the GSM signaling protocol is assembled into three general layers:
Layer 1 : The physical layer. It uses the channel structures over the air interface.
Layer 2 : The data-link layer. Across the Um interface, the data-link layer is a modified version of the Link access protocol for the D channel (LAP-D) protocol used in ISDN, called Link access protocol on the Dm channel (LAP-Dm). Across the A interface, the Message Transfer Part (MTP), Layer 2 of SS7 is used.
Layer 3 : GSM signalling protocol’s third layer is divided into three sublayers:
Radio Resource Management (RR),
Mobility Management (MM), and
Connection Management (CM).
