Previous Visitor Location Register

Description

The Previous Visitor Location Register (PVLR) is a critical data element within the GSM/UMTS/EPS core network architecture, specifically within the circuit-switched (CS) domain. It is not a standalone network node but a parameter, typically an address or identifier, stored in the Home Location Register (HLR) and used during mobility management procedures. When a mobile subscriber moves from one MSC/VLR service area to another, the new VLR sends a location update request to the HLR. This request includes the identity of the new VLR. The HLR then stores this new VLR address and also retains the address of the previous VLR (the PVLR) for a period. This historical record is essential for network operations.

The primary functional role of the PVLR emerges during subsequent signaling procedures. A key use case is the restoration of a subscriber's data. If the location update to the HLR is successful but the subsequent insertion of subscriber data into the new VLR fails, the HLR can instruct the new VLR to restore the data from the old VLR, which is identified by the PVLR parameter. This mechanism enhances robustness. Furthermore, during inter-MSC handovers or call delivery to a roaming subscriber, the PVLR information can be used to help route signaling messages through the correct network path, especially in complex roaming scenarios involving multiple network operators.

Architecturally, the PVLR is a field within the subscriber's data record in the HLR. Its value is the SS7 global title or network node address of the VLR that previously served the subscriber. The management of this parameter is governed by protocols such as MAP (Mobile Application Part). The HLR updates the PVLR field whenever it processes a successful location update from a new VLR, moving the current VLR address into the PVLR field and storing the new one as the current serving VLR. This two-step history is a simple but effective tool for network reliability and efficient signaling, preventing issues like orphaned subscriber contexts in VLRs and ensuring calls are correctly delivered even during transient network failures or rapid subscriber movement.

Purpose & Motivation

The PVLR concept was introduced to solve specific problems related to subscriber mobility and data consistency in early GSM networks and has been maintained through subsequent 3GPP releases. Its creation was motivated by the need for fault tolerance during the location update procedure. In a purely sequential process where a subscriber's profile is deleted from the old VLR and then inserted into the new VLR, a failure during the insertion phase could leave the subscriber without serviceable data in any VLR, causing a service outage. The PVLR provides a fallback mechanism.

By retaining the address of the previous VLR, the HLR enables a rollback or recovery procedure. If the new VLR reports a failure in populating the subscriber data, the HLR can command it to retrieve the data directly from the previous VLR (identified by the PVLR), restoring service without needing to fetch everything again from the HLR itself. This improves service reliability and reduces signaling load on the HLR. Furthermore, in complex call routing scenarios, especially for roaming subscribers, knowledge of the previous serving network node helps in optimizing and correctly routing signaling messages, ensuring efficient call setup and handover completion. It addresses limitations of a stateless mobility approach by adding a minimal layer of historical context to subscriber location tracking.