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

ISO/OSI Model 1-7
Layer n + 1
Definition of protocols
Layer n
Logical connection
Peer protocol
Layer n΄
Remote station
Layer n – 1
SAP: Service access point
Definition of services
FIGURE 1.6
Services, protocols, and service access points.
Peer-to-peer
protocol
SAP
Primitive
FIGURE 1.7
The SAP is a logical interface between the service user (SU) and the service provider.
it rather requests a service from the slave; the slave itself can again be the master of underlying slaves
(Figure 1.7). This is also called a cascaded system. The service user can also be seen as an instance, which
runs a process with the opposing instance (i.e., the peer user).
A communication stack according to ISO/OSI is therefore a parallel working system, which does not
have priority control (at least in the first definition of the model). This has been softened later by adding
a management, which will be described later.
The SAP can be implemented in software (e.g., as a subroutine) or in hardware (e.g., as memory). The
decision is up to the developer.
The vertical communication uses predefined primitives to unify the sequence of communication
in all systems (Figure 1.8). The sequence of primitives is as follows: Request, Indication, Response, and
Confirmation. Based on these primitives, we can derive different communication relations. Three examples
n-Entity SU
n΄-Entity SU
Request
Confirm
Response
Indication
SAP
(n – 1)-Entity
SAP
FIGURE 1.8
Predefined service primitives.
© 2011 by Taylor and Francis Group, LLC