Visited MSC of the B subscriber

Description

The VMSCB is a core network element within the GSM and UMTS circuit-switched domain, specifically the Network Switching Subsystem (NSS). It is the MSC that currently provides service to the B-subscriber, the party being called. When a call is initiated (by the A-subscriber), the call routing process must locate the B-subscriber, who may be roaming outside their home network. The home network's Home Location Register (HLR) for the B-subscriber provides routing information, which typically includes the address of the VMSCB currently serving that subscriber. The VMSCB is responsible for establishing the connection to the B-subscriber's terminal. It performs paging to locate the mobile station within its service area, manages radio resource allocation for the call, and handles call control signaling. It also executes any call-related supplementary services (like call forwarding, barring) as per the subscriber's profile and interacts with other network entities like the Visitor Location Register (VLR) associated with it for subscriber data and the Gateway MSC (GMSC) for inter-network routing.

The VMSCB's role is critical during the call setup phase. Upon receiving a call request from a GMSC or another MSC, it authenticates the request, checks the B-subscriber's local status and service profile from its VLR, and initiates the process to alert the mobile station. It manages the establishment of the traffic channel (in GSM/UMTS) for voice communication. Furthermore, the VMSCB is involved in call handling during the active call, managing aspects like handover if the B-subscriber moves within its coverage area, and it is the point of termination for the call leg towards the B-subscriber. At call termination, it releases the resources and may generate charging records.

In the architecture, the VMSCB is a standard MSC with all its typical functions: switching, call control, mobility management, and interconnection to other networks. Its designation as 'Visited' simply indicates it is not the subscriber's home MSC; it is the MSC in the network where the subscriber is currently roaming. The 'B' specifies it is for the called party. The VMSCB works in conjunction with the VMSCA (Visited MSC of the A subscriber) for the calling party, and the overall call path involves coordination between these MSCs, possibly via a GMSC. Its operation is defined in 3GPP TS 23.018 (basic call handling) and TS 23.066 (support of Mobile Number Portability), illustrating its role in fundamental and enhanced routing scenarios.

Purpose & Motivation

The VMSCB concept exists to enable efficient routing and completion of circuit-switched calls to mobile subscribers, especially when they are roaming. In the early GSM system, a key challenge was locating and connecting to a subscriber who could be anywhere in the world. The home network (HLR) knows the subscriber's subscription but not their real-time location. The VMSCB solves this by being the dynamically assigned point of contact in the network where the subscriber is physically present. It provides the necessary local switching and radio access control to complete the call.

Prior to such structured roaming architecture, inter-network calling would be cumbersome or impossible. The VMSCB, as part of the standardized MSC/VLR/HLR architecture, allows any network to query the home HLR, obtain the address of the current serving MSC (VMSCB), and route the call directly to it. This solves the problems of subscriber mobility and network interoperability. It also centralizes call control and service execution for the called party at the local point of service, ensuring consistent application of features like call barring or forwarding based on the subscriber's profile fetched from the associated VLR.

The motivation for defining this specific functional role (VMSCB) in standards was to clarify call routing procedures and the responsibilities of different network entities in a multi-MSC, multi-network environment. It provides a clear reference point in signaling diagrams and protocol specifications for where certain actions (like paging, alerting) must be performed. This precision is essential for reliable, scalable global mobile telephony.

Key Features

• Serves as the local switching and call control point for the called (B) subscriber
• Obtains routing information from the B-subscriber's HLR via the GMSC
• Performs paging and radio channel establishment for the B-subscriber
• Executes call-related supplementary services for the B-subscriber based on VLR data
• Manages the call leg towards the B-subscriber, including in-call handovers
• Generates charging records for the call segment it handles

Evolution Across Releases

Defining Specifications