Dual Active Protocol Stacks

Description

Dual Active Protocol Stacks (DAPS) is a sophisticated handover mechanism introduced in 5G to achieve make-before-break connectivity. Unlike traditional handovers where the User Equipment (UE) releases the connection to the source cell before establishing one with the target cell (break-before-make), DAPS allows the UE to maintain two fully active and independent protocol stacks simultaneously. This includes duplicate Packet Data Convergence Protocol (PDCP), Radio Link Control (RLC), and Medium Access Control (MAC) entities for the source and target cells. The core network establishes a split bearer where downlink data can be duplicated and sent to both the source and target gNBs. The UE receives this data from both cells, using mechanisms like PDCP duplication to ensure at least one copy is successfully delivered, thereby eliminating packet loss during the handover execution phase.

Architecturally, DAPS involves coordination between the source gNB, target gNB, and the core network's User Plane Function (UPF). The source gNB initiates the handover preparation by signaling the target gNB and the core network to establish a temporary dual connectivity-like path. The UPF is instructed to duplicate downlink packets and forward them to both gNBs. On the UE side, the protocol stack for the target cell is established and activated while the stack for the source cell remains fully operational. The UE continues to transmit uplink data exclusively to the source cell until a specific switch command is received, ensuring uplink continuity and order. This dual-reception window persists until the handover is finalized, at which point the source protocol stack is released.

The key operational principle is the decoupling of data reception from the handover execution command. The UE can receive downlink data from the target cell even before it has sent a handover confirmation message (e.g., RRCReconfigurationComplete) to that cell. This is a fundamental shift from legacy procedures. For uplink, the UE transmits data and control messages to the source cell until it receives an explicit 'UL Switch' indication within the RRC reconfiguration message, after which it switches uplink transmission to the target cell. This controlled switchover ensures no uplink packets are lost. DAPS handover is managed via enhanced RRC signaling (e.g., in NR) and Xn-AP procedures between gNBs, with support in the NG interface for core network coordination.

DAPS plays a pivotal role in the 5G radio access network by providing a foundation for ultra-reliable mobility. Its primary role is to guarantee service continuity for demanding applications like industrial automation, autonomous vehicles, and real-time remote control, where even milliseconds of interruption or a single lost packet can be catastrophic. By making the handover procedure virtually invisible to the application layer, DAPS is a key enabler for the 5G vision of supporting mission-critical communications and fulfilling the stringent requirements of URLLC service categories.

Purpose & Motivation

DAPS was created to solve the fundamental problem of data interruption and packet loss during handovers in cellular networks. Traditional LTE and early 5G handovers follow a break-before-make principle, where the radio link with the source cell is broken before a new link with the target cell is fully secured. This results in a handover interruption time, typically ranging from tens to hundreds of milliseconds, during which no data can be transmitted or received. This interruption, along with potential packet loss due to forwarding delays or failures, is unacceptable for emerging 5G use cases such as factory automation, tele-surgery, and vehicle-to-everything (V2X) communication, which demand 99.9999% reliability and sub-10ms latency.

The historical context stems from the limitations of existing enhancements like Packet Data Convergence Protocol (PDCP) status reporting and data forwarding between base stations, which mitigate but do not eliminate packet loss and latency spikes. Techniques like conditional handover improve reliability but do not address the core interruption time issue. DAPS was motivated by the need for a radical architectural change in the handover procedure to support Ultra-Reliable Low-Latency Communication (URLLC), a cornerstone of 5G Phase 2 (Release 16 and beyond). It directly addresses the limitation of having only one active radio link protocol stack at a time during mobility events.

By enabling a make-before-break paradigm, DAPS solves these problems by allowing the UE to prepare and activate the connection to the target cell while maintaining the active connection to the source cell. This ensures that data flow is never halted. The purpose is thus to provide true zero-millisecond interruption handovers, eliminate packet loss, and drastically reduce the latency impact of mobility, thereby unlocking the full potential of 5G for industrial and mission-critical IoT applications.