Description
The Transport Network Layer (TNL) in 3GPP systems refers to the foundational network infrastructure responsible for carrying all control plane signaling and user plane data traffic between different network functions and nodes. It is a logical layer that abstracts the physical transmission links (e.g., fiber, microwave) and switching/routing equipment. The TNL provides the transport service that interconnects elements of the Radio Access Network (RAN), the Core Network (CN), and between RAN and CN. Its primary role is to offer a reliable, scalable, and often quality-of-service (QoS)-aware packet delivery service.
Architecturally, the TNL is not a single entity but a collection of technologies and protocols. In modern 3GPP networks (from 3G onwards), it is predominantly based on Internet Protocol (IP). For user plane traffic in the RAN, the TNL utilizes the GPRS Tunneling Protocol for the user plane (GTP-U) over UDP/IP to create tunnels between nodes like the gNB and UPF, ensuring traffic isolation and forwarding based on Tunnel Endpoint Identifiers (TEIDs). For control plane signaling, protocols like Stream Control Transmission Protocol (SCTP) over IP are commonly used for reliable signaling transport, such as on the NGAP interface between gNB and AMF. The TNL also encompasses lower-layer technologies like Ethernet, MPLS, or optical transport (OTN) for the physical and data link layers.
How it works involves the higher 3GPP protocol layers (e.g., RRC, NGAP, F1-AP) using the services of the TNL. They pass Protocol Data Units (PDUs) to the TNL, which is responsible for their delivery to the peer entity. The TNL handles functions like routing, congestion control, fragmentation, and in some cases, security (e.g., IPsec). In the context of RAN, specific TNL associations (TNLAs) are established between nodes to provide redundancy and load distribution. The performance of the TNL—its latency, jitter, packet loss, and bandwidth—directly impacts the performance of the mobile services it supports, making its design and management critical for network operators.
Purpose & Motivation
The concept of a distinct Transport Network Layer has been fundamental since the early days of digital mobile networks. Its purpose is to separate the concerns of the radio-specific and service-specific protocol layers from the general problem of data transport. This abstraction allows the 3GPP radio and core network architectures to evolve independently of the underlying transport technology. Initially, in 2G and early 3G, transport was often based on TDM circuits. The shift to a packet-based TNL (IP) from 3GPP Release 5 onwards was driven by the need for greater efficiency, flexibility, and cost-effectiveness to handle growing data traffic.
The TNL solves several critical problems. It provides a unified, scalable backbone for aggregating traffic from thousands of base stations. It enables network sharing and virtualization by providing a common transport fabric. By defining standard transport protocols (like GTP, SCTP), it ensures multi-vendor interoperability between network equipment. The evolution towards an all-IP TNL addressed the limitations of circuit-switched transport, which was inefficient for bursty data traffic and cumbersome to scale. The ongoing purpose of the TNL is to support ever-increasing demands for capacity, lower latency (for URLLC), synchronization, and network slicing by incorporating advancements in transport technologies like Segment Routing, Time-Sensitive Networking (TSN), and enhanced QoS mechanisms.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (38 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-5, normative work from Rel-15.
In Release 15, the TNL (Transport Network Layer) saw the introduction of TNL Address discovery specifically for EN-DC (E-UTRA-NR Dual Connectivity) operations. The release also clarified protocols by specifying the use of TCP for reliable NAS transport between the UE and the N3IWF and provided clarifications on both the GTP-u protocol and SCTP association establishment. Furthermore, it introduced Transport Level Packet Marking as a new capability.
- Triggering UE capability info retrieval using DL NAS TRANSPORT (Stage 2) TS 36.300CR1160
- Using TCP for reliable NAS transport between UE and N3IWF TS 23.501CR0692
- Transport Level Packet Marking TS 23.501CR0942
- Clarification on GTP-u protocol TS 23.501CR1012
- Clarification on SCTP association establishment – for 36.300 TS 36.300CR1186
- 36.300 CR on Correction of Physical Layer Resource to Cell Resource TS 36.300CR1211
+ 4 more changes
In Release 16, key TNL enhancements included the introduction of QoS and GTP-U path monitoring for URLLC services, along with clarifications and corrections for GTP-U redundancy and reordering requirements. The release also specified procedures for updating the NGAP UE-TNLA-binding and for configuring Transport Level Marking values. Furthermore, it defined mechanisms for SCTP association changes upon association failure.
- QoS monitoring based on GTP-U paths TS 23.501CR1414
- NRIIOT Higher Layer Multi-Connectivity TS 38.300CR0253
- Clarification on reordering requirement with GTP-U redundancy TS 23.501CR1490
- Correction of NAS transport for LCS TS 23.501CR1578
- Correction on TNLA binding TS 23.501CR1770
- Requested NSSAI provided at the AS layer TS 23.501CR2038
+ 5 more changes
In Release 17, key TNL updates included enhanced support for the Multimedia Priority Service (MPS) for Data Transport Service with additional authorization functionality, and clarifications on the TSCTSF functionality for transport protocols. The release also introduced the transport of PMF information via the N4 interface, provided updates on PCC rule triggered GTP-U path monitoring, and enabled NAT traversal at the layer below IPsec for TNGF and N3IWF access.
