Visited Mobile Switching Center of the A-subscriber

Description

The Visited Mobile Switching Center of the A-subscriber (VMSCA) is a specific instance of a VMSC, identified within signaling protocols to pinpoint the MSC serving the calling party. In telecommunications terminology, the 'A-subscriber' is the party initiating a call (the originator), while the 'B-subscriber' is the called party (the recipient). The VMSCA is therefore the VMSC in the network where the A-subscriber is currently roaming when they place a call. This precise identification is crucial for accurate call routing, service execution, and charging in scenarios that involve multiple networks or advanced call-handling features.

Architecturally, the VMSCA is not a different physical node than a standard VMSC; it is a role assumed by a VMSC based on the call flow. Its key components are identical to a generic VMSC, including its switch fabric and associated VLR. The distinction is purely functional and contextual within the signaling messages of protocols like ISDN User Part (ISUP) or Mobile Application Part (MAP). When an A-subscriber initiates a call, their serving VMSC becomes the VMSCA for that call session. It is responsible for collecting dialed digits, performing initial analysis, and launching the call setup signaling towards the B-subscriber's network.

The role of the VMSCA becomes particularly important in complex call scenarios such as Call Forwarding. If the B-subscriber has activated call forwarding, the initial call setup attempt may be redirected. In such cases, charging records (Call Detail Records or CDRs) must accurately reflect the chain of events. The VMSCA's identity is included in these records to ensure the A-subscriber's serving network can be correctly identified for billing purposes, especially in inter-operator settlements. Furthermore, for optimal routing and to avoid tromboning (suboptimal routing paths), signaling messages may carry the VMSCA address to inform subsequent nodes in the call path about the originator's point of entry into the network.

Purpose & Motivation

The VMSCA concept exists to provide granularity and precision in call handling and accounting within multi-operator, roaming-enabled mobile networks. A simple 'VMSC' identifier is insufficient when a single call leg involves multiple MSCs (e.g., in forwarding scenarios) or when distinguishing between the networks serving the caller and the callee is critical. It solves the problem of ambiguous attribution in charging and routing logic.

Historically, as supplementary services like call forwarding became standardized, it became clear that a call could traverse several networks. For instance, a subscriber roaming in Network X (VMSCA) calls a subscriber whose phone is off and forwarded to a number in Network Y. The call path involves the VMSCA, the GMSC/HLR of the B-subscriber, and finally the MSC in Network Y. Without explicitly tagging the VMSCA, it could be difficult for billing systems to determine which network originated the call for the purpose of inter-operator revenue sharing.

The purpose of defining VMSCA in specifications is to ensure unambiguous signaling and data recording. It allows for the correct application of originating-side services and tariffs for the A-subscriber, even when they are roaming. It also enables more efficient routing decisions downstream in the call setup chain, as nodes aware of the VMSCA can potentially choose paths that minimize international or inter-network trunk usage. This level of detail was motivated by the need for fair and accurate financial settlements between operators in the increasingly complex global roaming ecosystem.

Key Features

• Identifies the specific VMSC serving the calling party (A-subscriber)
• Used within call signaling protocols (e.g., ISUP, MAP) for routing information
• Critical field in Call Detail Records (CDRs) for accurate inter-operator charging
• Enables correct application of originator-based call services and restrictions
• Supports optimal routing in complex call scenarios like forwarding
• Provides context for mobility management related to the call originator

Evolution Across Releases

Defining Specifications