Mobile Terminating in VMSC

Description

VT, or Mobile Terminating in VMSC, is a core call handling procedure defined within the 3GPP circuit-switched (CS) domain, primarily for GSM and UMTS networks. It describes the architectural model and signaling flow for delivering a mobile-terminating call—such as a voice call from another subscriber—to a mobile station (MS) that is currently being served by a Visited Mobile Switching Center (VMSC). The VMSC is the MSC in the network where the subscriber is currently roaming, as opposed to their Home MSC (HMSC) in their subscription's home network. The VT model is integral to the mobility management and call routing subsystems.

The process begins when a call is routed to the subscriber's home network based on their Mobile Subscriber Integrated Services Digital Network Number (MSISDN). The Gateway MSC (GMSC) in the home network queries the Home Location Register (HLR) to obtain routing information. The HLR, which maintains the subscriber's current location (the address of the serving VMSC), then queries the Visitor Location Register (VLR) associated with that VMSC to obtain a temporary roaming number called the Mobile Station Roaming Number (MSRN). This MSRN is returned to the GMSC via the HLR. The GMSC then uses this MSRN to route the call directly to the VMSC serving the subscriber. The VMSC, upon receiving the incoming call setup, performs paging and authentication procedures via its associated VLR and the radio access network to locate and alert the specific mobile station.

Key components involved in the VT procedure include the GMSC, HLR, VLR, and VMSC. The GMSC acts as the entry point and routing decision point. The HLR is the central subscriber database. The VLR is a temporary local database in the visited network that holds a copy of the subscriber's service profile and location area information. The VMSC is the switch that handles the call setup, radio resource management, and switching for the call termination. This architecture offloads the home network from handling the radio-related aspects of the call and allows charging records to be generated in the visited network (for the termination leg) and the home network (for the origination and routing legs), which is crucial for inter-operator settlements.

The role of VT in the network is foundational for global roaming. It enables seamless call delivery to subscribers anywhere in the world by dynamically determining their point of attachment. It separates the call control and mobility management functions, allowing the home network to maintain control over subscriber data and authentication while delegating the physical call connection to the local visited network. This model is efficient as it establishes a direct trunk connection between the GMSC and the VMSC, optimizing transmission paths and reducing post-dial delay. The VT concept is a cornerstone of the GSM/UMTS CS core network and is specified in detail across multiple 3GPP technical specifications covering service requirements, call control procedures, and codec specifications.

Purpose & Motivation

The VT call model was created to solve the fundamental problem of delivering circuit-switched voice calls to mobile subscribers who are roaming outside their home network's coverage area. Prior to standardized cellular systems, telephony was fixed-line and location-based. The advent of GSM introduced personal mobility, necessitating a network architecture that could find and connect to a subscriber whose location was variable and unknown to the caller. The VT model provides this solution by introducing a level of indirection through the HLR and dynamic number allocation (MSRN).

It addresses the limitations of a simplistic approach where all calls might be routed to the home network first, which would then need to extend the call leg internationally to the subscriber's location. This would be inefficient, costly, and increase latency. The VT model optimizes the routing path. By obtaining a temporary local number (MSRN) in the visited network, the call can be routed directly from the GMSC to the VMSC, often over more direct international trunks. This reduces transmission costs and improves voice quality by minimizing the number of network hops.

Furthermore, the VT model establishes a clear framework for inter-operator charging and settlements. The visited network that provides the radio resources and switching for the termination can generate Call Detail Records (CDRs) for billing the home operator. This created the commercial and technical foundation for global roaming agreements. Its creation was motivated by the need for a scalable, efficient, and commercially viable architecture for international mobile telecommunications, which was a key selling point of GSM over earlier cellular systems.

Key Features

• Dynamic call routing via HLR query for subscriber location
• Use of Mobile Station Roaming Number (MSRN) as a temporary routing address
• Termination of the call at the Visited MSC (VMSC) in the roaming network
• Separation of home subscriber control (HLR) from visited network call handling (VMSC/VLR)
• Enables generation of charging records in both home and visited networks for settlement
• Foundation for seamless international roaming for circuit-switched services

Evolution Across Releases

Defining Specifications