Because of the real-time nature of telephone signaling, both the PNS and PAC should be implemented with multi-threaded architectures such that messages related to multiple calls are not serialized and blocked. The transit delay between the PAC and PNS should not exceed one second. The call and connection state figures do not specify exceptions caused by timers. The implicit assumption is that since the TCP-based control connection is being verified with keep-alives, there is less need to maintain strict timers for call control messages.
Establishing outbound international calls, including the modem training and negotiation sequences, may take in excess of 1 minute so the use of short timers is discouraged.
If a state transition does not occur within 1 minute (except for connections in the idle or established states), the integrity of the protocol processing between the peers is suspect and the ENTIRE CONTROL CONNECTION should be closed and restarted. All Call IDs are logically released whenever a control connection is started. This presumably also helps in preventing toll calls from being "lost" and never cleared.