BLE - NUS and characteristics

RE: BLE - NUS and characteristics

Karl Ylvisaker — Wed, 14 Jul 2021 09:00:22 GMT

[quote user="Roei"]OK, now it's more cleared, thanks![/quote]

Great! :)

[quote user="Roei"]What about changes in the values? Let's say - a temperature value which is changed from time to time - how does the server sends this update to the client?[/quote]

Normally, the client have to poll the characteristics it would like to see, by reading them from time to time. This might however lead to a lot of unnecessary long transmissions, because the value has not changed.
This is where notifications and indications comes into play. By enabling notification on a characteristic, the client asks the server to notify it whenever the value changes.

You can read more about notifications and indications here.

Best regards,
Karl

RE: BLE - NUS and characteristics

Roei — Tue, 13 Jul 2021 16:14:55 GMT

OK, now it's more cleared, thanks!

What about changes in the values? Let's say - a temperature value which is changed from time to time - how does the server sends this update to the client?

RE: BLE - NUS and characteristics

Karl Ylvisaker — Tue, 13 Jul 2021 15:58:38 GMT

[quote user="Roei"]the video is very helpful![/quote]

I am happy to hear that! :)

[quote user="Roei"]I didn't understand how does the attributes which represents service/characteristics are sent to the client.[/quote][quote user="Roei"]For example - if we have the heart rate service - how does it send its attributes and rate data? How often? Does it send the whole attributes in the same message (service, characteristics, value, descriptors)?[/quote]

When the a connection is made, the GATT client will initiate attribute discovery of the GATT server, to learn what the GATT server is offering (what profiles, services and characteristics it exposes).
You can read more about this in the Attribute Discovery section of this blogpost(almost at the bottom).

Best regards,
Karl

RE: BLE - NUS and characteristics

Roei — Tue, 13 Jul 2021 15:28:42 GMT

Thanks, the video is very helpful!

After watching the GATT part of the video, I didn't understand how does the attributes which represents service/characteristics are sent to the client.

For example - if we have the heart rate service - how does it send its attributes and rate data? How often? Does it send the whole attributes in the same message (service, characteristics, value, descriptors)?

RE: BLE - NUS and characteristics

Karl Ylvisaker — Tue, 13 Jul 2021 09:33:05 GMT

Hello again,

[quote user="Roei"]Thanks, it helped me a lot![/quote]

I am happy to hear that you found it helpful! :)

[quote user="Roei"]I know that attributes contain data, but what kind of data Rx and Tx characteristics contain? Those characteristics should be attributes as well, right?[/quote]

Yes, all sections in the GATT (general attribute) table are attributes. Please see this stack overflow answer for an elaboration on this. The answer also contains a link to one of our videos which covers the essential parts of the whole BLE protocol more in depth. You can skip to the parts of the video that covers the GATT section, to learn more in depth about it.

Please do not hesitate to ask if you should have any more questions about this, or any other related topic!

Sorry for my short answers, the summer holidays have already begun here in Norway, and we are thus operating DevZone with reduced staffing.

Best regards,
Karl

RE: BLE - NUS and characteristics

Roei — Tue, 13 Jul 2021 08:18:12 GMT

Thanks, it helped me a lot!

I still have a question regarding Rx and Tx characteristics - how does it align with attributes?

I know that attributes contain data, but what kind of data Rx and Tx characteristics contain? Those characteristics should be attributes as well, right?

RE: BLE - NUS and characteristics

Karl Ylvisaker — Fri, 09 Jul 2021 18:01:40 GMT

Hello,

Please have a look at this blogpost for an in-depth explanation of how ATT and GATT relates to any service that a peripheral may be offering. The theory part in the beginning covers the topic very well, but you wont need to complete the tutorial part.

Best regards,
Karl

RE: BLE - NUS and characteristics

Roei — Fri, 09 Jul 2021 12:25:17 GMT

Hi,

How does attribute protocol related to this UART service? Can you please clarify that?

Thanks!

RE: BLE - NUS and characteristics

Karl Ylvisaker — Mon, 28 Jun 2021 12:45:38 GMT

[quote user="Roei"]now it's cleared - thanks![/quote]

Great, I am happy to hear that!

