How to write a "big" data characteristic (for example 64kB)

[nastiliano]

Hello everyone and happy new year!

I'm trying to send a "data" characteristc (like data sockets) and I was reading something like this but it doesn't work:

    async with asyncio.timeout(10):
        while characteristic.max_write_without_response_size == 20:
            char = "19b10002-e8f2-537e-4f6c-d104768a1214"
            data = 
"dfdlkfjwjflwdkjfwlfkjwflñwedkjfdñlfkdsjfñldskdfkñldñljñddkjdlkjldskdlkjñlkdñlksñldffndfdndfdsnfdñfldkfdñflkdfdñlfdnfdñlfdknfñdlfkdnfñdslfkdnfñdlfdknfdñlfkndfñldfkdnfñdslfdksnfdsñlfdkfndsñlfkdnñdsfknfñdskfnñflkfndfñdlkfdnfñlfkdnfñdlkfndfñdlkfndfñldfkdnñdlkfdfñdlsknfñdlkfndñlkfdnfñldskfnfñdslkfndfñdlfkdnfñdlkfndfñdslkfnñdslkfdnfñdlfkdnsñdlkfndsfñldkfsjdfdñslfkjfñldkjfdñflkjfdsñfldksjfdsñflkfjdslkfjsdñflkjsdñfldskjfdsñfldksdsñkdsjdsflkjdsñljdfñdslkfjdfñlkdjfñdlkfjdñflfkdjsfñdslfkjdfñldskjdsñlfkjdsñflkdsdñfkjdfdñlfkdjfñlfkjdsñlfkjsdñlkdjfdñfjdsñlfkjdñlkdslñdjfdslfjfjlfdffdfdldfkjdñdkjdflskfdfoirforefjefrlvknjfr`goihòigj`fsoigjòeje`rogijre`gooerjgèrogijreòigjèroijgreogijgèrgoiejrgèorirjgreògijerògijergeogrigjre`goijre`goirejgòerijgèrgoijg`roigjgòreigjre`goijr`gorgjreògriejgreògirjgrèogjg`rogijgrèogijre`jgiorg`reogjgèorgjireo`gijreo`gijrego`rgijg`rflñkfdgjfglfkdjgfñglkdfjgfñlgkfdjgfñdlkgjgfñgkgñlfdkñfkgñlgfkjgfñlgfgfñlgkjgñfdkfñgfkjfdfkjfñlgkfdflgkfñfdkjgñlfdkfgfkdfdñfkgfñdgkdñfdkfglfdkfñkjñflgkjgñlfkgjfglfjfñlfdjdfkjfdlkfdflgkjfñlfkgjopgetrijgtropitgtrogitrogitggpoigjpgotijgtrogtrpjlfdgkjfdñgfdlkfdñfkfdñkffñlgkfjfñlfgdjlfgfgpofgijgpogigjpofgijfpoijgopgfijgpogfijgpogfijgpofgdijpgfoigfjpoigfjgpofgdijgpogfijgpofgijpgfoigjgfpoifgjpgfoifgjgpoigfjgdpofigfjgfpoigfjgfoijgpgofijgpogfigjgpfoigfjgfpoigfjgfdopigjopgfijgpogfijgoigfjpoigfjgpfoigfjpgofijgpogfijgpogfijgopgfijgpogfijdpogfijgpofgijpogfigfjgopigfjgpoigjgpfoijggpofijgpgofijggfopijgpgfoijgpogfijgpgfpgfpogfopgfofgojfgoijgfjiogfjiogfjiogfjiogfjpoigfojpigfopjfgpogfjgfopgfjpoigfjfpoigjfpgofijfopgifjgfpoigjfgofigjfgpfdgjofgfpoigjfpogfigjfgpofigjfpogfijgfofigjgfopigjgfdopgijgfpodgijgpofgijfgopfigjfopgifjgfopgijgfpogifgjfpogifjgfpogifjgfpogfigjfpgoifgjfdogpifjgfgfoigjfgpfoigjgpofidjgpogifjgfdpogifjgpgoifjgpogfdigjfdpogijfdopgigjdfgpfoigjfdpgofijgfdpogfijgfofgigjfdpogifjgpofdigjfopigjfdpogifjgfpogifjgpofigjfdpogijfpogfijgfpogijfdpogifjgpfoigjgpoigfdjfogipeoigjròiggjerogijreògrjigèogireijgrògregjigèoigjròeijrèoj`goeijgòerijggèroiregjerògijgèroijfjñdskdlfkdñdfkjd".encode()
    
    await client.write_gatt_char(char_specifier = char , data = data)
    await asyncio.sleep(0.5)

Apperars an error in terminal:

Protocol Error 0x09: Prepare Queue Full

If I do simply:

await client.write_gatt_char("19b10002-e8f2-537e-4f6c-d104768a1214" , "1".encode())

works fine

Could you help me?

Thank you very much.

[dlech]

This is not what BLE was designed to be used for.

But if you want to do it anyway, you need to build your own protocol on top of GATT.

Writing with response is limited to 512 bytes and writing without response is limited to characteristic.max_write_without_response_size. So you need to divide your data into chunks of that size and send one at a time. If you do write without response, you will also want to send a chunk number to be able to detect dropped packets and it will need some kind of notification to provide backpressure for flow control.

[nastiliano]

Hello, thanks for your fast response!

I've been reading and I figure out that in ESP32 exists an BLE UART mode.
May be it would be easier implement that mode?

Thank you very much.

[dlech]

It depends on the requirements of your application. If you don't mind a slow data rate, this could be just fine. If you want to maximize throughput and not lose data, then you might need something else.

[nastiliano]

Hi again.

I don't really need high speeds or high amounts of data.
Tomorrow I'll look into the 2 modes to see if I can find something. I'll look into data slicing and I'll also try to see if I can implement the UART version.

Thank you very much.

[nastiliano]

It worked with slicing data!
I didn't use the UART mode.

Thank you very much, best regards.