What is the OSI Model?

The OSI model is a reference framework for defining interconnection architectures for communication systems. It is a functional guide for communication tasks and therefore does not specify a communication standard for such tasks. However, many standards and protocols comply with the guidelines of the Open Systems Interconnection Model.

What is the OSI Model?

What is OSI (Open Systems Interconnection)?

Many network technologies were created in the 1960s and 70s. Each is based on a specific Hardware design. These systems are made in one piece; what we can call monolithic architecture. This means that designers must deal with all the elements involved in the process.

We can assume that these elements form a transmission chain with several sections: physical connection devices, software and hardware protocols used in communication, application programs that perform communication.

This model, which sees the chain as a monolithic whole, is impractical because the slightest change can involve changing all its elements. The original Internet design of the U.S. Department of Defense provided a four-tier plan. Although it dates from the 70s, it is more or less used: Physical or Network Access Layer.

In any case, it is responsible for sending information about the hardware system used. Depending on the type of physical network, a different protocol is used. The network (NET) layer is also called the Internet Layer. It is responsible for sending data through different physical networks that can connect a source machine to the target machine of data.

Transmission protocols such as IP are closely related to this Transfer layer (Host-to-Host Layer). Controls the establishment and end of the connection; data flow control; retransmission of lost data and other details of transmission between the two systems. The most important protocols at this level are TCP and UDP (mutually exclusive Application Layer).

It consists of protocols that serve direct user programs; Browser, Email, FTP, TELNET, etc. Responding to the general theory prevalent in the computer world, modular hardware and layered software design, the ISO organization (International Standards Organization) in 1978 proposed a communication model for networks. They called it “The reference model of Open Systems Interconnection”, which is commonly known as the OSI model.

Its philosophy is based on separating the functionality of the transmission chain into various modules that standardize the interface with adjacent ones. This design philosophy has two advantages: replacing a module does not affect the entire chain. Also, given that the limits and interfaces are perfectly defined, there may be some interoperability between the various products and hardware/software manufacturers. This means, for example, that two different communication software can use the same physical communication tools.

Main Components

The OSI model has two main components:

1. A network model called the Basic Reference Model or Server layer.
2. A series of special protocols.

Although the network model is inspired by the Internet model, it has no more similarities than this. Primitive Internet was based on 4, while it was based on a seven-layer model. Currently, all developments are based on the following 7-level model:

  • Physical
  • Link
  • Network
  • Transport
  • Session
  • Presentation
  • Application

Each level performs a specific function and differs from adjacent ones with known interfaces, without any other aspect of total communication. Remember that this model, which all the books about networks are talking about, is a conceptual abstraction in which only physical reality (more or less) is adapted. However, the implications of applying this perspective are very useful. The basic concept is the same concept that allows us to put the address and information in a letter.

The letter follows a series of processes in the postal service, without anyone having to worry about what comes before or will happen. Finally, the letter is delivered to the recipient only in the mailbox he needs to read. In general, devices used in networks limit their work to one or more of these levels. For example, a Hub that raises and retransmits the signal from all ports only works on layer 1, while a Switch works on layers (1 and 2); a Router works on layers 1, 2 and 3. Finally, a user workstation usually processes layers 5, 6, and 7.

Regarding the software, it should be noted that each layer uses a specific protocol to communicate with adjacent layers and adds some additional information to the packet header Protocol Header.


The schematic description of the various layers that make up this model is as follows:

   Physical Layer

It is responsible for transmitting information bits over the line or medium used for transmission. It deals with the physical properties and electrical properties of various components; with transmission speed if it is unidirectional or bidirectional (unidirectional, bidirectional or full-duplex). In addition, as a summary of the mechanical aspects of connections and terminals, including the interpretation of electrical signals, and the tasks of this layer, we can say that a binary information packet is responsible for the successful conversion of Frame to the physical environment used in transmission.

These pulses can be electricity (cable transmission); electromagnetic (Wireless transmission) or luminous (optical transmission). When working in the receive mode, operation reverses; It is responsible for converting these pulses into binary data packets to be transmitted to the link layer. For example, this level defines the measurements of the coax Ethernet cable and the BNC connectors used. Another example of the standards for this layer is RS-232 (H2.5.1) and X.21 for serial communication.

   Link Layer

It can be said that this layer transfers messages from the physical layer to the network layer. It specifies how data is organized when transmitted in a particular medium. This layer defines the frames, addresses, and checksums of Ethernet packets.

In addition to local addressing, it deals with the detection and control of errors occurring in the physical layer, control of access to this layer, and data integrity and transmission reliability.

