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

1-8 Industrial Communication Systems
n
IDU
ICI
SDU
SAP
Interface
(Schnittstelle)
ICI
n – 1
ICI
SDU
PDU
SDU SDU PCI
PCI
FIGURE 1.9 Communication between two consecutive layers (ICI, interface control unit information; IDU,
interface data unit; PCI, protocol control information; PDU, protocol data unit; and SAP, service access point).
are unconfirmed service (using only Request and Indication), confirmed service (using Request, Indication,
Response, and Confirmation), and confirmed service with local significance (using all four primitives, but
without being able to link Response and Confirmation, because the communication channel is only unidirectional).
In the latter case, a Confirmation is initiated by a Request, which has the big disadvantage that
the user is led to believe that a confirmation has been sent—which is not the case.*
The process that happens between the layers is described in Figure 1.9 [Leo 00]. Layer n of a communication
stack shall send a piece of information to an opposing layer. This information is called service
data unit (SDU). The interface process in layer n attaches an interface control unit information (ICI)
to the head of the SDU, which together builds the interface data unit (IDU) that is transferred over the
SAP. The layer below the ICI is decoded and a new process is triggered. The following sequence (i.e.,
the execution of a protocol) is to be seen as a logical process, with the exception of the lowest layer, the
physical layer. For all other layers, we can say that layer n − 1 packs the SDU into a PDU by following a
procedure defined by the ICI by prefixing a protocol control information (PCI) to the SDU. The PDU is
now transmitted, which means that the layer n − 1 uses its layers underneath and acts as a service user.
Finally, layer 1 physically transmits information as shown in Figure 1.10. The SDUs can be physically
measured as well as all header, i.e., the PCI of all layers.
PCI (n – 4) PCI (n – 3) PCI (n – 2) PCI (n – 1) PCI (n) SDU (n)
PDU (n)
PDU (n – 1)
PDU (n – 2)
PDU (n – 3)
PDU (n – 4)
FIGURE 1.10
Data frame that can be physically measured on the connection between two end devices.
* For some implementations this is sufficient, assuming that errors occur rarely and can be caught on higher layers.
© 2011 by Taylor and Francis Group, LLC