Книга: C# 2008 Programmer
Traditional ASMX Web Services use the request/response communication model. This model has some disadvantages. In some cases, the client might want to call the service without waiting for a response from the service. For example, you might want to call a service rapidly to turn on and off a switch and you do not need a response from the service. Using the request/response model, all requests made by the client have to wait for a reply from the service (even if the request does not return a result). The result is unnecessary blocking on the client side, especially if there are many queued requests on the service's end.
WCF supports three communication models (also known as messaging patterns):
? One-way (simplex)
? Two-way (duplex)
The one-way messaging pattern allows clients to fire off a request and forget about it; no response is needed from the service. The two-way messaging pattern allows both the service and the client to send and receive messages.