For this, it groups the information to be transmitted in blocks and includes a checksum to each of them to check the integrity of the receiver. The received datagrams are verified by the buyer. If any datagram is corrupted, a control message is sent, requesting it to be sent to the sender. The PPP protocol is an example of this layer.

The link-layer can be divided into two sub-layers:

1. Logical Link Control – LLC: It defines how data is transferred over the physical medium by serving the upper layers.
2. Medium Access Control: This substrate acts as the controller for the underlying hardware (network adapter). In fact, the network card driver is sometimes called the “MAC driver” and the physical address contained in the card hardware is known as the MAC address (“MAC address” H12.4).

Its main task (calling it access control) is to arbitrate the use of physical tools to make it easier for several teams to compete simultaneously for the use of the same means of transportation.

CSMA/CD (Carrier Sense Multiple Access with Collision Detection) mechanisms used in Ethernet (H12.4) is a typical example of this substrate.

IEEE 802.11 Standard

The IEEE 802.11 standard defines the use of two sub-levels of the OSI architecture (physical and data link layers) by specifying the rules of operation in WLAN. The protocols of the 802.x branch define the technology of local area networks and metropolitan area networks.

802.11i was approved on June 24, 2004, to address security in wireless networks. It is based on the TKIP encryption algorithm like WPE, but it also supports the more secure AES (Advanced Encryption Standard).

   Network Layer

This layer deals with the transmission of datagrams (packets), a task that can be complicated in large networks such as the Internet, and redirecting each of them to the appropriate route, but not at all with packet errors or losses, for example, it defines the Internet address and route structure.

At this level, two types of packages are used: data packages and route update packages. As a result, this layer can be considered divided into two:

  1. Transport: It is responsible for encapsulating the data (user) to be transmitted. Uses data packets with IP protocol (Internet Protocol).
  2. Switching: This section is responsible for sharing certain network connection information (its effectiveness is rarely perceived by the user).

Routers are devices that operate at this level and take advantage of these route update packages. This category includes ICMP (Internet Control Message Protocol), which is responsible for generating messages when transmission errors occur and a special eco-mode that can be verified using PING.

Two of the most common protocols in this layer are X.25 and IP.

   Transport Layer

This layer is concerned with ensuring the reliability of the service, defining the quality and quality of data delivery. This layer defines when and how to use retransmission to provide the route. To do this, it divides the message received from the session layer into pieces (datagrams), enumerates them sequentially and transmits them to the network layer for sending.

At the time of reception, if the network layer uses the IP protocol, the Transfer layer is responsible for reordering the received layers in order. It can also work in the opposite direction by duplicating a transport link between various data connections. This allows data in various applications to share the same stream as the network layer.

A typical example of a protocol used in this layer is the UDP (Universal Datagram Protocol) transfer layer, which is also used on the Internet by some application programs.

   Session Layer

It is an extension of the transport layer that offers dialog control and synchronization, but few applications actually use it. For example, Internet communication does not use this. Note: Some authors refer to the session layer as a model that has practically no utility.

   Presentation Layer

This layer takes the semantic aspects of communication (defines the syntax of the data to be transmitted), and creates the necessary arrangements for machines that use different internal notations for data to communicate. explain how floating-point numbers can be transferred between computers using different mathematical formats.

This layer is a good candidate for implementing crypto applications. Theoretically, this layer “presents” the data to the application layer, takes the received data and converts it into formats such as text, image, and sound.

In theory, the client and server should negotiate the format to be used, and the corresponding formatting of this function and data will be the object of this layer. In the 1970s, however, it made sense that most of the networking work was about entering and outputting data on large computers using “dumb” terminals.

Currently, the landscape has changed; There is only one option for the data format, but the OSI protocol continues to agree on a coding scheme. On the Internet, the only service using this layer is TELNET, a service for accessing servers from remote terminals. In this case, the presentation layer is responsible for configuring the terminal to connect to a server with certain features.

   Application Layer

This layer explains how application programs do their job. For example, this layer implements the process with system files. On the one hand, they interact with the presentation layer; On the other hand, it represents the interface with the user, transmits information and receives commands that direct communication. Protocols used by the programs in this layer; HTTP, SMTP, POP, IMAP, Modbus Communication Protocol.

Network Layer Operation in the OSI model

The network layer provides its services to the transport layer, a complex layer that allows choosing the best way to connect and communicate between machines that can be geographically located on different networks.

It is responsible for information switching and routing functions (logical addressing) that provide the necessary procedures for data exchange between the source and destination, so it needs to know the topology of the network to determine the optimal route.

