC off WiFi speed goes back up. However, if I unplug the Ethernet cable from the PC the WiFi speed stays at 210MB/s regardless whether the PC is powered on.”​
@ColinTaylor You are correct, it appears that the active Ethernet connection is where our focus should be.
 
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@JemTheWire Now that @ColinTaylor has set me straight we need to determine if this is a physical or logical issue.

@ColinTaylor has previously suggested a cable swap out. Please do.

@JemTheWire Has this always been an issue? If not, what changed?

I agree that if you have a spare NIC, you should try it.
 
To recap.

PC connected to router directly. I have swapped out the cable. Same result.

WiFi speed is measured from my iPad Pro and iPhone and Dell Laptop.

Speed test from PC (Ethernet) is always around 210.

Speed test from WiFi is same at 210 without PC on but when switched on, WiFi speeds drop to anything from 30-70.

I realise that some people would be happy with that, I still have connectivity after all, but I just want to know why the WiFi speed drops and is it an induction of an underlying issue that will get worse.

Difficult for me to test further at the moment as I am in hospital Accident and Emergency with my 88yo dad. Just waiting for Dr so had time to check messages.
 
Please take care of your dad, he is your top priority.

I am going to assume that your PC is running Windows 10. When you have an opportunity please try the following but realize that if you’re running another version of Windows the steps may vary.
  • Right click the Start icon
  • Select Device Manager
  • Select Network adapters
  • Right click on your Ethernet controller
  • Right click on Properties
  • Click on Advanced
  • Under Property click on Speed & Duplex
  • Under Value you will probably find Auto Negotiation, replace it with 1 GBPs Full Duplex (or highest speed it will let you)
  • Click OK
  • Close Device Manager
See if this resolves issue. If not try using your spare NIC.

@ColinTaylor I have a 200 Mbps to my ISP too, though it tests slightly higher on my WiFi and Ethernet connected devices. I believe that for @JemTheWire the Ethernet connection may have a negotiation issue that is making the router busy but with an ISP speed that is ⅕ of the Ethernet a speed test on PC may not report a slowdown. What bothers me is that we don’t know the history of this issue.
 
Before I read ApexRon's post about forcing AutoNeg off, I had disabled my mobo onboard NIC, and installed the spare Intel Gigabit NIC. Low and behold, the wifi speed drop issue has gone away!

I double checked and re-enabled the onboard NIC, connected to it, and ran speed test again, and wifi throughput was back down to 80m or so.

Again, disabled onboard NIC, connected my Ethernet lead to the Intel Card NIC, wifi speed test was same as Ethernet at 210M.

I ran all speed tests from my iPhone 7 Plus and without changing its postiion/orientation etc.
 
Well that's progress I guess. At least it's not a problem with the PC as a whole.

I'm not sure I buy ApexRon's auto-negotiation idea. If that were the case I would expect the effect to be very obvious on the overall responsiveness of the PC. But then again, anything's possible. You could probably tell by looking at the network adaptor's status, or in the syslog on the router.

Maybe the on-board network socket is physically damaged. Look inside it to see if any of the contacts are bent or dirty. Maybe there are dry solder joints between the RJ45 socket and the motherboard.

Or you could just ignore it and use the other network card.:D
 
Low and behold, the wifi speed drop issue has gone away!
Hope your dad is okay.

Glad to hear that the cause of your issue has been identified.

@ColinTaylor As I pointed out, you and I don’t know the history. A NIC just doesn’t start having these symptoms. The NIC wasn’t dead per say.
  • Physical connection? Unlikely because cable connections had been jiggled when cable was swapped and new NIC is using same cable.
  • Electrical? Power surge, lightning?
  • Software or firmware update on PC or router?
  • How long had the issue been going on? Recent or since forever but just recently noticed? Was it a new PC?
What ever it was, it busied up the router to the point that it slowed down the router.
 

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