=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-1 OSI Model
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The OSI (Open Systems Interconnection) 7-Layer Model, which was established by International Standard Organization (ISO), is the design policy of network structure to realise data communications between different terminals. In this model, the substances of network equipments are not described. It divides network functions into 7 layers and defines standard function modules (data handling) in each layer. It is also called "OSI Basic Reference Model" or "OSI layer model".
The use of this model has the following advantages.


Simplification of processing in networks

Various element technologies such as route decision, data guarantee and data transfer are needed for the network technology (the technology to assure data transmission). When trying to realise them as one function, the system will be complex and sizable. However, by designing route decision, data guarantee and data transfer as separate function modules, simplification is possible.


Standardization bridges the gap between makers

Various functions are needed to transfer data. However, when each maker defines and implements these functions individually, it is highly likely that the system has to be configured using equipments of the same maker (for example, LAN). Therefore, functions are separated into layers and standardized to make multiple equipments available in a network. The gap between makers adopting this layer model can be minimized.


Improve development process

When route decision, data guarantee and data transfer are executed as one module, it is highly likely that the action to improve the route decision function adversely affects other functions. Moreover, it is difficult to develop concurrently. However, the development process is improved by designing the functions in modules and separating with the high individual independence.


Because of the above advantages, all the engineers engaging in networks have to learn the OSI 7-layer model. Any aspects such as server construction, network application development, security configuration and network design have to be considered based on the OSI 7-layer model.
Concretely, there are the term unifications, the creation of design drawing and the sharing of knowledge when establishing concepts, among the engineers engaging in networks, but they are not the inflexible definitions which can be shared completely. Because the interpretations depend on the maker or developer, gaps can be generated.

Even if there are network equipments for the same purpose, the specifications and descriptions are a little different between makers or within the same maker depending on where one stands, in many cases. Therefore, not only the correct design of a network but also the performance evaluation of each network equipment cannot be understood. Moreover, even if it can be temporarily designed or evaluated, it is difficult to reuse the results well when the persons in charge are changed. However, the design and evaluation on the basis of the OSI 7-layer model have high values.

The following six points will be shown in this chapter:

 ・ OSI 7-Layer Model and the role of each layer
 ・ Function modules and Header Additions
 ・ Concepts of Data transmission
 ・ Names and IP Addresses
 ・ Comparison with TCP/IP
 ・ Evaluation points of network equipments