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Comment by arghwhat | Hacker News Reader

Well, this is about USB 3.2 Gen 2x2, which is a mess created by USB IF for good old, blue USB A connectors. Not USB-C complexity.

USB 3.2 Gen 2x2 is the very rarely supported 20Gb/s variant of USB 3, and making devices now that require that for full performance is a weird decision, with high-speed capable ports generally having wider support for either USB4 or Thunderbolt3+. I imagine the reason would be that some chip with an otherwise poor market fit got cheap...

Throwing this into the mix definitely doesn't improve the USB-C "what does this port support" conundrum, but this specific one predates USB-C and is not at all something you'd normally hit.

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> Not USB-C complexity

3.2 Gen 2x2 (and the occasionally relevant 1x2 if you have a weak cable) are USB C only.

USB C ports and cables have 4 USB 3 "superspeed" lanes rather than two. When you use an A to C cable only one pair of those connects. The point of the "x2" modes is that they use the second pair of lanes that would otherwise go unused.

Except of course they don't always go unused. DisplayPort Alternate Mode sends DisplayPort over those two "unused" lanes getting you USB 3 data alongside a half speed DisplayPort connection. (or alternatively full speed DisplayPort on all four and only USB 2), and then of course Thunderbolt 3 and modern USB4/TBT4 use all four lanes and tunnel everything.

> The point of the "x2" modes is that they use the second pair of lanes that would otherwise go unused.

Thank you for answering a question I didn't know I had

Hmm, yes I was conflating a few things there.

However, I'd hold that this is an extension of the USB 3 mess that predates USB-C, even if the x2 mode specifically was a USB-C addition.

The important thing for people to know is that support for USB 3.2 Gen 2x2 is practically non-existent: TB3 came before as a well-established (but premium) solution, and USB4 just two years after to commoditize it. A complicated solution with mid-tier bandwidth and none of the flexibility didn't attract attention, so support is poor.

I mostly agree with you -- 2x2 is a relative rarity, especially before Intel's Barlow Ridge chipsets that actually added 2x2 support.

Which leads to the funny point that, as far as I am aware, the very first 2x2 capable hub chipset was USB4v2/TBT5 that happened to also support 2x2.

10 Gb/s Ethernet interfaces do not require 20 Gb/s USB ports for reaching maximum performance, they already reach that on 10 Gb/s USB ports, despite of what the writer of TFA believes.

The main application of 20 Gb/s USB ports is to connect external NVMe SSDs, when faster USB 4 or Thunderbolt ports and SSDs are not available.

For an external NVMe SSD on USB, a 20 Gb/s USB port will double the throughput, unlike for a 10 Gb/s Ethernet interface where any improvements are completely negligible.

I do not think that 20 Gb/s USB Type C ports are "very rarely supported". Every mini-PC or desktop motherboard that I have bought during the last 10 years had at least one such USB port.

Such ports appear to be rare only on laptops, because most laptops have very few USB ports.

What about overhead? Can you truly do 10Gb/s networking on a 10Gb/s USB port? Would having such NIC on a 20Gb/s USB port not result in higher speeds?

It uses 128b/132b encoding so 10Gb/s USB ≈ 9.69Gb/s you do then have USB framing overhead but it's probably around 2% on typical 1500B ethernet frames. So all in you are losing probably 5% or so to overhead.

I am of the opinion that 5Gbe is a much more sensible speed for a laptop adapter right now as it uses half the power and can obviously run full wack on 10Gb/s USB so you're looking at like 5Gbe vs ~9.4Gbe.

Stop insisting on Cat.6A (and related) copper cables for speeds beyond 1000BASE-T (maybe beyond 2.5G by now), just use dumb multi mode fiber it's way easier technology-wise and if you want power you can have that as well.

At distances where Cat.6A is even an option the demands on the fiber are very low. And it uses less power than the BASE-T PHY. The cable at least without integrated power is very thin as well, unless you can't respect it enough to not kink it, in which case you'd want a thicker one just to prevent you from being able to break the fiber.

In fact, just to for single mode fiber. Looking on fs.com, single mode cables are slightly cheaper, and the optics (for 10G) are $30 to MMF's $25.

And you get much better future proofing with SMF. And if you do need a long fast run, SMF is what you want.

I kind of a agree, but it's not going to happen for a long long time. The practicalities are just a nightmare.

How do I power an access point with fiber? Ok we add an AC wall socket to the ceiling but now we need a 'brick' to convert to DC. How do I remotely hard reboot an access point if it were to crash?

