Installation guide

Network Infrastructure for EtherNet/IP™
B-6
Appendix B Overview of the OSI Model, EtherNet/IP and CIP
The Logical Link Control (LLC) sublayer of the data link layer manages communications between
devices over a single link of a network. Defined in IEEE Std 802.2, LLC supports both connectionless
and connection-oriented services used by higher-layer protocols. IEEE Std 802.2 defines a number of
fields in data link layer frames that enable multiple higher-layer protocols to share a single physical data
link.
The
Media Access Control (MAC) sub-layer of the data link layer manages protocol access to the
physical network medium. The IEEE MAC specification defines MAC addresses, which enable multiple
devices to uniquely identify one another at the data link layer.
Network Layer
The network layer defines the network address, which differs from the MAC address. Some network layer
implementations, such as the Internet Protocol (IP), define network addresses in a way that route selection
can be determined systematically by comparing the source network address with the destination network
address and applying the subnet mask. Because this layer defines the logical network layout, routers can
use this layer to determine how to forward packets. Because of this, much of the design and configuration
work for inter-networks happens at Layer 3, the network layer.
Transport Layer
The transport layer accepts data from the session layer and segments the data for transport across the
network. Generally, the transport layer is responsible for making sure that the data is delivered error-free
and in the proper sequence. Flow control generally occurs at the transport layer.
Flow control manages data transmission between devices so that the transmitting device does not send
more data than the receiving device can process.
Multiplexing enables data from several applications to
be transmitted onto a single physical link.
Virtual circuits are established, maintained, and terminated by
the transport layer.
Error checking involves creating various mechanisms for detecting transmission
errors, while
error recovery involves acting, such as requesting that data be retransmitted, to resolve any
errors that occur. The transport protocols used on the Internet are TCP and UDP.