A backflow prevention device includes at least one backflow prevention unit. At least one segment is associated with a plurality of links to other segments. The delay can be chosen such that the transmitted symbol arrives at the access point simultaneously with a further symbol modulated into the frequency channel and transmitted by a further user terminal. The ink drops jetted by activating both the main and the peripheral actuators and the ink drops generated by activating only the main actuator have the same drop weight and drop velocity. The users can build this type of 3-dimensional puzzle maze according to individual creativity and imagination.