## 4 TSN for Industrial Communications
**Bounded End-to-End Latency**: In an industrial network, TSN adopts several mechanisms to provide the required end-to-end latency for different traffic classes, including time-critical messages. Special packet-handling approaches, such as traffic scheduling and traffic shaping techniques, are introduced to guarantee bounded latencies. In the early days, the development of TSN standards considered Ethernet as the physical layer, and IEEE 802.1Q is a standard that supports VLANs on an Ethernet network. According to this standard, the IEEE 802.1Q tag (\"Priority Code Point\" field--3 bit) defines eight types of traffic classes, each having its own separate queue per Ethernet port. Various mechanisms (defined by IEEE 802.1 Qav, IEEE 802.1 Qbv, IEEE 802.1 Qbu, IEEE 802.1 Qch, and IEEE 802.1 Qcr) [55] can be used to achieve different QoS levels across the eight queues in multi-hop switched networks. IEEE 802.1 Qbv (Time-Aware Shaper) manages time-critical traffic flows based on a time-triggered scheduling approach. Each queue has a transmission gate  
Figure 3: TSN: Features, key mechanisms, and associated standards [56]. that is opened/closed in a cyclic fashion for a particular time duration based on a gate control list (GCL). IEEE 802.1 Qav (Credit-Based Shaper) defines the operation of traffic flows with relaxed latency bounds. A \"credit\"-based transmission strategy is adopted to allocate bandwidth (logically) to all traffic classes and thus prevent the starvation of lower-priority traffic [58; 59]. IEEE 802.1 Qpu (Frame Preemption) specifies the suspension/holding of an ongoing low-priority best-effort traffic (preemptable) transmission by a high-priority traffic flow (express traffic) to meet latency bounds. The associated IEEE 802.3 br amendment defines two MAC interfaces: express and preemptable. Only a frame mapped to an express interface can preempt the frame mapped to a preemptable interface. IEEE 802.1 Qch (Cyclic Queuing and Forwarding) specifies assigning message frames to egress queues according to their arrival time. IEEE 802.1 Qcr (Asynchronous Traffic Shaper) also shapes the incoming traffic with the help of a \"credit\" counter and schedules prioritized over non-prioritized traffic, per hop, without the need for synchronization among various network elements. The IEEE P802.1 Qdq amendment describes the recommended shaper parameter settings for bursty traffic needing bounded latency, and IEEE P802.1 DC specifies the QoS features for a networked system rather than a bridge. **Reliability**: This is another critical requirement for industrial networks in the smart manufacturing domain. Most communication technologies handle reliability through re-transmission mechanisms, which is not an option for time-critical traffic flows. IEEE 802.1 CB (Frame Replication and Elimination) ensures reliability in TSN networks through redundant frame transmissions across disjoint paths. IEEE 802.1 Qca (Path Control and Reservation) handles the creation of multiple paths in the network by extending the application of Intermediate System to Intermediate System (IS-IS). It also specifies explicit path control, data flow redundancy, and distribution of control parameters for scheduling and time synchronization. IEEE 802.1 Qci (Per-Stream Filtering and Policing) specifies the procedures for filtering frames of a particular data stream (using rule matching to filter StreamIDs). It also addresses policing aspects to ensure that the systems/devices conform to the agreed configurations (e.g., allocated bandwidth) to guarantee QoS to different data streams. **Resource/Network Management**: Proper configuration/management of various network elements (e.g., bridges, switches, end devices) and resources, such as bandwidth, communication paths, and schedules, are crucial to achieving TSN's low latency and highly reliable communication capabilities. IEEE 802.1 Qat (Stream Reservation Protocol) deals with reserving resources and schedules between the source and the destination to achieve the desired end-to-end QoS.