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27 July 2020

Fundamentals of 100Base-T1 Ethernet

100Base-T1 toplogy
100Base-T1 Topology
The term Automotive Ethernet can be used to refer to any Ethernet-based network for in-vehicle electrical systems. It encompasses 100Base-T1, as well as several other variants/speeds of Ethernet (e.g., 10Base-T1, 1000Base-T1). Here, we’ll describe 100 Mb/s Automotive Ethernet as defined by the IEEE in its 802.3bp specification, which is nearly identical to Broadcom’s variant, BroadR-Reach.

100Base-T1 offers higher bandwidth than do most of the prevalent automotive serial-data standards. Because it relies on a single, unshielded twisted pair, it also provides a low-cost cabling scheme, with connectivity cost savings of about 80% (according to Broadcom’s press release). 100Base-T1 also meets the stringent EMC/EMI requirements as well as the temperature-grade requirements of the automotive application space. 

A final benefit is that all the software interfaces for the upper layers of the Ethernet stack are exactly the same as for standard Ethernet. If you've ever worked with Ethernet in the past, you'll probably already have all of the software and test tools covered.

One significant challenge, though, is debugging the link startup, which we’ll cover in a future blog post. Our 100Base-T1bus TDME trigger and decode software is designed specifically to assist with this task.

How It Works

100Base-T1 utilizes a point-to-point topology directly connecting two nodes. The “-T1” signifies that the signal is carried over one, twisted pair of cables—in this case, unshielded cables. Unlike “normal Ethernet” (100Base-Tx), 100Base-T1 is a full duplex signal, so the same twisted pair will carry a bi-directional signal from a Master and Slave. If this signal were to be observed using an oscilloscope alone, it would not be possible to discern which signal is from the Master and which is from the Slave, since signals are transmitted from both directions simultaneously.

100Base-T1 utilizes PAM3 signaling. Pulse Amplitude Modulation, or PAM, uses the amplitude of the signal to encode the message information. PAM3, as the name implies, uses three distinct levels. The receiver sets a high and low threshold to determine the levels. Any samples above the high level is a +1, below the low level is a -1, and between the two levels is a 0.

Signaling with three discrete values is called a ternary signal or, in the case of Automotive Ethernet, a ternary symbol. In 100Base-T1, two ternary symbols are combined to form a code group. When a code group is representing data, it represents 3 bits of data. The 100Base-T1 specification defines how these code groups are mapped to the 3 bits.

Upon power up, the Master and Slave initiate a handshaking process to establish the link, called the link startup or link training process, wherein they exchange signals enabling each to train their echo cancellers while refining the timing, equalizers and scramblers. The link startup concludes with an exchange of status messages that determine whether packet transfer can proceed, or the link training must be repeated.

Once the link is established, Master and Slave trade packets that are nearly identical to traditional Ethernet, with a few additions.

100Base-T1 Frame Structure

100Base-T1 frame structure
100Base-T1 frame is much like traditional Ethernet, with addition of SSD and ESD (red bars at ends).

One unique aspect of the 100Base-T1 data frame is that it is marked by a Start-of-Stream Delimiter (SSD) and End-of-Stream Delimiter (ESD) (red bars at ends of image).

The SSD denotes the beginning of the data stream. It is always represented by 00, 00, 00. The code group 00 is reserved especially for the SSD and ESD and is not used anywhere else in data or idle mode.

The SSD occupies the first 9 bits of the Preamble, which in 100Base-T1 is shortened due to the insertion of the SSD. While the preamble is included, it does not serve a function like it does in the traditional Ethernet frame. Normally in Ethernet, the Preamble provided a mechanism for synchronization at the beginning of the frame, useful for large networks with a bus connection so that devices could easily synchronize their receiver clocks. In 100Base-T1, it is only present for backwards compatibility but is not required because of the continuous connection of the point-to-point topology.

The Preamble is followed by the Start-of-Frame Delimiter, or SFD, which signifies the end of the Preamble and the beginning of the traditional Ethernet frame.

As with all Ethernet, the frame proper starts with the Header, including the Destination (Dest-Address) and Source (Src-Address) MAC Addresses, which aren’t so critical for 100Base-T1 given its point-to-point topology. The Header also includes the EtherType (Type_Len) field, which provides directions on how to interpret the forthcoming data payload.

Next comes the DATA.

The data payload is followed by a Frame Check Sequence (FCS), which is a 32-bit CRC used to detect any corruption of data.

The 100Base-T1 packet ends with the ESD, which is not present in the traditional Ethernet frame. The ESD can be transmitted in two different manners, depending on whether the MII has indicated there was a tx_error during the data frame. An error-free ESD is represented by code group 00, 00, 11, while a frame containing an error will end with 00, 00, -1-1. Like the SSD, these specific sequences are reserved for this purpose, and you will not find them used anywhere else.

After the ESD, Idle symbols are again transmitted. The presence of the ESD in 100Base-T1 shortens the interframe gap (IFG) by 9 bits.

To learn more about 100Base-T1, see our instructional videos on: 

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