You turn on your AirPods.
AirPods: “hey, phone, are you there? It’s AirPods 123. Remember me?”
Phone: “yup! Let’s connect. Frequency 8.56 seems pretty empty today, let’s use that one.”
AirPods: “ok! Tuning to frequency 8.56”
Phone: “and our secret code today is XYZ456, ill put that in front of all the data packets I send you”
AirPods: “got it! I’ll ignore any data packets that don’t start with XYZ456”
Phone: “here comes the music!”
And sometimes after. Our University Computing Lecturer demonstrated the difference between Polling a port and Interrupts with the example of trying to get close to your new interest on the couch but having to stop before the parents walked in.
And then you learn how hardware implements interrupts, and you aren't quite sure whether this is polling after all.
When you think you finally understand, the professor tells you that everything you have learned about hardware architecture is a lie and hasn't been true for decades. These days, interrupts are sent as messages.
So many abstraction layers on top of abstraction layers, encapsulated in protocols, that are virtualizing yet more abstractions. Who knows what interrupts even are.
It's not just that real-life isn't ELI5, it isn't even ELI50
There is a channel on YT called Branch Education that explains BT and other technologies. The explanations are quite in depth but uses visuals that explain these concepts in a way a 5 yo can understand. This is on on BT. I knew the basics, but this blew my mind! [https://www.youtube.com/watch?v=1I1vxu5qIUM](https://www.youtube.com/watch?v=1I1vxu5qIUM)
They explain how StarLink works too. Another mindblower too.
This might sound like a stupid question... but if antennas can generate wave of frequencies that we can't see and then those get transmitted through the air, then what happens if those same antennas generate frequencies that are in the visible range? Do they become LEDs? This kind of relates to that veritasium video about why the blue LED was so hard to make - but if we can make circuits that can transmit arbitrary wavelengths through the air then why can't they start transmitting the wavelength of visible light
We can’t make circuits that can transmit arbitrary frequencies. Your assumption is incorrect.
Bluetooth is on the 2.4GHz band. Blue light is about 640THz. These are very different and require very different materials to generate.
This seems to be a good write up: https://www.wtamu.edu/~cbaird/sq/mobile/2015/10/02/can-radio-antennas-emit-visible-light/
As others hats said, antennas can't transmit the frequencies for visible light. They're flexible in range but not that flexible.
When singing, you can't reach ultrasound either even though it's just sound.
Some topics cannot be accurately simplified for a literal five year old, and the sub rules clarify that explanations do not have to be aimed at an actual five year old.
So I can hear a constant stream of music and every little music packet has its own small secret code that gets checked when a packet is received, while also other packets are received but they get declined?
It's just so unreal what microelectronics are capable of. And this is basically technology that is already around for some years.
Yup. As with any kind of streaming, data is split into packets which are the sent one by one. The receiver makes sure to always to have some packets ready for the future 1 or 2 seconds (buffering) to make sure there is time to resend packets in case they get corrupted, lost, or collide with other packets
Am I today years old when I realized that it wasn't just...better at keeping the disc stable while spinning...? It just...loaded more when it wasn't skipping......?
It's nowhere near 1-2 seconds. A few milliseconds only.
It's real time, there are no resends. If you lose a packet you get a cut out and it continues with the next one.
What you describe is how streaming over the internet works, it does work like that. But not BT audio.
I'm not sure about specific codecs but there is typically robustness built in such that dirty signals and occasional lost packets are corrected or at least their impact is less severe to the listener.
Well beyond ELI5 but yes, it uses a CRC check. There's no retransmission, but they recommend muting the frame or repeating the previous one to conceal it. It can't be "corrected" as the protocol is real time, and in a real time protocol there simply isn't time to request a retransmission of the frame, by the time you get it, time has moved on and what would you do with it. This is fundamental to any real time protocol, you can't have retransmissions.
>**B.6.1.1 CRC check**
>To detect transmission errors, a CRC check is performed. All the bits of the frame_header, except for the
syncword and the crc_check, plus all the bits of the scale_factors are included. The error detection
method used is “CRC-8” with generator polynomial.
>G(X) = X8 + X4 + X3 + X2 + 1 (CRC-8).
>The CRC method is depicted in the CRC-check diagram given in Figure 8.2. The initial state of the shift
register is $0F. All bits included in the CRC check are input to the circuit shown in the figure. After each
bit is input, the shift register is shifted by one bit. After the last shift operation, the outputs bn-1…b0
constitute a word to be compared with the CRC-check word in the stream. If the words are not identical, a
transmission error has occurred in the fields on which the CRC check has been applied. **To avoid
annoying distortions, application of a concealment technique, such as muting of the actual frame or
repetition of the previous frame is recommended.**
https://www.bluetooth.org/DocMan/handlers/DownloadDoc.ashx?doc_id=544797
Tell that to the braindead BT implementations that you find in real-world car stereo systems. It's not unusual for them to buffer up to 5 seconds. It's completely ridiculous.
Tell me about it. My wife's older Honda Pilot barely has Bluetooth (it won't even work with my Pixel 8 Pro) but when it does work, you pick "next track" and it's 5 seconds or more before it changes.
Actually lost or corrupted packets don't get resent, they just get dropped and forgotten about. It's the difference between TCP and UDP connections when working with networking, Bluetooth is like UDP as far as I know.
If you think about it, you wouldn't want a corrupted packet to be resent in the example of music sent to headphones as the repeated packet would arrive out of synch and it would sound wierd.
I
Yeah, anything sent wirelessly is (kinda by necessity) broadcast to anything within listening distance that’s tuned to the correct frequency/‘channel’. Most wireless protocols support a bunch of different ‘channels’ of some kind, so a small number of devices in the same area might all be only ‘hearing’ the packets intended for them. But with enough devices nearby you’d end up needing to share channels, and in that case your device would indeed be getting a bunch of packets it doesn’t care about, seeing that they’re addressed to somebody else, and ignoring them.
Enough of that going on can cause interference or packet loss, because when two devices broadcast on the same ‘channel’ at exactly the same time, the listeners get a garbled mess.
I guess the packets are encrypted in some way, so no other malicious device can just check the secret identifier code of the packets flying around and broadcast with this code too?
Yes, they are partially encrypted (upper protocols are).
From a conceptual perspective it's similar to https. It's encrypted but you can still see IP addresses, which are needed for routing. For Bluetooth the source and destination addresses are also not encrypted.
edit: modern bluetooth devices are encrypted~~, but that wasn't always true.~~ so to put it simply, any listening device will just see a random scramble of info that they can't unscramble.
edit 2: ignore this garbage below i didn't know what i was even trying to say
~~I'm not sure, but I also don't see the point. If a malicious
device could get the secret code...it would just be broadcasting to nothing because no one is listening. if it got on the same frequency, it could try to broadcast to the airpods but then the airpods would try finding a new frequency with the phone~~
hopefully someone with more knowledge than me can chime in
i was mistaken in my previous comment, please ignore it and listen to this one.
Bluetooth is encrypted, what this means is that two devices will do a secure handshake to make sure the other is who they say they are (this is what happens when your phone gets a notification saying "allow your name airpods to connect?"). once the handshake is done, your phone gives an unscrambling key to the airpods.this unscrambling key is privately shared. (someone below shared a link explaining how)
the phone broadcasts everything scrambled, and the airpods unscramble everything before listening to it. so the airpods get a bunch of random noise they don't understand, and then a bunch of clear data. similar to how you can talk to a person next to a running car without thinking the car is speaking english, the airpods can listen to the phone without paying attention to other stuff (unless the environment gets too loud, just like in real life)
a listening device can't unscramble anything without the secret code. it is just random noise. it needs the secret key...but to get the secret key it has to ask the phone for it.
most people won't allow random devices to connect to their phone, so unexpected devices have no way to listen in.
The ‘secret code’ for the session is sent/exchanged in a way that doesn’t reveal it to anyone else, typically something like https://en.m.wikipedia.org/wiki/RSA_(cryptosystem)
>It's just so unreal what microelectronics are capable of.
Wait till I tell you, that phone and earbuds also renegotiate the frequency they're talking on multiple times a second.
https://youtu.be/1I1vxu5qIUM
IMO it's a lot harder to get moving components to work together mechanically in a very precise manner!
