THE OSI MODEL
The seven layers of the OSI model
The purpose of this Reference Model of Open Systems Interconnection is to provide a common basis for the coordination of standards development for the purpose of systems interconnection while allowing existing standards to be placed into perspective within the overall Reference Model.
The term Open Systems Interconnection (OSI) qualifies standards for the exchange of information among systems that are “open” to one another for this purpose by virtue of their mutual use of the applicable standards.
Cooperation among open systems involves a broad range of activities of which the following have been identified:
a) interprocess communication, which concerns the exchange of information and the synchronization of activity between OSI application processes;
b) data representation, which concerns all aspects of the creation and maintenance of data descriptions and data transformations for reformatting data exchanged between open systems;
c) data storage, which concerns storage media, and file and database systems for managing and providing access to data stored on the media;
d) process and resource management, which concerns the means by which OSI application processes are declared, initiated and controlled, and the means by which they acquire OSI resources;
e) integrity and security, which concern information processing constraints that have to be preserved or assured during the operation of the open systems; and
f) program support, which concerns the definition, compilation, linking, testing, storage, transfer, and access to the programs executed by OSI application processes.
The Open System Interconnection model (OSI) is a seven-layer model used to visualize computer networks.
Each of the seven layers goes up in increments of one as it gets closer to the human user. Layer one — the application layer, is closest to the person while layer seven — the physical layer is where the network receives and transmits raw data.
The OSI model belongs to the International Organization for Standards (ISO) and is maintained by the identification ISO/IEC 7498–1.
Layer 7 — Application
The application layer is where the user inputs data and data is output to the user. The application layer is usually software that is run off the local machine, however, this depends on the network architecture. The software could be cloud-based, meaning it runs off a server in a remote location and data is transferred over the internet, or the software could be run on a local server. The application layer provides services for email, Telnet, and File Transfer for example. An example of the application layer is an internet browser, an FTP client, or even Microsoft Word.
The application layer identifies communication components, determines resource availability, and ensures that communication runs smoothly. This layer is what allows access to network resources, so you'll likely recognize its most common protocols:
- Hypertext Transfer Protocol (HTTP)
- File Transfer Protocol (FTP)
- Simple Mail Transfer Protocol (SMTP)
Layer 6 — Presentation
The presentation layer is where the operating system lies. This operating system could be Windows, OS X, a Unix-based operating system, or one of the many others available. Where the human user interacts with the application layer described above, the application layer interacts with the presentation layer. This may be done directly or through a runtime environment such as the Java Runtime Environment (JRE).
Layer 5 — Session
The session layer is responsible for creating and maintaining sessions between the operating system on the presentation layer and other, third-party machines. For example, when a user is browsing the internet, they are interacting with the application layer, the application layer is interacting with the presentation layer and the session layer allows the operating system to interact with the web server.
Layer 4 — Transport
The transport layer is responsible for the logistics of the session. In the example used above, the transport layer would be responsible for determining what and how much information is transferred between the operating system and the web server.
Layer 3 — Network
The network layer is where routers operate. A router is a hardware device that forwards packets of information between computers on a network. This is where your IP address comes from and in the example used above, the router is responsible for sending packets of information out into the internet and receiving them. These packets' origin and destination are determined by the IP address of your router.
Layer 2 — Datalink
The data link layer is where switches operate and provide a reliable link between two directly connected nodes. The data link layer is also responsible for detecting and possibly fixing any packet errors that may form on the physical layer. The data link layer is divided into two separate layers, the Media Access Control (MAC) and Logical Link Control (LLC) layers. The MAC layer is responsible for controlling how devices connected to the network gain access. The LLC layer controls error checking, possible fixes, and packet synchronization.
Layer 1 — Physical
The physical layer is literally the physical hardware that makes up the network. This layer has several major functions:
- Defining physical specifications
- Defining protocols
- Defining transmission mode (half duplex & full duplex)
- Defining the network’s topology
Hardware such as the physical components of Ethernet cables and Bluetooth are just some examples of the physical layer.