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OSI Model

4 min read

OSI means Open System Interconnection.

Definition

  • A model for understanding and designing a network architecture that is flexible, robust, and interoperable
  • It is not a protocol
  • It is a guideline

Purpose

To show how to facilitate communication between different systems without requiring changes to the logic of the underlying hardware and software.

Layers in the OSI Model

  1. Application Layer
  2. Presentation Layer
  3. Session Layer
  4. Transport Layer
  5. Network Layer
  6. Data Link Layer
  7. Physical Layer

Please Do Not Throw Sausage Pizza Away

Example

Application Layer

my password is nesoacademy989#

Presentation Layer

Modifies or converts the content into a specified format.

ofrgrudiofbrd*gbrjkvd

Session Layer

Carries out session activity.

ofrgrudiofbrd*gbrjkvd

Transport Layer

Adds transport layer information.

TL INFO - ofrgrudiofbrd*gbrjkvd

Network Layer

Adds network layer information.

NL INFO - TL INFO - ofrgrudiofbrd*gbrjkvd

Adds data link layer information.

DL INFO - NL INFO - TL INFO - ofrgrudiofbrd*gbrjkvd

Physical Layer

Converts the data to bits to be sent.

010110001001101010100110011110010010

  • This layer knows the medium
  • If the medium is air, it converts the bits into waves
  • If the medium is cable, it converts the bits into electrical signals

Application Layer

Enables the user to access network resources.

Services provided:

  1. File Transfer and Access Management, or FTAM
  2. Mail Services
  3. Directory Services

Presentation Layer

Concerned with the syntax and semantics of the information exchanged between two systems.

Services provided:

  1. Translation: Converts the data from the sender into a format that all devices accept
  2. Encryption: Converts plain text into unreadable text. Only the sender and receiver can understand it through decryption
  3. Compression: Reduces the number of bits in the information, such as multimedia messages with audio and video

Session Layer

Establishes, maintains, and synchronizes the interactions among communicating devices.

Services provided:

  1. Dialog Control: Allows two devices to enter a dialog. A dialog is communication between two processes in half-duplex, full-duplex, one-way, or two-way communication
  2. Synchronization: Inserts a checkpoint in large files to make sure that files are sent correctly. The receiver confirms what it received. The sender and receiver must have a good relationship for the sender to provide valuable information to the receiver

Transport Layer

Responsible for the delivery process of the entire message.

Services provided:

  1. Port Addressing: Provides the path for devices to exchange port numbers for data transportation
  2. Segmentation and Reassembly: Segments data when it is too large and reassembles the data after it reaches the receiver
  3. Connection Control: Lets the devices select a connection type from the switching chapter
  4. End-to-End Flow Control: Lets the sender and receiver agree on a common speed for sending and receiving data
  5. Error Control: Checks for transmission errors

Network Layer

Responsible for delivering data from the original source to the destination network.

Services provided:

  1. Logical Addressing: Helps the router identify the source and destination IP Address
  2. Routing: Finds the best route for packet transmission

Responsible for moving data, or frames, from one node to another node.

Services provided:

  1. Framing: Groups a sequence of bits as a Frame
  2. Physical Addressing: Includes the source MAC Address and destination MAC Address
  3. Flow Control: A duty of the Data Link Control sublayer and a speed-matching mechanism
  4. Error Control: Identifies broken frames
  5. Access Control: When two or more devices connect to the same link, Data Link protocols find which device controls the link at that time
  • Responsible for transmitting bits over a medium
  • Provides electrical and mechanical specifications

Services provided:

  1. Physical Characteristics of the Media: Defines the type of media
  2. Representation of Bits: Defines the type of encoding
  3. Data Rate: Defines the transmission rate, or number of bits each second
  4. Synchronization of Bits: Keeps the clocks of the sender and receiver synchronized
  5. Line Configuration: Defines a point-to-point or point-to-multipoint configuration
  6. Physical Topology: Defines how devices connect in a star, ring, or mesh topology

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