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Chasing a Wifi Ghost

For quite a while, I have not been so confused (and excited) by a technical issue.

The problem is: I got very slow download speed (~10Mbps) on my Windows laptop, when connected to Wifi.

To make it a bit more puzzling:

- The upload speed is much better (~60Mbps).

- Sometimes the download speed may reach ~100Mbps for a few seconds, before quickly fall back to ~10Mbps.

Having been suffering from it for a few months, I finally decide to take on this issue today.


Beginning of the Journey

As usual, there are some troubleshooting routine I should do. Also as usual, they don't help much.

- Try other devices with the same Wifi
- Try to connect to other Wifi
- Try to connect to Ethernet
- Reset the Wifi router
- Run Windows troubleshooter

Well, eventually I did get some information. The download & upload speed is actually normal for all my other devices, which includes Android/iOS phones, MacBook Pro etc. 

This pieced of info narrowd the root cause down to the laptop itself: The router must be working well, and I'm not throttle by my ISP.


The Chase1: Killer

My machine has a Killer wifi card, which had some issues before. At the beginning, I try to examine all Killer componenets thoroughly:

- Try the latest Wifi driver
- Try some older Wifi drivers
- Install / Uninstall Killer software & services

Not surprisingly, nothing worked.

The Chase2: Windows

Next, I tried to exhaust my knowledge about Windows networking,  in order to examine all relevant aspects:
- run sfc /scannow
- Reset TCP settings
- Reset network stack (run ipconfig, netsh etc.)
- Disable SmartByte service (which is actually not available on my machine)
- Disable WMM/QoS (which is actually not available for my router)
- Disable power saving move for the adapter
- Altering Wireless mode (802.11) of the adapter
- Disable network discover
- Disable firewall
- Remove custom DNS servers
- Disable features (e.g. ipv6) of the adapter
- Disable other network adapters
- Disable bluetooth
- Disable Windows TCP autotuning
- Dissable WAN miniport

Again. Nothing worked.

The Ghost

After reading the Killer troubleshooting pages for a few times (one, two), I finally noticed an instruction, to set the "channel width" to 20MHz. This is relevant since I'm using a 5GHz Wifi.

Having absolutely no expection, I changed the settings in the router, and that's how I captured the ghost.
Now I have ~100Mbps download speed, finally. Still not as fast as the Ehternet, but good enough.

Summary

Previously the router had been configured to use choose from 20/40/80MHz automatically. The router has been using 80MHz according to my Wifi scanner. If I understand correctly, changing to 20MHz helps because there are many other Wifi signals "nearby": both "physically close" and "similar channel frequencies). This probably can also explain why the download speed was unstable.

It is yet unsolved why my Killer wifi card is not able to handle the broader width, while others can.

Anyway, I'm happy to meet this new cute ghost, and keep it with its friends together.
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