- Multimedia Priority Service (MPS) Phase 2 support for Data Transport Service TS 23.501CR2536
- Additional authorization functionality in support of MPS for Data Transport Service TS 23.501CR2971
- KI#3, clarification on the TSCTSF functionality and configuration for transport protocols TS 23.501CR3160
- PMF information transported via N4 TS 23.501CR3110
- Updates on PCC rule triggered GTP-U path monitoring TS 23.501CR2889
- Layer below IPsec to enable NAT traversal for TNGF/N3IWF access TS 23.501CR3442
In Release 18, key TNL advancements included enhanced support for interworking with Time Sensitive Networking (TSN) deployed in the transport network and the introduction of translation mechanisms for internal-external information to assist Application Layer AI/ML operations. The release also provided clarifications for GTP-U Path Monitoring procedures and introduced enhanced considerations for ATSSS multi-layer stack parameter settings, specifically for the MPQUIC protocol stack.
- Interworking with TSN network deployed in the transport network TS 23.501CR3811
- Clarifications for GTP-U Path Monitoring TS 23.501CR4057
- Translation of Internal-External Information for Assisting Application Layer AI/ML Operations TS 23.501CR4191
- ATSSS_Ph3 Enhanced Considerations of MPQUIC Multi-layer Stack Parameter Settings & Logics TS 23.501CR4777
- Correction on Support of Time Sensitive Networking (TSN) enabled Transport Network (TN) TS 23.501CR4955
- Correction on Transport Channels TS 38.300CR0892
In Release 19, key Transport Network Layer enhancements introduced mechanisms for more granular packet marking, specifically leveraging PDU Set QoS and Importance information for DSCP marking over the N3 and N9 interfaces. Furthermore, it added capabilities for recovery from N3mb path failures during unicast transport of multicast sessions. These improvements provide more dynamic transport-level packet marking and increased reliability for multicast data delivery.
- Leveraging PDU Set QoS information for DSCP marking over N3/N9 in the transport network TS 23.501CR5407
- XRM_Ph2_KI3 Leveraging PDU Set QoS information for DSCP marking over N3/N9 in the transport network TS 23.501CR6050
- Recovery of N3mb path failure for unicast transport of multicast session TS 38.300CR1032
- Triggering of Transport Level Marking based on PDU Set Importance in I-SMF TS 23.501CR6193
- Transport level packet marking considering PSI TS 23.501CR6478
Explore further
Broader topics and technologies where TNL plays a role.
Defining Specifications
3GPP specifications that define or reference TNL, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TR 21.905 vj00 | 3GPP Technical Terms and Definitions | Rel-19 |
| TS 23.501 vk00 | 5G System Architecture Stage 2 | Rel-20 |
| TS 25.401 vj00 | UTRAN Overall Architecture | Rel-19 |
| TS 25.415 vj00 | Iu Interface User Plane Protocol | Rel-19 |
| TS 25.424 vj00 | UTRAN Iur Interface Data Transport & Signalling | Rel-19 |
| TS 25.425 vj00 | UTRAN Iur Interface User Plane Protocols | Rel-19 |
| TS 25.435 vj00 | UTRAN Iub Interface User Plane Protocols | Rel-19 |
| TS 25.442 vj00 | Node B Implementation Specific O&M Transport via RNC | Rel-19 |
| TR 25.912 vj00 | Evolved UTRA and UTRAN Technical Report | Rel-19 |
| TS 28.874 vj10 | Study on Management Aspects of NTN Phase 2 | Rel-19 |
| TS 29.163 vj00 | Interworking between 3GPP IM CN and CS networks | Rel-19 |
| TS 32.860 ve00 | D-SON MLB OAM Enhancement Study | Rel-14 |
| TS 36.300 vj00 | E-UTRAN Radio Interface Protocol Architecture Overview | Rel-19 |
| TS 36.302 vj00 | E-UTRA Physical Layer Services | Rel-19 |
| TS 36.401 vj00 | E-UTRAN Overall Architecture Description | Rel-19 |
| TS 36.410 vj00 | S1 Interface: General Aspects and Principles | Rel-19 |
| TS 36.440 vj00 | E-UTRAN MBMS Architecture Description | Rel-19 |
| TS 36.456 vj00 | SLm Interface Introduction | Rel-19 |
| TS 36.459 vj00 | SLmAP for E-UTRAN Positioning | Rel-19 |
| TS 36.842 vc00 | Small Cell Enhancements for LTE Higher Layers | Rel-12 |
| TS 37.470 vj00 | W1 Interface Introduction for ng-eNB | Rel-19 |
| TS 37.480 vj00 | E1 Interface General Aspects and Principles | Rel-19 |
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |
| TS 38.401 vj10 | NG-RAN Architecture Specification | Rel-19 |
| TS 38.460 vj00 | E1 Interface General Aspects and Principles | Rel-19 |
| TS 38.470 vj10 | F1 Interface Introduction | Rel-19 |