I want to send as much serviceData as possible in ble advertisement , but when I set it into ServiceData, if data length is more than 18bytes, it did not send out any serviceData, and noting wrong occured, I am puzzled. I can not find any infomation from MircosoftDocs for max length for serviceData, is there any notice ？ code demo: auto advParameters = ref new Bluetooth::GenericAttributeProfile::GattServiceProviderAdvertisingParameters(); advParameters->IsConnectable = true; advParameters->IsDiscoverable = true; auto writer = ref new Windows::Storage::Streams::DataWriter(); writer->WriteString("123456789012345678"); // success //writer->WriteString("12345678901234567890"); // fail to send out servicedata, but noting wrong occured advParameters->ServiceData = writer->DetachBuffer();

Hello, Welcome to Microsoft Q&A! Based on the document Gatt Server , there is a note for publishing service with a parameter that said: When a service is both Discoverable and Connectable, the system will add the Service Uuid to the advertisement packet. There are only 31 bytes in the Advertisement packet and a 128-bit UUID takes up 16 of them! So, if the service UUID is added, there are only 15 bytes left for the payload that you could use. Thank you. If the answer is the right solution, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment". Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.

Hi @frank zhang , I set private data into ServiceData and advertise it(no matter the ServieData's length is over than 18bytes), but I can not parse it from the gatt client after received the advertisement.\ Is there any parameter should be enabled? Could you share the way to use GattServiceProviderAdvertisingParameters's serviceData section to sent the private data to client and how to parse it from the gatt client?

what is the max length for serviceData in Advertisement? (GattServiceProviderAdvertisingParameters)

Accepted answer

Roy Li - MSFT 31,766 Reputation points Microsoft Vendor

2022-05-09T02:59:34.563+00:00

Hello,

Welcome to Microsoft Q&A!

Based on the document Gatt Server, there is a note for publishing service with a parameter that said: When a service is both Discoverable and Connectable, the system will add the Service Uuid to the advertisement packet. There are only 31 bytes in the Advertisement packet and a 128-bit UUID takes up 16 of them!

So, if the service UUID is added, there are only 15 bytes left for the payload that you could use.

Thank you.

If the answer is the right solution, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
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frank zhang 121 Reputation points

2022-05-12T07:24:45.217+00:00

I know that not only advertisement can contain service data, but also scan response(also 31 bytes) can contain service data.
How to enable it , and put more service data into scan response as well? thanks.

Roy Li - MSFT 31,766 Reputation points Microsoft Vendor

2022-05-13T01:43:35.813+00:00

@frank zhang I've consulted the team about your requirement. According to the response I got, the scan response data is not directly controllable from the App. If you want more data payload, you could enable extended advertisement ( which is only available on systems that have radios that support this on both sending and receiving sides). The document is here: BluetoothLEAdvertisementPublisher.UseExtendedAdvertisement Property. This is a limitation of the technology unless extended advertisement is used.

frank zhang 121 Reputation points

2022-05-13T03:05:31.457+00:00

Thank you for your reply.
I know there is another api BluetoothLeAdvertisementPublisher can also start a advertisement publisher, but it can not set service uuid/data in advertisement, and can not publish ble service . It seems that advertisement from BluetoothLeAdvertisementPublisher can not be "connected" ?
So what I am puzzled is that why uwp api design two api to start advertisement ?( ServiceProvider->StartAdvertising and BluetoothLeAdvertisementPublisher->start()), should both of them start together ?

Roy Li - MSFT 31,766 Reputation points Microsoft Vendor

2022-05-13T06:15:24.997+00:00

@frank zhang For the first question, yes, the BluetoothLeAdvertisementPublisher.start() only publishes advertisements. You could not connect to that and respond to the advertisements. You could only receive advertisements for the publisher. It's one-way communication.

For the second question, Publishing advertisements and Publishing services are totally different functions. ServiceProvider does not publish advertisements, ServiceProvider advertises Bluetooth services. You could receive data from the service and respond to the service. This is two-way communication.

frank zhang 121 Reputation points

2022-05-13T10:45:26.06+00:00

For the second question, Publishing advertisements and Publishing services are totally different functions. ServiceProvider does not publish advertisements, ServiceProvider advertises Bluetooth services. You could receive data from the service and respond to the service. This is two-way communication.

Are you sure for that above? I have tested that my win10 pc , run program after set ServiceProvider->service then ServiceProvider->StartAdvertising() , it can publish advertisement, and can be discovered and connected by Android phone. And I think Android phone discover win10 pc by scan advertisements come from win10 pc.

demo code:
auto advParameters = ref new Bluetooth::GenericAttributeProfile::GattServiceProviderAdvertisingParameters();
advParameters->IsConnectable = true;
advParameters->IsDiscoverable = true;
if (serviceDataLen > 0) {
auto writer = ref new Windows::Storage::Streams::DataWriter();
for (int i = 0; i < serviceDataLen; i++) {
writer->WriteByte(serviceData[i]);
}
advParameters->ServiceData = writer->DetachBuffer();
}
serviceProvider->StartAdvertising(advParameters);

By the way, I found that server can not get official connected event from serviceProvider, it that right ? What I can do is let master do a notifyChanged(SubscribedClientsChanged) action to mention that a master is connected?
Thank you very much, waiting your reply. ..

Roy Li - MSFT 31,766 Reputation points Microsoft Vendor

2022-05-16T05:42:40.077+00:00

@frank zhang The Gatt server is publishing a Connectable and Discoverable service. The document GATT server has a clear statement about this.

When the GATT server publishes services, other GATT clients could send requests to the server. But the server does not have such an event like connected. It just handles the requests from the client. The server could read and write to the client as a response in the GattLocalCharacteristic.WriteRequested Event and GattLocalCharacteristic.ReadRequested Event. The GATT server document also give a full example about this. Please kindly check that.
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what is the max length for serviceData in Advertisement? (GattServiceProviderAdvertisingParameters)

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