OSI MODEL (last three layers)

OSI Reference Model and Cisco Three Layer Model

Each OSI layer contain a set of functions performed by programs to enable data to travel from a source to a destination on a network.

In this article I will provide brief descriptions of each layer in the OSI reference model.

Application Layer

The application layer is the OSI layer that is closest to the user. This layer provides network services to the user's applications. It differs from the other layers in that it does not provide services to any other OSI layer, but only to applications outside the OSI reference model. Applications layer provide a platform to access the data of remote computer.

The application layer protocols that you should know are as follows:

·         SNMP (Simple Network Management Protocol)— Communicates status and allows control of networked devices.

·         TFTP (Trivial File Transfer Protocol)— Simple, lightweight file transfer.

·         DNS (Domain Naming System)— Translates a website name (easy for people) to an IP address (easy for computers).

·         DHCP (Dynamic Host Configuration Protocol)— Assigns IP, mask, and DNS server (plus a bunch of other stuff) to hosts.

·         Telnet— Provides a remote terminal connection to manage devices to which you are not close enough to use a console cable.

·         HTTP (Hypertext Transfer Protocol)— Browses web pages.

·         FTP (File Transfer Protocol)— Reliably sends/retrieves all file types.

·         SMTP (Simple Mail Transfer Protocol)— Sends email.

·         POP3 (Post Office Protocol v.3)— Retrieves email.

·         NTP (Network Time Protocol)— Synchronizes networked device clocks.

presentation layer

The presentation layer is responsible for formatting data so that application-layer protocols (and then the users) can recognize and work with it. Presentation layer format the file extensions—such as .doc, .jpg, .txt, .avi, and so on. you realize that each of these file types is formatted for use by a particular type of application. The presentation layer taking the application layer data and marking it with the formatting codes so that it can be viewed reliably when accessed later. If necessary, the presentation layer might be able to translate between multiple data formats by using a common format.

The Session Layer

The session layer establishes, manages, and terminates sessions between two communicating hosts. It provides its services to the presentation layer. The session layer also synchronizes dialogue between the presentation layers of the two hosts and manages their data exchange. For example, web servers have many users, so many communication processes are open at a given time. Therefore, keeping track of which user communicates on which path is important.