The OSI (Open System Interconnection) model describes the interaction of networking equipment. In other words, devices such as PCs with network cards, switches, and routers communicate through it. The communications between computing systems are split into seven different abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application. In this article, we will describe all the OSI layers.
Physical
The lowest layer of the OSI model, where information transmission is performed directly. Here signals (radio signals, electrical, optical) go from the sender to the receiver. The current layer deals with cables, radio ether, coding zeros, and ones, etc. A first layer signal is a group of voltages of different amplitudes, waves, or radio frequencies.
One of the main standards among physical layer technologies is Ethernet.
Data Link Layer
After receiving a signal from the previous physical layer, the next (data link) layer verifies and corrects transmission errors. Here the terms "frame" and "MAC address" appear. MAC addresses take 48 bits in hexadecimal format and can be written, for example, such as this: 00:26:57:00:1f:02.
The second layer is more complex than the previous, physical. It involves the following sub-layers of control:
LLC (logical link)
MAC (medium access).
The link layer devices include bridges and switches.
Network layer
This layer is above the data link layer. The concepts of routing and IP address are introduced at this stage. The ARP protocol is used to transform MAC addresses into IP addresses.
The traffic routing is carried out here. For example, when the user wants to go to the website and enter his address, the DNS query is sent. The response is an IP address, which is included in the packet. Data packet is a new term that appears at layer 3 of the network.
The network layer device: router.
Transport layer
This is where the information is delivered through an external network. Data blocks in this case are divided into separate fragments. Their size depends on the protocol used - TCP or UDP. Which one is better to use depends on the type of data to be transmitted.
TCP (Transmission Control Protocol) – a protocol suitable for transferring traffic, when any loss of packets is sensitive. The transmission is controlled so that lost packets will be detected and retrieved. UDP (User Datagram Protocol) is used when the loss of a few packets is not crucial. For example, when transmitting video, images.
Session Layer
The session layer provides communication between the applications on the computers. It is involved in session creation and termination, data exchange, synchronization, and other processes.
The session layer protocols include X.225, ISO 8327, SMPP (through which SMS messages are sent), PAP.
Presentation layer
The sixth layer is the conversion of data formats, such as compression and encryption.
Application Layer
The top level of the model, containing the network services with which users interact directly. This layer describes the relationship between the applications on the PC and the external network. This includes protocols for browsing the Internet (HTTPS, HTTP), for working with mail services (SMTP, POP3), for transferring files (FTP, TFTP), and others.
When a transfer is made from the top to the bottom, it is called data encapsulation, and in reversed order - decapsulation.
