Lossless, bit-wise bus arbitration
Since every node can begin transmitting a message as soon as the bus is free in a CAN network, it is possible for several nodes to begin a transmission process at the same time. To prevent the nodes from destroying each others transmitted data, the message with the highest priority of all simultaneously arbitrating messages is determined in a so-called "arbitration phase". Only the network node that transmits this highest priority message may continue with the transmission of its message. The message with the lowest value of the message identifier has the highest priority. Image 1 shows the principle used for the bus arbitration. Each node monitors the signal level on the bus during the arbitration phase. The arbitration phase consists of the transmission of the message identifier and of the so-called RTR bit. If a network node detects a dominant bus level (dominant bit), although it has actuated a recessive level (recessive bit) itself, it aborts the transmission process immediately since, in this case, a message with higher priority is obviously being transmitted at the same time; it therefore goes into the receive state. Since a message is transmitted with every bus arbitration, the process ensures "lossless" bus access.