Can you do GATT caching with a 4.2 server and a 5.1 client?

[Andres]

We have a device based on NRF52832, SDK 13.0, Softdevice S132 v4.0.5. We can't upgrade the Softdevice at this time but we can modify the firmware if necessary.

We know that Bluetooth 5.1 has a feature to allow for GATT caching to speed up the connection between the server  and the client.

If the client, e.g., a smartphone, already has Bluetooth 5.1 and the server, e.g., our device, has Bluetooth 4.2, would it still be possible to have GATT caching?

What can the 4.2 device do to tell the 5.1 smartphone to do GATT caching?

[Andres]

Hi Kenneth,

Our product does not support bonding. And it is going to be very difficult to upgrade to a recent softdevice for a number of reasons.

My next option is the source of my question, namely, continue with a softdevice which supports BLE 4.2 and find out if a central running BLE 5.1 will be able to do the GATT caching. 

You raise an interesting point, is it correct that a softdevice that supports BLE 5.1 will still not be able to handle GATT caching? Is there a document that may explain more about what features Nordic supports for BLE 5.1?

Thank you,

Andres

[Kenneth]

Andres said:

You raise an interesting point, is it correct that a softdevice that supports BLE 5.1 will still not be able to handle GATT caching?

It's an optional feature, so you are right.

Andres said:

Is there a document that may explain more about what features Nordic supports for BLE 5.1?

You would need to go into the main page of each of the softdevice specifications to get the hightligthed supported features, e.g.:
https://infocenter.nordicsemi.com/topic/sds_s132/SDS/s1xx/s130.html

I think it's the second time I have seen a request for GATT caching so it doesn't seem as feature that is much used, so I guess the additional softdevice size, development and test time have not been prioritized as important enough to go into a softdevice release (at least not yet).

I am not sure if I have a quick fix for you either, you could possible do it the other way around, e.g. that the mobile phone is the GATT server and the nRF is the GATT client, then you could potentially do some "shortcuts", for instance if you reconnect to a previously connected mobile phone you can read a specific characteristic directly from the nRF to "see" if the GATT table have changed.

Kenneth