dsPIC30F Family Reference Manual
DS70070B-page 23-70 © 2004 Microchip Technology Inc.
The LLC sub layer is concerned with Message Filtering, Overload Notification and Error
Recovery Management. The scope of the LLC sub layer is:
• To provide services for data transfer and for remote data request.
• To decide which messages received by the LLC sub layer are actually to be accepted.
• To provide means for error recovery management and overload notifications.
The MAC sub layer represents the kernel of the CAN protocol. The MAC sub layer defines the
transfer protocol, (i.e., controlling the Framing, Performing Arbitration, Error Checking, Error
Signalling and Fault Confinement). It presents messages received from the LLC sub layer and
accepts messages to be transmitted to the LLC sub layer. Within the MAC sub layer is where it’s
decided whether the bus is free for starting a new transmission, or whether a reception is just
starting. The MAC sub layer is supervised by a management entity called Fault Confinement
which is a self-checking mechanism for distinguishing short disturbances from permanent
failures. Also, some general features of the bit timing are regarded as part of the MAC sub layer.
The physical layer defines the actual transfer of the bits between the different nodes with respect
to all electrical properties. The PLS sub layer defines how signals are actually transmitted and
therefore deals with the description of Bit Timing, Bit Encoding and Synchronization.
The lower levels of the protocol are implemented in driver/receiver chips and the actual interface
such as twisted pair wiring or optical fiber, etc. Within one network, the physical layer has to be
the same for all nodes. The driver/receiver characteristics of the physical layer are not defined in
the CAN specification so as to allow transmission medium and signal level implementations to
be optimized for their application. The most common example of the physical transmission
medium is defined in Road Vehicles ISO11898, a multiplex wiring specification.
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