Time-Sensitive Networking (TSN)

What Is TSN?

Time-Sensitive Networking (TSN) is a suite of protocols developed by IEEE 802.1 designed to bring deterministic minimal delay to the typically non-deterministic Ethernet. TSN introduces a set of universal time-sensitive mechanisms at the data link layer of Ethernet protocols, ensuring real-time, deterministic, and reliable data transmission over standard Ethernet. This technology enhances data transmission efficiency and allows both time-sensitive and non-time-sensitive data to coexist on the same network.

What Important Protocols Are Included in TSN?

Traditional Ethernet uses the carrier sense multiple access with collision detection (CSMA/CD) mechanism to prevent collisions when multiple stations transmit data simultaneously. However, when collisions do occur, they can cause significant delays in packet transmission, leading to unpredictable communication times. Historically, data requiring high real-time performance was transmitted via real-time Ethernet, necessitating separate networks for data with and without real-time requirements. This setup meant that controllers needed two network ports: one for real-time Ethernet and another for standard Ethernet, making it challenging to integrate information technology (IT) with operational technology (OT). TSN addresses this issue by ensuring real-time and deterministic data transmission while allowing both time-sensitive and non-time-sensitive data to coexist on the same network.

TSN offers a comprehensive set of protocol standards, known as the TSN protocol suite, to achieve real-time and deterministic data transmission within standard Ethernet environments. This suite ensures the reliable transmission of real-time data across various Ethernet scenarios. The flexibility of the TSN protocol suite allows users to choose specific protocols based on their application needs. The suite is divided into four main categories: clock synchronization, data scheduling and traffic shaping, reliability, and resource management.

Clock Synchronization

In contrast to the standard Ethernet defined by IEEE 802.3, clocks play a critical role in a TSN network. Achieving real-time communication requires precise management of end-to-end transmission delays, which is challenging without synchronized clocks across all devices in the TSN network. Currently, TSN employs the IEEE 1588 and IEEE 802.1AS protocols for clock synchronization. The IEEE 1588 protocol, also known as Precision Time Protocol (PTP), is designed to synchronize clocks in a computer network. Within a LAN, it can control clock accuracy to the sub-microsecond level, making it ideal for measurement and control systems.

Data Scheduling & Traffic Shaping 

TSN introduces various shaping mechanisms to keep data flow delays within specific limits, catering to scenarios that require low latency. Traditional Ethernet networks often experience unpredictable communication delays, necessitating large buffers at the receiving end to manage data flow. However, this lack of real-time capability is problematic for flows like audio and video. TSN addresses this by ensuring timely and low-latency transmission of time-sensitive flows. Traffic shaping is employed to optimize the transmission of time-sensitive, best-effort, and other data flows on the network, ensuring they meet their transmission time requirements.

TSN provides several standards—IEEE 802.1Qav, IEEE 802.1Qbv, IEEE 802.1Qbu, IEEE 802.1Qch, and IEEE 802.1Qcr—for data scheduling and traffic shaping. Among these, IEEE 802.1Qbv uses non-preemptive data scheduling, where traffic scheduling is managed through timeslots. Time-sensitive data is prioritized for transmission, while bandwidth is allocated for both best-effort and reserved data, allowing the coexistence of time-sensitive and non-time-sensitive flows on the same network. This guarantees real-time data transmission.

As illustrated by IEEE 802.1Qbv timeslot division, packets are scheduled for end-to-end transmission within a repetitive period. IEEE 802.1Qbv categorizes traffic into three basic types: time-sensitive flows, best-effort flows, and reserved flows. Time-sensitive flows refer to critical messages with stringent real-time requirements, best-effort flows are typical Ethernet traffic without specific QoS needs, and reserved flows require specific bandwidth reservations with soft real-time requirements.

IEEE 802.1Qbv uses a scheduling algorithm to control gates that enable or disable frame transmission. Ethernet communication is divided into periods of fixed length and continuous repetition, with these periods further divided into timeslots. Each timeslot assigns different priorities to data, ensuring that time-sensitive flows have dedicated slots for deterministic transmission on traditional Ethernet. Reserved flows are allocated dedicated bandwidth, while best-effort flows utilize the remaining bandwidth.

In the example provided, Timeslot 1 is exclusively reserved for time-sensitive flows, while Timeslot 2 is shared by best-effort and reserved flows, with dedicated bandwidth allocated to the latter. To ensure the precise transmission of different types of flows according to their designated timeslots, all network devices must be synchronized within nanoseconds.

Reliability 

For applications that require real-time data transmission, ensuring the timeliness and high reliability of data transmission is crucial. This capability helps mitigate issues such as bridge node failures, line disconnections, and external attacks, thereby maintaining functional and network security. To address these challenges, TSN implements specific performance standards using protocols like IEEE 802.1Qci, IEEE 802.1CB, and IEEE 802.1Qca.

IEEE 802.1CB introduces dual-link redundancy to Ethernet networks. This feature ensures that only one copy of a data frame is received by numbering and replicating each data frame within the network's source and relay systems and then eliminating duplicate frames at the target system and other relay points. This mechanism prevents packet loss due to network congestion, reduces the likelihood of packet loss from device failures, and shortens recovery time, ultimately enhancing network reliability.

Resource Management 

In a TSN network, each real-time application has specific performance requirements that must be met. Enabling a feature on a TSN network involves the configuration and management of available network resources. This process ensures that multiple TSN sub-protocols can be configured on the same network to allocate resources effectively along network paths, guaranteeing that the resources function as expected. TSN's resource management is governed by sub-protocols such as IEEE 802.1Qat and IEEE 802.1Qcc, with IEEE 802.1Qcc being an enhancement of IEEE 802.1Qat.

The IEEE 802.1Qat protocol is designed for flow reservation, where data admission control is based on the flow's resource requirements and the available network resources. This protocol reserves the necessary resources and notifies all network nodes between the data sender and receiver, ensuring that sufficient network resources are available throughout the entire transmission path for the specified flows.

Application Scenarios of TSN

Time-Sensitive Networking (TSN) is crucial in scenarios requiring highly reliable and low-latency data transmission. It plays a significant role in industrial automation, enabling precise machine control and robotic communication. Additionally, TSN ensures real-time communication between sensors and control units in automotive systems and optimizes synchronized data transmission in smart grids for power systems. TSN is also suitable for broadcast and audiovisual systems, ensuring seamless media transmission.