Layer 1, layer 2 , layer 3 , etc… it is not depending on one interface. Indeed, all interfaces are using those layers !
For instance layer 3 message on Air interface includes messages such as RACH ACCESS, HO ACCESS, MEASUREMENT REPORTS, etc.
Layer 3 on ABIS includes messages such as ESTABLISH REQUEST, MEASUREMENTS RESULTS, HO DETECTION, etc.
cf 3GPP 48.058, etc… you’ll find all standard layer 3 messages for GSM
Layer 1 and layer 2 are very rarely used for optimization, they describe the physical aspect of the transmission (bits, frames, bursts, timeslots, frequencies, etc). It is better for you to focus only on layer 3.
Check out OSI protocol stack 🙂