Protocol data unit: Difference between revisions

Revision as of 14:57, 16 October 2021

Revision as of 14:55, 19 October 2021

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In the context of [[packet switching]] data networks, a protocol data unit (PDU) is best understood in relation to a [[service data unit]] (SDU).

The features or services of the network are implemented in distinct "''layers''." The physical layer sends ones and zeros across a wire or fiber. The data link layer then organizes these ones and zeros into chunks of data and gets them safely to the right place on the wire. The network layer transmits the organized data over multiple connected networks, and the transport layer delivers the data to the right software application at the destination.

Between the layers (and between the application and the top-most layer), the layers pass service data units (SDUs) across interfaces. The higher layer understands the structure of the data in the SDU, but the lower layer at the interface does not; moreover, the lower layer treats the SDU as the [[Payload (computing)|payload]], undertaking to get it to the same interface at the destination. In order to do this, the ''protocol'' (lower) layer will add to the SDU certain data it needs to perform its function; which is called [[Encapsulation (networking)|encapsulation]]. For example, it might add a port number to identify the application, a network address to help with routing, a code to identify the type of data in the packet and error-checking information. All this additional information, plus the original service data unit from the higher layer, constitutes the ''protocol data unit'' at this layer.