We study channel systems whose behaviour (sending and receiving messages via unbounded FIFO channels) must follow given timing constraints specifying the execution speeds of the local components. We propose Communicating Timed Automata (CTA) to model such systems. The goal is to study the borderline between decidable and undecidable classes of channel systems in the timed setting. Our technical results include: (1) CTA with one channel without shared states in the form (A1,A2, c1,2) is equivalent to one-counter machine, implying that verification problems such as checking state reachability and channel boundedness are decidable, and (2) CTA with two channels without sharing states in the form (A1,A2,A3, c1,2,c2,3) has the power of Turing machines. Note that in the untimed setting, these systems are no more expressive than finite state machines. We show that the capability of synchronizing on time makes it substantially more difficult to verify channel systems.
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