How Time-Sensitive Networking Delivers Determinism

With Time-Sensitive Networking poised to bring determinism to standard Ethernet on the plant floor, it’s helpful to understand how the technology works to support your industrial networking decisions.

As work on Time-Sensitive Networking (TSN) for industrial applications is established in the IEC/IEEE 60802 standard and with TSN-capable products becoming increasingly available, it’s a good time to get up to speed with TSN-related terminology, associated devices, and their configuration.

Michael Zapke, industrial marketing lead at Xilinx (a supplier of programmable logic devices) for the company’s industrial, vision, healthcare & sciences operations, offers a list of the technical methods used in TSN. In his list below, the terms at the end of each bullet define the corresponding element in IEEE 802.1Q-2018. All of these methods are key to industrial automation use of TSN.

Zapke’s list is as follows:

Centralized Configuration

  • Identifies traffic relations and bridges between them and calculates a schedule—Central Network Configuration
  • Defines the departure and arrival time—Central User Configuration

Terminals/(End) Stations

  • All points are synchronized, i.e., they work with the same time of day—Generalized Precision Time Protocol
  • The network is aware of which conveyance belongs to which service and has awareness of the valid timetable for departures—Stream Identification
  • Sequence of components so that their individual timetables are met—Queues

Connections and Bridges

  • Awareness of timetables—Gate Control List
  • The network can stop other traffic in an urgent case—Pre-emption and Interspersing Traffic
  • The network knows methods and algorithms to control unplanned peaks—Per Stream Filtering and Policing

“The upcoming standard IEC/IEEE 60802 defines the profile for the selection of functions and their dimensioning for industrial applications,” says Zapke. “The term ‘TSN-IA’ has been introduced for this profile.”

Michael Zapke, industrial marketing lead, Xilinx.Michael Zapke, industrial marketing lead, Xilinx.For a time-aware network like TSN, Zapke notes that a common schedule is essential. “The Central Network Configuration has information about traffic types and it can retrieve information” about all points on the network (topology discovery), he says. “Also, travel times between network elements are known. There is exactly one active Central Network Configuration per network.”

A continuous connection between the Central Network Configuration and the bridges exists and allows access to topology information and the download of updated configurations to the bridges.

Zapke explains the Central User Configuration communicates with each station on the network. In TSN, stations are nodes that send and receive units to the connections. The Central User Configuration takes care to send the right service at the right time, which can include: Traffic with strict deadline for arrival, cyclic traffic with latency bounds, AVB (audio video bridging) traffic with bandwidth requirements, control traffic with strict priority and reliability requirements, and best effort traffic that is sent when possible, but which may be discarded.

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A microcontroller embedded in a station will have a prioritized service called Generalized Precision Time Protocol. “Two adjacent stations can achieve an accuracy of some 10 nanoseconds; between all stations in a network a few hundred nanoseconds should not be exceeded,” Zapke says.

“Because stations are both a source and a destination for traffic, it is their responsibility to label the transport units correctly,” he says. “This label is a tag in the Ethernet Frame (Layer 2). Most common is the use of VLAN Priorities to identify the type of traffic. In newer proposals, more options for tagging streams are also discussed so that any pattern in the Ethernet frame can be used. Beyond this, there are also methods to use the Layer 3 with IP Interception to determine the traffic type.”

Streams, queues, and bridges

The term used to assign characteristics to a frame is Stream Identification. “Every single frame must be scanned at every station for the tag to determine how to handle it at the egress point of the station,” says Zapke. “Stations sort the Ethernet frames and send scheduled traffic exactly at the right time to meet the given schedule.”

Up to eight parallel Queues at the exit point of a station can contain multiple Ethernet frames. Time-controlled gates let data leave the queue at the right time.

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“This all must be done for every single egress packet and may result in high processing load for the station,” Zapke says. “The use of stream identification, queue management and time-controlled gates are normally in dedicated logic to offload software and increase time accuracy. Such network elements use programmable logic in FPGAs (field programmable gate arrays), or microcode programmable SoCs (systems on chip) to realize this function.”

Connections and Bridges are the infrastructure used to connect stations. While the cabling between bridges is static, bridges actively handle the traffic distribution between multiple connections. Stream identification is required there as it is in stations. At every exit port, Ethernet frames must pass Time Controlled Gates that follow a Gate Control List (GCL). Intervals to control the gates are in the range of a few hundred microseconds to some milliseconds. Opening times for gates range from 50 µs to Milliseconds.

“A gate that is open for an unnecessarily long time reduces the capacity of the network,” Zapke explains. “That is why bridges normally know exactly when a scheduled frame arrives. Open gates with high time accuracy are a hardware function in modern TSN-enabled SoCs, as well as some FPGAs.”

Zapke says TSN introduces ‘Pre-emption and Interspersing Traffic’ to “reduce this loss of throughput. Traffic with lower priority can be cut into smaller fragments so that the guard band becomes very small. This allows traffic with higher priority to be transported, even if a long frame with lower priority is already using the connection.”

All of this works well, Zapke says, as long as there is no overload with high priority traffic on the network. An overload could be the result of a crashed application that exceeds its traffic limits. Compensating for this is ‘Per Stream Filtering and Policing’, which can apply metrics and filter traffic according to policies. “This is a network security feature that keeps the network operational,” says Zapke.

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