Communication Parameters 
Both message interfaces and imported interfaces can be designed to send a request or to respond to a request. You may or may not receive a direct response message to this request. For message interfaces, you define this using the characteristics synchronous or asynchronous and outbound and inbound.
Outbound and Inbound Interfaces
These characteristics determine the direction of an interface:
· An outbound interface sends a request message that is only used to make a communication partner aware of data sent, without waiting for a response message. In the case of the latter, we also refer to publishing interfaces.
· An inbound interface receives a request message that you reply to with a direct response message or whose data you can process in the system without a response.
You require an inbound and an outbound interface pair for two components to be able to communicate with each other. This means that an outbound message interface can be assigned to an inbound message interface as well as to an RFC interface that replaces the inbound counterpart semantically (and the other way around). Also see: Communication Partners.
Synchronous and Asynchronous Communication
A distinction is made between synchronous and asynchronous communication for both imported interfaces and message interfaces. You define the mode of communication when you define an interface:
· In synchronous communication, a response is expected from the receiver system on arrival of a query sent by the sender system. Once this message has been sent, no further messages can be sent until the answer to the query has arrived back at the sender system.
· However, in asynchronous communication the response is not immediate. A sending process can send multiple messages to a receiver in a bundle and then continue executing the process.
The pair of interfaces must have the same mode.
Message Exchange
The following graphic shows how the mode and direction of interfaces influence message exchange between interfaces:
