The OSI model is a basic reference model for the interaction of open systems. It is a system consisting of seven levels, each of which involves certain network protocols that provide data transfer at all levels of interaction.
General information
In order to facilitate understanding and easier to navigate in different directions of working with network protocols, a modular system adopted as a standard was created, making it much easier to localize the problem, knowing which of the network sections it is located on.
At each level of the OSI model, work is carried out with certain sets of protocols (stacks). They are clearly localized within each level, without going beyond its borders, while being connected in a clear and easy to read system.
So, how many layers are there in the OSI network model and what are they?
- Physical.
- Channel.
- Networked.
- Transport.
- Session.
- Executive.
- Applied.
The more complex the structure of a network device, the greater the number of opportunities it opens, while working simultaneously on more levels of the model. This also affects the speed of devices: the more levels are involved, the slower the work.
The interaction of the layers occurs through interfaces between two adjacent layers and through protocols within the same layer.
Physical level
The first level of the OSI network model is the data transmission medium. On it there is a data transfer as such. A bit is taken as a unit of load. The signal is transmitted via cable or wireless networks and the corresponding coding into information expressed by bits.
The protocols that are involved here: wire (twisted pair, optics, telephone cable and others), wireless data transmission media (for example, Bluetooth or Wi-Fi) and so on.
Also at this level are media converters, signal repeaters, hubs, as well as all mechanical and physical interfaces through which the system interacts.
Channel level
Here, information is transmitted in the form of data blocks called frames or frames; the link layer of the OSI network model implements their creation and transmission. It interacts, respectively, with the physical and network layers of OSI.
It is subdivided into two sublevels:
- LLC - manages the logical channel.
- MAC - work with access directly to the physical environment.
For ease of understanding, let us examine the following example.
There is a network adapter in the computer or laptop. So that it can work correctly, it uses software and drivers related to the upper sublevel - through them, interaction with the processor located on the lower sublevel is carried out.
The following protocols are used: PPP (direct connection of two PCs), FDDI (data transmission over a distance of less than two hundred kilometers), CDP (Cisco’s proprietary protocol used to detect and obtain information about neighboring network devices).
Network layer
This is the OSI model layer, which is responsible for the routes along which data is transmitted. Devices that operate at this stage are called routers. Data at this level is transmitted in packets. At the channel level, the device was determined using a physical address (MAC), and on the network, IP addresses begin to appear - the logical address of any network device, interface.
Consider the network layer features of the OSI model.
The main objective of this stage is to ensure data transfer between terminal devices.
For this purpose, a unique address is assigned for all these devices, encapsulation (supplying data with an appropriate header or labels, whereby the main unit of load - a packet) is created.
Next, the packet must be routed, namely delivered to the destination. The route can be set from a large number of intermediate points, which are called hop.
As soon as the packet reaches its destination, the decapsulation process takes place - the end node examines the received data to make sure that the packet is delivered to where it was needed and is passed to the next level.
Consider the list of OSI model network layer protocols. This is the previously mentioned IP, which is included in the TCP / IP stack, ICMP (responsible for the transfer of control and service data), IGMP (multicast data transmission, multicast), BGP (dynamic routing) and many others.
Transport level
Protocols of this level serve to ensure the reliability of the transfer of information from the sending device to the receiving one, and are directly responsible for the delivery of information.
The main task of the transport layer is to ensure that data packets were sent and received without errors, there were no losses, and the transmission sequence was observed.
This layer works with entire data blocks.
For example, you want to transfer a certain file by e-mail. In order for the correct information to reach the recipient, the exact structure and sequence of data transfer must be observed, because if at least one bit is lost when downloading a file, it will not be possible to open it.
There are two main protocols that work at this level: TCP and UDP.
UDP sends data without requesting a delivery response from the terminal device and does not retry if it fails. TCP, on the contrary, establishes a connection and requires a response on the delivery of data, if the information does not reach, it repeats the sending.
Session level
He is sessional. At this level of the OSI network model, installation and support of communication sessions between two terminal devices takes place. This level, like all subsequent ones, works directly with data.
For example, recall how videoconferences are held. In order for the communication session to be successful, appropriate codecs are needed that encrypt the signal, with the mandatory requirement of having them on both devices. If the codec is missing or damaged on one of the devices, the connection will not be established.
In addition, at the session level, protocols such as L2TP (tunneling protocol for supporting user virtual networks), PAP (sends user authentication data without encryption to the server and confirms their authenticity), and others, can be used.
Executive Level
Responsible for displaying data in the required format. The information is modified (for example, coding) in order for the data stream to be successfully transferred to the transport layer.
As an example, you can translate image forwarding by email. As a result of the SMTP protocol, the image is converted into a format that is easier to read at the lower levels, and for the user it is displayed in the familiar JPEG format.
Protocols of this level: image standards (GIF, BMP, PNG, JPG), encodings (ASCII, etc.), video and audio recordings (MPEG, MP3), etc.
Application level
The application layer, or application layer, is the highest level of the OSI model. It is distinguished by the largest variety of protocols and their functions.
There is no need to be responsible for building routes or guaranteeing data delivery. Each protocol performs its own specific task. Examples of protocols operating at this level include HTTP (it is responsible for transmitting hypertext, that is, it ultimately allows users to open web pages in a browser), FTP (network data transfer), SMTP (sending email) and others.
Protocol stacks
As already discussed above, there are a large number of network protocols that perform a wide variety of tasks. As a rule, most of them work in conjunction, performing their functions harmoniously, while simultaneously realizing their own functionality with each other.
Such bundles are called protocol stacks.
Based on the OSI network model, protocol stacks are conventionally divided into three groups:
- Applied (correspond to this level of OSI and are directly responsible for the exchange of data between different levels of the model).
- Network (responsible for ensuring and maintaining communication between terminal network devices, guarantee a reliable connection).
- Transport (their main task is to build a route for transmitting information, checking errors that occur during routing, and sending requests for retransmission of data).
Stacks can be configured based on the tasks and the necessary network functionality, adjust the number of protocols and attach protocols to server network interfaces. This allows for flexible network configuration.
Conclusion
In this article, we have provided basic information to familiarize yourself with the OSI network model. These are the basics that everyone who works in the IT field needs to know to understand how the data transfer system works.
In this article, at the level of the OSI network model for dummies, we tried to explain in simple language how data transfer is implemented, and most importantly, how the system for interacting network equipment at different levels is arranged.
You can tell a lot about each of the protocols. It is hoped that this article will arouse interest in further acquaintance with this interesting topic.