Mobile Station of the B subscriber

Description

The Mobile Station of the B subscriber (MSB) is a logical entity defined within 3GPP specifications for circuit-switched (CS) telecommunication services, primarily in GSM and UMTS networks. It specifically refers to the mobile terminal (or user equipment) that is the destination of a call or session—the called party. Within the network architecture, the MSB is not a separate physical device but a role assumed by a Mobile Station (MS) during call termination and control procedures. This role is crucial for the network's Call Control (CC) and Mobility Management (MM) entities, such as the Gateway MSC (GMSC) and terminating MSC, to correctly route the call, apply terminating subscriber policies, and trigger appropriate billing records.

The operational identification of the MSB is tied to the subscriber's International Mobile Subscriber Identity (IMSI) and the associated Mobile Station Integrated Services Digital Network Number (MSISDN). When a call is routed, the GMSC queries the Home Location Register (HLR) to obtain routing information (the Mobile Station Roaming Number, MSRN) to reach the serving MSC of the MSB. The terminating MSC then identifies the MS as the B-subscriber. This triggers specific signaling flows on the A-interface and within the Core Network using protocols like ISUP/BICC. The MSB's role influences the execution of terminating call screening, call forwarding services (CFB, CFNRy), and the application of any subscriber-specific barring or answering services.

In the context of supplementary services, the MSB is the entity upon which services like Call Forwarding Busy (CFB), Call Forwarding No Reply (CFNRy), or Call Hold are invoked as a terminating condition. The distinction between B-subscriber and A-subscriber (MSA) is architecturally significant for the correct application of call legs and states within the MSC, especially for features like Call Waiting and Multi-Party calls. While the concept is deeply rooted in traditional telephony, its principles underpin call state models and charging data records (CDR) generation, where parameters like 'called party number' and 'B-party charging identifier' are populated based on the MSB. With the evolution towards all-IP networks and IMS, the functional role of the B-subscriber is managed by the UE and the S-CSCF using SIP protocols, but the legacy MSB concept remains relevant for CS fallback, interworking, and legacy service emulation.

Purpose & Motivation

The MSB concept was created to provide a clear, standardized model for handling the terminating party in a circuit-switched mobile call. In early GSM systems, telephony was the primary service, and network elements needed an unambiguous way to identify the callee for fundamental operations like routing, service invocation, and billing. Defining the B-subscriber role allowed for consistent implementation of call termination logic across different vendors' MSCs and ensured that charging systems could accurately attribute costs or apply terminating charges.

Prior to standardized cellular systems, telephony switching relied on similar concepts, but mobility introduced the challenge of locating a subscriber who could be anywhere. The MSB, as part of the mobile station abstraction, solved the problem of dynamically routing a call to a subscriber's current location via HLR queries and MSRN assignment. It enabled advanced supplementary services to be executed correctly based on whether the subscriber was the recipient of a call, such as invoking call forwarding when the MSB is busy or not reachable. This distinction was essential for the commercial deployment of mobile networks, forming the basis for detailed billing records (e.g., for call termination) and lawful interception mandates related to call reception.

The motivation stems from the need for a robust administrative and operational framework symmetrical to the A-subscriber. By logically separating the B-subscriber, 3GPP specifications could define precise behaviors for call delivery, alerting, and answer/hang-up procedures. This model supported the growth of mobile telephony by ensuring interoperability and clarifying the network's responsibilities towards the receiving party, a foundation that later extended to services like voicemail (which acts as a B-subscriber proxy) and number portability resolution.