OSI model describe the functions of the OSI network model levels  

OSI model

OSI stands for Open Systems Interconnection

1. Physical Layer

Key words: bits, cable, Ethernet, Wi-Fi, Bluetooth, hub

The lowest layer of the OSI model is the physical layer. He is responsible for the physical connection between devices. The physical layer is responsible for the transfer of individual bits from one node to another.

Physical layer functions:

  • Data rate control.
  • Defines the network topology (bus, star, etc.).
  • Defines the data transmission mode (simplex, half duplex and full duplex).

2. Data Link Layer

Key words: frame, MAC address, switch

The Data Link layer is responsible for delivering a message from node to node. 

To deliver packets, the physical address of the device on the network is used - MAC address.

Data Link layer functions:

  • Framing allows the sender to transmit a set of bits that are meaningful to the receiver. This can be achieved by attaching special bit patterns to the start and end of the frame.
  • Physical addressing is performed by containing in the header of each frame the physical address (MAC address) of the sender and recipient.
  • An error control mechanism that detects and retransmits corrupted or lost frames.

3. Network Layer

Key words: packet, IP address, router

A segment of the network layer is called a packet.

The network layer is implemented by network devices such as routers.

The network layer works to transfer data from one host to another located on different networks. It also takes care of packet routing, i.e. about choosing the shortest path for transmitting a packet from among the available routes. For packet routing, the source and destination IP addresses are placed in the header of each packet.

Network layer functions:

  • Routing determines which route is suitable from source to destination using network layer protocols.
  • Logical addressing identifies each device on the internetwork with an IP address. This address makes each device unique and versatile.

4. Transport Layer

Key words: segment, TCP, UDP, port

The transport layer is called "the heart" of the OSI model.

The data in the transport layer are called segments.

The transport layer is controlled by the operating system. It is part of the OS and interacts with the application layer.

The transport layer receives data from the upper layers, performs segmentation, and controls flow and errors to ensure correct data transfer. It also adds the source and destination port number to its header and forwards the sharded data to the network layer.

The port number is usually specified by default, or manually.

The functions of the transport layer are :

  • Segmentation: This layer accepts the message from the (session) layer , breaks the message into smaller units . Each of the segment produced has a header associated with it. The transport layer at the destination station reassembles the message.
  • Service Point Addressing: In order to deliver the message to correct process, transport layer header includes a type of address called service point address or port address. Thus by specifying this address, transport layer makes sure that the message is delivered to the correct process.

5. Session Layer

Key words: data, session, authentication

This layer is responsible for establishing a connection, maintaining sessions, authenticating, and also providing security.

Session layer functions:

  • Session establishment, maintenance and termination: The layer allows two processes to establish, use and terminate a connection.
  • Dialogue controller: The session layer allows two systems to initiate communication with each other in half-duplex or full-duplex mode.

6. Presentation Layer

Key words: data, translation, encryption, types of files (mp3, avi, docx, jpg, etc.)

At the Presentation layer, data is retrieved from the application layer and processed according to the required format for transmission over the network.

Presentation layer functions:

  • Translation: for example, from ASCII to EBCDIC.
  • Encryption / Decryption: Data encryption translates data into another form or code. The encrypted data is known as cipher text, and the decrypted data is known as plain text. The key value is used for both encryption and decryption of data.
  • Compression: Reduces the number of bits that need to be transmitted over the network.

7. Application Layer

Key words: data, applications (browser, Skype, etc.), DNS, HTTP, POP, SMTP

The application layer produces data that needs to be transmitted over the network. This layer also serves as the interface between the user and services to access the network.
Example: Application - What's Up, Browsers, Skype Messenger, etc.

Application layers functions are:

  • Application layer helps you to identify communication partners, determining resource availability, and synchronizing communication.
  • It allows users to log in to a remote host
  • This layer provides various e-mail services
  • This application offers distributed database sources and access for global information about various objects and services.

Benefits of using the OSI model:

  • It will help you standardize your router, switch, motherboard, and other hardware.
  • Reduces complexity and standardizes interfaces
  • Helps ensure technology interoperability, accelerates evolution
  • Protocols can be replaced with new protocols as technology changes.
  • This is the standard model in computer networks.
  • Offers flexibility to adapt to different types of protocols.

Drawbacks of using the OSI model:

  • It takes time to configure the protocols.
  • You can only use it as a reference model.
  • Does not define any specific protocol.
  • In the OSI network layer model, some services are duplicated at many layers, such as the transport and data link layers.
  • Layers cannot work in parallel, since each layer must wait for data to be received from the previous layer.

Additionally TCP/IP stack


  1. Explain what OSI model is.
  2. How many layers does OSI model consist of?
  3. Name the order of layer for transmitting data/ receiving data.
  4. State software layers in OSI model.
  5. State hardware layers in OSI model.


Ex. 1

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Exam questions:

Категория: Types of networks | Добавил: bzfar77 (22.02.2021)
Просмотров: 137 | Теги: Presentation layer, Transport layer, Data link layer, Session layer, Physical layer, Application layer, Network layer, OSI model, Ip, UDP, TCP, Protocol | Рейтинг: 0.0/0
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