wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

1-6 Industrial Communication Systems
End-to-end
protocols
7
Applicationoriented
protocols
Point-to-point
4
1
Transportoriented
protocols
FIGURE 1.4
Layer build-up as blocks.
1.4 Dynamic Behavior of Services and Protocols
Figure 1.5 shows a common example of communication that runs over an intermediate station, e.g., a router.
One side, e.g., the left side, starts transmitting a message, which is done by the application using the application
layer. Communication runs vertically down to layer 1, horizontally through the router to the receiving
device, and vertically up to the application in the receiving device. This communication can also be seen as
parallel horizontal communications that run between each layer as peer-to-peer communications.
Due to the modularization that the OSI model requires, all layers operate simultaneously. Consequently,
the layers can operate independently, the interfaces can be defined easily and systematically, and the different
tasks can be clearly assigned to the different layers.
Each layer in the OSI model contains a definition of the protocol and the services. The protocol specification
contains the exact number of functions and messages (protocol data units, PDUs)—so to say
the “language” of the layer. The layer itself is characterized by the procedures that operate on the PDUs.
The service definitions can be explained as follows: two opposing entities (the service users, SU)
exchange information using protocols (peer-to-peer protocols) by using the service providers (SP). Thus,
a service provider offers services to a service user, so that the service user can fulfill its task. Service
communication runs vertically, where lower layers offer services to higher layers (with the exception of
the lowest layer).
Seen from a logical perspective, the layers of entities communicate with each other (i.e., layer n of
one entity communicates with layer n′ of the other entity). In the vertical direction, service primitives
are passed on using the service access points (SAP) as shown in Figure 1.6. There is no communication
defined between layer n − 1 and layer n + 1. The communication between two adjacent layers can
be compared to master–slave communication, because a master does not communicate with a slave,
Subnet a
Subnet a΄
7
6
5
4
3
2
1
Application layer protocol AL
Application layer protocol PL
Converter
3 3
2 2
1 1
Information flew
7 Application layer
6 Presentation layer
5 Session layer
4 Transport layer
3 Network layer
2 Link layer
1 Physical layer
Internal subnet-protocol
Network protocol NL
Link layer protocol LL
Physical layer protocol LL
FIGURE 1.5
Communication over subnets.
© 2011 by Taylor and Francis Group, LLC