Allen-Bradley PLC-5 MAP/OSI, Руководство пользователя, Страница: 10 / 135

Chapter 1
Overview of the MAP Communication
Environment
1-2
activities involved in communicating on the network
services required to perform those activities
The individual layer specifications dictate how the functions are
accomplished. The tasks within the layers are carried out by protocols.
Protocols are rules for how information is coded and passed between two
nodes. The protocols are actually the part that is implemented, the OSI
model serves merely as a reference to discuss the different aspects of
communication between devices. The following table lists the functions
that the protocols at each layer must accomplish.
This layer
:
Contains the functions that:
7 Application manipulate information to support applications. This layer's protocols
contain the most functionality.
6 Presentation ensure information is delivered in a form the receiving system can
understand and use.
5 Session manage communications between two application processes.
4 Transport transfer reliable data between communicating nodes.
3 Network route communication between the communicating nodes.
2 Data Link perform synchronization and error control for information passed over
the physical link (manages the access to the medium).
1 Physical activate, maintain, and deactivate the physical connection.
The OSI system enables many different vendors’ devices on the same
network to communicate with each other. In other words, as long as two
different vendors construct protocols that ‘‘fit” in the seven layer model the
same way, those two devices will be able to communicate (figure 1.2).
Figure 1.2
Different Vendors' Devices Can Communicate on OSI Networks
Allen-Bradley
PLC Controller
OSI interface
with OSI protocol
Another Vendor's
device with OSI
protocol.
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