In this article, you'll find a comprehensive overview of the key network security tools that are essential to ensuring your information infrastructure is protected in 2024.
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In this article, you will learn about the OSI network model, which is the foundation for understanding the structure and operation of computer networks. We will take a detailed look at each of the seven layers of the OSI model, explain their functions and role in the data transfer process. You will gain an understanding of how different network devices and protocols interact within this model, ensuring reliable and efficient communication between computer systems.
OSI stands for Open System Interconnection. In Ukrainian it sounds like this: Network model of interaction of open systems (reference model). This model can be called a standard. It is this model that manufacturers of network devices follow when developing new products. The OSI network model consists of 7 levels, and it is customary to start counting from the bottom.
Let’s list them:
Application layer (application layer)
Presentation layer or presentation layer
Session layer (session layer)
Transport layer (transport layer)
Network layer (network layer)
Channel layer (data link layer)
Physical layer (physical layer)
Presentation layer – it converts data into the appropriate format. For example, the image you see on your screen (all images) is sent as small pieces (bits) of 1’s and 0’s when you send a file. So if you send a photo to a friend by email, the SMTP application layer protocol sends the photo to a lower layer – presentation level.
There, the photo is converted into a data format convenient for the lower level, for example, into bits (1s and 0s). That way, when your friend starts receiving your photo, it’s sent to them in the same format of 1s and 0s. It is the representation layer that transforms the bits into a full-fledged photo, for example, a JPEG. This layer works in this way with such protocols (standards) as images (JPEG, GIF, PNG, TIFF), encoding (ASCII, EBDIC), music and video (MPEG).
Transport layer (transport layer) – this layer ensures the reliability of transmission from the sender to the recipient. In fact, everything is very simple, for example, you communicate using a webcam with your friend or teacher. Is there a need for reliable delivery of every bit of the transmitted image? Of course not, if a few bits are lost from the streaming video, you won’t even notice it, even the picture won’t change (for example, the color of one pixel out of 900,000 pixels will change, flashing at 24 frames per second).
Now let’s give an example: a friend sends you (for example, by mail) important information or a program in the archive. You download this archive to your computer. This is where 100% reliability is necessary, because if a couple of bits are lost when uploading the archive, you can unarchive it, i.e. extract the necessary data. Or imagine sending a password to a server, and one bit is lost on the way – the password will already lose its appearance and the value will change.
Thus, when we watch videos on the Internet, sometimes we see some artifacts, delays, noises, etc. And when we read text from a web page, the loss (or distortion) of letters is not acceptable, and when we download programs, everything also goes without errors.
At this level, we distinguish two protocols: UDP and TCP. The UDP protocol (User Datagram Protocol) transmits data without establishing a connection, does not confirm the delivery of data and repeats. The TCP protocol (Transmission Control Protocol), which establishes a connection before transmission, confirms the delivery of data, repeats it if necessary, guarantees the integrity and correct sequence of the data being downloaded.
So, for music, videos, video conferences and calls, we use UDP (transmit data without verification and without delays), and for text, programs, passwords, archives, etc. – TCP (acknowledgment data transfer, takes longer).
Network layer (network layer) – this layer determines the path by which data will be transmitted. And, by the way, this is the third level of the OSI Network Model, and there are such devices that are called third-level devices – routers. We’ve all heard of an IP address, that’s what the IP (Internet Protocol) does.
An IP address is a logical address on a network. There are quite a lot of protocols at this level, and we will discuss all these protocols in more detail later, in separate articles and examples. Now I will list only a few popular ones. Just as everyone has heard of the IP address and the ping command, the ICMP protocol works. The same routers (with which we will work in the future) use protocols of this level for routing packets (RIP, EIGRP, OSPF).
The IEEE (Institute of Electrical and Electronics Engineers) defines the channel layer as two sublayers: LLC and MAC.
LLC – logical link control (Logical Link Control), created for interaction with the upper layer.
MAC – media access control (Media Access Control), designed to interact with the lower layer.
Let’s explain with an example: your computer (laptop, communicator) has a network card (or some other adapter), so there is a driver for interaction with it (with the card). A driver is some program – the upper sub-level of the channel level, through which you can communicate with the lower levels, more precisely with the microprocessor (iron) – the lower sub-level of the channel level.
The TCP/IP model is commonly referred to as the DOD (Department of Defense) model.
The DOD (or TCP/IP) model differs from the OSI network model in the number of layers.
Application level
Transport level (Transport)
Network layer (Internet)
Network access level (Network Access)
Surprisingly, the four DOD layers are enough to cover the seven layers of the OSI model. Although there are certain differences. In general, if we do not go into details, the application layer of the DOD model corresponds to the three upper layers of the OSI model (application, presentation, session), the transport layer corresponds to the transport layer, and the network layer to the network, respectively, and the network access layer corresponds to the two lower layers of the OSI model ( data link, physical).
And if we go into details, we need to start with the fact that the TCP/IP network model was developed much earlier than the OSI model. The TCP/IP model was already formed on the basis of existing protocols, but OSI, on the contrary, first created a model, and then protocols for it, each of which has its own pros and cons. OSI is a more modern model, so it is discussed more often on the Internet, but the protocols used are the TCP/IP stack (groups, stacks (one on top of another)) based on the DOD model.
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