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What is exactly the reason Oculus Link requires at least USB 3.0?

KarstenS80
Adventurer
As the following measurement of Advanced USB Port Monitor shows, Oculus Link is using a maximum bandwidth of 110 - 120 MBits/s from the PC to the Oculus Quest and around 8.5 MBit/s from the Quest to the PC.

5438qre7vqux.jpg

USB 2.0 is supporting a bandwidth of upto 480 MBit/s. So thats more than enough, even for keeping the latency low.

Advantages would be:
  • less trouble with connection issues
  • possible longer cables
  • possible thinner cables
So why do we need an USB 3.0 connection?
17 REPLIES 17

nalex66
MVP
MVP


@KarstenS80
I know the difference, but you clearly don't, buddy. Mb means Megabits. MB means Megabytes. Your data show Megabits/sec, which is the same as Mb/sec. USB 2 has a 480 Mb/sec, which is 60MB/secs, which is not enough for a VR feed.


According to comments from Carmack, the Quest only handles 150 Mbps, not 150 MB/s. Here’s an article where he mentions it. From the article:

The video encoder Oculus is using is also limited to 150Mbps, which is easily handled by the USB 3.0 standard (and technically should be fine with USB 2.0 ports too). Carmack says the company would still like to optimize Link to work well over WiFi eventually.
...
Because Oculus controls both the hardware and desktop side of things, it's also able to implement remote rendering more efficiently than earlier attempts. Carmack says its implementation peels away a few layers of complexity, allowing the remote renderer to talk directly the Quest's Android OpenMAX driver. The big takeaway? There's far less latency than you'd expect while playing a VR video feed on the Quest.

They do a lot of tricks with the encoding and compression to squeeze a video signal through such a narrow bandwidth; they slice up the image to start sending it before it’s fully rendered, and they compress the periphery of the image more than the centre to reduce the effective image size. Here’s an article detailing how they achieved this.

DK2, CV1, Go, Quest, Quest 2, Quest 3.


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FelicityC
Adventurer
to chime in uselessly-

just because it can theoretically support a maximum of x Mb/s does not mean it practically can.

Just because 5ghz wifi can theoretically support 866Mbps does not mean it actually does.

Overhead exists and for any stable amount of data transfer that overhead must be clearly met, otherwise the torrent of 'why is my connection so bad/spotty!!!' would increase a thousandfold using ancient 2.0 standards that theoretically can be met.
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Anonymous
Not applicable
Correct - the longer the cable - the less bandwidth you also have as it takes time to travel even if its only an extra foot. Also, keep in mind that data is cut in half  for the duplex feature. USB 4 will be correcting this a bit by adding more lines thus the up/down can be a bit more mix, but it still will cut it by a percent of channels used.

Orodreth16
Expert Protege


@KarstenS80
I know the difference, but you clearly don't, buddy. Mb means Megabits. MB means Megabytes. Your data show Megabits/sec, which is the same as Mb/sec. USB 2 has a 480 Mb/sec, which is 60MB/secs, which is not enough for a VR feed.

I don't see why you're offended, I just said your data was wrong and explained why USB 2.0 was not enough. There are other ways to relieve stress than insulting someone you don't know, especially when it's someone answering your question.

Also, what nalex said.



While I do think that @KarstenS80 's reaction was somewhat exaggerated, his screenshot clearly states Mbit (or MBit, but come on, if you write it out, there really is no difference if it's a capital or lower case b) and he obviously knows what Bits and Bytes are. Also, his initial question, based on his own measurements shown in the screenshot, seems reasonable, since it does indeed show a significantly lower bandwidth usage than what USB2 is capable of.
However, I think this question has been answered comprehensibly by @nalex66 , so thanks for the explanation!

FelicityC
Adventurer
I like how in french, bytes are octets, and so there is no confusion on this matter.
There are 8 bits in an octet.
Makes perfect sense to any native latin-derived speaker.
btw there is a big difference between the capital and lowercase, a whole factor of a difference, it's how ISPs scam people with fast looking numbers. chiming in uselessly <---
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ClassicGOD
Protege


[...]


The point of using USB 3 cable is the transfer speed and better duplex capabilities (since Quest has to transfer positioning data as often as possible). It's not about average bandwidth used over a second but about how fast each of the frame packets can be transferred. It's not a sustained transfer of 120mbps but a series of bursts that average 120mbps over a second. Below are some approximate calculations I made some time ago when Carmac said they use up to 150mbits stream (and it's not Quest limit - it was chosen for best GPU encoder compatibility and can be increased in the future). Those calculations use theoretical max speeds in ideal world and don't take protocols overhead into the equation:

With 150mbps/72fps stream it takes just 0.4ms to transfer a single frame over USB3 (5gbps). It will take 2ms over 1gbps network cable, 2.5ms over 866mbps WiFi, >4ms over USB2.
At 72hz that 4ms over USB2 is 1/3 of the entire frametime and for VR every ms counts.
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KarstenS80
Adventurer
And not much later, it worked with USB 2.0

So how about all the "not possible" people?

But I already sold my Quest half year ago, as I switched to Index.

And with the changes to end support for Oculus Account, I'm happy with my decicion.