Understanding the OSI Model

What is the OSI Model?

• The OSI model is a reference model for how to design and implement network communication.
• It provides a framework for how data is transmitted between devices.
• It is commonly used in networking to understand and troubleshoot network problems.

OSI Model Layers

• The OSI model has seven layers in total, each responsible for a specific task.
• The layers of the OSI model are:
  • Physical Layer
  • Data Link Layer
  • Network Layer
  • Transport Layer
  • Session Layer
  • Presentation Layer
  • Application Layer

OSI Model Architecture

• The OSI model follows a layered architecture.
• Data travels through each layer, starting with the Application Layer and ending with the Physical Layer.
• Each layer performs a specific function on the data.

Application Layer

• The Application Layer is the top layer of the OSI model.
• It provides the interface between the user and the network.
• Most applications use this layer to interact with the network.

Presentation Layer

• The Presentation Layer is responsible for data presentation and format conversion.
• It converts data into a format that is understandable by both the sending and receiving systems.
• The Presentation Layer offers many services to the Session Layer, including:
  • Data Encryption
  • Data Compression
  • Data Format Conversion

Session Layer

• The Session Layer is responsible for managing the communication session.
• The Session Layer provides services for:
  • Establishing and terminating communication sessions
  • Maintaining an ongoing connection between devices.
  • Syncing communication dialogs between devices.
  • Error Handling

Transport Layer

• The Transport Layer provides reliable data transfer.
• The Transport Layer ensures data is:
  • Sent in the correct order
  • Free from errors
  • Error Handling

Network Layer

• The Network Layer is responsible for logical addressing.
• It also determines the best path for data to flow between devices.
• The Network Layer includes:
  • Addressing (IP Addresses)
  • Path Determination
  • Routing

Data Link Layer

• The Data Link Layer is responsible for providing access to the physical network.
• It provides a reliable connection between devices.
• The Data Link Layer provides services for:
  • Physical Addressing (MAC Address)
  • Framing of data for transmission
  • Error Detection/Correction

Physical Layer

• The Physical Layer is the lowest layer of the OSI model.
• It is concerned with the physical transmission of data over the network.
• The Physical Layer defines the:
  • Physical characteristics of the network (cable, connectors)
  • Transmission Techniques
  • Data Bit Representation
  • Signal Timing
  • Network Access Method.

Presentation Layer

• The presentation layer performs the opposite function of the presentation layer in a sending device.
• It decrypts the data received from the network layer.
• It converts machine-understandable data into human-understandable data or vice versa. It was previously converting human-understandable data into machine-understandable data.
• It may also decompress received data, although this is not always necessary.
• The presentation layer is responsible for formatting the data into a presentable form for the application layer.

Application Layer

• The application layer is responsible for interacting with the user and providing the interface for sending and receiving data.
• In the provided example, the application layer receives the decrypted and decompressed message from the presentation layer.
• The application layer then displays this message to the user, for example, in the form of "I love you."
• If the user wants to respond, the application layer will take the message and send it back through the network layers.

Network Layers

• When the user enters a message such as "I love you too", the application layer sends the message to the network layers.
• Each of the network layers, including the presentation and session layers, will perform their respective functions on the message to prepare it for transmission.
• The message will then be sent to the destination device, where it will be processed through the network layers in the reverse order.
• This will ultimately result in the original receiver receiving the message.

Cycle

• This process of sending and receiving messages is cyclical.
• The communication between devices is based on layers and each layer has its own role.