How the CAN network functions
Principles of data exchange
Broadcast transmission and acceptance filtering by CAN nodes
When data are transmitted by CAN, no stations are addressed, but instead, the content of the message (e.g. rpm or engine temperature) is designated by an identifier that is unique throughout the network. The identifier defines not only the content but also the priority of the message. This is important for bus allocation when several stations are competing for bus access.
If the CPU of a given station wishes to send a message to one or more stations, it passes the data to be transmitted and their identifiers to the assigned CAN chip (“Make ready”). This is all the CPU has to do to initiate data exchange. The message is constructed and transmitted by the CAN chip. As soon as the CAN chip receives the bus allocation (“Send Message”) all other stations on the CAN network become receivers of this message (“Receive Message”). Each station in the CAN network, having received the message correctly, performs an acceptance test to determine whether the data received are relevant for that station (“Select”). If the data are of significance for the station concerned they are processed (“Accept”), otherwise they are ignored.
A high degree of system and configuration flexibility is achieved as a result of the content-oriented addressing scheme. It is very easy to add stations to the existing CAN network without making any hardware or software modifications to the existing stations, provided that the new stations are purely receivers. Because the data transmission protocol does not require physical destination addresses for the individual components, it supports the concept of modular electronics and also permits multiple reception (broadcast, multicast) and the synchronization of distributed processes:
| Measurements needed as information by several controllers can be transmitted via the network, in such a way that it is unnecessary for each controller to have its own sensor! |
Principle of non-destructive bitwise arbitration
Non-destructive bitwise arbitration
For the data to be processed in real time they must be transmitted rapidly. This not only requires a physical data transfer path with up to 1 Mbit/s but also calls for rapid bus allocation when several stations wish to send messages simultaneously.
In real-time processing the urgency of messages to be exchanged over the network can differ greatly: a rapidly changing dimension (e.g. engine load) has to be transmitted more frequently and therefore with less delays than other dimensions (e.g. engine temperature) which change relatively slowly.
The priority at which a message is transmitted compared with another less urgent message is specified by the identifier of the message concerned. The priorities are laid down during system design in the form of corresponding binary values and cannot be changed dynamically. The identifier with the lowest binary number has the highest priority.
Bus access conflicts are resolved by bitwise arbitration on the identifiers by each station observing the bus level bit for bit. In accordance with the “wired and” mechanism, by which the dominant state (logical 0) overwrites the recessive state (logical 1), the competition for bus allocation is lost by all those stations with recessive transmission and dominant observation. All “losers” automatically become receivers of the message with the highest priority and do not re-attempt transmission until the bus is available again.
Efficiency of bus allocation
The efficiency of the bus allocation system is determined mainly by the possible applications for a serial bus system. In order to judge as simply as possibly which bus systems are suitable for which applications the literature includes a method of classifying bus allocation procedures.
Generally we distinguish between the following classes:
| Allocation on a fixed time schedule | Bus allocation on the basis of need |
|---|---|
| Allocation is made sequentially to each participant for a maximum duration regardless of whether this participant needs the bus at this moment or not (examples: token slot or token passing). | The bus is allocated to one participant on the basis of transmission requests outstanding, i.e. the allocation system only considers participants wishing to transmit (examples: CSMA, CSMA/CD, flying master, round robin or bitwise arbitration). |
| For CAN, bus allocation is negotiated purely among the messages waiting to be transmitted. This means that the procedure specified by CAN is classified as allocation on the basis of need! |
Another means of assessing the efficiency of bus arbitration systems is the bus access method:
| Non-destructive bus access | Destructive bus allocation |
|---|---|
| With methods of this type the bus is allocated to one and only one station either immediately or within a specified time following a single bus access (by one or more stations). This ensures that each bus access by one or more stations leads to an unambiguous bus allocation (examples: token slot, token passing, round robin, bitwise arbitration) | Simultaneous bus access by more than one station causes all transmission attempts to be aborted and therefore there is no successful bus allocation. More than one bus access may be necessary in order to allocate the bus at all, the number of attempts before bus allocation is successful being a purely statistical quantity (examples: CSMA/CD, Ethernet). |
In order to process all transmission requests of a CAN network while complying with latency constraints at as low a data transfer rate as possible, the CAN protocol must implement a bus allocation method that guarantees that there is always unambiguous bus allocation even when there are simultaneous bus accesses from different stations. The method of bitwise arbitration using the identifier of the messages to be transmitted uniquely resolves any collision between a number of stations wanting to transmit, and it does this at the latest within 13 (standard format) or 33 (extended format) bit periods for any bus access period. Unlike the message-wise arbitration employed by the CSMA/CD method this non-destructive method of conflict resolution ensures that no bus capacity is used without transmitting useful information.
Even in situations where the bus is overloaded the linkage of the bus access priority to the content of the message proves to be a beneficial system attribute compared with existing CSMA/CD or token protocols: in spite of the insufficient bus transport capacity, all outstanding transmission requests are processed in order of their importance to the overall system (as determined by the message priority). The available transmission capacity is utilized efficiently for the transmission of useful data since “gaps” in bus allocation are kept very small. The collapse of the whole transmission system due to overload, as can occur with the CSMA/CD protocol, is not possible with CAN. Thus, CAN permits implementation of fast, traffic-dependent bus access which is non-destructive because of bitwise arbitration based on the message priority employed.
Non-destructive bus access can be further classified into:
- centralized bus access control
- decentralized bus access control
depending on whether the control mechanisms are present in the system only once (centralized) or more than once (decentralized). A communication system with a designated station (inter alia for centralized bus access control) must provide a strategy to take effect in the event of a failure of the master station. This concept has the disadvantage that the strategy for failure management is difficult and costly to implement and also that the takeover of the central station by a redundant station can be very time-consuming. For these reasons and to circumvent the problem of the reliability of the master station (and thus of the whole communication system), the CAN protocol implements decentralized bus control. All major communication mechanisms, including bus access control, are implemented several times in the system, because this is the only way to fulfil the high requirements for the availability of the communication system.
In summary it can be said that CAN implements a traffic-dependent bus allocation system that permits, by means of a non-destructive bus access with decentralized bus access control, a high useful data rate at the lowest possible bus data rate in terms of the bus busy rate for all stations. The efficiency of the bus arbitration procedure is increased by the fact that the bus is utilized only by those stations with pending transmission requests.
These requests are handled in the order of the importance of the messages for the system as a whole. This proves especially advantageous in overload situations. Since bus access is prioritized on the basis of the messages, it is possible to guarantee low individual latency times in real-time systems.