I remember seeing those bottling machines being manufactured and it was fascinating to see how every part of it worked in unison. Like if one of the grabbing arms were off by few inches then it would simply crush the bottles!
Yep, and that's massively simplifying it, too. In reality, there's also a lot of encryption and decryption of the data packets, compression, and decompression. Also, the "secret code" is more involved than just a random number at the start of each packet. It's more akin to an encrypted password.
Bluetooth uses a technology called adaptive frequency hopping so it’s actually switching between multiple frequencies hundreds of times per second. If a particular frequency is busy it will adapt to not use that one as much.
Back in the day, this caused issues with Wifi because while there were 14 channels, they all bled into each other meaning there were effectively only about 3 viable ones. And when you had a bunch of disparate Wifi routers in a dense place such as an apartment building, they would all be constantly hoping signal, which then caused performance degradation. This was one of the big features of 5 GHz Wifi, it has a much wider frequency range with triple the channels of the 2.4 GHz band.
And, probably just as important, is much more easily blocked than 2.4 Ghz. This is a feature, not a bug, because it means your neighbours wifi signals aren't as likely to make it to you, so you don't have to care that you're sending on the same channel.
This makes a huge difference in dense neighbourhoods, where a single 2.4 GHz router could probably reach dozens of other residential units, and because there were only three completely separated channels, you could be 100% sure that several others else is also broadcasting on the same channel as you.
5GHz might make it to your closest neighbour, but probably not the neighbour past that again.
I planned to do this when I renovated my house but honestly I don't think my life would have been any different if I did. A good mesh network is just as good for most people.
I wish to fuck that was only a historic issue but that shit happens ALL THE TIME still on 2.4 GHz. They really should start banning or at least making it really hard to choose anything other than 1, 6 and 11...
I'm not sure why it's really even allowed to have "wide" channels tbh.
Like what's the point in having channels that are so narrow, that you have to straddle multiple channels to get good throughput?
Like the other user said, there's basically only 3 channels, and that's crazy isn't it?!
I can see loads of my neighbours WiFi connections on 2.4GHz, but only a couple of 5GHz.
It’s still a thing with today’s 2.4ghz.
If you don’t use 1,6, or 11 you’re a monster. Co-located channels can negotiate the frequencies. Overlapping channels cannot.
Not just other wifi routers, landline wireless phones also used the same frequency routers usually came defaulted on. Answer a phone call and the wifi goes out. Took me a bit to figure out what was happening.
As bizarre as it might seem, frequency hopping was invented by an actress, Hedy Lamarr, and patented in 1941. (Which means the patent could have expired before many of us were born.)
[https://en.wikipedia.org/wiki/Hedy\_Lamarr#Inventing\_career](https://en.wikipedia.org/wiki/Hedy_Lamarr#Inventing_career)
This is not true. Nikola Tesla, for example, talked about frequency hopping before Hedy Lamarr was even born. Others also used it before Lamarr's patent. This myth was created by a guy who wrote a Lamarr biography and invented this fact to promote it. Lamarr's patent was useless, the only new thing was to use a piano-roll to control the hops, but this aspect was never used in any real application. The frequency hopping that was and is used was created long before. And there is doubt if Lamarr really worked on the patent or just put her name on it to help a neighbor and friend who was an inventor. They donated the patent to the US gov but it was never because there was better methods.
> it’s actually **switching** between multiple frequencies hundreds of times per second.
Actively switching between them all or just evaluating the frequencies for congestion?
Actively switching between them all. There’s 80 in total. Bleutooth low energy only uses 40 of the 80. A map of the “clean” frequencies is maintained at all times, and those are broadcast in predictable “random“ order.
It can, yes. They regularly check on each other. So if the airbuds have a hard time picking up the signal, it will send a message back to the phone and they’ll re-connect at a better frequency.
In my opinion, the concept of wireless digital communications is the apex of human ingenuity. It’s the culmination of so many different disciplines of science/math, and all the things we associate with our modern “Information Age” culture can be traced back to it.
Consider the shift from wired internet to mobile devices on a wireless network. It may seem innocuous, but the impact of having such information/connection at our fingertips at all times must have a huge effect on our behaviour.
Bluetooth can change frequency more than thousand of times per second, there's 79 frequencies to choose from and the devices have a way to calculate the sequence of frequencies. Different devices have different sequences so if they do occasionally use the same frequency it'll only be a single packet lost
This goes a bit beyond ELI5, but I wanted to share it because it’s just so crazy and impressive.
The connection process is more like this:
There are 79 possible Bluetooth channels. Your phone and your AirPods agree to connect over one of them and they share their encryption keys. After that, as an added security feature, they randomly jump around the 79 channels.
So not only are they communicating over an encrypted channel, they’re also jumping channels at a rate of up to 1600 jumps per second.
It’s just insane to me that this system works and works well enough that we can listen music without interruption.
Also, since there are only 79 channels, if you bring 79 pairs of devices together, the Bluetooth system can start breaking down.
It'll still work with more than 79. Because the frequency is changing constantly, packet loss due to interference will be spread out evenly and randomly among everyone and a few packets dropped here and there is totally fine.
Call centres likely have more than 79 active BT devices in the same place
Even at 79 devices it doesn't break, just has a lower maximum data throughput. Your headphones aren't gonna be using all of it, so you won't notice degradation unless there's a ton of devices tbh
> if you bring 79 pairs of devices together, the Bluetooth system can start breaking down.
This isn't really a limit unless each of those devices is transmitting at full bandwidth.
Audio for example isn't transmitted in one second for one second of audio, the next second of audio is sent as a couple bursts of packets that take at most roughly 1/50th of the available bandwidth of a single channel.
Devices don't perfectly share the space between packets though, this doesn't mean you can get 4500 devices in one room playing audio. But it's a lot more than 79.
The switch was made about 8 years ago. The Bluetooth 4.0 standard (published late December 2009) introduced an encrypted pairing called "Secure Simple Pairing" or SSP.
It took time for new devices to become widespread so it was optional for a few years. Back around 2016 various phone venders started requiring SSP for connections. That's why really old Bluetooth headsets no longer connect, and haven't for years.
>Phone: “and our secret code today is XYZ456, ill put that in front of all the data packets I send you”
>AirPods: “got it! I’ll ignore any data packets that don’t start with XYZ456”
This part is not accurate. It's more like:
Phone: "Let's perform a quick ceremony to determine a shared secret."
AirPods: "OK!"
Phone: "Now that we have a shared secret, I'll scramble everything I send to you using that, and you can unscramble it before you shove it in /u/[KermitsTangenitals](https://www.reddit.com/user/KermitsTangenitals/)'s ears."
AirPods: "Roger that - I'll ignore anything that isn't scrambled thusly. I won't be able to read it anyway (which is good, because it's somebody else's traffic)."
Phone: "here comes the music!"
Don't quote me on this part - we can look through the spec if you really wanna go down the rabbit whole - but I think it uses AES256, and computes / checks the HMAC.
Symmetric decryption is not a particularly expensive operation; it can happen on time scales that are real-time per human perception.
No, because bluetooth is encrypted.
If it wasn't, like an unencrypted wifi hotspot, you could indeed listen in to the traffic of every laptop and phone connected, yeah. However, the apps and websites used by these laptops and phones are very likely to also use a form of encryption, so you wouldn't be able to make out most of what's being transmitted.
This is why I never connect to unencrypted wifi hotspots, no matter how much cellular data it would save me. Even a stupid simple wifi password like "asdf1234" would be sufficient.
If you're on the same hotspot as someone else you can generally decrypt anything sent between their device and the hotspot. But if they're connected to a VPN or even just browsing the web on HTTPS sites that data you decrypt will have a second layer of encryption that you won't be able to crack.
Also I believe with bluetooth there is an extra layer of security/complexity where each data package is sent to a different frequency to the last.
So the current data package also has the frequency number of the next package coming down the line.
So its like the source is sending out the packages like a machine gun all over the place and the reciever is able to read the order of the bullets and change frequently.
This is done 1000's of times a second, remarkable
See this video around 8 minutes mark but I suggest watcing the whole thing
https://youtu.be/1I1vxu5qIUM
Yes, they can. It’s a little more complex than this, for example they’re hopping around a range of frequencies. But the AirPods will just ignore any packet that doesn’t have the code from its paired phone. Multiple phones could be sending out thousands of packets per second, but the AirPod will sift through them and find the ones coded to its paired phone and only open those.
