0
\$\begingroup\$

Right now I'm trying to establish a network of 30+ transceivers with NRF24L01 modules - one of them acting as central. However there are big problems, whenever two or more nodes try to send data to one node.

As far as I know, WiFi devices are able to receive multiple frames at once with no problems. How is this possible? Are they listening on multiple (like 100) channels at once? And when there are two frames received in the same moment on one channel are there errors just like with NRF24l01 network?

\$\endgroup\$
5
  • 2
    \$\begingroup\$ Maybe high-end routers can, but this chip isn't one of them, it sits on whichever channel you set. If multiple devices transmit on the same channel you will get corrupted data (if their relative strengths are close enough). See section 6.3 of the product spec. Unless you want an ALOHA type system (random Tx timing and backoff) you'll have to go with a polling system or some other TDMA type configuration. \$\endgroup\$
    – user201365
    Nov 14, 2018 at 17:21
  • \$\begingroup\$ @isdi Wow, thanks. Can you propose any pages to read about WiFi systems? Like, how exatly are they dealing with receiving multiple frames in one moment? I'm a real newbie to this kind of stuff, please help... \$\endgroup\$
    – Em Ka
    Nov 14, 2018 at 17:46
  • 1
    \$\begingroup\$ Nice introduction here if you google: Wi-Fi_Overview_of_the_802.11_Physical_Layer.pdf Gives a decent history and a reasonable explanation of the different protocols without getting bogged down in the details. \$\endgroup\$
    – user201365
    Nov 14, 2018 at 18:03
  • \$\begingroup\$ In reality most networking schemes, especially radio ones, cannot handle overlapping packets - this is called a "collision" and measures are taken to avoid it, detect it when it happens, and then re-try at a hopefully non-colliding time. \$\endgroup\$ Nov 14, 2018 at 18:10
  • \$\begingroup\$ I should qualify the simultaneous transmission statement- you can transmit at the same time if your signal streams use different RF polarization patterns and this works well for simple systems at fixed locations/orientations. The high-tech equivalent is the MIMO spatial modulation technique which maybe is what you are thinking of, but again that's not going to be on a simple low power chip. \$\endgroup\$
    – user201365
    Nov 14, 2018 at 18:11

1 Answer 1

1
\$\begingroup\$

Talking about IEEE 802.11 a/g/n/ac… here.

As far as I know, WiFi devices are able to receive multiple frames at once with no problems.

That's wrong.

WiFi isn't designed to allow working with collisions. It has systems in place to avoid these.

So, a single WiFi device can only receive a single frame at once. It might be that complex devices were designed to work on multiple separate WiFi channels at once, but that's definitely not the norm, and it's actually like multiple devices doing multiple independent things at once; the frames don't collide.

How is this possible?

Aside from the system being neither specified nor designed to allow that:

Since a collision of two frames means that the preamble of the second one would be heavily damaged, I doubt it's overly possible practically.

Principally, even systems that weren't designed to allow collisions (those would typically be CDMA systems) can achieve reception of overlaying frames if the signal strength of one is significantly better than the other:

In that case, the receiver might be able to actually correctly receive one, recreate the "unininterrupted" signal as it would have sounded to itself, subtract that from the receive signal (that it has magically saved somewhere), and then receive the next weaker one. We call that technique Successive Interference Cancellation; again, as far as I know, that's not a part of any WiFi standard.

Also, MIMO/beamforming techniques can be used to "detangle" signals arriving from different partners/paths at the receiver, but that is a whole different kind of business.

\$\endgroup\$
2
  • \$\begingroup\$ So a Wi-Fi router to which there are 3 devices connected needs to have each one on different channel? Because if all 3 were on one channel there would be collisions all the time, right? \$\endgroup\$
    – Em Ka
    Nov 14, 2018 at 22:05
  • 1
    \$\begingroup\$ no, not at all. That's not how WiFi works: it's a same-channel collision avoidance system, as I said in my answer. Only one station can transmit at a time; it's that simple. \$\endgroup\$ Nov 14, 2018 at 22:30

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.