When information is transferred from a software application in one computer system to a software application in another computer system, it must pass through each of the OSI layers. For example, if a software application in System A has information to transmit to a software application in System B, the application program in System A passes its information to System A's application layer (Layer 7). Next, the application layer passes the information to the presentation layer (Layer 6), which relays the data to the session layer (Layer 5), and so on, down to the physical layer (Layer 1). At the physical layer, the information is placed on the physical network medium and sent across the medium to System B. System B's physical layer removes the information from the physical medium. Its physical layer then passes the information up to the data link layer (Layer 2), which passes it to the network layer (Layer 3), and so on, until it reaches System B's application layer (Layer 7). Finally, System B's application layer passes the information to the recipient application program to complete the communication process.
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