History
At the dawn of the global network, more specifically in the 1970s, the development of networking technologies and protocols was handled by government institutions and corporations that used their own proprietary standards. In other words, it was a complete mess. Fortunately, it was quickly understood that this was not sustainable, and ways were needed to “connect” incompatible protocols at different levels. Starting in 1977, the International Organization for Standardization (ISO) began a campaign to develop common standards for network interaction.
The OSI model (the Open Systems Interconnection model) was first presented in an initial form in Washington, D.C., in 1978 by Hubert Zimmermann, and the draft standard was published by ISO in 1980.
Model structure
The concept of the seven-layer model describes the following interaction layers:
- Physical (Physical);
- Data Link (Data Link);
- Network (Network);
- Transport (Transport);
- Session (Session);
- Presentation (Presentation);
- Application (Application).
For memorization
- A Penguin Said that Nobody Drinks Pepsi
Layers
7. Application layer
The Application layer is the top layer of the model and provides interaction between user applications and the network.
- Examples: HTTP, telnet, FTP.
6. Presentation layer
The Presentation layer provides protocol conversion and data encoding/decoding.
This is where it becomes more interesting. For example, we have a request from an application at the application layer, and it needs to be transmitted further through the network. What should be done? This is where the presentation layer helps. It usually acts as an intermediate protocol for converting information from neighboring layers. At this layer, requests are converted into a format suitable for network transmission, while data received from the network is converted into a format suitable for applications. This allows applications on heterogeneous computer systems to exchange data transparently. The presentation layer provides formatting and code conversion to ensure that the application receives information for processing in a form that makes sense to it. Presentation layer standards also define ways of representing graphical images.
- Examples: ASCII, TIFF, JPEG, GIF, ESBCDIC, PICT, MPEG, MIDI.
5. Session layer
The Session layer maintains communication sessions, allowing applications to interact with each other for long periods of time. The layer manages session creation and termination, information exchange, task synchronization, determining the right to transmit data, and maintaining the session during periods of application inactivity.
- Examples: NetBIOS, RPC, SQL.
4. Transport layer
The Transport layer is intended to provide reliable data transfer from sender to recipient. The level of reliability can vary widely. There are many classes of transport-layer protocols, ranging from protocols that provide only basic transport functions (for example, data transfer without acknowledgment of receipt) to protocols that guarantee delivery of several data packets to the destination in the proper sequence.
UDP is limited to checking data integrity within a single datagram and does not eliminate the possibility of packet loss or duplication.
TCP provides reliable continuous data transfer, eliminating data loss, order disruption, or duplication.
3. Network layer
The Network layer is intended to determine the path for data transmission. It is responsible for translating logical addresses and names into physical ones, determining shortest routes, switching, and routing. Network-layer protocols route data from source to destination.
- Examples: IPv4, IPv6.
2. Data Link layer
The Data Link layer is intended to provide network interaction at the physical level and control errors that may occur. It packages bits received from the physical layer into frames, checks them for integrity, and, if necessary, corrects errors (forms a retransmission request for a damaged frame), then sends them to the network layer.
- Examples: 802.11, ARP, Ethernet, VLAN.
1. Physical layer
The Physical layer is the lowest layer of the model. It defines the method for transmitting data represented in binary form from one device to another. It transmits electrical or optical signals through a cable or radio channel, receives them, and converts them into data bits according to digital signal encoding methods. Hubs, repeaters, and media converters also operate at this layer.
The physical layer defines transmission media such as fiber optic cable, twisted pair, coaxial cable, radio channel, and so on.
- Examples: RS-232, RS-485, RJ-45, WiFi.
