Mobile Switching Centre Server

Description

The Mobile Switching Centre Server (MSC-S) is a core network element introduced as part of the 3GPP All-IP network and IP Multimedia Subsystem (IMS) architecture. It represents the evolution of the traditional monolithic MSC by separating the call control (signaling) functions from the media switching (bearer) functions. The MSC-S is responsible for all call control and mobility management signaling for circuit-switched services, such as voice calls and circuit-switched data. It communicates with User Equipment (UE) and other network nodes using signaling protocols like BSSAP over the A interface for 2G access, RANAP over the Iu-CS interface for 3G access, and ISUP/BICC for interconnection with other networks.

Architecturally, the MSC-S works in conjunction with a Media Gateway (MGW) under the control of the Media Gateway Control Function (MGCF) or directly via the Mc interface using protocols like H.248 (Megaco). The MSC-S instructs the MGW to establish, modify, and release media connections (bearers) for user traffic, which is transported over IP or ATM networks instead of traditional TDM circuits. This separation allows the MSC-S to be deployed as a software-based entity on standard servers, enhancing scalability and flexibility. The MSC-S also contains the VLR functionality for managing visiting subscriber data, performing authentication, and handling location updates.

In operation, the MSC-S processes call setup requests, performs number analysis and routing, manages handovers (including SRNS relocation in UMTS), and executes supplementary services. It interfaces with the Home Subscriber Server (HSS) or HLR for subscriber data. For emergency calls, it may include specific routing functions. The MSC-S supports both legacy circuit-switched services and enables convergence towards IP-based networks, acting as a bridge during the transition from TDM to all-IP core networks.

Purpose & Motivation

The MSC-S was created to address the limitations of the traditional monolithic MSC, which combined control and switching in a single, often proprietary, hardware platform. This architecture was inflexible, costly to scale, and hindered the introduction of new services. The move towards All-IP networks demanded a separation of concerns to leverage IP transport and standard computing platforms.

Its development was motivated by the need for network modernization, reducing operational costs, and enabling service innovation. By separating the control plane (MSC-S) from the media plane (MGW), operators could scale and upgrade each independently, deploy services more rapidly, and transition from TDM to packet-based backbones. The MSC-S is a key element in the 3GPP's core network evolution strategy, facilitating the convergence of fixed and mobile networks and paving the way for IMS-based multimedia services.