# When Operation Technology Meets Information Technology: Challenges and Opportunities
## 3 State of the Art in I4.0
### Ultra-Low Latency  
The integration of Internet of Things (IoT) and Cyber-Physical Systems (CPS) with the principles of Industry 4.0 highlights the need for reliable and ultra-low latency communications in future industrial networks. The complex constraints required for ultra-high reliability combined with low latency make this technology very difficult to implement over standard Ethernet. The convergence of IT/OT technologies is bringing a whole world of possibilities for the configuration and management of real-time networks for the automation industry [36]. The use of ULL (Ultra-Low Latency) technologies is relevant for an issue that is deeply felt at an industrial level: safety systems in modern factories are among the most sensitive industrial cases, and reliable and deterministic communications are the backbone of a safety system. According to the Safety Integrity Levels (SIL) [37], there are requirements for the response time of safety applications such as medical or aerospace devices, for example, for level 3 (SIL-3). The panorama of ULL technology is quite crowded with proprietary protocols. For instance, industrial Ethernet systems, such as Sercos, PROFINET, and EtherCAT, are widely used in factory production lines. This dispersion of protocols is problematic in terms of duplication and inefficiencies from an industry point of view--that is, the so-called \"vendor lock-in\" usually creates difficulties in hardware and software interchangeability. One of the most promising standards is an open suite of protocols called TSN [38]. The Institute of Electrical and Electronic Engineers (IEEE) with the Time-Sensitive Networking Working Group 802.1 ([[https://www.ieee802.org/1/pages/tsn.html](https://www.ieee802.org/1/pages/tsn.html)]([https://www.ieee802.org/1/pages/tsn.html](https://www.ieee802.org/1/pages/tsn.html)), accessed on 29 December 2022) is currently working on improving the reliability and real-time capability of standard Ethernet (IEEE 802.3) [39]. Despite being based on best-effort packet networks, TSN is capable of providing real-time performance, low-latency synchronization, and the creation of contracts between transmitters and the network to achieve ultra-reliable packet delivery, bounded latency, and zero congestion loss [40]. The TSN standard set covers a wide range of Quality of Service (QoS) transmission requirements and can be classified into categories [41]. The most common and useful, from an industrial point of view, are as follows:  
* Time Synchronization: IEEE 802.1 AS Time Synchronization for Time-Sensitive Applications (AS). * Scheduling: IEEE 802.1 Qbv Enhancements to Traffic Scheduling: Time-Aware Shaper (TAS). * Control and Orchestration: IEEE 802.1 Qca Path Control and Reservation (PCR). * Policing and Redundancy: IEEE 802.1 Qci Per-Stream Filtering and Policing (PSFP). In this work, we will focus on the time synchronization capability between devices. As described, the TSN stack standardizes the time synchronization in the IEEE 802.1 AS documents. These documents aim to describe a specific protocol to obtain synchronization called the generic Precision Time Protocol (gPTP). This protocol is based on PTP (Precision Time Protocol), which is the backbone of the TSN stack. While the task of clock synchronization is usually performed by NTP (Network Time Protocol) in common scenarios (wall-clock time distribution), there are cases in which the performances of NTP are not sufficient [42], such as the one described in Section 4.2. The PTP standard spans over three main revisions: IEEE-1588 2008 is the second and most popular one, implemented in the majority of devices present in Italian industries; IEEE-1588 2019 introduces minor modifications over the previous standard. The standards define and classify the implementations using \"profiles\", which describe the minimum guaranteed features such as maximum latency or hardware time-stamping. gPTP is the profile required by TSN, specifically described in IEEE-802.1 AS.