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 ofTSN中使用的技术方法。在下面的列表中，每个子弹末尾的术语定义IEEE 802.1Q-2018中的相应元素。所有这些方法都是工业自动化使用TSN的关键。
Zapke’s list is as follows:
Centralized Configuration
- 识别它们之间的交通关系和桥梁并计算时间表 -中央网络配置
- Defines the departure and arrival time—中央用户配置
Terminals/(End) Stations
- All points are synchronized, i.e., they work with the same time of day—GeneralizedPrecision Time Protocol
- 网络了解哪些运输工具属于哪种服务，并具有对何时何种的有效时间表的认识 -Stream Identification
- Sequence of components so that their individual timetables are met—Queues
连接和桥梁
- Awareness of timetables—Gate Control List
- 网络可以在紧急情况下阻止其他流量 -抢先和散布交通
- 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.”
迈克尔·ZAPKE，工业营销铅，Xilinx。对于像TSN这样的时间感知网络，ZAPKE指出公共计划至关重要。他说：“中央网络配置有关于流量类型的信息，它可以检索信息”关于网络上的所有点（拓扑发现）。“此外，已知网络元件之间的旅行时间。每个网络都有一个主动中央网络配置。“
中央网络配置和桥梁之间的连续连接存在，并允许访问拓扑信息并将更新的配置下载到桥梁。
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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学习how TSN can improve industrial productivity. |
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 ns; 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 (Priority Code Point) 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 (IEEE 802.1CBdb). Beyond this, there are also methods to use the Layer 3 (IP packet) 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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阅读TSN的业务福利。 |
“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.”
连接和桥梁基础设施使用d 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 aGate 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 unnecessary 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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为什么工业网络协议正在围绕TSN协调。 |
