It's essential that timing synchronization always be maintained at all network nodes. Many network communication methods mandate that a signal line return to the zero state after each bit is transmitted. The resulting transitions are used to keep the serial data synchronized. But as mentioned before, CAN uses an NRZ serial data transmission method, so a string of repeating 1s and 0s contains no state transitions. Such strings, if they were long enough, might cause a loss of synchronization due to variations in the frequencies of the clocks at different nodes.