JNTU B.Tech IV WIRELESS NETWORKS AND MOBILE COMPUTING Previous Question Paper (October/November - 2020 - R15)

The GSM protocol architecture for signaling is structured into a hierarchical model based on the OSI reference model, specifically designed to handle the complex requirements of mobile communication, including mobility management, call control, and radio resource management. The signaling architecture is divided into the control plane (C-plane) and the user plane (U-plane), with the signaling protocols primarily residing in the C-plane.

The signaling protocol stack is organized into three distinct layers:

1. Layer 1 (Physical Layer): This layer handles the physical transmission of data over the air interface (Um interface) between the Mobile Station (MS) and the Base Transceiver Station (BTS). It involves channel coding, interleaving, burst formatting, and modulation.
2. Layer 2 (Data Link Layer): This layer is implemented using the Link Access Procedure on the Dm channel (LAPDm). It provides reliable data transfer by managing frame synchronization, error detection, and flow control between the MS and the Base Station Controller (BSC).
3. Layer 3 (Network Layer): This is the most critical layer for signaling and is divided into three sub-layers:
   • Radio Resource Management (RR): Manages the establishment, maintenance, and release of radio channels between the MS and the network.
   • Mobility Management (MM): Handles location updating, authentication, and security functions to track the user's location as they move between cells.
   • Connection Management (CM): Manages call control, supplementary services, and short message services (SMS).

The signaling flow involves communication across several interfaces: the Um interface (MS to BTS), the Abis interface (BTS to BSC), and the A interface (BSC to MSC). At the A interface, the signaling protocol uses the Message Transfer Part (MTP) and the Signaling Connection Control Part (SCCP) of the SS7 stack to transport BSSAP (Base Station System Application Part) messages, which encapsulate the RR, MM, and CM signaling information.

A conceptual sketch of the architecture shows the MS connected to the BSS (BTS and BSC) via the Um interface, and the BSS connected to the MSC via the A interface. The protocol stack at each node reflects the termination of these layers: the MS and the MSC terminate the CM and MM layers, while the RR layer is terminated between the MS and the BSC.