Description
Round Trip Time (RTT) is a fundamental network performance metric that quantifies the delay experienced by a bidirectional communication exchange. It encompasses propagation delays, transmission delays, processing delays, and queuing delays across the entire path, including radio access, transport networks, and core network elements. In 3GPP architectures, RTT is measured between user equipment (UE) and network nodes, such as base stations (gNB in 5G) or servers, using protocols like ICMP ping or dedicated measurement procedures defined in specs (e.g., 37.320 for self-organizing networks). The value is typically expressed in milliseconds and varies based on factors like distance, network congestion, and technology generation.
How RTT works involves timing the interval from when a request packet is sent until its corresponding response is received. For example, in LTE or 5G, RTT can be measured during radio resource control (RRC) procedures or data plane transmissions. Key components contributing to RTT include the air interface latency (e.g., frame structure and scheduling), backhaul latency, and core network processing (e.g., in the AMF or UPF). 3GPP specifications, such as 38.306 for 5G UE radio access capabilities, define requirements for maximum RTT to ensure service quality, with targets as low as 1 ms for ultra-reliable low-latency communications (URLLC) in 5G.
RTT's role in the network is pivotal for QoS management, influencing user experience in latency-sensitive applications like VoIP, online gaming, and autonomous vehicles. It is used in algorithms for congestion control, handover decisions, and network optimization. By monitoring RTT, operators can identify bottlenecks and deploy techniques like edge computing or network slicing to reduce delays. In 3GPP evolution, RTT metrics are integral to performance benchmarking and drive innovations in radio interface design and core network architecture.
Purpose & Motivation
RTT exists as a metric to quantify and manage network latency, addressing problems related to real-time communication reliability and efficiency. In early mobile networks, high RTT could degrade voice quality and data throughput, limiting service adoption. By measuring RTT, 3GPP standards enable optimization of network parameters to meet latency targets, solving issues like call drops or buffering in streaming services. Its introduction in Rel-4 provided a standardized way to assess end-to-end performance, supporting the transition to packet-switched services in UMTS.
Historically, the motivation for focusing on RTT grew with the rise of interactive applications; for instance, 3G networks needed lower latency for video conferencing. Previous approaches relied on simplistic delay measurements, but RTT offered a comprehensive view of bidirectional delay, essential for TCP performance and adaptive applications. It addressed limitations of one-way delay metrics by accounting for network asymmetry and feedback loops, crucial for congestion control mechanisms in evolving 3GPP releases.
In modern contexts, RTT's purpose extends to enabling technologies like 5G URLLC and IoT, where milliseconds matter for industrial automation or emergency services. 3GPP specs from Rel-15 onward define stringent RTT requirements to support these use cases, driving innovations in radio frame design and core network disaggregation. By continuously refining RTT measurement and reduction techniques, 3GPP ensures networks can deliver the low-latency experiences demanded by advanced digital societies.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (50 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-4, normative work from Rel-15.
In Release 15, enhancements were introduced for the Round Trip Time (RTT) function to support Ultra-Reliable Low Latency Communication (URLLC) services. These included specific QoS fixes related to attributes like Packet Delay Budget (PDB), Packet Error Rate (PER), and Maximum Data Burst Volume (MDB), as well as updates to the 5G QoS Identifier (5QI). Additionally, improvements were made to Channel Quality Indicator (CQI) and Modulation and Coding Scheme (MCS) configurations tailored for URLLC requirements.
In Release 16, the RTT (Round Trip Time) function was enhanced to support QoS monitoring for URLLC services, including end-to-end delay measurement. Specific corrections and clarifications were made to the protocol stacks for RTT measurements and to GTP-U path monitoring. Furthermore, support was defined for performing RTT measurements with TCP and for UPFs to conduct these measurements without requiring a Performance Measurement Function (PMF).
- New clause for URLLC supporting TS 23.501CR0810
- Introduction of QoS Monitoring to assist URLLC Service TS 23.501CR0990
- New Solution for Key Issue #7-URLLC Always on Control for the GBR QoS Flow TS 23.725CR0015
- QoS Monitoring support for URLLC TS 29.512CR0338
- E2E delay measurement for QoS monitoring for URLLC TS 38.415CR0012
- RTT measurements with TCP TS 23.501CR1168
+ 24 more changes
In Release 17, enhancements to the RTT (Round Trip Time) function included corrections and updates to the Multi-RTT procedure. Specifically, corrections were made to the applicability of the timing error margin for the RxTEG in the NR-Multi-RTT-SignalMeasurementInformation field descriptions. Additionally, a general "Correction to Multi-RTT" was implemented to improve the accuracy and reliability of the multi-round-trip time measurements.
