When data is erroneously transmitted and the error frame is missed, it means that the erroneous data is sent to the application layer. Unless the application layer has additional data identification measures, this data may cause unpredictable results. The CAN protocol claims to have a very low false frame miss rate (4.7 & TImes; 10-11 & TImes; error rate), and some promotional materials are released under certain conditions to have one missed inspection in 1000 years. This is not true. The error rate of missed frame is a very important indicator. Many applications use CAN when they see the instructions on the Bosch CAN2.0 specification. However, there is very little public information on the source of this indicator, and there is very little discussion, making it difficult for users to confirm or verify it, which poses risks to users. In this paper, we use the method of reconstructing the error detection example, and derive the lower limit of the missed error detection frame of CAN, which is several orders of magnitude larger than that claimed by CAN. In many applications, CAN is the choice of reliability and price balance, or has been produced and used for a long time. In the face of this newly discovered problem, there is an urgent need for a “patches†before the CAN itself is improved. To improve. Due to the limited space, we can only summarize the derivation process of the missed frame error rate, focusing on providing solutions.
1 Discussion on the literature of CAN miss detection error frame probability
The Bosch CAN 2.0 specification states that its missed error detection frame probability is less than the message error rate &TImes; 4.7 & TImes; 10-11. Its source is found in the reference, which does not provide an analysis algorithm that produces missed detection. It only mentions that the formula is obtained with a large number of simulations. To determine whether a frame will be missed after an error, at least 2 times of CRC calculation is required. For each bit, only several instructions are required for the assembly language. The frame of 80 to 90 bits considered in this paper, the CRC covers 58 to 66 bits. It is necessary to cycle 58 to 66 times. For the PDP11 or VAX machine commonly used in 1989, a machine instruction should be about 0.1 μs, and the judgment of one frame should be 0.07 ms. Even if it is not stopped for one year, it can be 2.20×1011 frames. 58 bits can form 258=2.88×1017 different frames, plus 58×57 different combinations of positions with 2-bit bit errors, so the simulation that can be done is only a small part of the possible situation (one millionth ). Since the sample is too small, it is difficult to conclude the influencing factors for the inductive formula.
In 1999, Tran also studied the missed frame error rate. In view of the difficulty of analysis, he also used a large number of computer simulation methods. For 11-bit ID and 8-byte data frames, he used a 600 MB Alpha server. As discussed above, although the amount of simulation is large, it is still a very small part of the possible situation.
Another standard CANopen Draft Standard 304 (2005) related to CAN gives a false frame miss detection rate of (7.2 x 10-9). The same data from the CAN Automation Association is unmatched.
2 The export of missed rate of new error frame
The research method in this paper is to construct an instance of missed detection, determine the probability that the instance takes up the possible frame, multiply the probability of multi-dislocation corresponding to the instance, and then find all possible instances to get the wrong frame of CAN. Missed detection rate. This paper analyzes the two dislocations that are most likely to cause missed detection and then expands to have multiple dislocations. The data field takes 8 bytes and assumes that errors occur in the data field. It does not take into account the scattered multi-bit error detection rate when the CRC check capability is exceeded, so the lower bound of the error detection error frame probability is obtained.
2.1 When there is an error in the CAN bit filling, the bit order is staggered.
When there is a bit error in the bit stream that may cause padding, it may cause the sender and the receiver to execute the padding rule only, causing confusion of the padding bit and the information bit. An error occurs in the third bit transfer of Fig. 1(a). As a result, the padding bit 1 of the sender is misinterpreted as data 1 by the receiver, and the entire received data is one bit longer than the transmitted data. The error in the third bit transfer of Fig. 1(b) causes the receiver to generate a condition for deleting the padding bit, so it deletes the transmitted data 1 and the received data stream is one bit shorter.
Figure 1 CAN bit filling rule makes the received bit stream change after an error
It can be known from the bit stream change that if the 2 bit errors that occur are exactly the type of Figure 1(a) and the type of Figure 1(b), the length of the transmitted data stream and the received data stream will still be equal. Both errors occur in the data field, and other tests by CAN cannot find them.
2.2 Conditions for missed detection
The transmitted bit stream and the received bit stream can be written in polynomial forms Tx(x) and Rx(x). The CRC check divides the two expressions by the CAN generator polynomial G(x), and the resulting remainder is called the CRC value. If the two remainders are the same, the CRC test passes. When a transmission error occurs, Rx (x) = Tx(x) + U(x) × G(x), the remainder obtained for Tx(x) and Rx(x) is the same, and an error occurs. Missing detection of the frame. So as long as U(x) is found, an instance of missed detection can be constructed.
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