3rd Generation Mobile Switching Centre

Description

The 3rd Generation Mobile Switching Centre (3G-MSC) is a core network element defined in the 3GPP UMTS architecture, specifically for the Circuit-Switched (CS) domain. It evolved from the 2G GSM Mobile Switching Centre (MSC) to handle the requirements of the 3G Universal Mobile Telecommunications System (UMTS). Architecturally, it sits within the Core Network (CN) and interfaces with the UMTS Terrestrial Radio Access Network (UTRAN) via the Iu-CS interface. This interface carries user plane traffic (e.g., voice encoded using AMR codecs) and control plane signaling for circuit-switched services. The 3G-MSC is responsible for the complete lifecycle of a circuit-switched call, including setup, management, and teardown. It performs critical functions such as digit analysis, routing, charging data generation, and interworking with other MSCs or external networks like the Public Switched Telephone Network (PSTN) or Integrated Services Digital Network (ISDN). A key component within the 3G-MSC is the Visitor Location Register (VLR), which is typically co-located. The VLR stores temporary subscriber data (like location area and service profile) for all mobile stations currently located in the geographical area controlled by the MSC. This integration allows the MSC to quickly access the information needed for call processing and mobility management without constantly querying the Home Location Register (HLR). For mobility, the 3G-MSC handles location area updates and is the anchor point for handovers within its service area and to other MSCs. It manages both intra-MSC and inter-MSC handovers, ensuring service continuity as a user moves. In the context of security, the 3G-MSC works with the Authentication Centre (AuC) via the HLR to authenticate subscribers and manage ciphering for the CS domain. It also interfaces with Equipment Identity Register (EIR) for equipment validation. The 3G-MSC's role expanded with 3G to support new services enabled by higher bandwidth, such as video telephony, though its core function remained centered on reliable, low-latency circuit-switched connectivity.

Purpose & Motivation

The 3G-MSC was created to serve as the cornerstone of the circuit-switched core network in 3G UMTS, ensuring backward compatibility and a smooth migration from 2G GSM networks. While 3G introduced a new radio interface (WCDMA) and the packet-switched domain for data, voice remained a primary service requiring the reliability and quality of circuit-switching. The 3G-MSC solved the problem of integrating the new UTRAN with the existing core network infrastructure, allowing operators to leverage their investments in GSM core networks while upgrading their radio access. It addressed the limitations of the 2G MSC by being designed to handle the specific signaling protocols (e.g., RANAP over Iu-CS) and traffic characteristics of UMTS, supporting higher capacity and the new mobility scenarios introduced with UTRAN. Historically, its development in 3GPP Release 99 was motivated by the need for a clear evolutionary path from GSM. Without a dedicated 3G-MSC, operators would have faced significant challenges in deploying 3G services, as a standard 2G MSC would not be optimized for the Iu interface or the potentially different traffic patterns and handover procedures of a WCDMA-based network. It ensured that foundational telephony services remained robust as networks evolved towards supporting multimedia.

Key Features

• Circuit-switched call control for voice and video telephony
• Mobility Management for location updating and handover execution
• Integration with Visitor Location Register (VLR) for subscriber data management
• Interfaces with UTRAN via the Iu-CS interface using RANAP protocol
• Interworking function for connectivity to PSTN/ISDN and other PLMNs
• Generation of charging data records for billing purposes

Evolution Across Releases

Defining Specifications