P-CSCF Proactive Unsubscription for CS Fallback Latency
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Solution Overview
Problem
Current Circuit Switched Fallback (CSFB) procedures introduce latency and inefficiencies when a UE loses coverage, as they rely on retrying SIP calls over the circuit-switched domain after detecting a released signalling bearer, leading to unnecessary network traversal and increased error messages.
Innovation Solution
The method involves proactive notification by the P-CSCF node to the S-CSCF about a released signalling bearer, allowing the IMS network to initiate CS fallback sooner, reducing latency and network load by unsubscribing from registration events and removing unnecessary registration data, and informing the application server about de-registration, thereby facilitating immediate CS fallback.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the P-CSCF keeps the registration of the called UE stored with a flag indicating not reachable, then the registration information is preserved, but the call setup time increases and network load increases
Solution Approach 1:
The P-CSCF performs preliminary action by proactively unsubscribing from registration events and removing registration data before a call attempt is made. This preliminary cleanup ensures that when a call is attempted, the network immediately recognizes the UE as unreachable and can initiate CS fallback without traversing the entire network, thereby reducing call setup time while maintaining reliable registration management.
2Reliability
If the network performs CS fallback by retrying SIP calls over circuit switched domain, then call connectivity is maintained, but network load increases and latency increases
Solution Approach 1:
The invention extracts and removes unnecessary registration data from the network before a call attempt. By proactively unsubscribing from registration events and deleting stale registration records, the network eliminates redundant data that would otherwise cause increased network load and latency during CS fallback procedures, while still maintaining reliable call connectivity through proper CSFB initiation.
3Speed
If the P-CSCF proactively unsubscribes from registration events, then CS fallback is initiated sooner, but the complexity of the signalling procedure increases
Solution Approach 1:
The P-CSCF implements feedback mechanisms by monitoring signalling bearer status and proactively unsubscribing from registration events when the called UE becomes unreachable. This feedback-driven approach enables faster CS fallback initiation by immediately responding to reachability changes, while the standardized subscription/unsubscription signalling protocols keep the procedure complexity manageable.
4Loss of information
If the network maintains registration data for called UE, then registration information is available, but unnecessary network traversal occurs and error messages increase
Solution Approach 1:
The P-CSCF discards stale registration data and unsubscribes from registration events when the called UE loses coverage. This discarding action prevents unnecessary network traversal and reduces error messages during subsequent call attempts. The registration information is recovered and properly managed through controlled subscription/unsubscription mechanisms, ensuring availability only when needed and preventing harmful redundant signalling.
Data Source
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AI summary
A method for enabling a calling User Equipment, UE, to retry a Session Initiation Protocol, SIP, call attempt to a called UE, over a Circuit Switched domain, in a telecommunication network, wherein said called UE has a SIP session established towards said telecommunication network, wherein said method comprises the steps of receiving, by a Proxy Call Session Control Function, P-CSCF, node, an indication that a signalling bearer between said called UE and said telecommunication network is released, wherein said signalling bearer is associated with said SI P session, transmitting, by said P-CSCF node, to a Serving Call Session Control Function, S-CSCF, node, triggered by said received indication, a subscribe message for informing said S-CSCF node that said P-CSCF unsubscribes to registration events pertaining to said called UE due to the released signalling bearer such that said S- CSCF node is able to initiate a CS retry when said telecommunication network receives a SIP call attempt from said calling UE to said called UE