Industrial Data Communications

Front 1

OSI principles:

Back 1

• A layer should be created where a different level of abstraction is required.
• Each layer should perform a well-defined function.
• The function of each layer should be chosen with thought given to defining internationally standardized protocols.
• The layer boundaries should be chosen to minimize the information flow across the boundaries.
• The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity and small enough that the architecture does not become unwieldy.

Front 2

7 Layers of OSI Model:

Back 2

Aplication
Presentation
Session
Transport
Network
Data Link
Physical

Front 3

PDU

Back 3

The system then passes the data physically down through the seven layers, adding headers (and possibly trailers), and
invoking functions in accordance with the rules of the protocol.
At each level, this combined data and header ‘packet’ is termed as protocol data unit or PDU.

Front 4

The effects of the addition of headers (protocol control information) to the respective PDUs:

Back 4

The net effect of this extra information is to reduce the overall bandwidth of the communications channel, since some of the available bandwidth is used to pass control information.

Front 5

OSI • Application (layer 7):

Back 5

=> is the topmost layer in the OSI model.
=> Responsible for giving application access to the network.

Examples of application layer tasks:
= File transfer.
= Eletronic Mail Services.
= Network Management.

Note: the user’s actual application programs do not reside here.

Front 6

OSI • Presentation (layer 6):

Back 6

The presentation layer provides services for the application layer above it and uses the session layer below it. In practice, the presentation layer rarely appears in pure form and is the least well-defined of the OSI layers.

Maps the data representations into an external data format that will enable correct interpretation of the information on receipt. The mapping can also possibly include encryption and/or compression of data.

Front 7

OSI • Session (layer 5):

Back 7

=> Responsible for synchronizing and sequencing the dialogue and packets in a network connection.
=> Responsible for making sure that the connection is maintained until the transmission is complete and ensuring that appropriate security measures are taken during a ‘session’ (i.e. a connection).


Control of the communications between the users. This includes the grouping together of messages and the coordination of data transfer between grouped layers. It also affects checkpoints for (transparent) recovery of aborted sessions.

Front 8

OSI • Transport (layer 4):

Back 8

The management of the communications between the two end systems.
The transport layer provides services for the session layer above it and uses the network layer below it to find a route between source and destination.
The layers below the transport layer are collectively known as the subnet layers.

Front 9

OSI• Network (layer 3):

Back 9

Responsible for the control of the communications network. Functions include routing of data, network addressing, fragmentation f large packets, congestion, and flow control.

Front 10

OSI • Data link (layer 2):

Back 10

Responsible for sending a frame of data from one system to another. Attempts to ensure that errors in the received bit stream are not passed up into the rest of the protocol stack. Error correction and detection techniques are used here.

Front 11

OSI • Physical (layer 1):

Back 11

Defines the electrical and mechanical connections at the physical level or the communication channel itself. Functional responsibilities include modulation, multiplexing, and signal generation. Note that the physical layer defines, but does not include the medium. This is located below the physical layer and is sometimes referred to as layer 0.

Front 12

OSI • Physical (layer 1):

Back 12

Defines the electrical and mechanical connections at the physical level or the communication channel itself. Functional responsibilities include modulation, multiplexing, and signal generation. Note that the physical layer defines, but does not include the medium. This is located below the physical layer and is sometimes referred to as layer 0.