22/05/2019 by Kvaser

BroadR-Reach vs. CAN FD: A Comparison


What is BroadR-Reach and how is it used?

BroadR-Reach is an ethernet physical layer standard that allows for more cost-effective applications in automobile computing, extending the range of twisted-pair connections without significant sacrifices to speed and connectivity.

One of the most prominent uses of BroadR-Reach is in advanced driver-assistance systems (ADAS). ADAS tasks are broken down into active or passive applications. Passive ADAS tasks simply provide a warning or alert to drivers. These applications include backup cameras or lane departure warnings. Active ADAS applications can take corrective action based on the warning, such as performing emergency braking or returning a vehicle to its lane.

BroadR-Reach maxes out at 100 Mb/s, making it best suited for passive ADAS. Active ADAS applications require lossless video (for, say, identifying and reacting to roadbound obstacles). The technology is also used for infotainment and in touchscreen displays, either for on-board operating systems or video screens.

Much of the cost savings is in the cabling, with BroadR-Reach offering the advantages of thinner, lighter-weight, twisted-pair cables, making it notably more cost-effective than Media Oriented Systems Transport (MOST) or low-voltage differential signaling (LVDS).

How Does BroadR-Reach Compare with CAN FD?

Ethernet is very efficient when used to transfer data from a camera to a central computer on a dedicated communication line. However, when it comes to real-time system control, Ethernet demands additional complex rules to compete with CAN FD.

Every CAN FD frame can have a maximum latency time before it is sent, while Ethernet doesn’t provide this function, requiring additional protocols (such as TDMA, Toke-bus, or Toke-ring) on top of Ethernet to provide predictable latency. This, unfortunately, can lead to problems during configuration, at start-up, and in recovering after failure.


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How do Ethernet and CAN FD with the same bit-rate compare?

Assuming they both have a 1 Mb/s bit-rate, the Ethernet frame has a 42-byte overhead, compared with 8 bytes of overhead in the CAN FD frame.

To get good real-time performance with Ethernet, the frames must become shorter in time, and the only way to do this is to increase the bit-rate. At the longest bit-rate, it is possible to send 18 of the shortest Ethernet frames during one CAN FD frame. While this makes Ethernet sound very promising, one needs to keep in mind that, if a node needs to send an Ethernet frame, there is no guarantee that the frame can be sent. This is typically solved by a restart of the communication.

BroadR-Reach certainly has its advantages in cost, weight, and efficiency when compared with other Ethernet physical layers, but the elegant simplicity and trusted reliability of CAN FD is still the better choice for real-time control.

Find out more about CAN FD