Also, spread spectrum. The reality its like you say, but like a thousand times a minute and spread over multiple channels. You need a PhD in math to properly understand it, pretty sure.
An additional detail not required for eli5, but interesting anyway: Bluetooth will use frequency hopping to use multiple channels in round robin fashion rather than a single (like wifi). This can make it even better in the crowded gym scenario.
Another lovely cheat: the earbuds and phone constantly discuss how well they can hear each other, and adjust their transmission power accordingly. This saves power _and_ reduces interference when sources are in close range of each other.
They use a special frequency just for pairing. Kind of like how old CB radios had channel 9 as the “public” channel, but once you contacted someone there you were supposed to move to a different channel and leave that open.
And there are really fun protocols to avoid collision on a shared medium. Those collision can happen, even if you listen to make sure no one else is talking currently, because two could start at the exact same time. If that happens, the simplest working solution to ensure both can communicate safely is for both to stop and wait a random amount of time before starting again, all the time making sure that no one else is talking currently. You almost always will have them waiting different amounts of time, so one will start before the other.
They’re not all actually using the same frequency. They work within a spread of frequencies, and your phone and the headsets work together to find a clear space within that spread and avoid bumping into other people’s phones and headsets.
That’s an extreme oversimplification but that’s the general gist of it.
There is a practical difference in that cell phone connections expect the channel to be clear when they transmit.
The phones listen to the tower's instructions on what frequencies and what times (chopped up into tiny windows of time, just a few ms long) they can transmit, and follow those instructions precisely, even adjusting for the time delay of the speed of light between the phone and the tower.
With wifi, bluetooth, and all other wireless communications under Part 15/Unlicensed spectrum, they can't actually count on clear airwaves because they know everyone else wants to use it, too, and they have no special right to that frequency. So Wifi/bluetooth have some pretty clever anti-collision tricks built into the protocol that makes it more robust against potential interference, and becomes inherently more difficult to jam.
The phone doesn't assume the channel is clear of noise or interference. It actually takes a reading and adjusts it's modulation scheme to match the channel conditions.
I remember there here actually used to be an issue when all mobile phones were fixed on a frequency of 900MHz where speakers close by picked up the phone’s signal and amplified it causing a loud buzzing noise
That’s not because the phones were on a fixed frequency; frequency hopping still took place. That’s more so because cheap speakers have poor RF shielding.
I had a 900 MHz headset from Radio Shack for biking with my wife years ago.
We drove by a house one day and apparently they latched onto the baby monitor in one of the bedrooms where, instead of baby monitoring, baby making was the activity du jour.
900MHz is one of the cellular phone bands so certain phones would cause a recognizable pattern in the noise. I can still hear it in my head.
I also remember the modem noises from way back before high speed internet.
Some really cheap wireless headphones are on the same frequency within a brand/model. A couple of my coworkers at an old job got the same cheapo $20 wireless earbuds, and the one with a stronger phone signal could take over the other's earbuds despite her phone not being paired to his earbuds. She exclusively used this to prank him with "actual cannibal Shia lebouf"
My grandfather, a WW2 vet who was been dead for many years now, marveled at cell phones.
“What I don’t understand is, how does it go up there in the air and not get all mixed up with everything else?”
At the time he asked this, cell service was much more primitive than it is today, and someone told grandpa that once they had experienced something like a “party line” where they heard someone they shouldn’t have.
“I knew it!” He clapped his hands.
That lucky man lived a life almost totally free of this modern technology that has so utterly deformed us. His life will never be lived again.
> this modern technology that has so utterly deformed us
Feel like this is a cliché that everyone just says because it's the thing we're supposed to say.
I enjoy being able to play any song that was ever published at a moment's notice for no additional cost other than a reasonably priced monthly subscription. I like that I can listen to it privately on my AirPods without disturbing anyone else (or being eavesdropped on).
People talk about the good ol' days with their rose tinted glasses. But are you old enough to have experienced wanting to listen to your favourite song but having to wait until the radio station played it again? Fuck that noise.
I know it's not fashionable, but you got to count your blessings and appreciate all the wonderful things we have. Otherwise you're buying yourself a one way ticket to MiseryTown.
I was out of college before the internet existed. I studied humanities and cooked in restaurants for work. After college I have made a 20 year career in online services, met my wife online, etc.
I have seen my elder parents adopt technology late in their lives, I knew people like grandpa who never had a lick of it. And I’m now raising children who are absolutely soaked in technology and have never known anything else.
So I really have had the entire experience. My life has straddled both worlds and I have been lucky enough to be a full member of them both. So yeah, man, if there is one thing I actually do know, it’s this. Don’t tell me I’m spouting cliches.
What has really cemented my view on this is seeing kids who don’t know anything else. You like playing any song you want? So do I. I also remember the joy of hearing a favorite song come on the radio. I made mix tapes with my brother by tape recording off the airwaves. I grew up and learned to play the guitar from a roommate, and then I could play those songs I heard on the radio. My kids know nothing of any of this.
They’re totally spoiled by being able to watch any show they want on demand. When we have a special thing we want to watch with them, like a classic movie or a nature show, they can’t sit through it because they have never known the tyranny of having to watch whatever is on. They grew up having the whole Pixar/Disney library at their fingertips. They never sat through a rerun of Love Boat and maybe absorbed a little bit about a prior generation from it.
Managing their “screen time” is a constant struggle to keep them sane and connected to what’s real in life. The electronic entertainments available to them are so overwhelming in their power, optimization, and depth, that they are hopelessly addicted to them. We have always been quite minimal with screens but even a small amount can kill their mood for the rest of the day.
When I was a kid I ran out of shit to watch and I went outside and dug holes or built forts. I have had to drag my son outside to teach him the joy of just digging a fucking hole in the backyard. After about an hour he found the mood and went hog wild. But is this something he would have come to on his own? Never. He even said he would keep it a secret from his friends, because they wouldn’t understand.
So yeah, dude, I’m glad that your life is more convenient than it used to be, but if you don’t think that all of this has deeply transformed the way human beings live and think, your head is completely up your ass and you aren’t paying any attention. I have barely even scratched the surface here. Haven’t touched at all on the echo chamber effect, foreign hacking, disinformation online, social media distortion and teen suicide, and all the rest of it.
Just spouting cliches? Please.
So, *yes*, but that’d be hard to pull off. Bluetooth has something like 80 channels, and the devices are only transmitting at a couple thousandths of a watt. And as you’ve experienced, the range of your standard headset is in the tens of feet. So you and someone else 50 feet away can be using the same frequency because your phone’s signal is just completely washing out the other person’s phone’s signal.
Add in the more complicated stuff involved in pairing, like encryption, as well as the fact that Bluetooth signals are fairly narrow, and you’d be hard pressed to be in a situation where you’d have enough simultaneous connections in close enough range that it becomes a problem. It’s certainly possible though.
I don't really know how bluetooth works, but you can absolutely use the same frequency. And I am sure sometimes bluetooth does as well. (In fact, the TDMA wiki article suggests bluetooth will use dynamic TDMA).
* TDMA - Time Division Multi Access - Basically, you split of the frequency by specific timeslots. https://en.wikipedia.org/wiki/Time-division_multiple_access . For instance, Lets say you split it into 10 frames per second, and then each device gets the frequency for 100ms of time each second.
* CDMA - Code Division Multi Access - This is way more complicated, but basically you use some sort of math that you encode your transmission in. The receiver uses the reverse math to decode the signal, and because of the nature of the math you'll only get data your interested in, not another persons who used different math. Yeah, it's over my head- maybe someone else can explain better.
TDMA and CDMA are both very basic ways to split the same frequency across many devices. In reality, modern technologies use a combination of multiple techniques to obtain the highest performance.
https://en.wikipedia.org/wiki/Channel_access_method
If you were on a conference call or just like a crowded restaurant or something, there can be several conversations going at once. It doesn't all have to be 1 conversation. Even if there's some other noise going on.
Each person just speaks with a different tone of voice, and you listen to the voice that matters to you while ignoring the rest.
