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).”