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14 June 2017

Fundamentals of the BroadR-Reach Protocol

BroadR-Reach delivers bandwidth of 100 Mb/s
Figure 1: BroadR-Reach
delivers bandwidth of
100 Mb/s
The burgeoning complexity of vehicular networks, the resultant high bandwidth demands, and the harshness of the automotive environment have driven the development of what we know today as Automotive Ethernet. Our last post began an overview of Automotive Ethernet technology, focusing on the physical/mechanical constraints and industry trends that influenced the protocol's development. Next, let's look more closely at the BroadR-Reach protocol.

BroadR-Reach, which was pioneered by Broadcom, leverages standard Ethernet technology but adapts it for use in automotive applications. It is capable of 100 Mb/s (Figure 1) over unshielded twisted-pair cabling with a reach of up to 15 meters. Need more reach? Going to shielded twisted pair extends BroadR-Reach to as much as 40 meters.

BroadR-Reach can be thought of as a hybrid of the 100Base-T and 1000Base-T protocols. The OPEN (One-Pair Ether-Net) Alliance, which oversees the BroadR-Reach specification, adopted the best aspects of both protocols and stressed those characteristics that make both suited for automotive networking applications.

Shown is an example of PAM-3 signaling as employed by BroadR-Reach
Figure 2: Shown is an example of PAM-3 signaling
as employed by BroadR-Reach
The 1000Base-T (Gigabit Ethernet) signal is a PAM-5 signal, which means there are five signal levels, while the 100Base-T signal is a PAM-3, three-level signal. BroadR-Reach went with the three-level scheme. Both 1000Base-T and 100Base-T have data rates of 125 Msymbols/s, whereas BroadR-Reach's data rate is 66.67 Msymbols/s. Bandwidth is reduced by a factor of 2X, which enables the protocol to accommodate lower-quality and lower-cost cabling while also permitting filtering that's required to meet stringent automotive signal-integrity needs. Additionally, BroadR-Reach operates full-duplex in emulation of 1000Base-T, but uses only a single twisted pair of wires.

In Figure 2 we see an example of PAM-3 signaling as it might look for a BroadR-Reach signal. Note that the +1 peaks have varying amplitudes, which is a result of settling time. The first peak on the left of the screen capture comprises some four or five +1 bits in succession, so it has reached full amplitude, while the next +1 peak, a single bit, has not.

Next time, we'll look at some of the benefits of Automotive Ethernet and at how it compares with some other serial protocols that have seen use in automotive applications.

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