BLE 5 compatibility with smartphones having BLE 4.0

[Deepak Patel]

Hie,

Is it compulsary for us to send only 20 bytes of data or less than that in one packet in BT5 as we used to do it in BT4 to make BT5 compatible in communicating with smartphones having BT4.

Thank You.

Regards Deepak

[RK]

No not at all. There are procedures involved in trying to increase the MTU and if you're talking to another device (whether BT4 or 5) which doesn't support longer MTUs, the procedures will fail and you'll need to fall back to the required minimum.

Just write your code to offer a larger MTU, attempt to negotiate a larger MTU and if they work, you use it, if they don't, you stick with the default.

[endnode]

Hi,

BT4.0 indeed supports PDU length extension (on Link Layer) as well as MTU extension on ATT/GATT layer. The fact that we all stick to default ATT_MTU size of 23B (from which 3 bytes are "eaten" by GATT header overhead) is just the way we deal with the fact that many deices (as well as Nordic stacks!) haven't supported these features originally. All these concepts (PDU and MTU extensions) are indeed valid in BT5.0 so there is no problem to be backward compatible (= support features from BT5.0 but by default use "legacy" modes to support connection with devices supporting only older BT versions).

Cheers Jan

[endnode]

Does BT5 allow starting connection directly with non-default PDU and ATT_MTU sizes?

[RK]

no, everything needs to be negotiated. Backwards compatibility is a blessing and a curse.