Simplex - Also known as one way communication. Source will send a message to the target, but target will not respond to the message.
Request/Replay - It is two way communications, source send message to the target, target will resend the response message to the source. But in this scenario at a time only one can send a message (that is), either source or destination.
Duplex -Also known as two way communication, both source and target can send and receive message simultaneously.
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