Synchronization User Plane Protocol

Description

The Synchronization User Plane Protocol (SYNC) is a layer 2 protocol defined within the 3GPP UTRAN architecture, specifically for the Iur interface (between RNCs) and the Iub interface (between RNC and Node B). It operates as part of the User Plane protocol stack, residing above the Transport Network Layer (e.g., using ATM or IP transport) and below higher-layer radio network control functions. Its primary role is to carry synchronization control information essential for the coordination of radio transmissions across different network elements. This includes the transport of timing information, such as Connection Frame Number (CFN) and Frame Sequence Number (FSN), which are used to align the timing of data frames between the controlling RNC (CRNC) and the serving Node B, or between RNCs during soft handover scenarios.

Architecturally, SYNC is one of several User Plane protocols defined for UTRAN interfaces, alongside others like the Frame Protocol (FP) for user data transport. It is typically implemented within the Radio Network Layer's User Plane. The protocol defines specific SYNC protocol data units (PDUs) that contain fields for synchronization parameters. These PDUs are exchanged between peer entities—for instance, between the CRNC and a Node B over the Iub interface—to establish and maintain timing alignment. The protocol handles the initialization of synchronization, continuous timing updates, and the management of synchronization status, ensuring that all involved nodes have a common understanding of frame timing, which is vital for processes like macro-diversity combining in soft handover.

In operation, SYNC works in conjunction with the Radio Resource Control (RRC) and Node B Application Part (NBAP) signaling protocols. For example, during radio link setup via NBAP, synchronization parameters are configured, and SYNC is then used to carry the ongoing timing alignment information on the established user plane transport bearer. The protocol supports both synchronous and asynchronous modes relative to the network's timing reference. Its correct functioning is fundamental for maintaining the quality of service, especially for delay-sensitive services, and for enabling seamless mobility through handovers. Without proper synchronization, radio transmissions could collide or become misaligned, leading to increased interference, dropped calls, and degraded system capacity.

Purpose & Motivation

SYNC was created to address the critical need for precise timing coordination in the UTRAN, which is a synchronous CDMA-based network. In such systems, accurate timing alignment between Node Bs and the RNC is essential for several key functions: successful soft handover (where a UE communicates with multiple cells simultaneously), efficient power control, and the proper reception and combining of signals. Prior to 3GPP standardization, proprietary solutions might have existed, but a standardized protocol was necessary to ensure multi-vendor interoperability across the open Iub and Iur interfaces.

The protocol solves the problem of distributing a common timing reference across geographically dispersed network elements that may have different internal clock sources. It provides a standardized mechanism to transport timing information (like CFN) over the transport network, which itself may introduce variable delays (e.g., over ATM or IP). This allows the RNC to control and adjust the transmission timing at the Node B, ensuring that data frames arrive at the UE from different cells within a very tight time window for effective combining. The creation of SYNC was motivated by the architectural shift in 3G UMTS, which separated the radio control (RNC) from the radio transmission (Node B), necessitating a reliable and low-latency timing coordination protocol over these standardized interfaces to maintain the performance benefits of WCDMA technology.

Key Features

• Transports synchronization control information (e.g., CFN, FSN) for timing alignment
• Operates over Iur and Iub interfaces in the User Plane protocol stack
• Supports initialization, maintenance, and status reporting of synchronization
• Works in conjunction with NBAP and RRC signaling for radio link management
• Enables macro-diversity and soft handover procedures in UTRAN
• Defines specific SYNC PDUs for reliable exchange of timing parameters

Evolution Across Releases

Defining Specifications