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

ISO/OSI Model 1-5
1.2.1.7 Layer 7: Application Layer
The application layer provides an interface that can be used by the application. It contains services for
the application, which can, for example, provide access to distributed databases or other high-level
services. The application layer strongly depends on what the applications (or the operating system)
above it need, and is therefore usually designed to meet the requirements of these applications.
As stated earlier, the seven layers cannot provide interoperability by themselves. Profiles, which create
a layer on top of the ISO/OSI model, can help to reach the level of interworkability.
1.3 Vertical and Horizontal Communication
Each communication node (i.e., a device that participates in communication) contains at least one
communication stack, which is defined by the function layers 1–7. Therefore, we distinguish between
horizontal and vertical communication as shown in Figure 1.3.
The layers communicate using a logical connection (horizontal communication) or the different layer
protocols (e.g., application protocol or session protocol), respectively. The term “protocol” is ambiguously
used for both the complete communication stack and the separate (seven) protocols of the stack.
The protocol contains not only data definitions that define the kind of information that is exchanged,
but it also defines the set of rules that communicating entities have to use, in terms of appropriate reactions
to incoming information, handling of error situations, or timing constraints. Information flows
vertically (vertical communication) through the layers, which offer services to higher layers. The following
section will go into more detail on this.
If we look at the protocols of the separate layers—on which we want to focus in the following—we see
a set of rules that are assigned to the same layer. They are defined by the type of control information, the
procedure, and its behavior; this is done in three definitions: (a) definition of the states, (b) definition of
the transitions, and (c) the definition of the timing. They should be realized in a way that they can each
be replaced separately in the stack.
The seven layers of the ISO/OSI model can be assigned to different block functions. The left side of
Figure 1.4 shows the separation between point-to-point connections, i.e., the connection between two units
without another unit (e.g., a router) is connected in between, and end-to-end connections. This means that
on a connecting line between two end devices, the three lower layers have to be processed by each device on
the way from end device to end device, while the upper four layers are only processed by the end devices.
Figure 1.4 on the right side separates between transport-oriented protocols and the upper three
application-oriented protocols. The transport-oriented protocols do not process payload data, while the
application-oriented protocols depend on the according application.
This indicates the importance of the transport layer: layer four is the first that shows the end-to-end
property of the communications. This layer shall guarantee that all data, which are sent, do receive the
other side completely and in correct order. On the other side, it is the lowest layer that can still be defined
independently from the application
7
6
5
4
3
2
1
Vertical communication
(services)
7
7
6
6
5
5
4
4
3
3
2
2
1
1
Horizontal (logical) communication
(protocols)
FIGURE 1.3
Layer architecture.
© 2011 by Taylor and Francis Group, LLC