Frame Sequence Number

Description

The Frame Sequence Number (FSN) is a field within a protocol data unit (PDU) header that identifies the sequential position of a frame within a data stream. In the context of 3GPP specifications for the UTRAN (Universal Terrestrial Radio Access Network), particularly in the Iur (interface between RNCs) and Iub (interface between RNC and Node B) protocol stacks, the FSN is used by the Frame Protocol (FP) and the Data Stream Protocol. Its primary function is to enable the receiving entity to reconstruct the original order of transmitted frames, which may arrive out-of-order due to network conditions, and to identify missing frames.

Architecturally, the FSN operates within the user-plane transport layer of these interfaces. When a higher-layer PDU (e.g., a MAC PDU containing user data) is passed down to the transport layer for transmission over the Iub/Iur, it is encapsulated into a Frame Protocol frame. The transmitting entity assigns a monotonically increasing FSN to each frame within a specific transport channel or data stream. The FSN space is cyclic, meaning it wraps around after reaching a maximum value (e.g., 0 to 4095), which requires the receiver to handle the wrap-around using window-based algorithms.

How it works involves both transmission and reception procedures. On the sender side, the FSN is incremented for each new frame on a given logical connection. The frame, with its FSN, is sent over the transport network (e.g., IP or ATM). On the receiver side, frames are buffered upon arrival. The receiver uses the FSN to place frames in the correct sequential order before delivering them to the higher layer. If a frame is received with an FSN that is not the next expected one, it indicates a gap—a potentially lost or excessively delayed frame. Depending on the specific protocol and configuration, this may trigger a retransmission request (if supported) or be reported to operation and maintenance systems. The FSN is thus a critical component for in-sequence delivery, a key aspect of the Quality of Service (QoS) for dedicated channels.

Purpose & Motivation

The FSN exists to solve the fundamental problems of packet-based transport networks: out-of-order delivery and packet loss. In the UTRAN architecture, the Iub and Iur interfaces use packet-switched transport (initially ATM, later IP). These networks do not guarantee the order or delivery of packets. Without sequence numbers, the Radio Network Controller (RNC) or Node B would be unable to correctly reassemble the user data stream, leading to corrupted information being passed to the radio interface or incorrect handover decisions.

Historically, earlier circuit-switched connections inherently maintained order. The migration to packet-based backhaul for cost and flexibility introduced this challenge. The FSN provides a simple, lightweight mechanism to reintroduce order and reliability at the layer where it is needed—between the RNC and Node B—without relying on complex transport layer protocols like TCP, which may introduce unacceptable latency for real-time radio traffic. It addresses the limitation of using a 'best-effort' transport network for carrying time-sensitive and order-critical radio interface data.

Its creation was motivated by the need for a robust, efficient control mechanism tailored to the real-time requirements of cellular networks. By enabling in-sequence delivery and loss detection at the Frame Protocol level, the FSN allows the RAN to maintain high data integrity and support seamless mobility (e.g., during soft handover where data is split over multiple Iub links). It is a foundational element that ensures the user plane data integrity across the RAN transport network, which is essential for maintaining end-user QoS.

Key Features

• Provides sequential numbering of frames within a data stream
• Enables receiver-side reordering of out-of-sequence frames
• Allows detection of lost or missing frames by identifying gaps in the sequence
• Typically uses a cyclic numbering space (e.g., modulo 4096)
• Operates within the user-plane Frame Protocol of Iub/Iur interfaces
• Supports QoS by ensuring in-sequence delivery for dedicated channels

Evolution Across Releases

Defining Specifications