Home Public Land Mobile Network of the B subscriber

Description

The Home Public Land Mobile Network of the B subscriber (HPLMNB) is a specific application of the HPLMN concept used within call and session control logic. It refers explicitly to the home network of the called party, or callee, in a telecommunications session. This identifier becomes relevant during the setup and control of a call, message, or data session where the originating party (A-subscriber) is attempting to communicate with the terminating party (B-subscriber). The network elements involved in routing and servicing the call, such as Mobile Switching Centers (MSCs), Service Control Points (SCPs), or IMS Call Session Control Functions (CSCFs), may need to identify the HPLMNB to apply specific routing rules, billing policies, or intelligent network services.

From an architectural perspective, the HPLMNB is derived from the B-subscriber's IMSI or Mobile Station International Subscriber Directory Number (MSISDN). When a call is placed, the originating network performs number analysis to determine routing. If the B-subscriber is a mobile user, the network identifies their HPLMN from their number or IMSI. This information is carried in signaling messages, such as ISUP or SIP, as the call is routed. In intelligent network architectures like CAMEL, the HPLMNB is a key parameter. For instance, the B-subscriber's HPLMN may have a CAMEL subscription that triggers a service logic instance in their home network to control how incoming calls are handled, such as applying call forwarding, barring, or prepaid charging for the receiving leg.

How it works involves several network interactions. In a simple mobile-to-mobile call, the Gateway MSC (GMSC) in the A-subscriber's network queries the B-subscriber's HLR/HSS (located in the HPLMNB) using the MAP_Send_Routing_Information operation. The HPLMNB's HLR responds with a roaming number, allowing the call to be routed to the MSC currently serving the B-subscriber. For value-added services, the HPLMNB's intelligent network platform may be invoked. The identification of the HPLMNB is therefore critical for determining jurisdiction—whether the call is on-net (within the same PLMN) or off-net, which directly impacts interconnection charges, billing tariffs, and the application of subscriber-specific service logic for the called party.

Purpose & Motivation

HPLMNB exists to enable accurate and feature-rich handling of the terminating side of a call based on the called subscriber's home network affiliation. Early telephony systems primarily focused on routing calls to a destination number. However, with the advent of mobile networks and intelligent services, it became necessary to treat the called party as a subscriber with their own service profile hosted in a specific home network. The HPLMNB concept allows the network to recognize this and incorporate the B-subscriber's home network policies into the call flow.

This solves several problems. First, it enables correct billing and accounting for the terminating leg of a call. Inter-operator settlement rates often depend on the networks involved. Identifying the HPLMNB allows the originating network to correctly calculate charges for delivering the call to that specific operator's subscriber. Second, it allows the B-subscriber's services to execute. Without identifying the HPLMNB, features like incoming call barring, personalized ringback tones, or prepaid call acceptance controlled by the B-subscriber's home network could not function when the subscriber is roaming. The HPLMNB parameter ensures that the call signaling can be routed to the appropriate service logic in the correct network, ensuring a consistent service experience for the called party regardless of the caller's location or network.

Key Features

• Identifies the home network of the called party (B-subscriber) in a communication session
• Derived from the B-subscriber's MSISDN or IMSI during call routing procedures
• Used by Gateway MSCs and IMS nodes for querying the B-subscriber's HLR/HSS for routing information
• Key parameter in CAMEL and other IN protocols for triggering B-party service control
• Determines jurisdictional boundaries for billing and inter-operator settlements on the terminating side
• Enables the execution of the called party's subscription services, such as call forwarding or barring

Evolution Across Releases

Defining Specifications