## 4 TSN for Industrial Communications
### TSN over Wireless
#### 4.1.2 TSN over 5G  
Private 5G networks and their capabilities, as discussed in Section 2, are emerging wireless solutions for the smart manufacturing domain. Combining TSN capabilities with 5G helps address the URLLC requirements of Industry 4.0. The architecture (e.g., considering a fully centralized TSN configuration model) and functionalities required for 5G-TSN integration are discussed in 3 GPP Release 16. The 5G system mainly consists of two components: a radio access network (comprised of UEs and gNBs) and a 5G core (5 GC) network. The core network comprises different network functions associated with the user plane and control plane elements. The 5G system (5 GS) is modeled as one or more logical/virtual TSN bridge(s) within the TSN network for 5G-TSN integration in 3 GPP Release 16, as shown in Figure 4. This logical 5 GS TSN bridge has TSN translator functionality to establish connectivity with the other elements in the network, both at the device side (DS-TT: Device-Side TSN Translator) and at the network side (NW-TT: Network-Side TSN Translator). The DS-TT acts as an Ethernet port at the UE-side user plane, whereas the NW-TT user plane, integrated with the 5 GC user plane function (UPF), acts as an Ethernet port for the 5 GS toward the external data network. The NW-TT control plane is implemented as an application function (AF) and manages the interface toward the CNC. The AF passes the TSN configuration/management-related information provided by the CNC to the 5 GC (control plane).  
Figure 4: Integration of 5G system into TSN as a logical bridge.  
Support for TSN time synchronization in 5G is defined by IEEE 802.1 AS. The 5 GS itself is a time-aware system with an internal synchronization mechanism that keeps all elements, such as gNB, NW-TT, DS-TT, etc., synchronized to the 5G internal clock. TSN synchronization represents another concurrent synchronization process in 5G-TSN integration. In this process, the TSN GM is outside the 5 GS, and when a gPTP message (involved with the TSN synchronization discussed earlier) enters the 5 GS through the NW-TT entity, the ingress timestamp (in terms of 5 GS time) is added and then forwarded to the UE through the UPF. When this gPTP message reaches the DS-TT, the residence time in the 5 GS is calculated with the help of an egress timestamp, included in the gPTP message, and sent to the external TSN elements. The 3 GPP Release 17 discusses uplink TSN synchronization and UE-UE synchronization, where the TSN GM can be from the UE side. In this case, the DS-TT acts as the ingress port and the NW-TT as the egress port for the TSN gPTP messages.  
The 5 GS features several mechanisms for enhanced reliability when integrated with TSN. These mechanisms include multi-antenna transmissions, the use of multiple carriers, and packet duplication over different radio links in the 5G network. Redundancy in PDU sessions, UEs, and RAN dual connectivity, with increased radio resource usage, ensures reliability within the 5 GS virtual bridge, an element of the TSN network. In addition to user plane redundancy, the 5G core network supports redundancy by enabling control plane network functions that can be deployed in multiple locations (e.g., edge, data centers).  
Flexible numerology and configuration of OFDM signals are vital for enabling low latency in 5G networks. Shorter subframe slots and mini-slots are key enablers of low latency in 5G-TSN systems. The 5G new radio also supports the preemption concept, allowing URLLC transmissions to preempt non-URLLC data transmissions. Network slicing techniques and the 5G LAN-type service can be employed to handle time-critical TSN flows.