=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-2 OSI 7-Layer Model and the role of each layer

It can be said that the OSI 7-layer model is the foundation of networks. To arrange and manage network nodes (generic name of all the equipments connected to networks such as personal computers, workstations and dedicated equipments) appropriately, the basic concept has to be understood. Moreover, it is also needed to design and construct networks, create the specifications when implementing servers and harmonize with venders.

The following figure is the structure of the OSI 7-layer model.

Figure A-2-1 OSI 7-layer model and function modules

The following table shows more properly the functions and roles.

Table A-2-1 OSI 7-layer model and roles of functions modules
Layer Name Functions / Roles
7 Application Layer It provides interfaces to use networks for software (server/client application) which uses networks (data communications). It defines the processes that the users directly interact with, such as telnet and ftp.
6 Presentation Layer It converts the data gotten from layer 5 into user-friendly formats and the data sent from layer 7 into formats suitable for communication. In short, it converts the formats of data gotten from layer 7.
It defines the data expression such as compression and character code.
5 Session Layer Based on the specification of each application which uses networks, it establishes sessions on communications. It defines connections, terminations and so on between computers.
4 Transport Layer It provides basic data transfer services, ensuring the reliability of data transfer. It provides mechanisms for the establishment, maintenance, and termination of virtual circuits, transport fault detection and recovery, and information flow control.
While the session layer is based on requests from applications, all the functions of the transport layer are provided for all data in common.
3 Network Layer It decides appropriate communication paths between two hosts (end systems) which communicate on networks, based on IP Addresses.
It only decides communication paths and is not involved in the quality of communications.
2 Data Link Layer It defines communication methods between computers connected to networks directly.
It realizes the specification of hosts to be communicated and reliable data transfer, based on MAC Addresses (physical addresses). In other words, It realizes the reliable transmission between two points based on MAC addresses.
The difference from layer 3 is that while the network layer decides paths including them between networks, the data link layer acquires the communication between adjacent nodes (exactly, broadcast segments and broadcast domains) within a network.
The difference from layer 4 is that the data link layer provides interfaces for applications (for example, provides services for the session layer) and realise the highly functional data transmission.
1 Physical Layer It is a protocol to decide physical signals and connection methods in networks. It defines function modules which define the maximum transfer rate, D/A-A/D conversion and maximum transfer (communication) distance to implement, cable materials and connector shapes. (Cables and connector are called network media.)

Terms of TCP/IP shown later and this OSI 7-layer model are provided as the important concepts.
As well as talking about mathematics without understanding four operation symbols, it means that not understanding (or memorizing) these functions has no foundation of network.
Advanced mathematics may be possible without rkeeping figures or four operation symbols in mind. However, excessive effort is taken because of no foundation and a big error may be noticed.
Protocols as computer terms mean the communication rules to communicate data through a network.