Can Automotive Ethernet contribute to the vehicle weight-loss plan?: Page 2 of 4

November 02, 2020 //By Klaus Neuenhüskes, Toshiba Electronics Europe
Can Automotive Ethernet contribute to the vehicle weight-loss plan?
The automotive cable harness is regularly claimed to be the third heaviest component in the vehicle, as well as the third most expensive. It combines a multitude of power cabling, standardised in-vehicle networks, and wiring for proprietary network protocols or bus systems. Without making substantial changes to the vehicle’s E/E Architecture, the only remaining approach to reducing weight is to move away from copper wiring to lighter alternatives.

Further issues arise when reviewing the cabling and signalling. Ethernet as implemented in homes and commercial buildings, if using CAT5e cabling, uses two of four pairs of wires for uni-directional data transfer inside an unshielded cable. This will obviously not improve the weight of the cable harness. If the existing technology were simply reduced to a single twisted-pair without shielding, the resultant solution would not fulfil existing electromagnetic interference (EMI) requirements.

Automotive Ethernet, AVB and TSN

To make Ethernet automotive-ready, several working groups have been developing standards to meet the automotive industry’s needs. At the physical level, a 100 Mbits/s full-duplex physical interface has been defined that can operate across unshielded twisted-pair cabling. Named 100BASE-T1, differentiating it from 100BASE-T, it is described in the IEEE 802.3bw standard. It makes use of PAM-3, 3-level signalling with a data rate of 66.67 Msymbols/s and can reach lengths of 15 m, or 40 m if shielded. A 1000 Mbits/s interface, known as 1000BASE-T1, is covered by IEEE 802.3bp. The result is two speed grades enabling an ultra-high-speed data backbone between key ECUs, and a more cost-effective, but still high-speed, interface to end nodes (Figure 1).

For simpler functions and comfort options, such as window openers, ambient lighting, and motorised seats, traditional in-vehicle networks, such as LIN, CAN and CAN-FD, will continue to play a vital role beyond the 100BASE-T1 enabled end nodes.

When it comes to transporting audio or video data for in-vehicle entertainment, it is essential to be able to operate within defined latencies and reserve bandwidth across the network. The Audio Video Bridging (AVB) working group developed a set of standards that deliver these functions. Many of these changes impact layer 2. IEEE 802.1Qav is responsible for defining the rules that ensure Audio/Video (AV) Bridges allow AV streams to pass through the network within a defined time constraint. It handles traffic shaping ensuring that the network is not overwhelmed by bursts of traffic. IEEE 802.1Qat guarantees the end-to-end resource necessary to support the transfer of a data stream and provide Quality of Service (QoS).

Further improvements are provided that ensures time synchronisation between network nodes (IEEE 802.1AS) and procedures for ensuring that multiple node present their data at the same moment in time (IEEE 1722). This is important in an application where a head unit is distributing audio data to be output at two or more loudspeaker nodes.

Although the AVB standards cover bandwidth reservation and fixed latency, there are other use cases where even shorter latencies are required. This is the case when closed-loop control is being implemented over the Ethernet interface. The Time Sensitive Networking (TSN) standards address these issues. These include IEEE 802.1Qbv-2015 that provides defined time windows to ensure end-to-end latencies by blocking low-priority traffic.

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