In dual rail encoded data, each boolean is implemented as two wires. This allows the value and the timing information to be communicated for each data bit. Bundled data, on the other hand, has one wire for each data bit and a separate wire to indicate the timing. Level vs. transition encoding. Level sensitive circuits typically represent a logic one by a high voltage and a logic zero by a low voltage.
Transition signalling uses a change in the signal level to convey information.
A transition on one wire indicates the arrival of a zero, a transition on the other the arrival of a one. The levels on the wires are of no significance. The most popular form in recent years has been dual-rail encoding with level sensitive signalling. In 1987, Ivan Sutherland chose the topic of asynchronous design for his Turing Award lecture. This approach uses bundled data with a transition signalled handshake protocol to control data transfers.