Automotive Bus Systems

Automotive Bus Systems - Today, a wide variety of vehicle communication systems is already used in the automotive area. Possible applications range from electronic engine control, several driving assistants and safety mechanisms up to the broad variety of infotainment applications.

As shown in Table 2, we distinguish the following five different vehicle communication groups according to their essential technical properties and application areas.

Local sub networks such as LIN (Local Interconnect Network) control small autonomous networks used for automatic door locking mechanisms, power-windows and mirrors as well as for communication with miscellaneous smart sensors to detect, for instance, rain or darkness.

Event-triggered bus systems like CAN (Controller Area Network) are used for soft real-time in-car communication between controllers, networking for example the antilock breaking system (ABS) or the engine management system.

Time-triggered hard real-time capable bus systems such as FlexRay, TTCAN (Time-Triggered CAN) or TTP (Time-Triggered Protocol) guarantee determined transmission times for controller communication and therefore can be applied in highly safety relevant Drive-by-Wire systems.

The group of multimedia bus systems like MOST, D2B (Domestic Digital Bus) and GigaStar arise from the new automotive demands for in-car entertainment that needs high-performance, wide-band communication channels to transmit high-quality audio, voice and video data streams within the vehicle.

The wireless communication group contains modern wireless data transmission technologies that more and more expand also into the automotive area. They enable the internal vehicle network to communicate with external devices surrounding the car as well as the reception of various broadcast stations (Location Based Services).

1. Bus Representatives

In the following, we give a short technical description of one appropriate representative from each identified vehicular communication network group (see Section 2). Further readings can be found in [Do02, He02, Kr02, Ra02, RT03].

a. CAN

The all-round Controller Area Network, developed in the early 1980s, is an event-triggered controller network for serial communication with data rates up to one MBit/s. Its multi-master architecture allows redundant networks, which are able to operate even if some of their nodes are defect.

CAN messages do not have a recipient address, but are classified over their respective identifier. Therefore, CAN controller broadcast their messages to all connected nodes and all receiving nodes decide independently if they process the message.

CAN uses the decentralized, reliable, priority driven CSMA/CD (Carrier Sense Multiple Access / Collision Detection) access control method to guarantee every time the transmission of the top priority message always first.

In order to employ CAN also in the environment of strong electromagnetic fields, CAN offers an error mechanism that detects transfer errors, interrupts and indicates the erroneous transmissions with an error flag and initiates the retransmission of the affected message.

Furthermore, it contains mechanisms for automatic fault localization including disconnection of the faulty controller.

b. LIN

The UART (Universal Asynchronous Receiver Transmitter) based LIN (Local Interconnect Network) is a single-wire sub network for low-cost, serial communication between smart sensors and actuators with typical data rates up to 20 kBit/s.

It is intended to be used from the year 2001 on everywhere in a car, where the bandwidth and versatility of a CAN network is not required. A single master controls the hence collision-free communication with up to 16 slaves, optionally including time synchronization for nodes without a stabilized time base.

LIN is similarly to CAN a receiver-selective bus system. Incorrect transferred LIN messages are detected and discarded by the means of parity bits and a checksum. Beside the normal operation mode, LIN nodes provide also a sleep mode with lower power consumption, controlled by special sleep respectively wake-up message.

c. FlexRay

FlexRay is a deterministic and error-tolerant high-speed bus, which meets the demands for future safety-relevant high-speed automotive networks. With its data rate of up to 10 MBit/s (redundant single channel mode) FlexRay is targeting applications such as Drive-by-Wire and Powertrain.

The flexible, expandable FlexRay network consists of up to 64, point-to-point or over a classical bus structure connected, nodes. As physical transmission medium both optical fibers and copper lines are suitable.

FlexRay is similarly to CAN a receiver-selective bus system and uses the cyclic TDMA (Time Division Multiple Access) method for the prioritydriven control of asynchronous and synchronous transmission of non-time-critical respectively time-critical data via freely configurable, static and dynamic time segments.

Its error tolerance is achieved by channel redundancy, a protocol checksum and an independent instance (bus guardian) that detects and handles logical errors.

d. MOST

The ISO/OSI standardized MOST (Media Oriented System Transport) serial high-speed bus became the basis for present and future automotive multimedia networks for transmitting audio, video, voice, and control data via fiber optic cables.

The peer-to-peer network connects via plug-and-play up to 64 nodes in ring, star or bus topology. MOST offers, similarly to FlexRay, two freely configurable, static and dynamic time segments for the synchronous (up to 24 MBit/s) and asynchronous (up to 14 MBit/s) data transmission, as well as a small control channel.

The control channel allows MOST devices to request and release one of the configurable 60 data channels. Unlike most automotive bus systems, MOST messages include always a clear sender and receiver address.

Access control during synchronous and asynchronous transmission is realized via TDM (Time Division Multiplex) respectively CSMA/CA.

The error management is handled by an internal MOST system service, which detects errors over parity bits, status flags and checksums and disconnects erroneous nodes if necessary.

e. Bluetooth

Originally developed to unify different technologies like computers and mobile phones, Bluetooth is a wireless radio data transmission standard in the license-free industrial, scientific, and medical (ISM) band at 2.45 GHz.

It enables wireless ad-hoc networking of various devices like personal digital assistants (PDAs), mobile phones, laptops, PCs, printers, and digital cameras for transmitting voice and data over short distances up to 100 meters.

Primarily designed as low-cost transceiver microchip with low power consumption, it reaches data rates of up to 0.7 MBit/s. Within the limited multi-master capable, so-called Piconets, single Bluetooth devices can maintain up to seven point-to-point or point-to-multipoint connections, optionally also encrypted.

2. Bus Interconnections

For network spanning communication, automotive bus systems require appropriate bridges or gateways processors to transfer messages among each other despite their different physical and logical operating properties.

Gateways processors read and write all the different physical interfaces and have to manage the protocol conversion, error protection and message verification.

Depending on their application area, gateways include sending, receiving and/or translation capabilities as well as some appropriate filter mechanisms. While so-called super gateways interconnect centralized all existing bus systems, local gateways are linking only two different bus systems together.

Therefore, super gateways require some kind of sophisticated software and plenty of computing power in order to accomplish all necessary protocol conversions, whereas local gateways realize only the hard- and software conversion between two different bus backbones.