Description
Timing Synchronization Status (TSS) is a parameter used in 3GPP radio access networks, particularly in NG-RAN (Next Generation RAN) for 5G, to indicate the synchronization accuracy of a base station (gNB) or a distributed unit (gNB-DU). It represents the quality of the node's timing source, which can be derived from Global Navigation Satellite Systems (GNSS like GPS), IEEE 1588 Precision Time Protocol (PTP), or through network-based synchronization from a master node. The TSS is typically communicated between network elements via Xn or F1 interface signaling messages, such as in the Xn Setup procedure or gNB Configuration Update.
Architecturally, TSS is part of the synchronization architecture defined for 5G NR, which supports requirements for time synchronization error as low as a few microseconds for features like Time Division Duplex (TDD) operation, carrier aggregation, and coordinated transmissions. Each gNB assesses its own synchronization accuracy, often categorized into levels (e.g., 'synchronized', 'not synchronized', or specific accuracy ranges like ±1.5 μs). This status is then shared with neighboring gNBs and potentially with the core network for management purposes. The TSS information is used in algorithms for cell selection, interference coordination, and especially in CoMP schemes where multiple transmission points jointly serve a UE and require precise timing alignment to avoid signal degradation.
In operation, a gNB continuously monitors its synchronization source. If it loses synchronization or detects degraded accuracy, it updates its TSS and notifies peers. Neighboring nodes receiving a 'not synchronized' or low-accuracy TSS may exclude that node from certain cooperative functions or adjust their scheduling to mitigate interference. For example, in dynamic TDD systems, unsynchronized cells could cause cross-link interference, so the TSS helps in applying interference management techniques. The TSS is also crucial for handover decisions, as a target cell with poor synchronization might not support latency-critical services reliably.
Purpose & Motivation
TSS was introduced to address the stringent synchronization requirements of advanced RAN features in LTE-Advanced and 5G NR. Earlier cellular systems (2G/3G) relied primarily on frequency synchronization, but technologies like TDD, CoMP, and New Radio (NR) with wide bandwidths require precise phase/time synchronization to function effectively. Without a mechanism to communicate synchronization health, networks risked performance degradation from misaligned transmissions.
The TSS parameter solves this by providing a standardized way for nodes to advertise their timing quality, enabling intelligent coordination. It allows the RAN to dynamically adapt cooperative behaviors based on real-time synchronization conditions, improving robustness and efficiency. This is particularly important in dense deployments and heterogeneous networks where not all nodes have equal access to high-quality timing sources. TSS supports network reliability by preventing synchronization-related failures from cascading.
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (66 CRs across 4 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-8, normative work from Rel-15.
In Release 15, the TSS (Timing Synchronization Status) function was introduced as part of enabling Time Sensitive Communication and Time Synchronization services, with a specific new UE capability for "support for network reconnection due to RAN timing synchronization status change". This allows a RAN timing synchronization status change event to be used as a trigger for presence detection within an Area of Interest for mobility event notification.
- Subscription status notification for Event Exposure service TS 23.501CR0273
- NF/NF service registration and status subscribe/notify description updates TS 23.501CR0335
- Clarification on reporting of PS Data Off status change TS 23.501CR0604
- Reporting PS Data Off status change when SM back off timer is running TS 23.501CR0628
- PS Data Off status update when congestion control is applied in AMF TS 23.501CR0853
- PS Data Off status update when UE in non-allowed area or out of LADN area TS 23.501CR0913
+ 1 more changes
In Release 16, the TSS function introduced new capabilities for handling network reconnection due to RAN timing synchronization status change, which can be used as an event trigger for Area of Interest monitoring. This was supported by a new UE 5GMM Core Network Capability indication and integration within the UE mobility event notification framework. Furthermore, the release formally defined the architecture and required functions, such as the TSCTSF, DS-TT, and NW-TT, to enable (g)PTP-based time synchronization services for TSN and other precision timing applications over Ethernet or IP PDU Sessions.
- Handling of Stored Small Data Rate Control Status at Subsequent PDU Session Establishment TS 23.501CR1252
- TSN Time Synchronization Traffic Handling TS 23.501CR1504
- Solving Synchronization issues, addressing IEEE LS response TS 23.501CR2396
- Event Configuration Synchronization between 4G&5G TS 23.501CR2482
- IMS voice over PS Session Supported Indication taking into account UE S1 mode status TS 23.501CR2600
In Release 17, the TSS function was enhanced by introducing a new architecture for AF-requested Time Synchronization services, which includes the new Time Sensitive Communication and Time Synchronization Function (TSCTSF) to control DS-TT and NW-TT for (g)PTP-based synchronization. Furthermore, the release added support for the 5GS to act as an IEEE 1588 Boundary Clock and introduced a new UE and network capability for "support for network reconnection due to RAN timing synchronization status change," enabling mobility event notification based on this status.
- KI#3B-1: Exposure of Time synchronization as a service TS 23.501CR2629
- Support for PTP in time synchronization service and BMCA TS 23.501CR2668
- Support for IEEE 1588 Boundary Clocks in time synchronization service TS 23.501CR2690
- Update for PTP in time synchronization service and BMCA TS 23.501CR2773
- Introduction of architecture for AF requested support of Time Sensitive Communication and Time Synchronization TS 23.501CR2833
- Addition of NR Timing Advance reporting for NR UL E-CID [NRTADV] TS 38.473CR0817
+ 21 more changes
In Release 18, the TSS (Timing Synchronization Status) function introduced new capabilities for subscription-based control of the time synchronization service and enhanced monitoring through RAN timing synchronization status change reporting from the AMF to the TSCTSF. It also added support for coverage area filters for the service and the reporting of network timing synchronization status, including specific enhancements for timing resiliency and URLLC. Furthermore, the release provided clarifications and corrections on the use of TSS information, service monitoring, and the Area of Interest (AoI) for the time synchronization service.
- Adding time synchronization service based on subscription TS 23.501CR3762
- Support for coverage area filters for time synchronization service TS 23.501CR3767
- Reporting the RAN timing synchronization status change from AMF to TSCTSF TS 23.501CR3807
- Support for network timing synchronization status and reporting KI1 - description TS 23.501CR3892
- Introduction of Timing Resiliency and URLLC enhancements TS 38.300CR0730
- Introduction of 5G Timing Resiliency and URLLC enhancements TS 38.413CR0972
+ 21 more changes
Explore further
Broader topics and technologies where TSS plays a role.
Defining Specifications
3GPP specifications that define or reference TSS, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.501 vk00 | 5G System Architecture Stage 2 | Rel-20 |
| TS 32.373 v1900 | IRP Security Services CORBA Solution | Rel-9 |
| TS 32.376 vj00 | Security services for IRP Solution Set | Rel-19 |
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |
| TS 38.413 vj10 | NG Application Protocol (NGAP) | Rel-19 |
| TS 38.473 vj10 | 5G F1 Application Protocol (F1AP) | Rel-19 |