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OSI Model Levels

In the last article, we got acquainted with the OSI model. It's time to take a closer look at each of the layers.


Physical layer


This layer of the OSI model is responsible for transmitting the bit stream over physical communication channels - various cables and wireless communication lines.


Physical layer functions are implemented on all devices connected to the network. For a computer, this function is performed by a network adapter, and from the side of intermediate network devices - routers, switches, etc.


There is no data analysis going on at this level. The main task is to deliver, without distortion, a stream of bits, in accordance with the chosen encoding and the interval between adjacent bits.


Datalink level


This layer provides, to the overlying, network, the following services:

  •    establishing a logical connection between interacting nodes;
  •    coordination within the connection of the speeds of receiving and sending information;
  •    ensuring reliable transmission, error detection and correction.


Datalink layer protocols are implemented both on end nodes (network adapters) and on all intermediate network devices (routers, etc.).


The data unit of this level is a frame. The frame data field contains a network layer message, and the header contains service information, including the destination address.


The datalink layer can ensure the reliability of transmission, for example, by fixing the boundaries of the frame, placing a special sequence of bits at its beginning and end, and then adding a checksum to the frame. On the receiver side, the channel layer groups the bits coming from the physical layer into frames, calculates the checksum of the received data again, and compares the result with the checksum transmitted in the frame. If they match, then the frame is considered correct. If the checksums do not match, then an error is fixed.


Network Layer


In my opinion, this level is one of the most significant levels of the model. It is a link for different networks that use their own channel layer technology, for example, Ethernet, FDDI, Token Ring networks. Without this layer, it is impossible to transmit a message from the Ethernet network to the FDDI network.


Network layer functions are implemented:

  •    protocol group;
  •    routers.


Routers can be either software supplied with the operating system, or hardware, separate devices that are most often responsible for this level.


A network layer data unit is a packet that is formed as a result of adding appropriate headers to the data of a higher level. The header of network layer packets, along with other information, contains data about the destination address of the packet.


In order for packets to be delivered to any node in a composite network, they must have unique addresses within it - network or global.


Determining the route is another task of this level, which describes the route through which the package should pass. As a rule, a route consists of a sequence of other routers.


The router stores information about the topology of connections in routing tables.


Transport level


This layer is responsible for ensuring reliable data transmission for the upper levels of the model with the degree of reliability that they require.


The OSI model defines five classes of transport service from the lowest 0 to the highest 4. The classes differ: urgency, connection renewability, availability of connection multiplexing facilities, the ability to detect data packet distortion.


The choice of a transport protocol depends on the needs of a particular application, for example, for media streaming services, possible packet losses are not so important, which cannot be said, for example, about file servers.


The brightest representatives of protocols of this level are TCP and UDP protocols of the TCP/IP stack.


Session level


This level controls the interaction of the parties: it records which of the parties is currently active and provides tools for session synchronization. The functions of this layer are often combined with the functions of the application layer and implemented in a single protocol.


Presentation level


Thanks to this layer, the information transmitted by the application layer of one system is understandable to the application layer of another. Data encryption and decryption can also be performed at this level, for example, using the SSL protocol, storing/unpacking information, encoding media files.


Application level


This is the level that applications interact with. Authentication, authorization, determination of session security requirements, etc. take place at this level.


For requests to this level, system calls of the operating system are used, forming an API.


Examples of application layer protocols: SSH, FTP, HTTP, etc.


Conclusion


We have reviewed all levels of the OSI model in general terms, which is useful knowledge. In the following articles, we will begin to delve into the TCP/IP stack.

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