Installation guide

Network Infrastructure for EtherNet/IP™
B-3
Appendix B Overview of the OSI Model, EtherNet/IP and CIP
OSI Layer Services
One OSI layer communicates with another layer to make use of the services provided by the second layer.
The services provided by adjacent layers help a given OSI layer communicate with its peer layer in other
computer systems.
Basic Elements of Layer Services
Three basic elements are involved in layer services: the service user, the service provider, and the
service access point (SAP).
In this context, the service user is the OSI layer that requests services from an adjacent OSI layer. The
service provider is the OSI layer that provides services to service users. OSI layers can provide services
to multiple service users. The
service access point (SAP) is a conceptual location at which one OSI
layer can request the services of another OSI layer. Figure B-4 shows how these three elements interact at
the network and data link layers.
Figure B-4 Service Users, Providers, and SAPs Interact at the Network and Data Link Layers.
OSI Model Layers and Information Exchange
In one computer, the seven OSI layers use various forms of control information to communicate with
their peer layers in other computer systems. This
control information consists of specific requests and
instructions that are exchanged between peer OSI layers.
Control information typically takes one of two forms: headers and trailers.
Headers are prepended to
data that has been passed down from upper layers.
Trailers are appended to data that has been passed
down from upper layers. An OSI layer is not required to attach a header or a trailer to data from upper
layers.
Headers, trailers, and data are relative concepts, depending on the layer that analyzes the information unit.
At the network layer, for example, an information unit consists of a Layer 3 header and data. At the data
link layer, however, all the information passed down by the network layer (the Layer 3 header and the
data) is treated as data.