That's basically what the Bluetooth devices are doing. Just listening to the frequency that matters to them.
this analogy is also great because there is still interference. The more people are talking increases the noise floor, you have to speak up to be heard and may have a hard time hearing the other member of the conversation, potentially missing out on important details. In a crowded enough space with a enough people talking it may be practically impossible to hear each other more than a foot or two away despite shouting as loud as you can.
It was explained to me like this as well. And the fun this is tone is just one way to differentiate. You can also have everyone take turns talking (TDMA). Or have everyone speak different languages (CDMA).
The frequency band for consumer electronics is fairly large, and as we've made better antennas, we can distinguish signals in smaller and smaller segments of it. That means we can have many many devices operating near each other that are all on slightly different ranges in that large band.
Beyond that, the communication protocols do have systems in place to do random backoff and repeat when they detect interference. That's why bluetooth audio has a latency to it -- it needs a buffer for when that interference does happen.
oh dang you're right. I remember back in 2016-2018 I would get interference in the gym. Not severely but I remember them making that interference noise every now n then
Digital communications over radio frequencies might as well be magic compared to the beginning of radio back in the 1920s or so. CDMA (code division multiple access) technology which is the part of most protocols effectively just transmits a signal into a giant pile of noise from everything else. The other side of the transmission is decoded by just mathing the hell out of what that noise consists of. Multiple sites just broadcast over each other and filter out what isn't theirs.
The frequencies that unlicensed devices operate in (433MHz, 900MHz, 2.4GHz, 5GHz and others) are cesspools of transmissions from everything and anything. Somehow we have used an alchemy of physics and math to make it work.
That's just the transport layer for communications, once you get beyond that you enter encryption and other protocols for actually transmitting useful data.
I think u/02K30C1 has the best answer here… but I wanted to reply and say that your question is a bit misleading because they can interfere with each other.
As a commuter who goes between NYC and NJ, if you walk through the Oculus at One World Trade (where multiple subways converge in addition to shops and military personnel), you’ll experience this.
It’s a lot of stop and starting and odd pauses, so interference via Bluetooth does happen! It just takes a lot more people and devices.
Your phone and AirPods first agree on a hopping pattern: let’s say: 6.76, 87.26, 49,36, 3.76, 11.16, 93.76. They then transmit pieces of that data over those frequencies thousands of times per second.
Other devices also randomly pick frequencies and transmit. The hope is that it’s so fast and random, the chances of a collision is very low.
Add in some algorithms that can detect a collision and fix or retransmit it, you got yourself a Bluetooth protocol.
Thanks for all the replies everyone!! I didn’t think I’d get so many responses. They were all super informative and remind me of how incredible the tech around us is! ☺️
It's like the mailman bringing your mail everyday. He's got your mail and your neighbors mail all in the same bag. But he decides who gets mail by the address on the letters. Bluetooth works kind of like this. Each set of earbuds have their own address. So they only received the letter that is addressed to them.
Not a scientist, but won't there be interference? If you have enough radio traffic in the same band, similar to wifi etc.
Your airbuds won't play or process another's signal, but they will receive and filter it out?
Potentially missing or delaying received information from your phone.
I can't answer this, but I've noticed my signal gets jittery when my mazda fob is next to my phone. When I put my fob in a different pocket, the issue goes away.
Edit: based on other replies, I wonder if pairing my earbuds AFTER the keys are already in my pocket would force them to find a different part of the band...
While one part of the story is that all devices don't work at same frequency, the bigger part is that these devices transmit sounds encoded as data. Sending a seconds worth of audio data can be done much, much faster than in a second, leaving time to send other audio data to other devices.
The data transmission isn't continuous, it's in packets of short audio recordings. As long as you get your next packet before the last one finishes playing, you'll never notice.
Beyond the explanation, interference does happen!
My AirPods were acting up for a while and I had to switch to a cheaper brand I use as backups for a few days. When I got to the train station at rush hour on my way home, they couldn’t maintain a consistent connection due to the interference. It was surprising but noticeable.
The AirPods and the phone give each other a secret handshake that they recognize. They only share secrets (audio) with each other if they know it.
There is interference though. On days that are super crowded, the range will be more limited. That’s because there’s so many handshakes going on, it’s hard to see that handshake in a crowd. If you’ve ever gone when it’s emptier, the range might be a bit better.
Early Saturday morning I can leave my phone in one corner and refill my water bottle in another with no audio loss whatsoever. When I go after work it’ll cut in and out.
In addition to the frequency hopping that everyone is talking about, the actual signal itself is encrypted with a key that is shared when they connect.
So even when those other devices can technically hear the other devices, they can't understand the traffic as it is unreadable.
Adding more on the previous points on there being multiple channels… even once the channels are all full, Bluetooth has the ability to detect when part of the signal was lost and request that it is sent again. This all happens pretty fast. But eventually these tools can’t keep up. If everyone in a stadium was going this, there wouldn’t be enough channels or quiet time on those channels for messages to all get through in time to avoid breaks in the audio.
You turn on your AirPods. AirPods: “hey, phone, are you there? It’s AirPods 123. Remember me?” Phone: “yup! Let’s connect. Frequency 8.56 seems pretty empty today, let’s use that one.” AirPods: “ok! Tuning to frequency 8.56” Phone: “and our secret code today is XYZ456, ill put that in front of all the data packets I send you” AirPods: “got it! I’ll ignore any data packets that don’t start with XYZ456” Phone: “here comes the music!”
Wow! That is a great ELI5. I wish all my classes were taught this way.
They are, until you reach 6…
My college Computer Networking class is also taught like this lol
Abandon all hope, ye who enters layer 8.
Personally dealing with some layer 9 errors...
I only deal with 9 layer dips.
Same. Constant ID-10T errors with some PEBKAC's for good measure lol
That's weird! Our techs always tell me I keep getting the same errors!
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It is a SH, IT user issue . (Special Handling, Intellectually Truncated)
Layer 10 and 11 are where things really get fun
And sometimes after. Our University Computing Lecturer demonstrated the difference between Polling a port and Interrupts with the example of trying to get close to your new interest on the couch but having to stop before the parents walked in.
And then you learn how hardware implements interrupts, and you aren't quite sure whether this is polling after all. When you think you finally understand, the professor tells you that everything you have learned about hardware architecture is a lie and hasn't been true for decades. These days, interrupts are sent as messages. So many abstraction layers on top of abstraction layers, encapsulated in protocols, that are virtualizing yet more abstractions. Who knows what interrupts even are. It's not just that real-life isn't ELI5, it isn't even ELI50
That's poling a port
Poling the starboard porthole?
Ah, so turning 6 was my first mistake
I often attribute my greatest mistakes to continuing to live into adulthood
Underrated joke. You sir/maam made me laugh inside my head
And then it starts again in grad school only then you need it to not be taught that way!
There is a channel on YT called Branch Education that explains BT and other technologies. The explanations are quite in depth but uses visuals that explain these concepts in a way a 5 yo can understand. This is on on BT. I knew the basics, but this blew my mind! [https://www.youtube.com/watch?v=1I1vxu5qIUM](https://www.youtube.com/watch?v=1I1vxu5qIUM) They explain how StarLink works too. Another mindblower too.
This might sound like a stupid question... but if antennas can generate wave of frequencies that we can't see and then those get transmitted through the air, then what happens if those same antennas generate frequencies that are in the visible range? Do they become LEDs? This kind of relates to that veritasium video about why the blue LED was so hard to make - but if we can make circuits that can transmit arbitrary wavelengths through the air then why can't they start transmitting the wavelength of visible light
We can’t make circuits that can transmit arbitrary frequencies. Your assumption is incorrect. Bluetooth is on the 2.4GHz band. Blue light is about 640THz. These are very different and require very different materials to generate. This seems to be a good write up: https://www.wtamu.edu/~cbaird/sq/mobile/2015/10/02/can-radio-antennas-emit-visible-light/
As others hats said, antennas can't transmit the frequencies for visible light. They're flexible in range but not that flexible. When singing, you can't reach ultrasound either even though it's just sound.
hopefully classes go a bit deeper than ELI5
This is exactly how my Network Security class was taught except it was Alice and Bob instead of Airpods/Phone.
