Visitor Location Register Fallback for Mobile Network Call Routing
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Solution Overview
Problem
In public land mobile networks, such as GSM and UMTS, the failure of the Home Location Register (HLR) disrupts the routing of mobile terminating calls, as it is responsible for providing location information essential for call setup, leading to call interruptions and authentication failures.
Innovation Solution
The method involves requesting information from a Visitor Location Register (VLR) when the HLR is unavailable, converting the Send Routing Info (SRI) procedure to a Provide Roaming Number (PRN) procedure, allowing the network to route calls even if the HLR is down, by utilizing the visitor database's temporarily stored subscriber data within the same service area.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the HLR is used to provide location information for routing calls, then call routing functionality is ensured, but the system becomes vulnerable to single point of failure when HLR is unavailable
Solution Approach 1:
The VLR pre-stores location information and subscriber data before HLR failure occurs. When HLR becomes unavailable, the VLR can immediately provide the stored location information without needing to query HLR, thus preventing service interruption and resolving the reliability issue.
Solution Approach 2:
The VLR acts as an intermediary database that can provide location information independently of HLR. It mediates between the MSC and HLR by storing copies of location data, allowing call routing to continue even when HLR is unreachable, thus resolving the single point of failure vulnerability.
2Reliability
If the network always queries HLR for location information, then accurate routing is achieved, but service interruption occurs when HLR fails
Solution Approach 1:
The VLR pre-loads and stores location information during normal operation. When HLR failure occurs, this pre-stored information is immediately available for call routing without requiring additional query time, thus maintaining service continuity and avoiding call setup delays.
Solution Approach 2:
The system prepares backup location information in the VLR in advance of HLR failure. This cushioning measure ensures that when HLR becomes unavailable, the network can continue routing calls using the pre-prepared information, preventing service interruption and maintaining operational continuity.
3Adaptability or versatility
If the VLR stores temporary subscriber data, then emergency call routing is enabled during HLR failure, but additional storage requirements increase system complexity
Solution Approach 1:
The VLR is designed to serve dual purposes: normal call processing during HLR availability and emergency call routing when HLR fails. By storing subscriber data and location information that serves both functions, the system achieves adaptability without requiring separate dedicated emergency storage systems, thus limiting the increase in complexity.
Data Source
AI summary
A method for providing an emergency functionality when setting up a call directed to a User Equipment in a public land mobile network, the public land mobile network having a home database and a visitor database, includes, in case of the occurrence of a mobile terminating call towards the User Equipment: requesting, by a requesting entity, that the home database provide information specific to the User Equipment to the requesting entity; and in case of a failure of the functionality of the home database, requesting, by the requesting entity, that the visitor database provide the information specific to the User Equipment.