The seven OSI layers use various forms of control information to communicate with their peer layers in other computer systems. This control information consists of specific requests and instructions that are exchanged between peer OSI layers.
Control information typically takes one of two forms: headers and trailers. Headers are prep ended to data that has been passed down from upper layers. Trailers are appended to data that has been passed down from upper layers. An OSI layer is not required to attach a header or a trailer to data from upper layers.
Headers, trailers, and data are relative concepts, depending on the layer that analyzes the information unit. At the network layer, for example, an information unit consists of a Layer 3 header and data. At the data link layer, however, all the information passed down by the network layer (the Layer 3 header and the data) is treated as data.
In other words, the data portion of an information unit at a given OSI layer potentially
can contain headers, trailers, and data from all the higher layers. This is known as encapsulation. Figure shows how the header and data from one layer are encapsulated into the header of the next lowest layer.
Figure Headers and Data Can Be Encapsulated During Information Exchange
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