Installation guide

Network Infrastructure for EtherNet/IP™
B-4
Appendix B Overview of the OSI Model, EtherNet/IP and CIP
In other words, the data portion of an information unit at a given OSI layer potentially can contain
headers, trailers, and data from all the higher layers. This is known as
encapsulation. Figure B-5 shows
how the header and data from one layer are encapsulated into the header of the next lowest layer.
Figure B-5 Headers and Data Can Be Encapsulated during Information Exchange.
Information Exchange Process
The information exchange process occurs between peer OSI layers. Each layer in the source system adds
control information to the data, and each layer in the destination system analyzes and removes the control
information from that data.
If System A has data from a software application to send to System B, the data is passed to the application
layer. The application layer in System A then communicates any control information required by the
application layer in System B by prepending a header to the data. The resulting information unit (a header
and the data) is passed to the presentation layer, which prepends its own header containing control
information intended for the presentation layer in System B. The information unit grows in size as each
layer prepends its own header (and, in some cases, a trailer) that contains control information to be used
by its peer layer in System B. At the physical layer, the entire information unit is placed onto the network
medium.
The physical layer in System B receives the information unit and passes it to the data link layer. The data
link layer in System B then reads the control information contained in the header prepended by the data
link layer in System A. The header is then removed, and the remainder of the information unit is passed to
the network layer.
Each Layer Performs the Same Actions
The layer reads the header from its peer layer, strips it off, and passes the remaining information unit
to the next highest layer. After the application layer performs these actions, the data is passed to the
recipient software application in System B, in exactly the form in which it was transmitted by the
application in System A.