Understanding the OSI Model: Layers and Functions

Posted on Dec 17, 2024 in Computers

The OSI Model: Layers and Functions

The International Organization for Standardization (ISO) establishes global agreements on international standards. The Open Systems Interconnect (OSI) model is an ISO standard covering all aspects of communication networks. It’s an open system model that allows communication between different systems regardless of the underlying architecture.

The objective of the OSI model is to enable communication between different systems without altering the underlying hardware or software logic. Its design includes a flexible, robust, and interoperable network architecture. The OSI model is a tiered architecture for network system design, facilitating communication between all types of computers.

It consists of seven related levels, each defining a segment of the process necessary to move information across a network.

1. Physical Layer

This layer deals with the electrical and mechanical interface and transmission medium. It defines procedures and functions for physical devices and interfaces to enable transmission.

Responsibilities:

  • Physical characteristics: Defines the type of transmission medium.
  • Bit representation: Defines the encoding type for data streams (sequences of zeros and ones).
  • Data Rate: Defines the transmission rate (bits per second).
  • Bit synchronization: Ensures sender and receiver are synchronized at the bit level.
  • Line Configuration: Connects devices to the environment.
  • Physical Topology: Defines network structure (e.g., ring, star, mesh).
  • Transmission Mode: Sets the direction of transmission (simplex, half-duplex, full-duplex).

2. Data Link Layer

This layer transforms the physical layer into a reliable link, responsible for node-to-node delivery. It presents the physical layer to the network layer as an error-free medium.

Responsibilities:

  • Framing: Divides the bit stream into manageable data units called frames.
  • Physical Addressing: Adds a header to define the physical address of the sender and/or receiver.
  • Flow Control: Implements mechanisms to prevent receiver overflow.
  • Error Handling: Adds reliability by detecting and retransmitting lost or defective frames.
  • Access Control: Determines which device controls the link when multiple devices connect to the same link.

3. Network Layer

This layer is responsible for delivering a packet from source to destination across multiple networks.

Responsibilities:

  • Logical Addressing: Adds a header with logical addresses of the sender and receiver when a packet crosses network boundaries.
  • