Figure 110 Three Basic Components Make Up a Network Layer Packet

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Network layer header

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The term drtrgrrm usually refers to an information unit whose source and destination are network layer entities that use connectionless network service.

The term segment usually refers to an information unit whose source and destination are transport layer entities.

Amessage is an information unit whose source and destination entities exist above the network layer (often at the application layer).

Acell is an information unit of a fixed size whose source and destination are data link layer entities. Cells are used in switched environments, such as Asynchronous Transfer Mode (ATM) and Switched Multimegabit Data Service (SMDS) networks. A cell is composed of the header and payload. The header contains control information intended for the destination data link layer entity and is typically 5 bytes long. The payload contains upper-layer data that is encapsulated in the cell header and is typically 48 bytes long.

The length of the header and the payload fields always are the same for each cell. Figure 1-11 depicts the components of a typical cell.

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