Ethernet Rises to Automotive Deadline Challenges

­The automotive sector is seeing a major shift in the computing architecture used in vehicles. This shift has major implications for the networking techniques OEMs use to coordinate the many different functions that require software control.

Manufacturers are deploying more advanced microprocessors to artificial intelligence (AI) functions that improve safety and driving performance, and advanced control to improve energy efficiency. These changes are leading to a move away from conventional designs where an electronic control unit is dedicated to each discrete function. Vehicle makers are moving to zonal architectures to take advantage of the high performance of multicore SoCs. In these architectures, the vehicle is divided into several zones that are identified by location rather than function. Applications, ranging from driving control to infotainment services, will be distributed among processors in each of these zones.

To ensure the reliability and security of data that moves around the vehicle, all communications are mediated by zonal gateways. These gateways help ensure that low-priority packets, such as email attachments delivered to passengers, do not cause vital real-time data needed for braking or lane control to be delayed. For this reason, automotive networks employed in zonal architectures have moved to forms of the Ethernet standard that enforce real-time behaviour.

Toshiba’s own tests have shown how important the changes made with the Time-Sensitive Networking (TSN) enhancements are to automotive systems compared with the best-effort design of traditional Ethernet. To demonstrate a realistic test environment, Toshiba engineers built a network that emulated the behaviour of a storage node connected by Ethernet to WiFi controllers used to implement the basis of an in-vehicle wireless hotspot. Code implemented on the host SoCs was designed to emulate the typical applications in automotive: a zonal gateway; audio-video streaming; wireless-hotspot support; and mass-storage transfers. The tests showed how video sent using the AVB protocol, which takes advantage of TSN features, remains synchronised and smooth while traffic relayed using best-effort protocols experiences longer delays as contention on the network increases.

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To improve the real-time behaviour of the network, the TSN extensions to Ethernet comprise a number of options that improve quality of service (QoS) and make various real-time architectures possible. The core of TSN lies in IEEE 802.1AS, also known as the Generalized Precision Time Protocol (gPTP).

By providing a mechanism to ensure all the nodes on an Ethernet network can agree on a common time, within a certain degree of error, gPTP provides the basis for a variety of protocols that enforce the timely delivery of packets. Rather than demand the use of high-precision clocks in each communicating node, gPTP lets endpoint devices simply agree on a common local time and to do so with sub-microsecond precision.

The protocol deals with the problems faced by controllers in the automotive environment where differences in voltage, temperature and other conditions can cause the clocks to drift apart from each other. By maintaining agreement between nodes, gPTP ensures all controllers on the network can agree on a consistent timeline.

With a common timeline, network controllers can support time-aware traffic shaping and packet scheduling. The idea behind traffic shaping is to prevent packets with latency constraints from being affected by packets that can be delivered on a best-effort basis. Nodes agree and enforce time boundaries for delivery of packets with a known end-to-end transmission latency. Time-sensitive packets - sitting in the dedicated output buffers of a sending node - are transmitted during the reserved timeslot and therefore benefit from stronger delivery-time guarantees.

The IEEE 802.1Qbv enforces strict schedules for time-sensitive traffic. The scheduler used by IEEE 802.1Qbv allocates periodic cycles to accommodate fixed-length time slices, each of which is allocated to a priority level. An application operating at the corresponding priority is granted exclusive use of the network for the length of that time slice. Best-effort traffic queues for access to periods when no higher-priority packet is being transmitted. In principle, use of IEEE 802.1Qbv reduces the variance in path delay for traffic that needs delivery guarantees.

Further improvements to real-time behaviour come with the IEEE 802.1Qbu and IEEE 802.3br extensions, which implement the ability for a node to interrupt the sending of a long packet with low priority and to insert one or more time-critical packets in its place before resuming transmission of the original data. The protocols differ in the way they target different layers of the Ethernet stack. Whereas IEEE 803.2br is a physical-layer standard, 802.1Qbu operates at the media-access control (MAC) layer and is used to manage frame pre-emption according to network and traffic-priority policies.

System designers do not need to divide all traffic into best-effort and strict scheduling. The credit-based traffic shaper was introduced with the IEEE 802.1Qav standard can be used to send packets that need better handling of time-sensitive data but where hard timing guarantees are not required. Nodes may use this credit-based system, for example, to deliver video frames where end-to-end latency is not critical and so take lower priority than the mission-critical sensor data that employs the IEEE 802.1Qbv priority-based time slices.

The issue that automotive OEMs face when creating networks for the zonal architectures in their latest vehicles is that though SoCs may support standard Ethernet, they do not possess the extra capabilities of TSN. Furthermore, though it is important that zonal gateways support TSN, many other endpoints will also have strict real-time requirements that cannot be supported by connecting to a gateway through a non-TSN interface. For example, audio devices such as microphone controllers and sensor hubs may have conventional Ethernet support but will need to be able to negotiate for credit-based or time-critical time slots on the network.

TSN capabilities can be provided through the use of dedicated real-time Ethernet interfaces such as Toshiba’s TC9562 and TC9563. Both are highly integrated Ethernet controllers with full support for gPTP, IEEE 802.1Qav, IEEE 802.1Qbv, and other elements needed for reliable real-time communications where high bandwidth is a key requirement. The TC9562 provides support for 1Gbps Ethernet, suiting it to endpoints that need high bandwidth capability. The TC9563 extends the networking capability to two ports, both able to support 10Gbps.

As well as the dual Ethernet interfaces, the TC9563 incorporates an Arm Cortex-M3 processor, delivering the ability to run monitoring and control software. This can be used to monitor errors and conditions on the network to help improve overall reliability. To support locally attached clusters of advanced sensors and compute modules, the TC9562 implements a PCIe Gen 2.0 interface and the TC9563 incorporates a PCIe switch with support for one upstream and two downstream Gen 3.0 ports.

Services that need the power of high-performance SoCs to deliver AI-enhanced control and superior multimedia are only possible if applications can rely on packets being delivered within programmed time windows. The TSN enhancements to Ethernet provide the basis for this behaviour. Their implementation in devices such as Toshiba’s TC9562 and TC9563 ensure that OEMs and systems integrators have access to the TSN support they will need to implement these advanced-vehicle designs.

Toshiba Electronics Europe