name: TNL
full_form: Transport Network Layer
context: When a UE connects to the 5GC via a 5G-AN node without providing any UE identities (i.e. a GUAMI or a 5G-S-TMSI), or the UE provides a GUAMI or a 5G-S-TMSI but the 5G-AN node cannot associate to any of its connected AMFs, the following steps are performed: 1. The 5G-AN node selects an AMF as defined in clause 6.3.5 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. 2. The 5G-AN node creates an NGAP UE-TNLA-binding for the UE by selecting a TNL association from the available TNL associations permitted for the initial message e.g. N2 INITIAL UE MESSAGE for the selected AMF, as defined in clause 5.21.1.3 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2] and forwards the UE message to the AMF via the selected TNL association. 3. The AMF may decide to use the TNL association selected by the 5G-AN or the AMF may modify the NGAP UE-TNLA-binding by triangular redirection. NOTE 1: This process could take place during the Registration procedure (for Initial Registration, Mobility Registration Update). 4. The AMF may decide to modify the NGAP UE-TNLA-binding toward other 5G-AN nodes such as N3IWF. This is done if the AMF is changed and old AMF have existing NGAP UE-TNLA-bindings toward other 5G-AN nodes. When a UE connects to the 5GC via a 5G-AN node with a 5G-S-TMSI or GUAMI associated with the AMF usable by the 5G-AN node, the following steps are performed: 1. The 5G-AN node creates an NGAP UE-TNLA-binding for the UE by selecting a TNL association from the available TNL associations permitted for the initial N2 message for the AMF identified by the UE's 5G-S-TMSI or GUAMI. 2. The AMF may decide to use the TNL association selected by the 5G-AN or the AMF may modify the NGAP UE-TNLA-binding by triangular redirection. NOTE 2: This process could take place during the Registration procedure or Service Request procedure.
description: The Transport Network Layer (TNL) is a critical component in the 5G system architecture, responsible for managing the transport of user plane and control plane data between different network functions. It plays a key role in establishing and modifying NGAP UE-TNLA-bindings, which are essential for routing user traffic and signaling messages within the 5G core network.
explanation: TNL is classified as a Network Function because it serves as an intermediary layer facilitating communication between various network elements, such as the Access and Mobility Management Function (AMF) and the User Plane Function (UPF), in accordance with the 3GPP's specifications for efficient data transport within the 5G system.
section_title: 4.2.7.2.1 Creating NGAP UE-TNLA-bindings during Registration and Service Request
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