yes, the term “layer 3” refers to the OSI model, which says what functions should be handled by the layer 3. But OSI doesn’t say exactly what messages or what mechanisms should be implemented.
the layer 3 for GSM is very particular, because it handles mobility, call management, etc.
The layer 3 is conveyed on SACCH, FACCH and SDCCH, and usually it contains information that are useful for the BSC (LapD protocol) and the MSC (DTAP protocol).
The layer 2… mmm… it contains some information, you can see them during drive tests, but i forgot what it is.
Why don’t you google it ?? it’s really easy to find good documents about this !
Layer 1 multiplexes the physical access to the radio channel and provides a number of logical channels
which can be used for signalling. Layer 2 is responsible for establishing a data link on these logical channels
to allow reliable transmission of Layer 3 signalling messages. It corresponds to the Data Link layer (layer 2)
in the OS1 model.
Layer 2 can operate in two modes.
0 Unacknowledged mode – used for broadcast data where acknowledgement is not possible and
measurement reports where acknowledgement is not essential.
0 Acknowledged mode – used for all connections where reliable transmission of Layer 3 messages is
important (e.g. call set-up).”