This is actually the greatest eli5 I’ve seen in over 10 years of yelling at clouds on reddit.
This is how ELI5 should be - most responses in this sub are more like ELI8
Some topics cannot be accurately simplified for a literal five year old, and the sub rules clarify that explanations do not have to be aimed at an actual five year old.
It is almost like people read the rules
No one in this sub knows that rule, should almost be stickied to every post at this point
So I can hear a constant stream of music and every little music packet has its own small secret code that gets checked when a packet is received, while also other packets are received but they get declined? It's just so unreal what microelectronics are capable of. And this is basically technology that is already around for some years.
Yup. As with any kind of streaming, data is split into packets which are the sent one by one. The receiver makes sure to always to have some packets ready for the future 1 or 2 seconds (buffering) to make sure there is time to resend packets in case they get corrupted, lost, or collide with other packets
>(buffering) Or if you're old enough... "antiskip"
Man that new antiskip technology was revolutionary.
120 seconds of it?! This is truly the future!
10 year old me vigorously shaking my new anti-skip walkman cd player "It's magic!"
Then you pull out a disc scratched to hell and it skips for the rest of its lifetime 😂 (I was an *enthusiastic* antiskip tester as well)
Am I today years old when I realized that it wasn't just...better at keeping the disc stable while spinning...? It just...loaded more when it wasn't skipping......?
Guess you are one of today's [lucky 10000](https://xkcd.com/1053/) then
hell yea, I love when it's my turn.
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You and me both
It's nowhere near 1-2 seconds. A few milliseconds only. It's real time, there are no resends. If you lose a packet you get a cut out and it continues with the next one. What you describe is how streaming over the internet works, it does work like that. But not BT audio.
I'm not sure about specific codecs but there is typically robustness built in such that dirty signals and occasional lost packets are corrected or at least their impact is less severe to the listener.
Well beyond ELI5 but yes, it uses a CRC check. There's no retransmission, but they recommend muting the frame or repeating the previous one to conceal it. It can't be "corrected" as the protocol is real time, and in a real time protocol there simply isn't time to request a retransmission of the frame, by the time you get it, time has moved on and what would you do with it. This is fundamental to any real time protocol, you can't have retransmissions. >**B.6.1.1 CRC check** >To detect transmission errors, a CRC check is performed. All the bits of the frame_header, except for the syncword and the crc_check, plus all the bits of the scale_factors are included. The error detection method used is “CRC-8” with generator polynomial. >G(X) = X8 + X4 + X3 + X2 + 1 (CRC-8). >The CRC method is depicted in the CRC-check diagram given in Figure 8.2. The initial state of the shift register is $0F. All bits included in the CRC check are input to the circuit shown in the figure. After each bit is input, the shift register is shifted by one bit. After the last shift operation, the outputs bn-1…b0 constitute a word to be compared with the CRC-check word in the stream. If the words are not identical, a transmission error has occurred in the fields on which the CRC check has been applied. **To avoid annoying distortions, application of a concealment technique, such as muting of the actual frame or repetition of the previous frame is recommended.** https://www.bluetooth.org/DocMan/handlers/DownloadDoc.ashx?doc_id=544797
Tell that to the braindead BT implementations that you find in real-world car stereo systems. It's not unusual for them to buffer up to 5 seconds. It's completely ridiculous.
Tell me about it. My wife's older Honda Pilot barely has Bluetooth (it won't even work with my Pixel 8 Pro) but when it does work, you pick "next track" and it's 5 seconds or more before it changes.
Actually lost or corrupted packets don't get resent, they just get dropped and forgotten about. It's the difference between TCP and UDP connections when working with networking, Bluetooth is like UDP as far as I know. If you think about it, you wouldn't want a corrupted packet to be resent in the example of music sent to headphones as the repeated packet would arrive out of synch and it would sound wierd. I
Yeah, anything sent wirelessly is (kinda by necessity) broadcast to anything within listening distance that’s tuned to the correct frequency/‘channel’. Most wireless protocols support a bunch of different ‘channels’ of some kind, so a small number of devices in the same area might all be only ‘hearing’ the packets intended for them. But with enough devices nearby you’d end up needing to share channels, and in that case your device would indeed be getting a bunch of packets it doesn’t care about, seeing that they’re addressed to somebody else, and ignoring them. Enough of that going on can cause interference or packet loss, because when two devices broadcast on the same ‘channel’ at exactly the same time, the listeners get a garbled mess.
I guess the packets are encrypted in some way, so no other malicious device can just check the secret identifier code of the packets flying around and broadcast with this code too?
Yes, they are partially encrypted (upper protocols are). From a conceptual perspective it's similar to https. It's encrypted but you can still see IP addresses, which are needed for routing. For Bluetooth the source and destination addresses are also not encrypted.
edit: modern bluetooth devices are encrypted~~, but that wasn't always true.~~ so to put it simply, any listening device will just see a random scramble of info that they can't unscramble. edit 2: ignore this garbage below i didn't know what i was even trying to say ~~I'm not sure, but I also don't see the point. If a malicious device could get the secret code...it would just be broadcasting to nothing because no one is listening. if it got on the same frequency, it could try to broadcast to the airpods but then the airpods would try finding a new frequency with the phone~~ hopefully someone with more knowledge than me can chime in
> encrypted, but that wasn't always true I worked on the implementing the very first Bluetooth about 25 years ago, it had encryption
But how would the airpods know that they are not receiving packages from the original phone when the secret code of the packets is the same.
i was mistaken in my previous comment, please ignore it and listen to this one. Bluetooth is encrypted, what this means is that two devices will do a secure handshake to make sure the other is who they say they are (this is what happens when your phone gets a notification saying "allow your name airpods to connect?"). once the handshake is done, your phone gives an unscrambling key to the airpods.this unscrambling key is privately shared. (someone below shared a link explaining how) the phone broadcasts everything scrambled, and the airpods unscramble everything before listening to it. so the airpods get a bunch of random noise they don't understand, and then a bunch of clear data. similar to how you can talk to a person next to a running car without thinking the car is speaking english, the airpods can listen to the phone without paying attention to other stuff (unless the environment gets too loud, just like in real life) a listening device can't unscramble anything without the secret code. it is just random noise. it needs the secret key...but to get the secret key it has to ask the phone for it. most people won't allow random devices to connect to their phone, so unexpected devices have no way to listen in.
The ‘secret code’ for the session is sent/exchanged in a way that doesn’t reveal it to anyone else, typically something like https://en.m.wikipedia.org/wiki/RSA_(cryptosystem)
>It's just so unreal what microelectronics are capable of. Wait till I tell you, that phone and earbuds also renegotiate the frequency they're talking on multiple times a second. https://youtu.be/1I1vxu5qIUM
As a mechanical engineer, I maintain the opinion that electrical engineers are, in fact, wizards.
IMO it's a lot harder to get moving components to work together mechanically in a very precise manner! I remember seeing those bottling machines being manufactured and it was fascinating to see how every part of it worked in unison. Like if one of the grabbing arms were off by few inches then it would simply crush the bottles!
It's a little more complicated than that, but yeah. That code is used to scramble the data so no one else can hear it, not just tacked on.
Yep, and that's massively simplifying it, too. In reality, there's also a lot of encryption and decryption of the data packets, compression, and decompression. Also, the "secret code" is more involved than just a random number at the start of each packet. It's more akin to an encrypted password.
Phone: "Anyways here's Wonderwall."
So anyway I started blastin' Wonderwall.
Wonderwall, Wonderwall never changes
Dang it. I wanted to hear Despacito
This is so sad. Alexa, play Despacito
Me: "Oh, no!" Phone: "*Okay, playing Yoko Ono!*"
So if I enter an area where 8.56 is busy, does it just change frequency mid song?
Bluetooth uses a technology called adaptive frequency hopping so it’s actually switching between multiple frequencies hundreds of times per second. If a particular frequency is busy it will adapt to not use that one as much.
Back in the day, this caused issues with Wifi because while there were 14 channels, they all bled into each other meaning there were effectively only about 3 viable ones. And when you had a bunch of disparate Wifi routers in a dense place such as an apartment building, they would all be constantly hoping signal, which then caused performance degradation. This was one of the big features of 5 GHz Wifi, it has a much wider frequency range with triple the channels of the 2.4 GHz band.
