# GSM Radio Stack

![Intro GSM Stack
](/files/HEqLtR7k3y2Iia2vCG76)

## MS to BTS Protocols

Based on the interface, 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**](/gsm-interfaces/um-interface.md) − 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).

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