A single radio frequency is capable of handling a limited number of radio messages in a specified time. The maximum number of messages will be determined by the length of each message and its response. Ideally, a pilot should be able to transmit a message at any time of his/her choosing, and receive an immediate reply. As radio traffic increases above the ideal, the frequency becomes congested. The pilot must wait for a break in transmissions to pass a message and may have to wait for a response from the ATCO, who has to judge different priorities.
Persistent (rather than transient) frequency congestion is a frequent contributory factor in communication breakdown, usually in association with other factors.
Pilot or ATCO is unable to pass an important message at the desired time.
Use English language, to assist other pilots to determine when communication with ATC is finished.
Do not talk continuously but give pilots the opportunity to check in on the frequency by building in rest blocks; In practical terms, it may be best to transfer several aircraft to their next sectors before checking in new aircraft - this is a matter of judgement for the ATCO.
Reduce the number of aircraft on a frequency or extend the number of controllers, so that the workload per frequency is lower.
Blocked transmissions can be partly overcome by anti-blocking devices.
Design a warning if the mike is pressed too long, for instance by a beep indicting the microphone is still transmitting. (Some aircraft already have such a feature).