Hi Pan, I will copy some stuff from our documents (note: i’m not talking about LapDm, but LapD !). Basically, it clearly says that the BTS and BSC are both in charge of layer 2 supervision.
Lapd is a protocol that operates at the layer2, Datalink layer of the OSI architecture to transfer the information between Layer3 entities (in BSC, BTS and TSC) across the user network interface.
On the BTS side each TRX handles the LAPD protocol for the RSL signalling link of this TRX and the OMU handles the LAPD protocol for the OML signalling link of the BTS.
In BSC, the LAPD protocol (Layer 1 and Layer 2) is handled in a TCU.
The reset procedure (Data Link reset in LAPD specification) is performed either by the TRX or the TCU or the OMU mainly when a layer 2 frame has been
transmitted N200 times without acknowledgement.
The lapdm is handled between MS and BTS, it doesn’t do over the abis :
The Link Access Procedure on the Dm channel, LAPDm, is the data link layer protocol, (layer 2 of the OSI model), used between the BTS and the MS, on the air interface.
Ok, so for once, I was closer to the truth 🙂 Perhaps the 3GPP doesn’t describe each protocol well enough and those are Alcatel Lucent implementation.