BLE Library - Add service data support

[julesDesjardin]

Hi !
I would like to use the BLE library to manage advertising data, but right now it's only possible to add service UUIDs. Would it be possible to implement service data as well ? I also have the same question for the scanning part (for this part, a direct access to the advertising data via a getAdvertisingData method on the AdvertisedDevice class would also be enough, and easier to implement I think)
Thanks !

[1technophile]

Hi,

Yes could be interesting to be able to read service data by an AdvertisedDevice, I think about examples like Mi flora who advertise sensors values on service data.

[chegewara]

Maybe im lack of knowledge and/or imagination, but how it suppose to looks like? It is implemented in Beacon already, but im guessing its not the same.

[1technophile]

Nop I don't think so : -) more a lack of explanation here is a screenshot of what i would like to be able to read with the library.
https://ibin.co/3lNX93QvWwn3.png

Currently I didn't found a method on BLEAdvertisedDevice.h to do that, maybe i m misding something.
Please note that i m using
https://github.com/nkolban/ESP32_BLE_Arduino

[chegewara]

I think it is implemented already. But it cant be done without connecting, its just implemented to do it out of box. But it is not added in library you are using. You can see this code:

esp32-snippets/cpp_utils/BLEDevice.cpp

Lines 197 to 205 in 3d39b22

	/* STATIC */ std::string BLEDevice::getValue(BLEAddress bdAddress, BLEUUID serviceUUID, BLEUUID characteristicUUID) {
	ESP_LOGD(LOG_TAG, ">> getValue: bdAddress: %s, serviceUUID: %s, characteristicUUID: %s", bdAddress.toString().c_str(), serviceUUID.toString().c_str(), characteristicUUID.toString().c_str());
	BLEClient *pClient = createClient();
	pClient->connect(bdAddress);
	std::string ret = pClient->getValue(serviceUUID, characteristicUUID);
	pClient->disconnect();
	ESP_LOGD(LOG_TAG, "<< getValue");
	return ret;
	} // getValue

[nkolban]

Howdy @julesDesjardin , @1technophile

Great questions and thanks for posting them. Lets see if we can't get this going for you. This is a technical discussion and I don't know how much or how little you know so please forgive me if it is too simple or too complicated ... let me know if I need to adjust.

I am assuming that we are thinking of the ESP32 in its role as a BLE Central (Client) where it is passively scanning/listening for arriving advertisement from BLE Peripherals (Servers) that are broadcasting/advertising.

When a BLE advert arrives at the ESP32, the advert contains up to 31 bytes of payload/advert data. On receipt of an advert, the ESP32 will parse its content and understand what arrived. Included in the advert is the Bluetooth Device Address (BDA) of the partner that sent the advert in the first place. The ESP32 can (should it choose and the partner allow) immediately send a message called a "Scan Request" which is basically "I just saw an advert from you, please send me one follow on message and tell me more" where a Scan Response message will be returned ... which is again just a second advertisement packet but containing more data.

Adverts being sent by BLE are not just arrays of bytes. An advert is composed of identified fields where a field has some set of allowable and understood values. The complete list of possible advertisement fields can be found here:

https://www.bluetooth.com/specifications/assigned-numbers/generic-access-profile

This so far is theory ...

In practice, the BLE C++ library found at this repository receives the incoming packet and decodes it to produce an object that is a logical representation of what was learned from the incoming advert. Since an advert may or may not contain certain fields ... it is 100% up to the advertiser as to what they include/exclude ... the resulting object will have some values filled in and others not set.

The object type we create is BLEAdvertisedDevice. An instance of this object can be asked:

1. Do you have a populated value for a given advertisement field?
2. What is the value for a populated received advertisement field?

So far we have provided support for fields of type:

• appearence
• manufacturer data
• name
• RSSI
• service UUID
• TX Power

However, as you will see in the BLE spec list, there are many more. We simply haven't implemented all of those because we don't know which ones folks need or want. If there is a field a device is advertising and the BLEAdvertisedDevice doesn't yet decode that field, simply let us know and we will go ahead and code it in as quickly as possible.

[chegewara]

http://www.cypress.com/blog/100-projects-100-days/project-008-gap-broadcaster

[1technophile]

@chegewara Thanks for the code extract I will try that and let you know if it answer my needs.

[julesDesjardin]

Hi,
Thanks @1technophile for your screenshot, that's exactly what I was talking about !
I managed to implement it in the library by adding methods to modify m_adv_data.p_service_data and m_adv_data.service_data_len in the BLEAdvertising. I can share it, it's pretty straightforward (only a few lines, it doesn't use the service UUID but instead directly the bytes that will be in the advertising packet) but may lack protections, and consistency with the other functions.
To be able to scan it I only added a getPayload method in the BLEAdvertisedDevice, and managed the parsing on the application side (actually I'm only using one 29 bytes long service data so I don't have to parse that much).
Anyway thank you very much @nkolban , this library works perfectly fine and it's much more easier to use an ESP32 than other bigger boards !

[chegewara]

Ok, now i see it and its easy to implement. Work in progress.

[1technophile]

@nkolban thanks for this detailled explanation !
@chegewara thanks for the PR I will give you some feedback soon.