Theory Of Operation

The Automotive Ethernet network is being used for communications between critical time sensitive and video related ECU's. Only two ECU's are ever connected to a Automotive Ethernet network. The Automotive Ethernet network use a twisted pair of insulated copper wires for the data bus. One wire is designated as Ethernet (+), the other is Ethernet (-). Connectors house the network circuit terminals in adjacent cavities and connect them to a module. The connectors also connect certain branches of wiring in-line along the wiring harness. A gateway module is used to link A.E. to other communication networks. A.E. messaging from multiple ECU's can be mirrored/reflected using an ethernet switch, that are internal to the Security GateWay (SGW) Module and Radio at the Diagnostic Port/Data Link Connector (DLC). The internal ethernet switch joins multiple physical connections and can independently forward messages from one A.E. network to another A.E. network.

The Automotive Ethernet voltages represent symbols that an ECU sender encodes based on a desired bit stream and an ECU receiver uses the resulting symbol stream to decode. There are several methods used for encoding and decoding the different voltages generated. Information can be transferred bidirectional on Automotive Ethernet at 100Mbit/s. Two interconnected ECU's can send and receive simultaneously (full duplex). As a sender, an ECU adds its own differential voltage to the two wires; while as a receiver, it subtracts its own voltage from the applied total voltage. The result of the subtraction corresponds to the voltage that was sent by the opposite ECU. This mechanism is a component of the echo cancellation method that is used in other Ethernet systems. For differential voltages to be added or subtracted, the two ECU's must know when a new symbol begins. This means that both ECU's must be synchronized to the symbol stream. This is done with the help of a Commander ECU and Responder ECU. The Commander ECU generates a continuous symbol stream to which the Responder ECU is synchronized.

The Automotive Ethernet uses a twisted pair cable on which symmetrical differential voltages are applied is used for the physical connection. The voltages represent symbols that a sender encodes based on the desired bit stream. A receiver uses the resulting symbol stream in turn to decode the contained bits. Only two ECU's are ever connected to one cable. Thus, only point-to-point connection topology is available. Information is transferred bidirectional on a wire pair at 100 Mbps. Two interconnected ECU's can send and receive simultaneously (full duplex). As a sender, a ECU adds its own differential voltage to the two wires; while as a receiver, it subtracts its own voltage from the applied total voltage. The result of the subtraction corresponds to the voltage that was sent by the opposite ECU. This mechanism is a component of the echo cancellation method that is used in other Ethernet systems.

The two ECU's must know when a new symbol begins for differential voltages to be added or subtracted so that both ECU's must be synchronized to the symbol stream. The synchronization is done with the help of a Commander (Sender) ECU and Responder (Receiving) ECU. The Commander (Sender) generates a continuous symbol stream to which the Responder (Receiver) is synchronized. The software of the microcontroller configures as a Commander (Sender) or Responder (Receiver). The Ethernet controller first listens on the physical medium before a message is sent, preventing another ECU in the network from overwriting the current message. When the medium is free, the Ethernet controller can begin its transmission.

The transmission connection is made through TCP (Transmission Control Protocol), known as a connection-oriented transmission. The connection between the two ECU's is identified using an IP address, port number, and a three-way handshake or three-step process before the actual data transmission as listed below:

• In the first step, the Commander (Sender) ECU wanting to establish the connection first transmits a segment containing a set synchronization flag. The synchronization flag signals the Commander (Sender) ECU's desire to connect with the Responder (Receiver) ECU. A sequence number is composed of a randomly generated number, the ISN (Initial Sequence Number), and a consecutive number. As a result, the order of all segments is known, and the data stream can be reassembled later by the Responder (Receiver) ECU. The window field gives information about the available memory of the ECU. It ensures that a Commander (Sender) ECU does not send more data to the Responder (Receiver) ECU than its available memory allows. Because communication can be bidirectional, both ECU's communicate their available memory to the opposite ECU.
• In the second step, the original Responder (Receiver) ECU sends a synchronization acknowledgment segment containing a set synchronization flag and a set acknowledgment flag. As a result, this ECU signals the original Commander (Sender) ECU that it has received the request to establish a connection and acknowledges positively. The ECU also sends its sequence number. Both ECU's now clearly know their opposite node, and all future exchanged segments can be associated with the corresponding Commander (Sender) ECU.
• The third and last step for establishing the connection includes an acknowledgment that the original Commander (Sender) ECU transmits. It uses this to signal the opposite ECU that the connection has been successfully established, and then data is exchanged between the two ECU's.

For additional information regarding the Automotive Ethernet in this vehicle. Refer to COMMUNICATION, DESCRIPTION AND OPERATION .