- Adding the usage of Redundant Transmission Experience analytics for URLLC service TS 23.501CR2581
- Multiple round-trips of AA messages during UUAA-MM TS 24.501CR3521
- Multiple round-trip of AA messages during UUAA-SM TS 24.501CR3522
- Update on energy efficiency of URLLC network slice TS 28.554CR0079
- Introduction of Rel-17 IIoT/URLLC to TS 38.300 TS 38.300CR0416
- Start drx-HARQ-RTT-TimerUL after last repetition [ulHARQ_RTT_Timer] TS 38.306CR0802
+ 4 more changes
In Release 18, the RTT function was enhanced with a focus on timing resiliency and URLLC, alongside clarifications for RTT measurement in RSM and for uplink/downlink policy control based on round-trip latency. The release also included corrections to key UE capabilities, specifically NR-Multi-RTT-MeasurementCapability and NR-DL-TDOA-MeasurementCapability, and to UAI for URLLC.
- Introduction of Timing Resiliency and URLLC enhancements TS 38.300CR0730
- Clarification of RTT measurement for RSM TS 23.501CR4342
- Clarification for UL and DL policy control based on Round-Trip latency requirements TS 29.512CR1250
- Correction of NR-DL-TDOA-MeasurementCapability and NR-Multi-RTT-MeasurementCapability TS 37.355CR0528
- Correction on UAI for URLLC TS 38.300CR0793
- Corrections to UE capabilities related to Rel-17 URLLC and RedCap TS 38.306CR1150
In Release 19, the standardization of Round Trip Time (RTT) was enhanced to support new reliability Key Performance Indicators (KPIs) for end-to-end downlink and uplink within URLLC Network Slices. Furthermore, the RTT function's requirements were expanded to include the specific feature of URLLC for direct UE-to-Satellite-to-UE communication scenarios.
Explore further
Broader topics and technologies where RTT plays a role.
Defining Specifications
3GPP specifications that define or reference RTT, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.271 vj00 | LCS Stage 2 Specification | Rel-19 |
| TS 23.436 vk00 | ADAEnabler Functional Architecture and Information Flows | Rel-20 |
| TS 23.501 vk00 | 5G System Architecture Stage 2 | Rel-20 |
| TS 23.700 vk00 | XR Services Application Enablement Layer | Rel-20 |
| TS 23.725 vg20 | Study on URLLC Architecture Enhancements | Rel-16 |
| TR 23.737 vh20 | Satellite Access in 5G Architecture Study | Rel-17 |
| TS 24.193 vj50 | ATSSS Procedures Specification | Rel-19 |
| TS 24.501 vj50 | 5G NAS Protocols Specification | Rel-19 |
| TS 25.305 vj00 | UTRAN UE Positioning Stage 2 | Rel-19 |
| TS 26.506 vj20 | Real-Time Media Communication Architecture for 5G | Rel-19 |
| TR 26.806 vi00 | Technical Report on Smartly Tethering AR Glasses | Rel-18 |
| TR 26.812 vi10 | Technical Report | Rel-18 |
| TR 26.910 vj00 | MTSI enhancements for RAN delay budget reporting | Rel-19 |
| TR 26.922 vj00 | Video Telephony Robustness Improvements Study | Rel-19 |
| TR 26.926 vj00 | Traffic Models & Quality Evaluation for Media/XR in 5G | Rel-19 |
| TR 26.928 vj00 | Study on eXtended Reality (XR) in 5G | Rel-19 |
| TR 26.938 vj00 | DASH Deployment Guidelines for 3GPP Networks | Rel-19 |
| TR 26.962 vj00 | ITT4RT Operation and Usage Guidelines | Rel-19 |
| TR 26.982 vj00 | Multiparty Real-Time Text Protocol Details | Rel-19 |
| TS 28.554 vk00 | 5G Network & Slice KPI Specification | Rel-20 |
| TS 29.165 vj10 | Inter-IMS Network to Network Interface (NNI) | Rel-19 |
| TS 29.244 vj40 | PFCP Specification for Control/User Plane Separation | Rel-19 |
| TS 29.512 vj40 | 5G Session Management Policy Control Service | Rel-19 |
| TR 29.893 vi00 | Technical Report on QUIC for 5GC SBI | Rel-18 |
| TS 36.300 vj00 | E-UTRAN Radio Interface Protocol Architecture Overview | Rel-19 |
| TS 36.355 vj00 | LTE Positioning Protocol (LPP) | Rel-19 |
| TS 36.855 vd00 | E-UTRA Positioning Enhancements Study | Rel-13 |
| TS 37.320 vj00 | Minimization of Drive Tests (MDT) Overview | Rel-19 |
| TS 37.355 vj20 | LTE Positioning Protocol (LPP) | Rel-19 |
| TR 37.901 vf10 | UE Application Layer Data Throughput Performance | Rel-15 |
| TR 37.910 vj00 | 5G SRIT and NR RIT Self-Evaluation Report | Rel-19 |
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |
| TS 38.306 vj00 | NR UE Radio Access Capability Parameters | Rel-19 |
| TS 38.415 vj10 | PDU Session User Plane Protocol | Rel-19 |
| TS 38.811 vf40 | Study on NR Support for Non-Terrestrial Networks | Rel-15 |
| TR 38.913 vj00 | Next Gen Access Tech Scenarios & Requirements | Rel-19 |