Introduction to Data Communications
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Layer Specific Communication

Each layer may add a Header and a Trailer to its Data. The combination of the header, data and trailer is called a Protocol Data Unit (PDU). A PDU is a generic term that is applied to a layer's data structure.

The header is information that procedes the data and the trailer is information that follows the data. Usually the header will contain source and destination addresses and some control information used to manage the communication. The trailer usually contains information such as error checking or a field to indicate the end of the PDU. The Application Header is indicated by the letters AH. The Application Trailer is indicated by the letters AT in the above figure.

Each layer's data field consists of the upper layers PDU. For example: the Network layer's data field consists of the Transport layer's PDU.

Generally, each layer only is aware of its own layer. The Network layer only communicates with other Network layers. It is not aware of the Transport layer or the Data Link layer.

Introduction to Data Communications
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Copyright Eugene Blanchard Jan 1998, January 2007