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Environment: Pi4 2gig, Buster, updated

The title pretty much says it all. I'm trying to understand how a device (WiFi adapter) that's rated for a speed of 867 Mbps ends up landing on Bus 01, which is limited to, at best, 480 Mbps. Yes, it's a USB 3 device, with a blue USB plug, and yes, it's plugged into one of the (tried both) blue 3.0 slots. I've plugged in another USB 3 device (storage) and it registered on Bus 02 and is much faster (~365Mbps), so the faster bus does work. Real-world throughput of the WiFi adapter is actually far lower than even the theoretical limit of Bus 01, and I'm wondering if this might have something to do with it.

$ iwlist wlan1 bitrate

gives:

wlan1     4 available bit-rates :
          1 Mb/s
          2 Mb/s
          5.5 Mb/s
          11 Mb/s
          Current Bit Rate:144.4 Mb/s

So unless I'm misreading that, the fastest speed I'm going to see is 144 Mbps. It seems like somewhere along the way, either the device is announcing an incorrect speed or it's not being properly negotiated. How might this happen?

This is a simpler question related to a previously posted question for which I'm still hoping for an answer:

[USB 3 device appears to be listed on the wrong bus

FWIW, I've done a lot of testing, including iperf3, to ensure to the best of my ability that other factors aren't interfering. The Pi is running as an AP (systemd-networkd), nothing else is connected except the test client, nothing outside of the basic processes are running, and CPU usage is low when running iperf3 and very low when idle. But at the end of the day it's connecting to Bus 01.

  • @Milliways, Can you briefly explain this It seems pretty pointless using USB3 for a WiFi dongle! please ? Because I can read for a Wi-Fi dongle in its description of the product sheet : With high-speed USB 3.0, [MODEL] breaks through the low-speed USB 2.0 limitation for 802.11ac high-speed experience. Thank you. (I'm confuse because, yes,I think the datas are not sent to the Wi-Fi client with the USB bus but with the Wi-Fi controller. Is it only an argument to sell?) – Ephemeral May 26 at 20:49
  • @Milliways, I totally understand how USB 3 works, and yes, this has 4 outer pins and 5 inner pins. I guess there's a fine line between typing out way too many meaningless details and/or looking like a possible idiot, but I assure you I'm not the latter. ;-) And as Ephemeral says, there's nothing pointless about USB3 for a WiFi dongle; in fact, the big sell of this particular adapter is that it supports 867Mbps. I personally have gigabit available, and it's getting more and more common, especially with so many people working from home. But my question was simply: how is this negotiated? – peter.rando May 27 at 0:20

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