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Automotive Ethernet, Time Sensitive Networking (TSN)

Time sensitive networking puts real time into automotive Ethernet.
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Description

Automotive Ethernet is a wired connection in or between systems in the automobile. Because Ethernet is an open standard, managed by IEEE 802.3 working group and has been around for over 40 years, the automotive version of Ethernet is seen as a way to reduce cost and complexity. Also, because the medium (the cabling) used in automotive Ethernet is a two-wire system with speeds of 2 Gbps, this is seen as reducing weight from all proliferation of cabling in cars. Ethernet in automotive could reduce the cacophony of networks in a car because networks vary by domain. “The networking infrastructure found inside a car is a domain-based architecture. There are different domains for each key function—one for body control, one for infotainment, one for telematics, one for powertrain, and so on. Often these domains employ a mix of different network protocols (e.g., with CAN, LIN and others being involved),” writes Marvell Semiconductor’s Christopher Mash, senior director of automotive applications and architecture. A switch to zonal architecture Ethernet negates the need for all the gateways between network systems. Again, it could also reduce weight by reducing extra wiring.

Time sensitive networking
The Ethernet for automotive that touches safety critical functions has to have low latency, must be deterministic and have guaranteed bandwidth, all features that Ethernet for home or office use do not need. A deterministic Ethernet that guarantees messages hit their destination in real time—or at least a guaranteed amount of time—is now possible. “If you have Ethernet in your car and Ethernet is the medium between your foot pedal and your brakes, you don’t want that to be a random amount of time when you step on the brake pedal until when your brakes react. You want to know that it is going to react in a certain amount of time,” said Cadence’s Jacobson in Part 3 of his Ethernet video series. This type of Ethernet, called time sensitive networking (TSN), is used in industrial control and automotive because it gives engineers the tools to design automotive networks with predictable latency and guaranteed bandwidth. As advanced driver assistance systems (ADAS) improve in cars, they need real-time networking. IEEE and industry have developed standards for TSN version of Ethernet and continue to work on it.

Source: Synopsys article “Ethernet Time-Sensitive Networking for Automotive ADAS Applications,” John Swanson, Sr. Product Marketing Manager, Synopsys, 2018.

ADAS has to often synchronize with multiple systems in the car, requiring a lot of data to be deployed over the network. “For example, the emergency braking system must account for the brake distance and human reaction time. When an obstacle is detected, the collision avoidance system notifies the driver and the braking system activates. Communication between the detection and braking system is critical and delays in applying the brake could be catastrophic,” writes Synopsys’ John Swanson.

TSN evolved with the introduction of Audio Video Bridging (AVB), which was intended for audio video applications including automotive infotainment and in-vehicle networking systems. “In an AVB network, audio from the radio, video from the infotainment system, data from the car’s command center, and file transfers from running diagnostics is streamed and bridged across a common network. The latency for such networking does not have a critical time constraint that data does for safety-critical ADAS application like an automatic braking system. Moreover, the amount of data needed for an ADAS application can be much greater. Because of these reasons, the IEEE committee expanded the capabilities and features originally defined as AVB and renamed the working group to TSN. The IEEE working group has created multiple TSN standards seen in Table 1.”

How TSN Works
TSN has these components:

  • Time-aware shaper
  • Preemption
  • Cycling queue and forwarding
  • Per stream filtering and policing
  • Frame replication and elimination
  • Enhanced generic precise timing protocol

Time-aware shaper*:
Automotive networks are designed to have predictable guaranteed latency; this type of network is known as an engineered network. The time-aware shaper is used in an engineered network and allows scheduling so a critical traffic queue is not blocked. This is done with a time gate that allows the time-critical data to stream and proceed unimpeded while blocking the non-time critical data; shown in Figure 1. The IEEE 802.1Qbv scheduler’s logic determines the time intervals that the gates must open and close. Time-aware shaper is implemented in the Ethernet MAC.

Preemption*:
Preemption can also be used to reduce the latency of time-critical data streams. On an Ethernet network, preemption allows a time-critical data frame to interrupt the transmission of a non-time critical data frame. Once the time-critical data frame reaches its destination, the non-time critical data frame resumes its transmission. Any fragmented data frame must be reassembled before they can continue their transmission. See Figure 2.

Cycling queue and forwarding*:
Cyclic queuing and forwarding supports known latencies regardless of the network topology. Its main role is to make the network latencies more consistent across the bridges. See Figure 3. According to the IEEE P802.1Qch standard, the cyclic queuing and forwarding amendment, “specifies a transmission selection algorithm that allows deterministic delays through a bridged network to be easily calculated regardless of network topology. This is an improvement of the existing techniques that provides much simpler determination of network delays, reduces delivery jitter, and simplifies provision of deterministic services across a bridged LAN.”

Per stream filtering and policing*:
Per stream filtering and policing enables a bridge, or endpoint component to detect whether components in the network are conforming to the agreed upon rules. For example, a node gets allocated a certain amount of bandwidth and when this bandwidth is exceeded due to a component failure, or malicious act, action can be taken to protect the network. This standard includes procedures to perform frame counting, filtering, policing, etc. Policing and filtering functions are especially valuable and can be used for the detection and subsequent elimination of disruptive transmissions, thus improving the robustness and security of the network.

Frame replication and elimination*:
Frame replication and elimination supports seamless data redundancy. It detects and mitigates issues caused by cyclical redundancy check (CRC) errors, broken wires, and loose connections. The time-critical data frame is expanded to include a sequence number and is replicated where each frame follows a separate path in the network. At any bridge or merge point in the network, when the separate paths join again, duplicate frames are eliminated from the stream, allowing applications to receive frames out of order. See Figure 4.

Enhanced generic precise timing protocol*:
Enhanced generic precise timing protocol supports clock redundancy by synchronizing clocks across the network, in two ways: with a single grand master or multiple grand masters. The system has a master that synchronizes the clock and a grand master that references the root timing across the network. In a single grand master model, the clock time information is transmitted to the listener on one segment of the network, and then communicated to the other segment on the same network. Only the grand master knows the accurate clock time. In a multiple grand master model, the clock time is transmitted throughout the network at different directions so in case of an interruption, the accurate clock time is still known throughout the network.

Transmission speeds for AVs
Automotive Ethernet connections will need to support higher rates of data transmission to meet the networking requirements of autonomous vehicles (AVs), Alan Amici of TE Connectivity wrote in this 2019 analysis. “Wireless networks could offer some advantages in internal and external AV communication, but AVs cannot rely on a network with any chance of experiencing a delay, making wired networks the safest bet,” he notes. “Cars with semi-autonomous features currently have network speeds ranging from 500 kilobits per second to 1 megabit per second — but fully autonomous cars will require networks capable of speeds approaching 10-20 gigabits per second.”

*Source of definitions: Synopsys article “Ethernet Time-Sensitive Networking for Automotive ADAS Applications,” John Swanson, Sr. Product Marketing Manager, Synopsys, 2018.

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