=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-4 Concepts of Data transmission

In the function of data transfer, the point that the understanding of layer models (such as OSI 7-layer model) is difficult is that functions and roles of the data link layer (layer 2), network layer (layer 3) and transport layer (layer 4) overlap. In particular, when comparing the overlapper roules of the network layer (layer 3) and transport layer (layer 4) with the roules of the data link layer (layer 2), both are involeved in route decision, data transmission, quality guarantee and so on.

This is shown in the following figure.
In the figure, the name of each layer is abbreviated as shown in Table A-4-1.

Table A-4-1 The meaning of abbreviations of Figure A-4-1
  4. Transport Layer     L4  
  3. Network Layer    L3 
  2. Data Link Layer    L2 

Figure A-4-1 The aspect of data transmission

The following table briefly sums up the above figure.

Table A-4-2 Summary of roles in data transmission
  4. Transport Layer It transmits data between the source and destination.
  3. Network Layer It decides the path across the network.
  2. Data Link Layer It decides the path and transfer the data between adjacent nodes.

The data link layer (L2) realizes the route decision and data transmission to an adjacent node. However, when connecting across the network, the data has to be passed through some nodes. In this case, the data cannot know what course to go through with only the adjacent node. Therefore, the network layer (L3) indicates the path of nodes that the data should go through (only indicates the direction), and the transport layer (L4) actually transmits the data.

The following figure compares the above to a mail delivery.

Figure A-4-2 Comparison of the data transmission

The letter is posted from home, picked up from the post by a postal worker, transported to the destination office, delivered from the office by a deliveryman and delivered to the destination mailbox.
In this case, a person posting the letter responsibly transports it from his/her home to the post. That is the data link layer (L2). After that,
each transportation between adjacent nodes (such as from the post to the local office, and from the originating office to the destination office) is realized by L2.

When an address is not written in the letter, it is not delivered even if a person responsibly posts it and a postal worker picks it up. The network layer (L3) corresponds to the address to be delivered. This layer realizes the function of deciding the delivery path by the address.

Thereby, the letter is delivered without any trouble.
The postal worker does not check whether the content of the letter is correct. Therefore, the content has to be checked by calling the partner and so on. (Actually, the partner checks it independently, and writes back when there is any question or failure.) The letter has to be sent again if the content is incorrect, and the addition has to be sent if there is any failure.
For example, if you have a claim or question about a commodity you purchased by mail, it is meaningless to tell a post office or postal worker about it. You have to tell the sender about it.
The transport layer (L4) plays such a role.