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Ethernet evolves for the connected car era

Posted: 25 Feb 2015 ?? ?Print Version ?Bookmark and Share

Keywords:computer networking? automotive bus? Ethernet?

Think of a late model sedan, for example, which in some cases can take hours to finish software or firmware updates, routine diagnostics and maintenance operations.

Clearly, in order to support today's demands and accommodate future advances, automotive manufacturers need to find a new "digital backbone." While various technologies have been considered, most leading automotive manufacturers are moving towards Ethernet as the basis for the car's main system bus.

Ethernet: Automotive-style
Ethernet owes much of its acceptance within the automotive community to the versatile nature of the IEEE 802.3 and 802.1 standards which define the technology. The standard's layered approach enabled development of new physical layer (PHY) specifications media access protocols (MACs) and other capabilities allowing it to keep pace with the demands of modern networks.

Extensions to the original IEEE 802.1/802.3 standards now define practical solutions for providing security, deterministic performance, traffic shaping and many of other application-specific functions, features and specifications.

As a result, Ethernet already has many of the capablities it needs to meet the unique demands of in-vehicle networking. Nevertheless, there were several technical challenges which had to be addressed before Ethernet could win acceptance as the bus of choice for tomorrow's connected vehicles.

A PHY is born
One of the biggest challenges in creating automotive-grade Ethernet was finding an alternative to the multi-pair unshielded twisted-pair (UTP) cabling commonly used by 100BASE-T and 1000BASE-T networks.

While well-suited for use in today's LANs, Category-5/6 cabling is simply too bulky and expensive to be practical in the tight confines and tight budgets of high-production automobiles.

In order to meet the unique cost and manufacturability requirements of automotive applications, the OPEN Alliance (One-Pair Ether-Net) Special Interest Group has developed a new PHY technology (figure 3).

Referred to as 100BASE-T1 or 1 Twisted Pair 100Mbps Ethernet (1TPCE), it uses digital echo cancellation and Decision Feedback Equalisation (DFE) techniques borrowed from 1000BASE-T Gigabit Ethernet to support full-duplex Ethernet at 100Mbps over a single pair of unshielded twisted wires (UTP).

Figure 3: Conventional 10/100BASE-TX Ethernet (b) The single-pair full-duplex PHY developed by the OPEN Alliance. (Source: the OPEN Alliance)

Analogous efforts for 1000BASE-T1/RTPGE are also ongoing. This is a single-pair version of Gigabit Ethernet intended as a high-speed aggregation backbone for 1TCPE traffic.

It could also serve as a direct connection for network backbone and diagnostics, ADAS cameras, infotainment video and other applications with data requirements above 100Mbit/s.

The standard which defines 100BASE-T1 was developed outside of the IEEE 802.3 committee's governance.

But since it is strictly based on the IEEE 802.3 framework, the PHY is fully compatible with a standard Ethernet MAC.

This allows seamless interoperability with nearly any Ethernet-capable microcontroller or embedded system element, provided its MAC's Media Independent Interface (MII) is compatible with the OPEN PHY.

Maintaining compatibility also eliminates the need for any special drivers, making it easily interoperable with Ethernet devices that support the IEEE 1588 remote timing and clock synchronisation protocol.

As you'll see in the next section, the precise timing and synchronisation which IEEE 1588 provides will be critical in automotive sub-systems such as braking, engine control, active suspension, and anywhere else which performs real-time operations.

Timing is everything
As it was originally conceived, Ethernet moves packets through networks on a best-effort, non-deterministic basis.

This makes it poorly-suited for use in mission-critical real-time systems used to control drivetrain or braking systems or support driver-assist functions. But later developments have added mechanisms like the IEEE 802.1tsn (time-sensitive networking) protocol which provides assured, non-conflicting delivery of data and command packets within strict time parameters.

Additional standards for specific timing-critical applications are also available, including the IEEE 802.1AS/802.1Q/802.1Qca protocols which support the transport, switching and management of latency-sensitive connections used in high-speed real-time control applications.

The next generation of infotainment systems will benefit from IEE 802.1AVB which enables the aggregation and distribution of audio/video streams.

All these capabilities rely on the IEEE 1588 precision timing protocol to provide the timing information required by each node to keep its clock synchronised with the network's master clock.

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