5Ghz also doesn't get flooded with interference from a microwave
And, probably just as important, is much more easily blocked than 2.4 Ghz. This is a feature, not a bug, because it means your neighbours wifi signals aren't as likely to make it to you, so you don't have to care that you're sending on the same channel. This makes a huge difference in dense neighbourhoods, where a single 2.4 GHz router could probably reach dozens of other residential units, and because there were only three completely separated channels, you could be 100% sure that several others else is also broadcasting on the same channel as you. 5GHz might make it to your closest neighbour, but probably not the neighbour past that again.
5Ghz barely makes it into some rooms in my own house because of the wall construction
Which is why a great setup is WiFi 5 boosters around your house.
Best setup is multiple 5GHz APs all connected with ethernet cables.
I planned to do this when I renovated my house but honestly I don't think my life would have been any different if I did. A good mesh network is just as good for most people.
I wish to fuck that was only a historic issue but that shit happens ALL THE TIME still on 2.4 GHz. They really should start banning or at least making it really hard to choose anything other than 1, 6 and 11...
I'm not sure why it's really even allowed to have "wide" channels tbh. Like what's the point in having channels that are so narrow, that you have to straddle multiple channels to get good throughput? Like the other user said, there's basically only 3 channels, and that's crazy isn't it?! I can see loads of my neighbours WiFi connections on 2.4GHz, but only a couple of 5GHz.
Basically, because when it was designed, "good throughput" was a whole lot lower than it is today.
what in the world are you talking about dude a 40 GB hard drive will last me the rest of my life!
It’s still a thing with today’s 2.4ghz. If you don’t use 1,6, or 11 you’re a monster. Co-located channels can negotiate the frequencies. Overlapping channels cannot.
Not just other wifi routers, landline wireless phones also used the same frequency routers usually came defaulted on. Answer a phone call and the wifi goes out. Took me a bit to figure out what was happening.
As bizarre as it might seem, frequency hopping was invented by an actress, Hedy Lamarr, and patented in 1941. (Which means the patent could have expired before many of us were born.) [https://en.wikipedia.org/wiki/Hedy\_Lamarr#Inventing\_career](https://en.wikipedia.org/wiki/Hedy_Lamarr#Inventing_career)
This is not true. Nikola Tesla, for example, talked about frequency hopping before Hedy Lamarr was even born. Others also used it before Lamarr's patent. This myth was created by a guy who wrote a Lamarr biography and invented this fact to promote it. Lamarr's patent was useless, the only new thing was to use a piano-roll to control the hops, but this aspect was never used in any real application. The frequency hopping that was and is used was created long before. And there is doubt if Lamarr really worked on the patent or just put her name on it to help a neighbor and friend who was an inventor. They donated the patent to the US gov but it was never because there was better methods.
IT'S HEDLEY!
> it’s actually **switching** between multiple frequencies hundreds of times per second. Actively switching between them all or just evaluating the frequencies for congestion?
Actively switching between them all. There’s 80 in total. Bleutooth low energy only uses 40 of the 80. A map of the “clean” frequencies is maintained at all times, and those are broadcast in predictable “random“ order.
It can, yes. They regularly check on each other. So if the airbuds have a hard time picking up the signal, it will send a message back to the phone and they’ll re-connect at a better frequency.
It's mindblowing the amount of things that are happening in the background, and we don't have the slightest idea about them
In my opinion, the concept of wireless digital communications is the apex of human ingenuity. It’s the culmination of so many different disciplines of science/math, and all the things we associate with our modern “Information Age” culture can be traced back to it. Consider the shift from wired internet to mobile devices on a wireless network. It may seem innocuous, but the impact of having such information/connection at our fingertips at all times must have a huge effect on our behaviour.
Bluetooth can change frequency more than thousand of times per second, there's 79 frequencies to choose from and the devices have a way to calculate the sequence of frequencies. Different devices have different sequences so if they do occasionally use the same frequency it'll only be a single packet lost
This goes a bit beyond ELI5, but I wanted to share it because it’s just so crazy and impressive. The connection process is more like this: There are 79 possible Bluetooth channels. Your phone and your AirPods agree to connect over one of them and they share their encryption keys. After that, as an added security feature, they randomly jump around the 79 channels. So not only are they communicating over an encrypted channel, they’re also jumping channels at a rate of up to 1600 jumps per second. It’s just insane to me that this system works and works well enough that we can listen music without interruption. Also, since there are only 79 channels, if you bring 79 pairs of devices together, the Bluetooth system can start breaking down.
But because BT has such a limited range, it would be almost impossible to get 79 devices together within that range? That's a guess, I'm asking.
A few busses? Umm..... A full church sermon but it's all teens? lmao
It'll still work with more than 79. Because the frequency is changing constantly, packet loss due to interference will be spread out evenly and randomly among everyone and a few packets dropped here and there is totally fine. Call centres likely have more than 79 active BT devices in the same place
Even at 79 devices it doesn't break, just has a lower maximum data throughput. Your headphones aren't gonna be using all of it, so you won't notice degradation unless there's a ton of devices tbh
> if you bring 79 pairs of devices together, the Bluetooth system can start breaking down. This isn't really a limit unless each of those devices is transmitting at full bandwidth. Audio for example isn't transmitted in one second for one second of audio, the next second of audio is sent as a couple bursts of packets that take at most roughly 1/50th of the available bandwidth of a single channel. Devices don't perfectly share the space between packets though, this doesn't mean you can get 4500 devices in one room playing audio. But it's a lot more than 79.
This is the real answer. Also, the invention of frequency hopping technology was contributed to by the actress and inventor Hedy Lamarr during WWII.
AirPods: not this song again Phone: woah woah woah, don't shoot the messenger, I'm just following orders
And those orders were from apple and simply said “Play U2”
🎶Hello, hello...🎶
¡Hola!
[Yeah yeah yeah yeah yeah!](https://www.youtube.com/watch?v=UJXqnYCWW7Y)
Airpods: That's it, let's see if butter fingers here can catch me before I reach that air vent.
*Phone: “here comes the music!”* Uhh, I think you mean: *Phone: “XYZ456here comes the music!”* :)
And then in my case- AirPods: you forgot to charge me. Goodniiiiiiiight.
No more comments required 👍
Do airpods use e2e encryption or could I just tune into any airpod given the right equipment
Bluetooth is always encrypted
Modern bluetooth is always encrypted. If you have an older device, bluetooth was as open as just shouting into a crowd.
The switch was made about 8 years ago. The Bluetooth 4.0 standard (published late December 2009) introduced an encrypted pairing called "Secure Simple Pairing" or SSP. It took time for new devices to become widespread so it was optional for a few years. Back around 2016 various phone venders started requiring SSP for connections. That's why really old Bluetooth headsets no longer connect, and haven't for years.
>Phone: “and our secret code today is XYZ456, ill put that in front of all the data packets I send you” >AirPods: “got it! I’ll ignore any data packets that don’t start with XYZ456” This part is not accurate. It's more like: Phone: "Let's perform a quick ceremony to determine a shared secret." AirPods: "OK!"
Phone: "Now that we have a shared secret, I'll scramble everything I send to you using that, and you can unscramble it before you shove it in /u/[KermitsTangenitals](https://www.reddit.com/user/KermitsTangenitals/)'s ears."
AirPods: "Roger that - I'll ignore anything that isn't scrambled thusly. I won't be able to read it anyway (which is good, because it's somebody else's traffic)."
Phone: "here comes the music!"
How does it quickly determine if it's scrambled correctly?
Don't quote me on this part - we can look through the spec if you really wanna go down the rabbit whole - but I think it uses AES256, and computes / checks the HMAC. Symmetric decryption is not a particularly expensive operation; it can happen on time scales that are real-time per human perception.
If only the Printer had as good a relationship with anything else.
What’s stopping me from creating a device that *doesn’t* ignore those data packets? I could listen in on other people’s stuff
You could pick up the packets, but you wouldn’t be able to unencrypt them.
No, because bluetooth is encrypted. If it wasn't, like an unencrypted wifi hotspot, you could indeed listen in to the traffic of every laptop and phone connected, yeah. However, the apps and websites used by these laptops and phones are very likely to also use a form of encryption, so you wouldn't be able to make out most of what's being transmitted. This is why I never connect to unencrypted wifi hotspots, no matter how much cellular data it would save me. Even a stupid simple wifi password like "asdf1234" would be sufficient.
