Most communication
environments use the OSI model to separate the communications
functions and applications processing into layers. Each layer in the
OSI model serves a set of specific functions. This semester's focuses on Layers 2, 3, and 4 of the OSI model.
Each layer within one
system uses its own protocols to communicate with peer layers in
another system. Each layer protocol exchanges information, called
protocol data units (PDUs), between peer layers. A given layer has
specific names for each of its PDUs. The Table gives examples of
specific PDUs for Layers 2, 3, and 4 and the device types that
process those PDUs.
Each peer-layer
protocol is serviced by the underlying layer. For example,
Transmission Control Protocol (TCP) segments are encapsulated in
Layer 3 packets, and Layer 3 packets are encapsulated in Layer 2
frames. A layer-specific device processes only those PDUs for which
it is responsible - this is done by inspection of the PDU header.
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