Although logical communication occurs between two corresponding layers, the actual communication happens through the layer. At the source, the data flows downwards through the layer. At the destination, the data flows up-words through the layer. In the intermediate systems, the data flows upward and then downward. Through this movement, headers and trailers are added to or removed from the PDU delivered by upper or lower layers. This process is called en-capsulation/de-capsulation because it looks as if the PDU from an upper layer is en-capsuled during the downward movement and de-capsuled during the upward movement.
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