Fairness, queues and time

In certain cases the CCS-laws presented in Section 3.5 are not sufficient to build a suitable abstraction function needed for verification (Section 3.1). One such case has to do with fairness; roughly speaking, the fairness property requires that some unwanted behaviour will not last forever. We shall illustrate the necessity for fairness using a suitable example: the alternating-bit protocol. Consider a sender and a receiver, communicating via a channel: