30
\$\begingroup\$

I've read that WiFi b/g/n is using 2.3995 to 2.4845 GHz.

Bluetooth is in between: 2.4000 to 2.4835 GHz.

So what will happen when I put a Bluetooth sending device next to a WiFi sending device?

Will the two devices be aware of each other (like two WiFi devices do)?

Or will they send their data as if they were alone (means interference)?

\$\endgroup\$
37
\$\begingroup\$

Yes, WiFi and Bluetooth can disturb each other.

But both are equipped to handle that. A standard that is not capable to handle disturbance and/or interference will simply be unusable under many circumstances.

The 2.5 GHz ISM band is also used by Microwave ovens and other wireless standards like Zigbee.

Wifi transceivers are able to detect when certain data has been lost and can ask for a re transmission. It is also possible to lower the datarate which makes the link more "robust" in the sense that it is less sensitive to disturbances.

Bluetooth uses frequency hopping, it changes channel (frequency) 1600 times per second. That way if one channel is disturbed only part of the data is lost. Also a re-transmit of data is possible.

So yes, interference happens, it is a fact that the standards simply have to deal with.

\$\endgroup\$
  • 13
    \$\begingroup\$ So Bluetooth and WiFi don't have a common CA/CD (collision avoidance/collision detection) system. They simply don't know each other and it's just noise for the other party. That's what I wanted to know. Is that right? \$\endgroup\$ – somega Nov 11 '19 at 15:31
  • 3
    \$\begingroup\$ When I do large wifi transfers from my laptop TO an external device, suddenly my mouse becomes jerky. I think that when the laptop is transmitting a lot, it disturbs significantly the mouse. The same thing doesn't happen when receiving. \$\endgroup\$ – Thomas Nov 12 '19 at 0:05
  • 6
    \$\begingroup\$ @slebetman and wifi uses frequency hopping. No it doesn't, it is not part of the standard. For WiFi a channel is selected and then that is used, see en.wikipedia.org/wiki/IEEE_802.11 The channel is not changed dynamically so there is no "hopping". But feel free to prove me wrong. \$\endgroup\$ – Bimpelrekkie Nov 12 '19 at 7:35
  • 8
    \$\begingroup\$ @Thomas the explanation is simple: when sending a file, your laptop transmits a signal which is strong at your laptop (and then disturbs the mouse) but weaker at your WiFi access point. When receiving, the situation is reversed, the signal is strong at the access point but weaker at your laptop. Such a weak signal does not disturb the mouse. \$\endgroup\$ – Bimpelrekkie Nov 12 '19 at 7:38
  • 4
    \$\begingroup\$ @Bimpelrekkie correct, WiFi doesn't frequency-hop, but in the early days of 802.11b, many people confused DSSS with FHSS and I think that is how this confusion was born. \$\endgroup\$ – hobbs Nov 12 '19 at 12:08
17
\$\begingroup\$

Wifi uses a listen before transmit system. If the channel is busy, it holds off transmitting. Eventually it gets through. Each channel is fixed. If it tries to use a channel that is busy, from bluetooth, then it will wait. This may reduce the data speed for wifi if it has to wait too much.

Bluetooth for over a decade now uses adaptive frequency hopping (Bluetooth 1.2) so it will actively check each of it's channels to see if they are good or bad, and blacklists them for a while. This is on top of it's normal frequency hopping. If it tries to use a channel(s) that is busy, from say wifi, then it will move on to the next and won't use those wifi channels, essentially not interfering.

So both try to actively prevent interfering with others and suffer data loss or speed loss for it. But because of these techniques, they can coexist. In a quiet environment, the wifi and the bluetooth networks won't even bother each other. In a noisy environment, there goes your bandwidth.

Here is a nice if old (2006!) writeup on interference techniques for wifi and bluetooth (and ZigBee and wireless usb) https://www.eetimes.com/document.asp?doc_id=1273359#

And even newer (but still old) the development of all in one wireless networking ICs that handle both Wifi and bluetooth for the same device, allow interoperability by communicating to each other so that they know when the other is transmitting and what channel, so they can avoid each other. Teamwork. https://www.marvell.com/wireless/assets/Marvell-WiFi-Bluetooth-Coexistence.pdf

\$\endgroup\$
  • \$\begingroup\$ What do you mean by "This is onto of its normal frequency hopping."? \$\endgroup\$ – Peter Mortensen Nov 12 '19 at 11:18
  • 1
    \$\begingroup\$ I think he means "on top". Bluetooth has a fixed hopping sequence even without interference. \$\endgroup\$ – patstew Nov 12 '19 at 16:07
  • \$\begingroup\$ @patstew correct. Damn apple autocorrect. \$\endgroup\$ – Passerby Nov 12 '19 at 22:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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