Resynchronization Marker

Description

The Resynchronization Marker (RM) is a control mechanism used within the data link layer protocols of 3GPP systems, most notably in the Radio Link Control (RLC) layer operating in Acknowledged Mode (AM). Its primary function is to provide a known, unambiguous point in the sequence of transmitted Protocol Data Units (PDUs) from which the receiver can restart correct processing after a synchronization failure. A synchronization failure can occur due to severe error conditions, such as the loss of a significant number of PDUs, protocol ambiguities, or timer expiries, which cause the transmitter and receiver to have inconsistent views of which data has been successfully delivered.

Operationally, when the transmitting RLC entity determines that resynchronization is necessary (often triggered by a protocol error or a control procedure), it will include an RM control PDU or flag within the data stream. This marker carries specific sequence number information that defines a new anchor point. Upon receiving and recognizing the RM, the receiving RLC entity uses the information it contains to reset its state variables, such as the expected sequence number for incoming PDUs. It will then typically discard any ambiguously received PDUs that were queued prior to the marker and begin accepting and processing PDUs starting from the new sequence number indicated by the RM.

The RM is a key component for ensuring the robustness and reliability of the RLC Acknowledged Mode service, which guarantees in-sequence, error-free delivery of upper-layer data. Without such a mechanism, a persistent de-synchronization between transmitter and receiver could lead to a permanent failure of the radio bearer, requiring a higher-layer connection re-establishment. By providing a controlled 'reset' mechanism within the layer, the RM allows the link to recover from transient but severe error conditions autonomously and quickly, maintaining service continuity. Its implementation is tightly coupled with RLC sequence numbering, window management, and retransmission procedures, forming a critical part of the link layer's error recovery toolbox.

Purpose & Motivation

The RM was introduced to address a fundamental problem in reliable data link protocols: protocol synchronization recovery after catastrophic failure. In early wireless data services, error rates were high, and complex stateful protocols like RLC Acknowledged Mode could enter deadlock or undefined states if the sequence number space became ambiguous between sender and receiver due to lost acknowledgements or control packets. Prior to standardized resynchronization, the only recovery from such a state was to tear down and re-establish the entire radio bearer or even the RRC connection, which is a slow, resource-intensive process disruptive to the user experience.

The creation of the RM provided an in-band, lightweight recovery mechanism. It solves the problem by allowing the transmitter to unilaterally (or through negotiation) declare a new starting point for the sequence number space, effectively 'wiping the slate clean' for a segment of the data flow. This was motivated by the need for more robust and efficient radio bearers for emerging packet-switched services in 3G and later systems, where maintaining connection integrity for services like VoIP, video streaming, and interactive gaming was paramount. The RM enables the system to maintain a high level of reliability without resorting to connection re-establishment for every major error event.

Its specification ensures interoperability between different vendor implementations of RLC entities. By standardizing the format and procedures for resynchronization, it guarantees that a receiver from one vendor can correctly interpret an RM sent by a transmitter from another, maintaining the end-to-end reliability of the service across multi-vendor networks. This is essential for the healthy ecosystem of cellular equipment.

Key Features

• Provides a defined resynchronization point within the RLC AM data stream
• Carries sequence number information to reset receiver state variables
• Enables recovery from protocol synchronization failures without higher-layer connection release
• Implemented as part of RLC control PDUs or specific fields within data PDUs
• Triggered by protocol error conditions or specific control procedures
• Ensures interoperability for reliable data delivery in multi-vendor networks

Evolution Across Releases

Defining Specifications