Distinguishing BroadR-Reach and 100Base-T1

Figure 1: BroadR-Reach provides full-duplex operation
over a single twisted pair of wires

The world of Automotive Ethernet can be a little confusing in that there are two dominant specifications that serve the application space: BroadR-Reach and 100Base-T1. Both are explicitly intended for automotive use and there's quite a bit of overlap between them. In this installment, we'll look a little more closely at BroadR-Reach applications and also explain the differences between it and 100Base-T1.

The configuration of a BroadR-Reach link (Figure 1) shows that the protocol offers full-duplex operation at a data rate of 100 Mb/s over a simple twisted pair of wires. At both ends of the link are BroadR-Reach PHY chips that send and receive data in both directions at once. Going above the PHY chips to the MAC interface level, we find plain-vanilla IEEE 802.3 Ethernet. Thus, the only difference between BroadR-Reach and ordinary Ethernet is from PHY to PHY. At the upper levels of the Ethernet stack, the topology is identical to that of 802.3.

What are some prominent applications for BroadR-Reach? One is Advanced Driver Assistance Systems (ADAS). ADAS can be implemented in passive or active fashion. A passive ADAS system only provides drivers with a warning or alert if, for example, they stray outside of their lane. In contrast, an active ADAS goes beyond a warning, taking control to perform a corrective action (returning the vehicle to its lane or performing automated emergency braking). Another passive ADAS application is backup cameras; an active ADAS provides parking assistance.

At 100 Mb/s, BroadR-Reach is best suited for passive ADAS applications. Active ADAS requires lossless video so that image-recognition algorithms can function appropriately. For example, if the vehicle is trying to recognize potential hazards in the near distance, the algorithms will focus more accurately with full lossless video input.

Another popular application for BroadR-Reach is infotainment. BroadR-Reach is more cost-effective than Media-Oriented Systems Transport (MOST) or low-voltage differential signaling (LVDS). Much of the cost savings is in the cabling. LVDS cables are quite thick, so that means higher cost as well as more weight. LVDS was used often with analog cameras connected with coaxial cables. With the move to digital cameras, BroadR-Reach offers the benefits of the twisted-pair cable and higher bandwidth.

How 100Base-T1 Differs From BroadR-Reach

100Base-T1 is the IEEE's specification for 100-Mb/s Automotive Ethernet. You might see it referred to as IEEE 802.3bw, but both names address the same specification. The "-T1" appendage is an important distinction that lets you know the name refers to Automotive Ethernet, which is distinct from 100Base-T.

The -T1 variant is interoperable with OPEN Alliance BroadR-Reach. They are very nearly identical (the names are often used interchangeably), but for two small exceptions. In the physical-layer electrical (physical medium attachment or PMA) test suite, the 100Base-T1 specification defines a test for the maximum Transmit Peak Differential Output. This parameter is not explicitly defined in the BroadR-Reach specification.

The second exception is that the 100Base-T1 specification has some differences in the protocol timing for wake-up commands to make those periods shorter. But those are the only differences worth noting.

Why, then, one wonders, did the IEEE 802.3bw specification come about? The driver was applications for Automotive Ethernet in other application domains, such as industrial automation and avionics. The benefits of BroadR-Reach were enticing enough for the IEEE to create its own version of the specification. As a result, the two Automotive Ethernet specifications share a common environment and ecosystem.