IN VEHICLE NETWORKS CH3a CAN : OSI MODEL 1/6

Transcription

1 IN VEHICLE NETWORKS CH3a CAN : OSI MODEL 1/6 OSI NETWORK REFERENCE MODEL... 2 LAYERED ARCHITECTURE OF CAN... 3 ( CAN TRANSCEIVER)... 4 ( CAN CONTROLLER)... 5 CAN BASED HIGH LAYER PROTOCOLS... 6

2 IN VEHICLE NETWORKS CH3a CAN : OSI MODEL 2/6 OSI NETWORK REFERENCE MODEL HIGH LAYER PROTOCOLS (HLP s) CONTROLLER AREA NETWORK BUS Most network applications follow a layered approach to system implementation. This systematic approach enables interoperability between products from different manufacturers. A standard was created by the International Standards Organization (ISO) as a template to follow for this layered approach. It is called the ISO Open Systems Interconnection (OSI) Network Layering Reference Model and is shown for reference. The CAN protocol itself implements most of the lower two layers of this reference model. The communication medium portion of the model was purposely left out of the Bosch CAN specification to enable system designers to adapt and optimize the communication protocol on multiple media for maximum flexibility (twisted pair, single wire, optically isolated, RF, IR, etc.). With this flexibility, however, comes the possibility of interoperability concerns. To ease some of these concerns, the International Standards Organization and Society of Automotive Engineers (SAE) have defined some protocols based on CAN that include the Media Dependant Interface definition such that all of the lower two layers are specified. ISO is a standard for high-speed applications, ISO (replaces ISO11519) by is a standard for low-speed applications, and J1939 (from SAE) is targeted for truck and bus applications. The rest of the layers of the ISO/OSI protocol stack are left to be implemented by the system software developer. Higher Layer Protocols (HLPs) are generally used to implement the upper five layers of the OSI Reference Model. HLPs are used to: 1) standardize startup procedures including bit rates used, 2) distribute addresses among participating nodes or types of messages, 3) determine the structure of the messages, and 4) provide system-level error handling routines. This is by no means a full list of the functions HLPs perform, however it does describe some of their basic functionality.

3 IN VEHICLE NETWORKS CH3a CAN : OSI MODEL 3/6 LAYERED ARCHITECTURE OF CAN Application layer Data link layer ISO Physical layer ISO ISO SAE J 2411 Layered Architecture of CAN according to the OSI Reference Model When referring to the "CAN standard" or "CAN protocol" we understand the functionality which is standardised in ISO This standard comprises the Physical (Layer 1) and Data Link Layer (Layer 2) according to the OSI-reference model. Whereas Layer 1 is responsible for functions like physical signalling, encoding, bit timing and bit synchronisation, Layer 2 performs functions like bus arbitration, message framing and data security, message validation, error detection and signalling and fault confinement. The CAN standard does not specify the medium attachment unit and the medium upon which it resides, nor an Application Layer. The Layer 2 of the CAN protocol offers two types of connectionless transmission services to the user:. Transfer of a CAN-message and. Remote request of a CAN-message Connectionless transmission means that no data link connection has to be established before performing a message transfer or request. Reception of messages is supported by the CAN chips in form of different type object filtering and object buffering methods. A Layer 2 CAN data message according to the CAN Specification V 2.0 is determined by the message identifier, standard/ extended format indication, data length and the data to be transmitted. Since the CAN-Protocol specifies no rules for the assignment of message-identifiers, a variety of different, application-specific usages is possible. Assignment of the CAN message identifiers therefore is one of the most important decisions when designing a CAN-based communication system. Assignment and allocation of message identifiers also is one of the main items of a higher Layer approach.

4 IN VEHICLE NETWORKS CH3a CAN : OSI MODEL 4/6 ( CAN TRANSCEIVER) BIT TIMING BIT ENCODING SYNCHRONIZATION The Physical Layer The Physical Layer defines how signals are actually transmitted and therefore deals with the description of Bit Timing, Bit Encoding, and Synchronization. Within this specification the Driver/Receiver Characteristics of the Physical Layer are not defined so as to allow transmission medium and signal level implementations to be optimized for their application. Driver/Receiver Characteristics o Media Attachment o Transceiver Characteristics o Bus Cable Characteristics

5 IN VEHICLE NETWORKS CH3a CAN : OSI MODEL 5/6 ( CAN CONTROLLER) LLC ACCEPTANCE FILTERING OVERLAOD NOTIFICATION RECOVERY MANAGEMENT MAC DATA ENCAPSULATION DECAPSULATION FRAME CODING (STUFFING/DESTUFFING) MEDIUM ACCES MANAGEMENT ERROR DETECTION ERROR SIGNALLING ACKNOWLEDGMENT SERIALIZATION/DESERIALIZATION Data link layer There are 2 SUB LAYERS within the data link layer of the CAN controller: LLC (Logical Link Control) (object layer) MAC (Medium Access Control) (transfer layer) The MAC sub layer represents the kernel of the CAN protocol. It presents messages received from the LLC sub layer and accepts messages to be transmitted to the LLC sub layer. The MAC sub layer is responsible for Message Framing, Arbitration, Acknowledgment, Error Detection and Signalling. The MAC sub layer are supervised by a management entity called Fault Confinement which is self-checking mechanism for distinguishing short disturbances from permanent failures. The LLC sub layer is concerned with Message Filtering, Overload Notification and Recovery Management.

6 IN VEHICLE NETWORKS CH3a CAN : OSI MODEL 6/6 CAN BASED HIGH LAYER PROTOCOLS HIGH LAYER PROTOCOLS (HLP s) HIGH LAYER PROTOCOLS SAE J1939 ISO11992 ISOBUS Proprietary HLP s : Volcano (Volvo),GMLAN, IVLAN (GM) CANopen CONTROLLER AREA NETWORK BUS The main items of CAN-based higher layer protocols are message identifier assignment system, method of exchanging Process Data, peer-to-peer communication, method of establishing Process Data connections, network management, principle of device modelling and device profiles Some HLP (high layer protocols): CANopen, SAE J1939, ISO 11992, ISOBUS, proprietary HLPs: Volcano (Volvo), GMLAN, IVLAN (GM)