315-4PN33

7.2 Basics - ISO/OSI reference model
The ISO/OSI reference model is based on a proposal that was devel-
oped by the International Standards Organization (ISO). This repre-
sents the first step towards an international standard for the different
protocols. It is referred to as the ISO-OSI layer model. OSI is the
abbreviation for Open System Interconnection, the communication
between open systems. The ISO/OSI reference model does not rep-
resent 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 communica-
tion systems 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 communica-
tion tasks. In this manner the complexity of the communication
between different systems is divided amongst different layers to sim-
plify 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 - Physical Layer
Depending on the complexity and the requirements of the communi-
cation 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, electrical and the proce-
dural 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 communicating partners. This includes the recognition
and correction or flagging of communication errors and flow control
functions. The security layer (data link layer) converts raw communi-
cation 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 (Media Access Control) is the lower level and con-
trols how senders are sharing a single transmit channel. The LLC
(Logical Link Control) is the upper level that establishes the connec-
tion for transferring the data frames from one device into the other.
Overview
Layer 1 - Bit communi-
cation layer (physical
layer)
Layer 2 - Security layer
(data link layer)
VIPA System 300SDeployment Ethernet communication - productive
Basics - ISO/OSI reference model
HB140 | CPU | 315-4PN33 | GB | 14-43 108