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8.2 Basics - ISO/OSI reference model
The ISO/OSI reference model is based on a proposal that was developed by the Interna-
tional Standards Organization (ISO). This represents the first step towards an interna-
tional standard for the different protocols. It is referred to as the ISO-OSI layer model. OSI
is the abbreviation for O pen S ystem I nterconnection, the communication between open
systems. The ISO/OSI reference model does not represent a network architecture as it
does not define the services and protocols used by the different layers. The model simply
specifies the tasks that the different layers must perform. All current communication sys-
tems are based on the ISO/OSI reference model, which is defined by the ISO 7498
standard. The reference model structures communication systems into 7 layers that cover
different communication tasks. In this manner the complexity of the communication
between different systems is divided amongst different layers to simplify the task.
The following layers have been defined:
n Layer 7 - Application Layer
n Layer 6 - Presentation Layer
n Layer 5 - Session Layer
n Layer 4 - Transport Layer
n Layer 3 - Network Layer
n Layer 2 - Data Link Layer
n Layer 1- Physical Layer
Depending on the complexity and the requirements of the communication mechanisms a
communication system may use a subset of these layers.
The bit communication layer (physical layer) is concerned with the transfer of data bits via
the communication channel. This layer is therefore responsible for the mechanical, elec-
trical and the procedural interfaces and the physical communication medium located
below the bit communication layer:
n Which voltage represents a logical 0 or a 1?
n The minimum time the voltage is present to be recognized as a bit.
n The pin assignment of the respective interface.
This layer performs error-checking functions for bit strings transferred between two com-
municating partners. This includes the recognition and correction or flagging of communi-
cation errors and flow control functions. The security layer (data link layer) converts raw
communication data into a sequence of frames. This is where frame limits are inserted on
the transmitting side and where the receiving side detects them. These limits consist of
special bit patterns that are inserted at the beginning and at the end of every frame. The
security layer often also incorporates flow control and error detection functions. The data
security layer is divided into two sub-levels, the LLC and the MAC level. The MAC ( M edia
A ccess C ontrol) is the lower level and controls how senders are sharing a single transmit
channel. The LLC ( L ogical L ink C ontrol) is the upper level that establishes the connection
for transferring the data frames from one device into the other.
The network layer is an agency layer. Business of this layer is to control the exchange of
binary data between stations that are not directly connected. It is responsible for the log-
ical connections of layer 2 communications. Layer 3 supports the identification of the
single network addresses and the establishing and disconnecting of logical communica-
tion channels. Additionally, layer 3 manages the prior transfer of data and the error pro-
cessing of data packets. IP (Internet Protocol) is based on Layer 3.
Layer 4 connects the network structures with the structures of the higher levels by
dividing the messages of higher layers into segments and passes them on to the network
layer. Hereby, the transport layer converts the transport addresses into network
addresses. Common transport protocols are: TCP, SPX, NWLink and NetBEUI.
Overview
Layer 1 - Bit communica-
tion layer (physical layer)
Layer 2 - Security layer
(data link layer)
Layer 3 - Network layer
Layer 4 - Transport layer
System 300S +
Deployment Ethernet communication - productive
Basics - ISO/OSI reference model
HB140 | CPU | 315-4PN23 | en | 18-02 101