Lossless Packet Handover via SNDCP Sequence Management
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Solution Overview
Problem
Current packet switched base station handover methods in GPRS and UMTS communications face challenges in achieving lossless data transfer without introducing additional delay or overhead, particularly in unacknowledged LLC modes, which can result in data losses and throughput reduction.
Innovation Solution
The implementation of a new mode of operation for the SNDCP protocol that combines N-PDU Send and Receive Sequence numbers with LLC operating in unacknowledged mode, along with management of downlink and uplink status with or without source BSS assistance, to minimize packet loss and delay during handover, allowing for lossless packet switched handover without requiring LLC/SNDCP to operate in acknowledged mode.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If LLC operates in unacknowledged mode during packet switched handover, then data transfer delay is reduced and throughput is improved, but packet loss occurs during base station change
Solution Approach 1:
The source BSS performs preliminary actions by buffering downlink data packets before handover occurs and actively managing uplink packet transmission status. This preliminary preparation ensures that when handover happens in unacknowledged mode, the network already has the necessary packet information ready, preventing packet loss while maintaining the low-delay benefits of unacknowledged operation.
Solution Approach 2:
The system implements feedback mechanisms where the source BSS provides status information about uplink packet transmission to the network, and the target BSS acknowledges received downlink packets. This feedback loop allows the network to track which packets have been successfully transmitted and which need to be retransmitted or buffered, enabling lossless handover in unacknowledged mode.
2Reliability
If acknowledged mode is used for LLC/SNDCP during handover, then packet loss is prevented, but additional delay and overhead are introduced
Solution Approach 1:
The handover process is segmented into distinct phases: preparation phase (before handover) where the source BSS buffers downlink data and tracks uplink status, execution phase (during handover) where packets are forwarded efficiently, and completion phase (after handover) where acknowledgment is finalized. This segmentation allows the system to operate in unacknowledged mode during the critical execution phase to minimize delay, while still ensuring reliability through the preparation and completion phases.
3Reliability
If source BSS buffers downlink data during handover, then packet loss is reduced, but buffer management complexity increases
Solution Approach 1:
The source BSS applies local quality management by buffering downlink data selectively based on handover status and packet priority. Not all packets are treated equally - critical packets are buffered while less critical packets may be discarded or delayed. This localized approach to buffer management reduces overall complexity compared to buffering everything, while still preventing packet loss for important data during handover.
Data Source
AI summary
A mobile station (MS) and method for base station handover from a source cell of a source base station to a target cell of a target base station in a packet-switched cellular radio network. While the MS is still in the source cell, the MS receives from the network, a handover command message and information regarding a next expected uplink packet for a packet flow subject to lossless packet-switched handover. Upon arrival of the MS in the target cell, the MS transmits an uplink sequence number status message to the target base station providing a downlink sequence number status for the packet flow, and starts uplink data transmission to the target base station beginning with the next expected uplink packet.