OSI Layers

1-Application Layer

It provides a real interface between the computer application and the network. This layer
the one closest to the user. Only this layer does not provide services to other layers.
The application layer is the program that will use the network service. This layer meets the user’s requirements. For example, a database application or e-mail application, etc. as.

2-Presentation Layer

In the presentation layer, the data is translated. Presentation Layer Sends data to the application layer. In this layer, arrangements are made about the structure and form of the data.
The format of the data in the presentation layer is determined. In addition, the data is encrypted and opened in this layer. In this layer, compression of the data is done.

3-Session Layer

In the session layer, the communication link between the two points is established, started, and terminated.
It is terminated. Session Layer represents the session between applications. The session layer communicates the data to be sent to the presentation layer.
In Cisco applications, the following protocols are called session layer protocols. Network File System (NFS): Used on TCP / IP and Unix workstations.
Remote Procedure Call (RPC): Client / Server routing protocol.
Apple Talk Session Protocol (ASP): Used to communicate between Apple Talk clients and server computers.

4-Transport Layer

Provides data transfer between two devices (computers). To ensure reliability in data transmission, one device organizes a connection session. Then both sides of the preparedness for transmission are checked and data transfer is started.
The preparation process of this communication session is called a connection-oriented session.
When the two devices exchange data between them, they control each other regularly.
The control of the sent data is done by the acknowledgment process. Waiting for approval
The amount of data sent is called a window. Windowing controls how much information is transferred from one device to another.

5-Connection-Oriented Protocols

Connection-based flow control for data transmission, error checking protocols
. Connection-based protocols must establish a connection before data is sent:
• Connected
• Data is transferred
• Connection terminated
Unbounded protocols do not define a path. Packages are called “datagram”.
IPX and IP protocols are a connectionless protocol.
Although connection-based protocols seem safer; connection-based
the network stops when there is a problem with the services. Unconnected services do not prevent the data from being transmitted.

6-Network Layer

The main task of the network layer is routing. Routing enables packets to be sent to other networks outside the local network. In the network layer, data is transmitted in the shortest way between two stations.
Thanks to this layer, data is routed through routers. Network stage
messages are addressed and logical addresses are converted to physical addresses. At this stage
Network traffic, routing operations are also done.
When a packet is received by the Router, the destination IP address is checked.
Two types of packages are used in the network layer:
• Ver I packages
• Routing packages.
Data packets are used to move the user’s information. These packages are like IP and IPX
transported by routable protocols.
Routing packets are used to update routers in the network. Protocols such as RIP, EIGRP and OSPF perform this function. Made to update routers
the main process is to update the routing tables found on each router.

7-Data Link Layer

In the Data Link layer; how to transmit the data on the electronic media provided in a sub-stage, or how the data will be placed on this media. The data in this layer is arranged as data frames.
This layer works with access methods known as Ethernet or Token Ring. These access methods process data in accordance with their own protocols. In the data line layer, data is sent from the network layer to the physical layer. At this stage, the data is divided into certain parts. These parts are called packages or frames.
The frames are packages that send the data in a certain control.