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

1-4 Industrial Communication Systems
following sections will show that this is still an insufficient requirement for connecting devices. This can
only be achieved by profiles, which have been introduced as an additional layer on top of the ISO/OSI
model during the development of field buses (virtually an additional layer 8). If it can be proven that
devices cooperate, we have reached interoperability.
1.2.1 Layer Functionalities
The OSI model defines seven layers, each of which has dedicated functions. A brief description of these
functions is given in the following.
1.2.1.1 Layer 1: Physical Layer
This layer covers the physical part of the communication. It contains all hardware specification data,
including the signals used, the electrical and mechanical characteristics of the connection, and all functional
parameters that are necessary, which include tasks like activating, maintaining, and terminating
the physical connection.
1.2.1.2 Layer 2: Link Layer
The link layer is responsible for providing an error-free connection from one node to another node in
the same network segment (point-to-point communication). It has to correct errors that occur during
the physical transmission by using, for example, error-correction codes. For that, it needs error-correction
algorithms and redundant information in the received data. It also adds source and destination address
to the packets that are transmitted.
1.2.1.3 Layer 3: Network Layer
The network layer defines the path that packets take on their way through the network. A packet that is
addressed to a destination address will not always be transmitted directly to its receiver but will rather
be passed from one part of the network to the other until it reaches its destination. This is done by routing
the packets, an algorithm that can be implemented in different ways, depending on the capabilities
of the components. Layer 3 defines addresses, which are not related to the addresses on layer 2 (if they
are implemented). The network layer also is responsible for establishing and terminating network
connections and reestablishing broken network connections.
1.2.1.4 Layer 4: Transport Layer
The transport layer is responsible for the flow control of data that is sent from one end user to the other
(end-to-end connection) and for assigning logical addresses to the physical addresses that are used by
the network layer. It uses the network layers’ ability to establish network connections in order to guarantee
that messages really reach their end users, which also includes retransmission of lost packets.
1.2.1.5 Layer 5: Session Layer
In order to establish a session, the session layer has to make sure that all the end users agree on the same
session protocol; therefore, the participants first have to negotiate a common protocol, which is then
used throughout the session. The session layer defines how a session is started and terminated, describes
how data-exchange is established, and is responsible for end-user identification (e.g., by password).
1.2.1.6 Layer 6: Presentation Layer
The presentation layer defines how the information shall be formatted in order to make it understandable
for the end user. If, for example, an integer number is transmitted, the presentation layer knows
how to interpret the bytes that make up the number and is able to provide a mathematical value to the
application layer (e.g., by first converting big endian to little endian). Conversion of data is covered here
as well as optional encryption of information.
© 2011 by Taylor and Francis Group, LLC