Communicating Between Layers
Each layer of the OSI model uses its own protocol
to communicate with its peer layer in the destination
device. The OSI model specifies how each
layer communicates with the layers above and
below it, allowing vendors to focus on specific layers
that will work with any other vendor’s adjacent
layers.
Information is exchanged between layers using
protocol data units (PDU). PDUs include control
information (in the form of headers and trailers)
and user data. PDUs include different types of
information as they go up or down the layers
(called “the stack”). To clarify where the PDU is
on the stack, it is given a distinct name at each of
the lower levels.
At-a-Glance: OSI Model
In other words, a PDU that is a segment (Layer 4)
includes all the application layer’s information. A
packet (Layer 3) includes network layer control
information in addition to the data and control
information contained at the transport layer.
Similarly, a frame (Layer 2) is a PDU that includes
data link layer control information in addition to
the upper layer control information and data.
Finally, PDUs at the physical layer (Layer 1) are
called bits. 24