=OSI Model and Transmission Path=
 A.OSI 7-Layer Model
 B.Signals and Network Media
 C.Data Link Layer
 D.Bridges and Repeaters
 E.ARP and ARP tables
 F.Topologies and Access Methods
  Related Items
 A-1 What is the OSI model?
 A-2 OSI 7-Layer Model and the role of each layer
 A-3 Function modules and Header Additions
 A-4 Concepts of Data transmission
 A-5 Names and IP Addresses
 A-6 Comparison with TCP/IP
 A-7 Evaluation points of network equipment
 

 A-3 Function modules and Header Additions
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The following figure shows briefly the roles of function modules expressed in the OSI 7-layer model.


Figure A-3-1 Input and Output of Massage


The details of the above figure are as follows.

If the application layer of a source receives a message "Good morning. It is fine today.", the host processes this message in the following order and converts it into a bit string: 7. Application Layer --> 6. Presentation Layer --> 5. Session Layer --> 4. Transport Layer --> 3. Network Layer --> 2. Data Link Layer --> 1. Physical Layer. When this bit string arrives at a destination host connected to the network, it is processed in reverse order and converted into the message "Good morning. It is fine today." and the message is outputted.

In the OSI 7-layer model, the source and destination exchange with the function modules which realize each layer. The following figure shows the data unit on the moment.

Figure A-3-2 Names of data in Layer 1 to 4



For example, the data created by the transport layer of the source is called a segment. The transport layer of the destination also analyzes the segment.
The network layer, data link layer and physical layer also process likewise.


In the processing in the source of Figure A-3-1, a header is added whenever going through each function module.
A function module adds the unique data when it is given a data from the upper function module. At this time, it is not involved in the content of the data given from the upper layer. (It does not change the processing according to the content.)

The following figure shows the aspect of adding headers.

Figure A-3-3 Header additions



    


The data which the transport layer is given from the session layer is placed on the transport layer, added TH (TransportHeader -- TCP Header, UDP Header) and passed to the network layer.
The data on the moment is called a segment.
The data which the network layer is given from the transport layer is placed on the network layer, added NH (Network Header -- IP Header) and passed to the data link layer.
The data on the moment is called a packet.
The data which the data link layer is given from the network layer is placed on the data link layer, added FH (Frame Header) and FT (Frame Tailer) and passed to the physical layer.
The data on the moment is called a frame.

The important point here is that each function module merely handles the data given from the upper module independently of the content, as shown above.
The network layer is not concerned about whether the data given from the transport layer is a Web data, FTP data or mail data. It merely handles the data as "the data from the transport layer". It sometimes handles it by dividing into some data, but does not "handle it as the one data because of one mail". Moreover, the upper layer does not also request, adding such a condition.


The following figure shows the aspect when the data arrives at the destination.

Figure A-3-4 Header deletions

As shown in the above figure, the headers added in the source are deleted and analyzed by the function module corresponding to each header, in the destination. Then, the data is passed to the upper layer.
The important point to understand the foundation of communication is as follows:
While the source adds a header each time the data goes through a function module, the destination processes the data, removing headers added to the data.