First Missing PDCP SN

Description

The First Missing PDCP SN (FMS) is a key information element within the PDCP (Packet Data Convergence Protocol) status report, defined in 3GPP TS 36.323. It operates in the context of RLC (Radio Link Control) Acknowledged Mode (AM) for user plane data, where reliable delivery is required. The PDCP layer, situated above RLC, is responsible for header compression, ciphering, and in-sequence delivery. When out-of-sequence packets are received, the receiving PDCP entity uses status reports to inform the transmitter about missing packets, with the FMS field pinpointing the start of any gap in the sequence.

Architecturally, the PDCP entity maintains a reception window to track incoming PDCP Protocol Data Units (PDUs). Each PDU carries a PDCP SN. Upon detecting a missing SN (e.g., due to RLC retransmission failures or air interface errors), the receiver generates a PDCP status report. This report includes the FMS, which is the lowest SN that has not been successfully received and for which all lower SNs have been received or are also missing in a contiguous block. It essentially marks the beginning of a loss event in the sequence space.

How it works involves the receiver monitoring the PDCP SNs of incoming PDUs. When a gap is detected, the receiver sets the FMS value to the SN of the first missing PDU. This status report is then sent back to the transmitting PDCP entity, typically triggered by specific events like periodic reporting or upon detection of missing data. The transmitter uses the FMS, along with a bitmap of subsequent missing SNs (if included), to identify exactly which PDUs need retransmission. This mechanism is integral to the PDCP layer's lossless handover support and reliable service for certain QoS flows.

Its role in the network is to enhance data integrity and efficiency. By precisely identifying the first missing packet, it minimizes retransmission ambiguity and reduces overhead compared to full sequence listings. This is particularly important for latency-sensitive applications and during handovers where packet loss must be recovered swiftly. The FMS is a fundamental component of the PDCP ARQ (Automatic Repeat Request) mechanism, ensuring robust data transfer in LTE and 5G NR radio access networks.

Purpose & Motivation

The FMS was introduced to address the need for efficient and reliable packet loss detection and recovery at the PDCP layer in 3GPP systems, starting with LTE in Release 8. Prior approaches in earlier mobile standards often relied solely on RLC for retransmissions, which could be insufficient during certain mobility events like handovers where RLC contexts might be reset. PDCP-layer recovery provides an additional safeguard for data integrity.

The primary problem it solves is enabling lossless handover and reliable data service for specific bearers. During an inter-eNodeB handover in LTE or inter-gNB handover in NR, the source node forwards unacknowledged PDCP PDUs to the target node. The receiving PDCP entity in the target or UE must identify any gaps in the sequence to request retransmissions, and the FMS provides a compact way to signal the starting point of these gaps. This ensures seamless data continuity without packet loss, which is critical for applications like VoIP or real-time gaming.

Historically, without PDCP status reporting and FMS, recovery from packet loss during mobility relied entirely on higher-layer protocols like TCP, which could introduce significant latency due to slow-start and retransmission timeouts. The FMS mechanism allows faster, layer-specific recovery, improving user experience and network efficiency. It addresses limitations of relying only on RLC ARQ, which might not survive handovers, by providing a persistent, end-to-end (between UE and anchor node) loss detection capability.