Why is Bluetooth so slow?

Hey,

A few weeks ago, I tried transferring a “large” file (0.5 GB) from device A (phone from 2019) to device B (laptop from 2020) via Bluetooth. I don’t know how much time it took… Maybe 45 minutes… Device A was literally on top of device B. The distance between both was zero.

Since then, I’m asking myself: WHY?

My devices can up- and download stuff (via WLAN) pretty fast. Even if my “WLAN network speed” was only 2 megabytes/second (it’s better than this), it’d take around 4 minutes.

Was this just “bad luck” (bad hardware, buggy hardware, broken hardware) or is the Bluetooth protocol actually a lot slower than the alternative (Transferring it “through the router” to a different device)?

If yes, why? Instead of WLAN (Device A -> 4 meters distance -> router -> 4 meter distance -> Device B), it’s just (Device A -> Device B) and I’d expect it to be faster TBH…

accideath,

Bluetooth is not really made for transferring large amounts of data. While the current WiFi standard, WiFi 6e, supports theoretical data rates of up to 9.6Gbps, the current Bluetooth standard, 5.1, only supports up to 50mbps.

This is because Bluetooth is focussing on low energy data transmission and in fact, the LE profile of Bluetooth 5 only supports up to 2mbps. For the kinds of devices it is made for (headphones, smartwatches, etc) that is plenty and the advantage to battery life has priority.

WiFi on the other hand is purpose built for a higher data throughput. Bluetooth was never really meant for transferring large files, it just happens to be capable of doing so. There’s a reason Apple for example implemented their AirDrop feature years ago because making the handshake via Bluetooth yet sending the actual data via WiFi is a much better solution.

The distance to the other device doesn’t matter so much for speed as long as it’s not too far away and not blocked. You can imagine it like that: if you take a 10km hike from place A to B, that is the shorter distance. But if you get into your car and drive to town C that’s 30km away from either A and B and then on town B, all on a highway, you’ll be in town B 4x as fast as on foot but you‘ll have burnt 5 litres of gasoline.

ndsvw,

That AirDrop fact sounds interesting I had never questioned how it works…

kpw,

Why didn't you use the WLAN for data transfer?

HarkMahlberg,
HarkMahlberg avatar

(Transferring it “through the router” to a different device)?

Yep. I transfer files between my phone and my computer by starting up an SFTP server on the phone (Amaze, Android) and connecting the computer to the phone (Filezilla, Windows).

moody,

Bluetooth is meant for low-power applications. We’re talking 2.5 milliwatts of power on the high end. For that purpose, it’s a low-bandwidth, short range protocol.

Teknikal,

I dunno Bluetooth kinda blew me away recently but it was when I bought a new phone. The range of my earbuds suddenly started working perfectly even when I think I was about 50 meters away.

I presume the newer Bluetooth version made this happen when before it wad like maybe a room away maybe 15 meters at best.

shortwavesurfer,

Bluetooth is not meant for large file transfers. It is meant for very small amounts of data and uses very narrow bandwidth channels, which makes the speed quite slow.

SlurpDaddySlushy,

It was designed for low power short range use. This made it ideal for electronics back in the day when small batteries with huge capacity didn’t exist. It’s also pretty easy to interfere with a bt signal and lower the transfer speed even more.

The reason it stayed this way is because it’s not really worth the investment to make it have high transfer speeds because there are already better alternatives.

agressivelyPassive,

Because Bluetooth is simply a different protocol for a different purpose. It’s intended for low latency, short range, low power, high stability. And this comes at a cost.

A given protocol can’t just arbitrarily increase speeds. It’s limited by factors like bandwidth, modulations, etc.

ndsvw,

I knew about short range, not about the others…

StreetKid,

Because that is how Bluetooth is specified. Bluetooth is primarily used for audio, where high throughput is not the most important parameter. Expected Bluetooth throughput is less than 2 Mbps, see more here bluetooth.com/…/exploring-bluetooth-5-how-fast-ca…

WiFi on the other hand, has peak throughput above 1 Gbps. So your observations are as expected.

LWD,

deleted_by_author

    •
    cmnybo,

    It’s not possible to get zero latency with bluetooth, but you can get around 30-40 ms when using AptX-LL or AptX Adaptive with a hardware AptX transmitter.

    ABCDE,

    Comparing distance from devices is not important (within reason), but limitations of the hardware. Bluetooth has a low limit, WLAN is high (two megabytes/bits is likely your internet speed, are you talking about sending from devices to your computer?). If you are uploaded to the internet, four minutes at 2MB/s sounds correct.

    If you are sending from a 2019 device, it will top out at the max speed it can do, which may be under 1Mbps.

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