An evolution of the Ethernet standard enable time-sensitive networking with the predictable latencies and guaranteed bandwidth necessary for automotive applications.
Advanced driver assistance systems (ADAS) such as lane keeping/departure warning, emergency braking, collision avoidance and, eventually, fully autonomous driving features, all demand predictable latency and guaranteed bandwidth in the network to ensure that safety-critical data can flow from sensors to processors and back quickly enough to be effective.
To meet this demand, Ethernet has become increasingly popular for automotive networking, and so the IEEE’s Working Group on Time Sensitive Networking (TSN) has released a set of TSN standards, and continues to define new specifications, to support real-time networking in vehicles. Automotive SoC developers need to understand the capabilities of Ethernet with TSN, as well as what it takes to implement the standard in an automotive context.
The evolution of time-sensitive networking
The evolution of TSN started with the introduction of Audio Video Bridging (AVB), which was meant to enable the use of Ethernet in audio video systems. As automotive designers began to look to using Ethernet in ADAS applications, their much stricter latency requirements could not be met with AVB alone. This led the IEEE TSN working groups to expand the AVB specifications to meet the requirements of using Ethernet for control applications with predictable latency and guaranteed bandwidth. The initial set of TSN standards is shown in Table 1.
Table 1 The TSN working group has introduced multiple standards (Source: Synopsys)
Automotive networks are designed to have predictable, guaranteed latency. A network with these characteristics is known as an engineered network. The time-aware shaper is used in an engineered network and enables scheduling to ensure that a critical traffic queue is not blocked. This is done with a ‘time gate’ that enables time-critical data to stream unimpeded while blocking non-time critical data, as shown in Figure 1. The IEEE 802.1Qbv scheduler’s logic determines the time intervals at which the gates must open and close. The time-aware shaper is implemented in the Ethernet MAC.
Figure 1 Time-aware shaper allows scheduling (Source: Synopsys)
Pre-emption can also be used to reduce the latency of time-critical data streams. On an Ethernet network, pre-emption enables a time-critical data frame to interrupt the transmission of a data frame whose transmission is not time critical. Once the time-critical data frame reaches its destination, transmission of the non-time critical data frame resumes. Any fragmented data frame must be reassembled before its transmission can continue. See Figure 2.
Figure 2 Preemption reduces latency of time-critical data streams (Source: Synopsys)
Let’s use an emergency braking system as an example. Two Ethernet MACs transmit the time-critical data frame (green) and non-time critical data frame (orange). The preemptable MAC lets the green frame travel ahead of the orange frame to get to its destination in time. The brake is then applied, regardless of the other data frames on the network. The time-critical data frame preempts the non-time critical data frame, which reduces the latency for the time-critical data and makes it more predictable.
Cyclic queuing and forwarding
Cyclic queuing and forwarding supports known latencies regardless of the network topology. Its main role is to make network latencies more consistent across 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.”
Figure 3 Cyclic queuing and forwarding supports known latencies regardless of the network topology (Source: Synopsys)
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 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 useful for detecting and enabling the elimination of disruptive transmissions, thus improving the robustness 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. If the separate paths rejoin at a bridge or merge point in the network, duplicate frames are eliminated from the stream, allowing applications to receive frames out of order. See Figure 4.
Figure 4 Frame replication and elimination detects and mitigates issues caused by CRC errors, broken wires, and loose connections (Source: Synopsys)
For example, when an adaptive cruise control system sends a signal to the control system to maintain a certain speed and distance from the car ahead, separate paths are created across the network to enable this signal and signals from other applications to travel seamlessly. Once the signals merge together, duplicate frames are eliminated to allow uninterrupted signal transmission. The IEEE defines three implementations of frame replication and elimination where the talker sends the signal and the listener receives the signal:
- Talker replicates, listener removes duplicates
- Bridge replicates, listener removes duplicates
- Bridge replicates, bridge removes duplicates
Enhanced generic precise timing protocol
The enhanced generic precise timing protocol supports clock redundancy by synchronizing clocks across the network in two ways: with a single grand master, or with multiple grand masters. The system has a master that synchronizes the clock and a grand master that references the root timing of 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 in different directions so that, in case of an interruption, an accurate clock time is still known throughout the network.
Figure 5a Single grand master transmitting two copies using separate paths (Source: Synopsys)
Figure 5b Multiple grand masters transmitting two copies using separate paths (Source: Synopsys)
The TSN specifications, and other standards such as IEEE P802.1Qcc and P802.1Qcr, have evolved to help automotive designers be more certain that the vehicles they design will be safe. Designers producing SoCs for automotive applications, especially for ADAS, can take advantage of the standards to ensure that safety-critical data flows through their designs as necessary to ensure correct operation.
Automotive SoC designers also need to respect other, stringent standards, such as ISO 26262 covering functional safety, AEC-Q100 covering reliability, and advanced quality management strategies. For example, achieving ISO 26262 certification demands defining and documenting all processes, development efforts, standards, and safety plans for the Automotive Safety Integrity Level (ASIL, A thru D) that the designer has chosen to implement. Similarly, the SoC and IP must be tested to meet very low defect densities measured in defects parts per million.
Synopsys already offers automotive-certified IP such as Synopsys’ DesignWare Ethernet Quality-of-Silicon IP. This ASIL B Ready ISO26262 certified IP with automotive safety package supports Ethernet speeds up to 2.5Gbit/s, real-time networking, the original IEEE AVB specification, and now TSN.
John Swanson is a senior product marketing manager at Synopsys.