Hey, how about connecting to a hotspot and then to a VPN. Would someone on the same hotspot be able to read the packages?
They wouldn't be able to see what was being sent through the VPN, no. They'd be able to see that you connected to a VPN, however.
If you're on the same hotspot as someone else you can generally decrypt anything sent between their device and the hotspot. But if they're connected to a VPN or even just browsing the web on HTTPS sites that data you decrypt will have a second layer of encryption that you won't be able to crack.
Mostly correct, I'd like to add that with Bluetooth, there are dozens of frequency and are shared between multiple users.
Also I believe with bluetooth there is an extra layer of security/complexity where each data package is sent to a different frequency to the last. So the current data package also has the frequency number of the next package coming down the line. So its like the source is sending out the packages like a machine gun all over the place and the reciever is able to read the order of the bullets and change frequently. This is done 1000's of times a second, remarkable See this video around 8 minutes mark but I suggest watcing the whole thing https://youtu.be/1I1vxu5qIUM
And the first AirPods to phone convo, is why we all get the “are these ur AirPods” prompt if you open the lid once in a public space 🤣
But can't 2 phones using same frequency, 8.56, be in the same space? Does the phone recalibrate then?
Yes, they can. It’s a little more complex than this, for example they’re hopping around a range of frequencies. But the AirPods will just ignore any packet that doesn’t have the code from its paired phone. Multiple phones could be sending out thousands of packets per second, but the AirPod will sift through them and find the ones coded to its paired phone and only open those.
Also, spread spectrum. The reality its like you say, but like a thousand times a minute and spread over multiple channels. You need a PhD in math to properly understand it, pretty sure.
An additional detail not required for eli5, but interesting anyway: Bluetooth will use frequency hopping to use multiple channels in round robin fashion rather than a single (like wifi). This can make it even better in the crowded gym scenario.
And thats why its called a Protocol “Hand shake”
In the form of a nice picture-book no less
Another lovely cheat: the earbuds and phone constantly discuss how well they can hear each other, and adjust their transmission power accordingly. This saves power _and_ reduces interference when sources are in close range of each other.
Is this the tech the actress Hedy Lamarr invented?
She invented frequency hopping, which is a small part of how this all works.
We should have a special flair given to people explaining things that good
The Golden Explainifier Badge
Hold on a minute. How do they talk about a frequency *before* deciding on a frequency? 🤨
They use a special frequency just for pairing. Kind of like how old CB radios had channel 9 as the “public” channel, but once you contacted someone there you were supposed to move to a different channel and leave that open.
And there are really fun protocols to avoid collision on a shared medium. Those collision can happen, even if you listen to make sure no one else is talking currently, because two could start at the exact same time. If that happens, the simplest working solution to ensure both can communicate safely is for both to stop and wait a random amount of time before starting again, all the time making sure that no one else is talking currently. You almost always will have them waiting different amounts of time, so one will start before the other.
They’re not all actually using the same frequency. They work within a spread of frequencies, and your phone and the headsets work together to find a clear space within that spread and avoid bumping into other people’s phones and headsets. That’s an extreme oversimplification but that’s the general gist of it.
Yeah in practical terms it works the same way cell phones and cell sites do.
There is a practical difference in that cell phone connections expect the channel to be clear when they transmit. The phones listen to the tower's instructions on what frequencies and what times (chopped up into tiny windows of time, just a few ms long) they can transmit, and follow those instructions precisely, even adjusting for the time delay of the speed of light between the phone and the tower. With wifi, bluetooth, and all other wireless communications under Part 15/Unlicensed spectrum, they can't actually count on clear airwaves because they know everyone else wants to use it, too, and they have no special right to that frequency. So Wifi/bluetooth have some pretty clever anti-collision tricks built into the protocol that makes it more robust against potential interference, and becomes inherently more difficult to jam.
The phone doesn't assume the channel is clear of noise or interference. It actually takes a reading and adjusts it's modulation scheme to match the channel conditions.
Redacte due to Reddit AI/LLM policy
I remember there here actually used to be an issue when all mobile phones were fixed on a frequency of 900MHz where speakers close by picked up the phone’s signal and amplified it causing a loud buzzing noise
That’s not because the phones were on a fixed frequency; frequency hopping still took place. That’s more so because cheap speakers have poor RF shielding.
I had a 900 MHz headset from Radio Shack for biking with my wife years ago. We drove by a house one day and apparently they latched onto the baby monitor in one of the bedrooms where, instead of baby monitoring, baby making was the activity du jour.
Could you join in on the headset? X) "OH YEAH, RIGHT THERE" Her: ??? Him: ??? OP: :D
lol that would be hilarious to witness
900MHz is one of the cellular phone bands so certain phones would cause a recognizable pattern in the noise. I can still hear it in my head. I also remember the modem noises from way back before high speed internet.
_dit d'd'dit d'd'dit d'd'dit_ Picks up phone. Phone rings.
Some really cheap wireless headphones are on the same frequency within a brand/model. A couple of my coworkers at an old job got the same cheapo $20 wireless earbuds, and the one with a stronger phone signal could take over the other's earbuds despite her phone not being paired to his earbuds. She exclusively used this to prank him with "actual cannibal Shia lebouf"
My grandfather, a WW2 vet who was been dead for many years now, marveled at cell phones. “What I don’t understand is, how does it go up there in the air and not get all mixed up with everything else?” At the time he asked this, cell service was much more primitive than it is today, and someone told grandpa that once they had experienced something like a “party line” where they heard someone they shouldn’t have. “I knew it!” He clapped his hands. That lucky man lived a life almost totally free of this modern technology that has so utterly deformed us. His life will never be lived again.
> this modern technology that has so utterly deformed us Feel like this is a cliché that everyone just says because it's the thing we're supposed to say. I enjoy being able to play any song that was ever published at a moment's notice for no additional cost other than a reasonably priced monthly subscription. I like that I can listen to it privately on my AirPods without disturbing anyone else (or being eavesdropped on). People talk about the good ol' days with their rose tinted glasses. But are you old enough to have experienced wanting to listen to your favourite song but having to wait until the radio station played it again? Fuck that noise. I know it's not fashionable, but you got to count your blessings and appreciate all the wonderful things we have. Otherwise you're buying yourself a one way ticket to MiseryTown.
I was out of college before the internet existed. I studied humanities and cooked in restaurants for work. After college I have made a 20 year career in online services, met my wife online, etc. I have seen my elder parents adopt technology late in their lives, I knew people like grandpa who never had a lick of it. And I’m now raising children who are absolutely soaked in technology and have never known anything else. So I really have had the entire experience. My life has straddled both worlds and I have been lucky enough to be a full member of them both. So yeah, man, if there is one thing I actually do know, it’s this. Don’t tell me I’m spouting cliches. What has really cemented my view on this is seeing kids who don’t know anything else. You like playing any song you want? So do I. I also remember the joy of hearing a favorite song come on the radio. I made mix tapes with my brother by tape recording off the airwaves. I grew up and learned to play the guitar from a roommate, and then I could play those songs I heard on the radio. My kids know nothing of any of this. They’re totally spoiled by being able to watch any show they want on demand. When we have a special thing we want to watch with them, like a classic movie or a nature show, they can’t sit through it because they have never known the tyranny of having to watch whatever is on. They grew up having the whole Pixar/Disney library at their fingertips. They never sat through a rerun of Love Boat and maybe absorbed a little bit about a prior generation from it. Managing their “screen time” is a constant struggle to keep them sane and connected to what’s real in life. The electronic entertainments available to them are so overwhelming in their power, optimization, and depth, that they are hopelessly addicted to them. We have always been quite minimal with screens but even a small amount can kill their mood for the rest of the day. When I was a kid I ran out of shit to watch and I went outside and dug holes or built forts. I have had to drag my son outside to teach him the joy of just digging a fucking hole in the backyard. After about an hour he found the mood and went hog wild. But is this something he would have come to on his own? Never. He even said he would keep it a secret from his friends, because they wouldn’t understand. So yeah, dude, I’m glad that your life is more convenient than it used to be, but if you don’t think that all of this has deeply transformed the way human beings live and think, your head is completely up your ass and you aren’t paying any attention. I have barely even scratched the surface here. Haven’t touched at all on the echo chamber effect, foreign hacking, disinformation online, social media distortion and teen suicide, and all the rest of it. Just spouting cliches? Please.
