Image Processing Reference
In-Depth Information
Layer (Session Layer)
The main task of layer is to bring together several end devices into a session and to synchronize
the “conversation.” his also involves identification or authentication (e.g., password check) and the
handling of very large messages. Close cooperation between this layer and the operating system is
of vital significance. For this reason, layers and are implemented in fieldbus practice with high
transparency for these particular layer- functions, or a separate channel is created to bypass them. It
isalsothetaskoflayertointroduceanynecessarysynchronizationmarkers,sothatitknowswhen
to resume after a breakdown in communication.
Layer (Presentation Layer)
The presentation layer interprets the incoming data and codes the data to be transmitted. his means
that level carries out syntactic and semantic tasks. hese include, e.g., the meaning of the sequence
of bits of a character, to be interpreted as a letter, interpretation of currency as well as physical units,
cryptographic tasks, etc. For encoding, ISO defined the standards Abstract Syntax Notation or Basic
Encoding Rules which are frequently used.
Layer (Application Layer)
Layer is a boundary layer (interface to the application) and with that occupies a special position. It
forms the interface between the application and the communication unit. In this level, the procedures
or protocol processes of various application functions are defined, for calling up data, file transfer,
etc. The purpose of layer is a transparent representation of the communication. If, e.g., a system
accessesdatabasesviathecommunicationunit,layermustbedesignedsothatitdoesnotrequire
any knowledge of the individual tasks of the underlying layers. With that, an efficient communication
system allows a database that is distributed among various different locations to be viewed as a single
interconnected database. An intelligent switch that transmits information via an OSI communica-
tion unit, such as “turn light on,” does not need to know anything about the actual communication
protocol;itsimplyknowsthe“name”ofthelightsaswellasthefunctions“turnlighton,”“turnlight
off, ” or “dim light by %,” etc.
Based on these definitions, it is possible to set up complex communication systems in a structured
way. Moreover, the strictly hierarchical layout of the model allows interconnection of heterogeneous
systems on different layers. Through the use of repeaters, one can overcome the limitations of a
given physical layer. The interconnecting device shares a common data link layer. Bridges inter-
connect different networks by translating data and protocols on layer . Routers link networks
on layer , whereas gateways (or more precisely, application layer gateways) interconnect entirely
different communication systems on the application layer.
20.4.2 Communication Services
Figure . shows in detail how data are exchanged between service user and service provider or
between two peer layers. he interface between two neighboring layers is called service access point
(SAP). he vertical and horizontal communication is made up of two important units:
•
Service Data Unit
(SDU). Communication of layer N
with the underlying layer N
occurs via its services, or more accurately via the interfaces of these services. For layer N,
the transferred data represent pure user data that are passed on to the next lowest layer
for further processing.
•
Protocol Data Unit
(PDU). Communication between two peer layers is implemented
viaso-calledPDUs.heserepresentthecoreelementoftherulesetthatcanonly
be understood and correctly interpreted by the peer layers. A PDU consists of the
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