Message Exchange Pattern describes how the client and server should exchange messages, and also the MEP will make clear that the client is going to wait for any response from the service or just simply sending is enough ,or the client is expecting any callbacks from the service.
WCF supports three Message Exchange Patterns (MEPs). They are as follows.
The below given diagram show how Duplex (One-Way) works.
Below given diagram shows how a Duplex (Request-Reply) MEP works,