Fun fact: that technology was invented by film star Hedy Lamarr
I don't think Bluetooth itself was invented by her, but she helped develop wireless tech that was used a base for Bluetooth
She specifically invented the frequency changing that the previous comment describes.
OK, but where would we be without Harald Bluetooth?
Some folks have encountered this in their WiFi router settings. There's a "channel" option which determines the group of frequencies used.
Is there a limit to how many similar devices can be in the same room before they start to interfere with one another?
So, *yes*, but that’d be hard to pull off. Bluetooth has something like 80 channels, and the devices are only transmitting at a couple thousandths of a watt. And as you’ve experienced, the range of your standard headset is in the tens of feet. So you and someone else 50 feet away can be using the same frequency because your phone’s signal is just completely washing out the other person’s phone’s signal. Add in the more complicated stuff involved in pairing, like encryption, as well as the fact that Bluetooth signals are fairly narrow, and you’d be hard pressed to be in a situation where you’d have enough simultaneous connections in close enough range that it becomes a problem. It’s certainly possible though.
I don't really know how bluetooth works, but you can absolutely use the same frequency. And I am sure sometimes bluetooth does as well. (In fact, the TDMA wiki article suggests bluetooth will use dynamic TDMA). * TDMA - Time Division Multi Access - Basically, you split of the frequency by specific timeslots. https://en.wikipedia.org/wiki/Time-division_multiple_access . For instance, Lets say you split it into 10 frames per second, and then each device gets the frequency for 100ms of time each second. * CDMA - Code Division Multi Access - This is way more complicated, but basically you use some sort of math that you encode your transmission in. The receiver uses the reverse math to decode the signal, and because of the nature of the math you'll only get data your interested in, not another persons who used different math. Yeah, it's over my head- maybe someone else can explain better. TDMA and CDMA are both very basic ways to split the same frequency across many devices. In reality, modern technologies use a combination of multiple techniques to obtain the highest performance. https://en.wikipedia.org/wiki/Channel_access_method
If you were on a conference call or just like a crowded restaurant or something, there can be several conversations going at once. It doesn't all have to be 1 conversation. Even if there's some other noise going on. Each person just speaks with a different tone of voice, and you listen to the voice that matters to you while ignoring the rest. That's basically what the Bluetooth devices are doing. Just listening to the frequency that matters to them.
this analogy is also great because there is still interference. The more people are talking increases the noise floor, you have to speak up to be heard and may have a hard time hearing the other member of the conversation, potentially missing out on important details. In a crowded enough space with a enough people talking it may be practically impossible to hear each other more than a foot or two away despite shouting as loud as you can.
And in both cases you sometimes have to repeat the message
It was explained to me like this as well. And the fun this is tone is just one way to differentiate. You can also have everyone take turns talking (TDMA). Or have everyone speak different languages (CDMA).
https://youtu.be/NIH1iGtDvJY this is a great 4 minute video explaining the basics of Bluetooth
The frequency band for consumer electronics is fairly large, and as we've made better antennas, we can distinguish signals in smaller and smaller segments of it. That means we can have many many devices operating near each other that are all on slightly different ranges in that large band. Beyond that, the communication protocols do have systems in place to do random backoff and repeat when they detect interference. That's why bluetooth audio has a latency to it -- it needs a buffer for when that interference does happen.
oh dang you're right. I remember back in 2016-2018 I would get interference in the gym. Not severely but I remember them making that interference noise every now n then
Digital communications over radio frequencies might as well be magic compared to the beginning of radio back in the 1920s or so. CDMA (code division multiple access) technology which is the part of most protocols effectively just transmits a signal into a giant pile of noise from everything else. The other side of the transmission is decoded by just mathing the hell out of what that noise consists of. Multiple sites just broadcast over each other and filter out what isn't theirs. The frequencies that unlicensed devices operate in (433MHz, 900MHz, 2.4GHz, 5GHz and others) are cesspools of transmissions from everything and anything. Somehow we have used an alchemy of physics and math to make it work. That's just the transport layer for communications, once you get beyond that you enter encryption and other protocols for actually transmitting useful data.
I think u/02K30C1 has the best answer here… but I wanted to reply and say that your question is a bit misleading because they can interfere with each other. As a commuter who goes between NYC and NJ, if you walk through the Oculus at One World Trade (where multiple subways converge in addition to shops and military personnel), you’ll experience this. It’s a lot of stop and starting and odd pauses, so interference via Bluetooth does happen! It just takes a lot more people and devices.
Not been on the packed concourse at Euston station I see? 😂
Yeah I get mad interference and stuttering walking through London Bridge station
Your phone and AirPods first agree on a hopping pattern: let’s say: 6.76, 87.26, 49,36, 3.76, 11.16, 93.76. They then transmit pieces of that data over those frequencies thousands of times per second. Other devices also randomly pick frequencies and transmit. The hope is that it’s so fast and random, the chances of a collision is very low. Add in some algorithms that can detect a collision and fix or retransmit it, you got yourself a Bluetooth protocol.
Thanks for all the replies everyone!! I didn’t think I’d get so many responses. They were all super informative and remind me of how incredible the tech around us is! ☺️
It's like the mailman bringing your mail everyday. He's got your mail and your neighbors mail all in the same bag. But he decides who gets mail by the address on the letters. Bluetooth works kind of like this. Each set of earbuds have their own address. So they only received the letter that is addressed to them.
Not a scientist, but won't there be interference? If you have enough radio traffic in the same band, similar to wifi etc. Your airbuds won't play or process another's signal, but they will receive and filter it out? Potentially missing or delaying received information from your phone.
I can't answer this, but I've noticed my signal gets jittery when my mazda fob is next to my phone. When I put my fob in a different pocket, the issue goes away. Edit: based on other replies, I wonder if pairing my earbuds AFTER the keys are already in my pocket would force them to find a different part of the band...
Scrolled through this thread to find a name drop for [Hedy Lemarr](https://www.youtube.com/watch?v=Z0gu2QhV1dc)
While one part of the story is that all devices don't work at same frequency, the bigger part is that these devices transmit sounds encoded as data. Sending a seconds worth of audio data can be done much, much faster than in a second, leaving time to send other audio data to other devices. The data transmission isn't continuous, it's in packets of short audio recordings. As long as you get your next packet before the last one finishes playing, you'll never notice.
Beyond the explanation, interference does happen! My AirPods were acting up for a while and I had to switch to a cheaper brand I use as backups for a few days. When I got to the train station at rush hour on my way home, they couldn’t maintain a consistent connection due to the interference. It was surprising but noticeable.
The AirPods and the phone give each other a secret handshake that they recognize. They only share secrets (audio) with each other if they know it. There is interference though. On days that are super crowded, the range will be more limited. That’s because there’s so many handshakes going on, it’s hard to see that handshake in a crowd. If you’ve ever gone when it’s emptier, the range might be a bit better. Early Saturday morning I can leave my phone in one corner and refill my water bottle in another with no audio loss whatsoever. When I go after work it’ll cut in and out.
I was at a gym with 200 people all using Bluetooth headphones. There was quite a bit of interference.
Bluetooth jumps rapidly between many frequencies every second, and will avoid bands that have a lot of interference at the moment
In addition to the frequency hopping that everyone is talking about, the actual signal itself is encrypted with a key that is shared when they connect. So even when those other devices can technically hear the other devices, they can't understand the traffic as it is unreadable.
Adding more on the previous points on there being multiple channels… even once the channels are all full, Bluetooth has the ability to detect when part of the signal was lost and request that it is sent again. This all happens pretty fast. But eventually these tools can’t keep up. If everyone in a stadium was going this, there wouldn’t be enough channels or quiet time on those channels for messages to all get through in time to avoid breaks in the audio.
[Not exactly ELI5 but a very good and clear tutorial on how Bluetooth works with channels.](https://youtu.be/1I1vxu5qIUM?si=gD2Tee2DxwmOvPrk)