lower layer provides a service to the next higher layer. Consider the receipt of data by the host
on the right side of Figure 3-2:
places the received binary pattern into a buffer. It notifies the data
link layer that a frame has been received after decoding the
incoming signal into a bit stream. Therefore, Layer 1 has provided
delivery of a stream of bits across the medium.
the trailer to determine whether errors occurred in transmission
(error detection). If an error has occurred, the frame is discarded.
(Some data link protocols perform error recovery, and some do
not.) The data link address(es) are examined so that Host B can
decide whether to process the data further. If the data is addressed
to host B, the data between the Layer 2 header and trailer is given
to the Layer 3 software. The data link has delivered the data across
that link.
the address is Host B's address, processing continues (logical
addressing) and the data after the Layer 3 header is given to the
transport layer (Layer 4) software. Layer 3 has provided the
service of end-to-end delivery.
(Layer 4), the counters identifying this piece of data are encoded
in the Layer 4 header along with acknowledgment information
(error recovery). After error recovery and reordering of the
incoming data, the data is given to the session layer.
of messages is completed. For example, this data could be
meaningless if the next four exchanges are not completed. The
Layer 5 header could include fields signifying that this is a middle
flow in a chain, not an ending flow. After the session layer ensures
that all flows are completed, it passes the data after the Layer 5
header to the Layer 6 software.
formats. For example, if the data is binary instead of character
data, the header denotes that fact. The receiver does not attempt to
convert the data using the default ASCII character set of Host B.
Typically, this type of header is included only for initialization
flows, not with every message being transmitted (data formats).