Fiber termination requires a fusion splicer and a trained engineer, sharps box etc. The power socket needs an electrician. It's just such a nightmare in comparison, install is going to be more expensive, longer to fix faults, less flexible to move a socket etc

5GBASE-T interfaces often use 3x less power than 10GBASE-T

Yes very true, 2X is with the most modern 10G chipsets only.

Both 10 Gb/s Ethernet and 10 Gb/s USB have bit data rates that are 3% lower than 10 Gb/s, due to encoding (64/66 bits for Ethernet, 128/132 bits for USB).

So the their maximum speed is approximately 9.7 Gb/s.

Then for Ethernet there is a protocol-dependent overhead, e.c. depending on whether TCP or UDP is used, and depending on whether standard packets or jumbo packets are used.

The TCP overhead can reach in the worst case up to close to another 3%, reducing the achievable TCP throughput to around 9.4 Gb/s.

The USB frames add some extra overhead, but it is normally not important in comparison with other factors that can reduce the throughput.

All that a 20 Gb/s USB port can do is to reduce the overhead of the USB frames, but that is a negligible improvement. Using jumbo Ethernet frames (which are 6 times bigger than standard frames), if both ends support them, is likely more useful for increasing the throughput, than using a 20 Gb/s USB port.

10 Gig ethernet is 10GBps usable rate (before packet overhead). The line rates are higher to accommodate this. For 10GBase-R, it's typically 10.3125 GBps, with a 64/66 encoding. For 10GBase-T, it's 4 lanes with PAM-16 at 800 MBaud -> 12.8 Gbps raw.

>Every mini-PC or desktop motherboard that I have bought during the last 10 years had at least one such USB port.

Are you talking about USB 3.2 Gen 2x2 though? Because I've never seen any MiniPC with this port and as for motherboards, I checked my local retailer and only ~15% of currently sold ones have Gen 2x2 (mostly high-end ones).

Most of my mini-PCs have been Intel NUCs (or more recently an ASUS NUC). I also had some Gigabyte and Zotac mini-PCs and a few others from less well-known vendors. IIRC almost all had one such 20 Gb/s USB Type C port, unless they had one or two faster Thunderbolt ports.

With mini-PCs, I frequently use external SSDs, so I certainly used those ports at their full speed.

The only mini-PCs that I had in recent years without such a fast USB port were Arm-CPU based, as those are typically starved in fast peripheral interfaces in comparison with the Intel/AMD CPUs.

> 10 Gb/s Ethernet interfaces do not require 20 Gb/s USB ports for reaching maximum performance, they already reach that on 10 Gb/s USB ports, despite of what the writer of TFA believes.

While this may be theoretically (almost) possible, I’m quite sure this is absolutely not the case in practice.

For example see these benchmarks of one of the more recent USB to Ethernet chipsets [1], that can reach ~9.5 Gb/s on USB 3.2 Gen 2x2 but only between ~6.2 to ~7.3 on 3.2 Gen 2x1 laptops.

1. https://www.jeffgeerling.com/blog/2026/new-10-gbe-usb-adapte...

Edit: Haha, didn’t realise TFA was by the same author as these benchmarks but he’s done a lot of testing and benchmarking of these kind of devices over a long time, and it agrees with all the other benchmarking from other people I’ve seen too!

Ethernet is duplex though. 20Gb/s is the max throughput a 10Gb NIC can achieve.

So is usb superspeed. The tx and rx don't flip around like low/full/high speed

If you read carefully (emphasis mine):

> The main problem is USB-C's bandwidth complexity - especially when paired with the Realtek RTL8159 Ethernet controller, which requires USB 3.2 Gen 2x2 (20 Gbps) to get the full rated 10 Gbps speeds

Jeff's statement wasn't that 10 Gb/s Ethernet requires 2x2. It's that that requirement comes from a very specific controller.

In Ethernet, "10 Gbps" refers to the actual Ethernet frame throughput. The raw physical coding rate is usually somewhere around 10.3125 Gbps to account for this.

In USB 3.2 Gen 2x1, the actual USB packet throughput is 9.697 Gbps and the "10 Gbps" refers to the raw encoding rate.

This difference means you are guaranteed to lose at least a few hundred Mbps off maximum performance. It's not really a practical concern, but it's not an error to say 10 Gb/s USB ports lack the bandwidth needed to support the maximum performance of a 10 Gbps USB Ethernet adapter.

> 10 Gb/s Ethernet interfaces do not require 20 Gb/s USB ports for reaching maximum performance, they already reach that on 10 Gb/s USB ports, despite of what the writer of TFA believes.

The first half is true, the second half is not. Remember overhead. You don't need 20GB/sec, but you need to take into account the USB overhead.

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