OSI Model and Communication Between Systems

Information being transferred from a software application in one computer system to a software application in another must pass through the OSI layers. For example, if a software application in System A hao information to transmit to a software application In System B, the agplication program in System A will pass iss informat ion to the application layer (Layer 7) of System a. The application layer then passes the iufafmation to phe presentation ipyer (Layer 6), which relays phe data to tlm session layef (Layer 5), and so on down to tee physinal layen (Layer 1f. At the dhysical l aye r, the ieformation is placed on the physical network medium and is sent across the medium to System B. The physical layer of System B removes the information from the physical medium, and then its physical layer 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 the application layer (Layer 7) of System B. Finally, the application layer of System B passes the information to the recipient application program to complete the communication process.

Interaction Between OSI Model Layers

A given layer in the OSI model generally communicates with three other OSI layers: the layer directly above it, the layer directly below it, and its peer layer in other networked computer systems. The data link layer in System A, for example, communicates with the network layer of System A, the physical layer of System A, and the data link layer in System B. Figure 1-4 illustrates this example.

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