150/051 Reference Model - Motorola MC68824 User Manual

II 1.2 LOCAL AREA NETWORK STANDARDS
Until recently, proprietary local area networks were the only way to tie equipment together to
share information and resources. These proprietary local area networks allowed the user to con-
nect small segments of one vendor's computers together. This was all the user needed or wanted.
Today, with the increasing complexity of factories and offices, there is a significant increase in
the requirements for local area network functionality. Users need to connect more than just a
small segment of one vendor's equipment. Since each vendor has its own proprietary protocol,
connecting equipment together from more than one vendor or placing a different vendor's ma-
chine into a proprietary network can cost up to 80% of the price of the actual equipment. This
cost is mainly attributed to the time necessary to write special software programs.
Users began to see that this situation left them with two choices; to buy everything from one
vendor, or to buy standard equipment. Since it is obviously not reasonable to buy everything
from one vendor, local area network standards began to receive attention. General Motors has
specifically addressed the standardization of the factory local area network through the Manu-
facturing Automation Protocol (MAP) specification based on the 150/051 reference model.
1.2.1 150/051 Reference Model
The industry accepted communications model is the Inter-
national Standards Organization (ISO) reference model of
Open Systems Interconnection (051). The model specifies
seven layers, with each layer responsible for providing spe-
cific services to the layer above it. These layers are inde-
pendent of each other and peer-to-peer layer interfaces are
defined and standardized (see Figure 1-1).
Application Layer
Provides the network services necessary for the user to
run his application program.
Presentation Layer
Provides the services necessary to format data ex-
change and to manage session dialog.
Session Layer
ISO LAYER
APPLICATION
PRESENTATION
SESSION
STATION
TRANSPORT
MANAGEMENT
NETWORK
OATA LINK
PHYSICAL
I
()
PHYSICAL MEOlA
)
Figure 1-1. 105/051 Model
Handles connection establishment, connection termination, and arbitrates session user rights
to services.
Transport Layer
Provides the functions necessary for error free delivery of messages (flow control, acknowl-
edgement, error recovery).
Network Layer
Routes frames between multiple network segments.
Data Link Layer
Is responsible for sending frames across the physical link in a reliable manner.
Physical Layer
Transmits data bits across the physical medium.
MOTOROLA
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MC68824 USER'S MANUAL