Please do not hesitate to open another ticket if you should encounter any issues or questions in the future.

Good luck with your development!

Best regards,
Karl

RE: BLE - NUS and characteristics

Roei — Mon, 28 Jun 2021 11:58:18 GMT

Ok, now it's cleared - thanks!

RE: BLE - NUS and characteristics

Karl Ylvisaker — Mon, 28 Jun 2021 11:56:51 GMT

[quote user="Roei"]Does discovery service related to NUS or device information service? Or is it a separate service which doesn't need to be declared?[/quote]

There is no separate 'discovery service', the service discovery is a process for discovering what services that a connected device provide and support.

Best regards,
Karl

RE: BLE - NUS and characteristics

Roei — Mon, 28 Jun 2021 11:37:17 GMT

Does discovery service related to NUS or device information service? Or is it a separate service which doesn't need to be declared?

Thanks!

RE: BLE - NUS and characteristics

Karl Ylvisaker — Mon, 28 Jun 2021 10:58:09 GMT

[quote user="Roei"]

I think the service discovery is the thing I don't understand well.

How does it happen?

[/quote]

The discovery of services and characteristics happens the same way as most other exchanges in the BLE communication - one of the devices sends a request packet to the other device, which fulfills the request by providing information about its services and characteristics. This is done until the entire GATT database has been discovered.
This sequence diagram shows how the exchanges for discovering characteristics work, for example.

[quote user="Roei"]Or via advertisement channels?[/quote]

Service and characteristics discovery does not happen on the advertising channels, no. This only happens when already in a connection.
The primary thing transferred on the advertisements channels is advertisements, scan responses, scan requests and connection requests.

[quote user="Roei"]Should the other side ask for the Nordic device to transmit this data?[/quote]

Yes, the central device will need to initiate the discovery, like shown in the message sequence charts I referenced earlier.
You can see this done in most of the central examples from the SDK as part of the BLE_GAP_EVT_CONNECTED event handler.

Best regards,
Karl

RE: BLE - NUS and characteristics

Roei — Fri, 25 Jun 2021 08:12:54 GMT

I think the service discovery is the thing I don't understand well.

How does it happen? Does the Nordic device transmit those services via data channels, so they will be discovered? Or via advertisement channels? Should the other side ask for the Nordic device to transmit this data?

Thanks!

RE: BLE - NUS and characteristics

Karl Ylvisaker — Fri, 25 Jun 2021 08:07:59 GMT

[quote user="Roei"]Thanks for your answer, it helps me a lot![/quote]

No problem at all, I am happy to help!

[quote user="Roei"]What about other characteristics which are discovered by other side? How can they be transmitted if not via BLE NUS?[/quote]

I am not sure that I understand your questions here. Could you elaborate?
The services and characteristics are discovered during the "service discovery" phase of the connection, and afterwards the devices may for example enable notification on a characteristics (such as done in the BLE NUS examples). Notifications / notify means that the connected device will be sent the new value of the characteristic any time it is updated.

Best regards,
Karl

RE: BLE - NUS and characteristics

Roei — Fri, 25 Jun 2021 07:58:57 GMT

Thanks for your answer, it helps me a lot!

Actually - I have another question:

What about other characteristics which are discovered by other side? How can they be transmitted if not via BLE NUS?

RE: BLE - NUS and characteristics

Karl Ylvisaker — Fri, 25 Jun 2021 07:32:02 GMT

Hello,

[quote user=""]

I know that the characteristics are used to expose data, but we also have BLE nus which is used to send and receive data.

Is there any connection between them?

[/quote]

Yes, BLE NUS is actually short for BLE Nordic UART Service, which implements a service with two characteristics simulating the RX and TX pins of a physical UART interface. You can see the BLE NUS demonstrated in the BLE NUS example from the SDK.
The service contains the two characteristics which are used to actually exchange the information between the devices, by notifying data on the two RX and TX characteristics.

Please do not hesitate to ask if anything still should be unclear, or if you have further questions!

Best regards,
Karl