The main functions are:

  • It divides the messages of the transport layer (s) into more complex units called packets, where the computers that communicate assign logical addresses.
  • Know the network topology and address the situation where the source machine and target machine are in different networks. It routes the information over the network according to the packet addresses and determines the switching and routing methods through intermediate devices (routers).
  • Sends packets from node to node using a virtual circuit or datagram. It installs the packages to the target computer. This layer is where routers, data packets are routed from the source to the destination, or the responsible devices work along the best possible path between them.

IP Layer in the OSI Model

IP protocol is the basic basis of the internet. It makes it possible to send data from the source to the destination. The transfer level divides the data stream into datagrams. During transmission, a datagram can be divided again.

IP packets:

  • Version 4 > Allow updates.
  • IHL > Length of the header with 32-bit words. The maximum value is 15 or 60 bytes.
  • Service Type > Determines whether data transmission and speed are reliable.
  • Total Length > Up to 65,535 bytes. To determine which datagram a part belongs to.
  • DF (Fragmentation) > Cannot reconnect target datagram.
  • MF (More Parts) > Not determined in the last part.
  • Relocation of Part > Which part of the datagram this part belongs to. The size of the base part is 8 bytes. Lifetime decreases with each jump.
  • Protocol > The transport protocol on which the datagram is based. Options include strict routing (full route specified), loose routing (only a few routers are specified in the route), and road recording.

The technical process in which the data is transmitted over the network can be divided into two separate, systematic steps. At each step, certain actions are performed that cannot be performed in another step. Each step includes its own rules and rules/procedures/protocols. The protocol steps should be performed in an appropriate order and should be the same on each computer on the network. On the source computer, these steps should be performed from top to bottom.

On the target computer, these steps should be carried out from the bottom up.

Protocols on the source computer:

  1. They are divided into smaller sections called packages.
  2. Information about the IP address is added to the packets so that the target computer can determine whether the data belongs to it.
  3. It prepares the data to transmit via NIC and sends it with a network cable.

Target computer: The protocols on the target computer consist of the same sequence of steps, but vice versa.

  1. It takes the data packets from the cable and places them on the computer via the NIC.
  2. It removes all information transmitted from the data packets, eliminating the information added by the source device.
  3. Copies package data to the buffer to reorganize and send it to the application. Source and target teams must perform each step in the same way, so they have the same structure when they are received while the data is being sent.

Processing of TCP/IP Packets in the OSI Model

Protocols such as TCP/IP determine how PCs communicate with each other over networks such as the Internet. These protocols work together and are often superimposed on what is known as the protocol stack. Each protocol stack is designed to achieve a special purpose in sending and receiving computers.

The TCP stack combines application, presentation, and session stacks into a stack also called the application stack. In this process, the properties of the packaging to transmit data are given: The TCP application stack formats sent data so that the sub stack can be transported. The TCP application stack performs equivalent operations performed by the top three OSI stacks: applications, presentations, and sessions.

The next stack is the transport stack, which is responsible for data transfer and ensures that the data sent and received are actually the same, in other words, ensuring that no errors occur while sending the data. TCP breaks down the data it receives from the application stack. Adds a title that contains information to be used when the data is received to ensure that the data is not changed while on the road and that the segments can be combined correctly with their original forms.

The third stack prepares the data for delivery by entering the IP datagrams and determining the full Internet address. The IP protocol runs on the Internet stack, also called the network stack. Places an IP container with a header in each segment. The IP header contains information such as the IP address of the sending and receiving computers, the length of the datagram and the order of its order. Sequential sorting is added because the datagram can possibly exceed the allowable size for network packets and therefore need to be divided into smaller packets. The inclusion of the ordered order will ensure that they are reassembled appropriately.

Main Reference

One of the most urgent needs of a communication system is the creation of standards that can communicate with each other only without the equipment of the same manufacturer and using the same technology. The connection between electronic equipment has been gradually standardized, the OSI Model is the main reference for network communication.

Although there are other models, most network manufacturers today associate their products with the OSI model, especially when they want to teach users how to use their products. Manufacturers consider it the best tool available to teach how to send and receive data over a network.

The OSI reference model allows users to see the network functions that occur on each layer. It is an understandable model for users. Also, the study defined and explained the IP in both version 4 and the new version IP version 6.

Since the need to create a new version is based on the exhaustion of the previous IP addresses, the OSI model was explained and emphasis was placed on layer 3. Because the Internet protocol works in this layer. At the same time, communication is established in this layer and the path of the data is determined.

How Does It Work?

The basic operating principle of OSI is to compare mutual stack levels to communicate with the opposite device. For example, when the application is intended to send data, the sending device observes and communicates with the target application stack.

This communication happens regardless of the network structure such as Ethernet or Token Ring. TCP combines the application, presentation, and session areas of the OSI model into a model called the application stack.

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