Description
VPLMNB is the counterpart to VPLMNA, representing the Visited Public Land Mobile Network where the B-subscriber—the party receiving a call or session—is registered at the time of call termination. Defined in 3GPP TS 23.018, this parameter is dynamically determined during the call delivery and termination signaling process. When a call is routed towards a roaming subscriber, the network element responsible for locating that subscriber (typically the Home Location Register or HLR and Gateway MSC) may include or determine the VPLMNB. This identifier is then used in signaling messages sent to the Visited MSC (VMSC) serving the B-subscriber to complete the call setup.
Architecturally, VPLMNB functions within the mobility management and call control procedures for mobile-terminated calls. The process begins when a Gateway MSC (GMSC) queries the HLR for routing information (e.g., a Mobile Station Roaming Number or MSRN). The HLR, which knows the current serving network of the B-subscriber from the VLR (Visitor Location Register) address, can provide or imply the VPLMNB. This information is propagated through the signaling path. In the core network, it is carried in similar protocols as VPLMNA, such as ISUP/BICC or SIP, providing context to all involved nodes about the termination network of the call.
The role of VPLMNB is multifaceted. Primarily, it ensures the call is correctly routed to the exact MSC serving the roaming subscriber. It is also vital for generating accurate charging records on the terminating side, allowing the visited network operator to charge the home operator for the use of its radio and core resources to complete the call (terminating access charge). Furthermore, it enables specific service treatments for the called party based on their roaming status. For example, different call forwarding rules or announcement tones may be applied if a subscriber is roaming in a particular country. The parameter works in conjunction with the Mobile Station Roaming Number (MSRN) and the IMSI to uniquely identify the subscriber's current point of attachment to the network. Its consistent use is a cornerstone for reliable global call delivery to roaming users.
Purpose & Motivation
VPLMNB was created to solve the symmetrical problem addressed by VPLMNA, but for the called party. In early mobile networks, delivering a call to a roaming subscriber required the network to find the subscriber's current location, but the context of *which foreign network* they were in was not always explicitly signaled. This could lead to inefficiencies and complexities in billing for the termination of calls to roaming subscribers.
The parameter's purpose is to explicitly identify the network where a call will be terminated, which is essential for several operational and commercial functions. It allows the home network operator to apply the correct charging agreement for calls terminated to their subscriber while roaming. For the visited network operator, it provides clear identification that the terminating subscriber is a roamer, triggering the appropriate resource usage accounting and potential inter-operator charges. It also supports advanced call features, like ensuring that a call to a roaming subscriber does not inadvertently trigger a second, unnecessary international leg due to suboptimal routing decisions.
From a historical perspective, as roaming became ubiquitous, the need for a complete and symmetric set of signaling parameters became apparent. VPLMNB completed the picture by providing the visited network context for the destination of a call, just as VPLMNA did for the origin. This allowed for fully automated, accurate, and auditable call handling and financial settlement for both ends of a mobile call, which was critical for scaling international mobile services profitably and reliably.
Key Features
- Dynamic network identification for the called (B) party during roaming
- Used in mobile-terminated call delivery and routing procedures
- Critical for accurate termination charging and inter-operator settlements
- Enables location-based service logic for the called party (e.g., conditional forwarding)
- Carried in signaling between HLR, GMSC, and VMSC
- Ensures correct call termination to the serving MSC in the visited network
Evolution Across Releases
Introduced alongside VPLMNA in TS 23.018 as part of the standardized call handling for UMTS. It was defined to explicitly signal the visited network of the called subscriber within ISUP signaling, providing a complete context for call termination and enabling symmetric roaming procedures for both call origination and termination.
Defining Specifications
| Specification | Title |
|---|---|
| TS 23.018 | 3GPP TS 23.018 |