Installation guide

Network Infrastructure for EtherNet/IP™
B-2
Appendix B Overview of the OSI Model, EtherNet/IP and CIP
The upper layers deal with application issues and generally are implemented only in software. The
highest layer, the application layer, is closest to the end user. Both users and application layer processes
interact with software applications that contain a communications component. The term “upper layer” is
sometimes used to refer to any layer above another layer in the OSI Model.
The
lower layers handle data transport issues. The physical layer and the data link layer are implemented
in hardware and software. The lowest layer, the physical layer, is closest to the physical network medium
(the network cabling, for example) and is responsible for actually placing information on the medium.
Communication between Systems
Information being transferred from a software application in one computer system to a software
application in another must pass through the OSI layers. For example, if a software application in System
A has information to transmit to a software application in System B, the application program in System A
will pass its information to the application layer (Layer 7) of System A. The application layer then passes
the information to the presentation layer (Layer 6), which relays the data to the session layer (Layer 5),
and so on—down to the physical layer (Layer 1). At the physical layer, the information is placed on the
physical network medium and is sent across the medium to System B.
The physical layer of System B removes the information from the physical medium, and then its physical
layer passes the information up to the data link layer (Layer 2), which passes it to the network layer
(Layer 3), and so on, until it reaches the application layer (Layer 7) of System B. Finally, the application
layer of System B passes the information to the recipient application program to complete the
communication process.
Interaction between OSI Model Layers
A given layer in the OSI model generally communicates with three other OSI layers: the layer
directly above it, the layer directly below it, and its peer layer in other networked computer systems.
The data link layer in System A, for example, communicates with the network layer of System A, the
physical layer of System A, and the data link layer in System B as shown in Figure B-3.
Figure B-3 How OSI Layers Communicate.