I am explaining clock timing and synchronization, scheduling, and traffic shaping, which are closely related to the real-time performance of TSN. In terms of clock synchronization due to the difficult to negotiate time limits of end-to-end transmission delay, all devices in the network need a common time reference to complete clock synchronization. For fields such as communication and industrial control, since all tasks are based on a time reference precise clock synchronization is the fundamental standard. TSN first needs to solve the clock synchronization and delay calculation problems in the network to ensure high consistency in task scheduling for the entire network. Time synchronization can be achieved through different technologies. In theory, GPS clocks can be equipped for each terminal device and network switch, and synchronization can be based on GPS second pulses. This method is costly and cannot guarantee that the GPS clock can always be connected to satellite signals. The time of TSN networks usually comes from a central clock source, which is based on the IEEE 1588 Precision Time Protocol (also known as the Precision Clock Synchronization Protocol for Network Measurement and Control Systems) to achieve time synchronization. The basic idea of IEEE 1588 is to synchronize the clocks of net work devices with the master clock of the central device through hardware and software, achieving synchronization establishment time of less than 10 µs. Compared to the pure software-based Network Time Protocol (NTP), IEEE 1588 has much higher synchronization accuracy.
Due to the limitations of switch port forwarding mechanism, real-time performance is difficult to guarantee in standard Ethernet. Especially when the network Utilization is high, latency becomes very severe. The storage and forwarding strategies and bandwidth reservation capabilities commonly used in multi-port switches are insufficient. If the network traffic is too large, it can reject datagrams. Even if Ethernet frame priority identification is used, high priority datagrams may still be lost. Scheduling and traffic shaping allow for the coexistence of traffic of different priority categories on the same network, with each category defining different requirements for available bandwidth and end-to-end latency.
The implementation ideas of scheduling and traffic shaping have some similarities with PROFINET IRT. PROFINET IRT divides the time of each cycle into deterministic and open channels, forcing the time to be split into two time slices within one cycle. Through time triggering, deterministic channels are used for data with strict time requirements to ensure real-time and deterministic data transmission. IEEE 802.1 Qbv defines a Time Awareness Shaper (TAS), which establishes multiple software data queues within the hardware and introduces the concept of transmission gates. The gates have two states: on/off, and each queue is controlled by a pre-set periodic gate control list, dynamically providing on/off control for the data queue. During the transmission process, only the information with the gate of the data queue in the ‘open” state is selected and the transmission port is opened to ensure that the queue with strict time requirements is protected form interference from other network information. TSN integrates real-time industrial control, OT and IT, audio and video into a single network, which sounds very attractive, but at the same time, it means that the product is relatively complex, and complexity often affects product cost, quality, and reliability, which may become an obstacle to implementing TSN. Currently, TSN related equipment costs are still high.
Post time: Apr-11-2025