The WCF runtime provides a set of classes responsible for sending and receiving messages. For example, formatting messages, applying security, transmitting and receiving messages by using various transport protocols, as well as dispatching received messages to the appropriate operations; all these fall within the WCF runtime scope.
The following diagram shows the WCF runtime (http://msdn.microsoft.com/en-us/magazine/cc163302.aspx):
The sequence of a WCF service call might look as follows:
A client application calls a WCF service through a WCF proxy.
The proxy translates the call to one or more WCF messages.
The messages are passed through channels and transported to the service.
On the service side, the messages are transported through channels.
Service dispatchers pick up the messages, making calls to WCF services and then returning the results back to the client.
As you can see from this diagram, there are multiple layers and many entry points for a WCF service call...