Sequence Number

Description

A Sequence Number (SN) is a critical field in many 3GPP layer 2 and layer 3 protocols, providing a mechanism to impose order on a stream of data units. It is a counter, typically incremented for each new packet or Service Data Unit (SDU) submitted for transmission. The primary role of the SN is to allow the receiver to reconstruct the original order of packets, which may arrive out-of-sequence due to network conditions like multi-path routing or radio link layer retransmissions. Furthermore, by detecting gaps or repeats in the sequence, the receiver can identify lost or duplicate packets, triggering retransmission requests in reliable protocols.

In the architecture of radio access networks, SNs are employed extensively. For example, in the Packet Data Convergence Protocol (PDCP) layer (specified in 36.323 for LTE and 38.323 for NR), a PDCP SN is assigned to each data packet. This SN is used for in-sequence delivery to the upper layers, duplicate detection, and as a basis for ciphering and integrity protection. Similarly, the Radio Link Control (RLC) layer uses its own RLC SN for segmentation, reassembly, and Automatic Repeat Request (ARQ) operations. In the core network, sequence numbers appear in signaling protocols to ensure the correct order of operations.

The operation relies on the sender and receiver maintaining state about the expected sequence numbers. The receiver acknowledges received packets, often using the SN, allowing the sender to discard successfully delivered data from its buffers. The size of the SN field (e.g., 12-bit, 18-bit) determines the maximum window of packets that can be 'in flight' before wraparound occurs, which is a key factor in link efficiency and throughput, especially over high-latency connections.

Purpose & Motivation

The Sequence Number is a fundamental concept borrowed from computer networking and adapted to meet the stringent reliability and efficiency requirements of cellular systems. In early mobile networks, as data services emerged, the simple, connection-oriented circuits of voice calls were insufficient. Packet-switched data introduced the problems of out-of-order delivery, packet loss, and the need for efficient retransmission over unreliable radio links. The SN provides a lightweight yet powerful tool to manage these issues.

It solves the core problem of maintaining data integrity and order across the air interface, where conditions are highly variable. Without sequence numbers, higher-layer protocols like TCP would bear the full burden of recovery, which is inefficient over the radio link due to its different error characteristics. By implementing SN-based mechanisms at the radio link layer (RLC) and the convergence layer (PDCP), 3GPP systems can perform localized retransmissions and reordering, hiding radio-induced impairments from upper layers and significantly improving overall throughput and latency. Its creation was motivated by the need for a robust data delivery service over the inherently challenging wireless medium.