OSI Model Layers and Information Exchange

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 prepended 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 trailer to data from upper layers.

Depending on the layer that analyzes the information unit, headers, trailers, and data are relative concepts. An information unit, for example, consists of a Layer 3 header and data at the network layer. 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 can potentially contain headers, trailers, and data from all the higher layers. This is known as encapsulation. Figure C-5 illustrates how the header and data from one layer are encapsulated to become the data of the next lowest layer.

Figure C-5 Headers and Data Are Encapsulated During Information Exchange

System A

Information units

Header 4

Data

Header 3

Data

Header 2

Data

Network

System B

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