You need to test, we're here to help.

You need to test, we're here to help.

24 August 2017

Introduction to Automotive Ethernet Compliance Testing

As with most any networking scheme, Automotive Ethernet is subject to standardization to ensure that the various components of a given system reliably pass signals among themselves. Where there is a standard for a protocol, there must also be testing for compliance with that standard. This will be the first in a series of posts detailing compliance test of the Physical Media Attachment (PMA) aspect of the Automotive Ethernet standard.

We've covered extensively the basics of Automotive Ethernet in a previous series of posts, so we won't repeat much of that here. Let it suffice to say that "Automotive Ethernet" is an umbrella term for both BroadR-Reach (or OPEN Alliance BroadR-Reach) and 100Base-T1 (the IEEE's 802.3bw-2015). In either case, Automotive Ethernet is specifically tailored to enable faster data communication for in-vehicle networking.

There are various categories of Automotive Ethernet compliance testing, and we will focus on test of electrical signaling to determine whether a product conforms to the electrical transmit and receive specification. Other types of testing include the physical coding sublayer (PCS) and PHY control to evaluate the functionality of the protocol itself.

The PMA, PCS, and PHY control tests are spelled out in the 100Base-T1 specification. Having said that, the IEEE did not write a test specification. The specification might say, for example, that jitter must not exceed a given value, but the specification does not detail how to perform jitter testing in the way that specifications in the high-speed serial data world do.

For its part, the University of New Hampshire's InterOperability Laboratory (UNL-IOL) has traditionally written test documents that describe how they perform compliance tests. The OPEN Alliance has licensed UNL-IOL to create and maintain "test suites" for each group of Automotive Ethernet compliance testing. These documents serve as "pseudo" test specifications for PMA, PCS, and PHY control compliance. But given the lack of a true test specification, it is incumbent on OEMs, Tier 1 automotive suppliers, and PHY vendors to work together with the test manufacturer or testing house to ensure that the appropriate level of testing takes place and is conducted correctly.

Physical Media Attachment compliance test breaks down into two groups: Electrical Measurements and PMA Receive Tests. In these posts on compliance test, we'll take a closer look at the former group, and in particular those that are performed with an oscilloscope (some MDI tests are done using a vector network analyzer).

Why Is PHY Compliance Important?

But to backtrack for a moment, let's look at why PHY Control compliance testing is so important. PHY testing is a protocol-level affair that is typically performed by silicon vendors. The OEM development cycle for an automotive ECU is a long one, so the OEMs must be assured of the PHY chip's compliance before implementation.

Even though the PHY chip has passed compliance testing, the ECU should be tested again for compliance after PHY implementation. That testing can be a full compliance suite or some subset thereof. Why repeat compliance test for the ECU? Because in the process of integration, changes in the layout might impact the electrical specifications. Thus, it's important to validate the design after integration of the PHY chip.

Further, compliance with the 100Base-T1 specification does not guarantee interoperability. Although the transmit requirements are well defined, the receiver design is left up to the implementer. So after the full system has been assembled, another round of testing ensures that data communications are solid and packets are not being dropped.

In the next post in this series, we'll begin with an overview of the five 100Base-T1 test modes.

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