Why does L2CAP exist in BLE stack?

[JoonDong]

I've found out that softdevice S130 cannot handle the length longer than default MTU(23).

But I know L2CAP is responsible for assembling and disassembling packet.

So why does L2CAP exist in BEL stack, If softdevice cannot handle this?

Is this a problem only for softdevice?

[endnode]

Hi,

L2CAP role is to provide "logical channels" so you could run multiple protocols on top of BT LE link at the same time. However today you have basically only "ATT" and "Security Manager" messaging defined in the specification (the latter means methods used to establish and manage Secure connection, otherwise all "meaningful" communication should happen on top of "ATT". There are "free" channel IDs (CIDs) which can be dynamically assigned to other protocols but that would mean something proprietary (at least according to BT SIG v4.0/4.1) and I've never seen it. So practically L2CAP is here mainly to be able to manage Security Manager layer (and not mix it with ATTribute and higher layers).

Cheers Jan

[JoonDong]

So does the answer means that L2CAP provide some(logical channel) of original features in BLE stack e.g. except for assembly and disassembly features?

[endnode]

Yes. Have you read the BT SIG Core specification or at least some training slides like these? I highly recommend it, might be useful.

[endnode]

And also note that L2CAP does assembly/disassembly on PDU level, ATT_MTU lives on higher layer then L2CAP so it has nothing to do with it.

[JoonDong]

thank you. It's very helpful.