Image Processing Reference
In-Depth Information
Open system 1
Open system 2
Layer N + 1
Layer N + 1
(N+1)-ICI
(N + 1)-SDU
(N + 1)-SDU
(N + 1)-ICI
(N + 1)-IDU
(N + 1)-IDU
SAP
SAP
Layer N
Layer N
(N)-PCI
(N + 1)-IDU
(N + 1)-IDU
(N)-PCI
(N) -PDU
(N)-PDU
(N)-ICI
(N)-SDU
(N)-SDU
(N)-ICI
(N)-IDU
(N)-IDU
SAP
SAP
Layer N
-
1
Layer N
-
1
(N - 1)-ICI
(N)-IDU
(N)-IDU
(N - 1)-ICI
(N - 1) -PDU
(N - 1)-PDU
FIGURE .
Communication in the OSI model.
transmitted user data supplemented with parameters of the interface (interface control
information [ICI]) and unique control information (protocol control information [PCI]).
The exact procedure is highlighted in Figure .. The user data of layer N that are to be transmit-
ted to its peer layer are put into an SDU. he interface process of layer N adds an ICI to the message
header (the SDU header), whereby the ICI and SDU together form the interface data unit (IDU).
The IDU is then transferred via the SAP. In the underlying layer, the ICI is now decoded and a cor-
responding process is initiated. he following process (execution of a protocol) should be viewed as
a logic process, except in the lowest layer, the physical layer. In the procedure initiated by the ICI,
layer N
packs the SDU into a PDU. his involves adding a PCI to the SDU. he assembled PDU is
now transmitted, which means that layer N
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in turn uses the lower layers and itself now behaves as
a service user. Only layer actually physically transmits the information.
In even more detail, the interaction between the individual layers is governed by a series of oper-
ationsdeinedintheOSImodel.heseoperationsarecalledserviceprimitives.herearebasically
four different service primitives: Request, req; indication, ind; confirmation, con; and response, res.
Each service primitive that is called up by the service user is termed a request. With the request, layer
N receives the order to execute a specific task. he respective task and corresponding data are con-
verted into a corresponding PDU. In accordance with the OSI model, the service provider uses the
services of the underlying layer N
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, to carry out its task. his interaction continues until the lowest
layer entrusts the data to the physical medium. On the receiver side, the peer layer is activated via an
indication (or sometimes even a whole series of indications). After the remote layer N has decoded
thePDUandextractedthecontrolinformation,theuserdataarepassedontotheabovelyinglayer
(i.e., the layer directly above or in the case of the application layer, the actual user process) also by
means of an indication. he way back from the receiver side is composed from responses generated
by the service users and confirmations issued by the service providers. However, services need not
always comprise each of these four service primitives. here are many possible and practically used
combinations with varying degree of reliability, as the following examples show:
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