PS to CS Handover Indicator for Voice Call Continuity
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Solution Overview
Problem
The existing handover procedures from a packet switch network to a circuit switched network in EUTRAN to UTRAN/GERAN networks face a race condition that can result in premature termination of voice calls due to incorrect bearer management, leading to unnecessary call losses and increased complexity in QoS updates.
Innovation Solution
Introducing a new indicator in the GPRS tunneling version 2 protocol to signal the handover of EPS bearers to the CS domain, specifically across S3, S16, S4, and S5/S8 interfaces, ensuring that voice bearers are correctly handed over and avoiding the reuse of preservation indications to prevent QoS updates and complexity in the PGW.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the existing handover procedures are used without a specific PS to CS handover indicator, then the general handover process can be maintained, but a race condition occurs where step B (PGW reporting to PCRF) may happen before step A (PCRF updating P-CSCF), causing the PCRF to incorrectly assume the voice bearer is lost and trigger unnecessary IMS session release
Solution Approach 1:
A new indicator in the GTPv2 protocol is introduced as an intermediary signal to explicitly convey PS to CS handover status from source MME to target S4-SGSN and subsequently to PGW. This intermediary mechanism ensures that the PCRF receives accurate bearer status information, preventing incorrect assumptions about bearer loss and avoiding unnecessary IMS session releases.
Solution Approach 2:
The handover indicator is set in advance by the source MME before the handover completion, allowing the target S4-SGSN and PGW to be pre-informed about the upcoming PS to CS transition. This preliminary action ensures that the PCRF receives accurate status information before making policy decisions, preventing race condition-related errors.
2Adaptability or versatility
If the QoS information element is sent to indicate bearer preservation, then the preservation function can be supported, but this adds considerable complexity to the PGW for handling QoS updates from MME/S4-SGSN
Solution Approach 1:
The handover indication function is extracted from the QoS information element and implemented as a separate, dedicated indicator in the GTPv2 protocol. This extraction removes the complexity of QoS update handling from the PGW while preserving the bearer preservation functionality, as the indicator provides sufficient information without requiring complex QoS processing.
Solution Approach 2:
Instead of using the general QoS information element for multiple purposes (including preservation indication), a specific local quality indicator is introduced solely for handover status signaling. This specialization simplifies the PGW's task by providing targeted information without the overhead of processing complete QoS updates.
Data Source
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AI summary
The present invention relates to a solution for handling handover of connections for a user equipment from a packet switch network to a circuit switched network. This is provided as a method implemented in a node and system. The method is provided in a telecommunications network (100) connecting user equipment, i.e. UE, (101) communicating wirelessly (111) with the network. The method comprising steps of detecting (301) that handover is required from a packet switched, i.e. PS, based network (102) to a circuit switched, i.e. CS, based network (103), initiating (302) in a source mobility management node (104) a single radio voice call continuity, i.e. SVRCC, initiating (303) a PC to CS handover for non-voice components with information about voice related parameters and a PS to CS handover indicator, executing (304) hand over, sending (305) an update request to a serving gateway, i.e. SGW, (107) from a target mobility management node (105) with non-voice related parameters and the PS to CS handover indicator, forwarding (306) the update request from the SGW to a packet data network gateway, i.e. PGW (108), receiving the update request in the PGW, detecting the PS to CS handover indicator, and handling (307) in the PGW the PS to CS handover indicator.