There are many communication systems that are being used in vehicles today. Here, we briefly describe these systems and provide a view on the emerging use of Ethernet in the Automotive space.
Here are some communication systems used in Automotive today:
CAN (Controller Area Network): This is the bedrock for communication in Automobiles. CAN is a Bus-based communication system. Physically it is two wires twisted and unshielded (UTP). There can be a total of 30 nodes connected to the bus. Maximum bitrate is 1 Mbps for CAN and 8 Mbps for CAN-FD. Bus Arbitration is based upon initial bits which also decide the priority of the messages. Max frame size is 8 bytes for CAN and 64 bytes for CAN-FD.
MoST (Media Oriented Serial Transport): This is mainly targeted at transporting media. MoST is mainly bus-based with Ring Topology. Physical options include optical fiber, co-axial cable, and UTP. There can be a total of 64 nodes connected to bus. A master enables synchronous communication. Maximum bitrate is 150 Mbps in MOST150. Frame rate is fixed and at typical audio rates like 48 KHz.
LIN (Local Interconnect Network): This is mainly targeted at low cost, low speed applications. This is bus-based with Master/Slave topology. It is a single wired bus. There can be a total of 16 nodes connected to bus. Maximum bit rate is 20 kHz.
Ethernet has been an established standard in other industries for several decades. It had not been used in Automotive industry mainly due to electro-magnetic interference. Broadcom with its BroadR-Reach technology enabled a lesser spectral bandwidth reducing EMI. This provided a 100 Mbps rate on a single unshielded twisted pair. This has now been adapted as 802.3bw standards by IEEE. For increased rates, IEEE standards 802.3bp – a 1Gbps over a single unshielded twisted pair has been specified.
Regular Ethernet frames are not interruptible, 802.3br was specified, so that higher priority packets can preempt the regular frames. 802.3bu is specified for power over data lines.
Open SIG Alliance is promoting the use of Ethernet in Automotive. They have come out with the Test specifications right from Media up to TCP/IP for Automotive. They have also specified the diagnostic features and extended the 802.3bw to implement Sleep and Wake-up features.
Ethernet AVB supports Transmission of Audio, Video directly over Ethernet. They provide for time synchronization between Audio and Video and also provide for reserving bandwidth. The specifications relevant are 802.1AS, 802.1BA, 802.1Qx, 1722. AVNu alliance creates open standards for interoperable AVB devices. They have worked on whitepapers and specified an Automotive profile for AVB. NHSTA specified Audio and Video to be available within two seconds of an Automotive switching on. The Automotive profile created by AVNu alliance specified the mechanisms for setting up the AVB network for the same.
AUTOSAR specified Ethernet from version 4.0, latest version specified TCP/IP also. AUTOSAR is based on PDU-based communication paradigm, whereas TCP/IP is socket based. AUTOSAR has specified SoAD, a socket adaptor for transforming between PDU and sockets.
Some of the use cases that are being used or can be used for Ethernet in Automotive are: Diagnostics over IP, Calibration via XCP over Ethernet, AVB, as a backbone communication mechanism, etc.
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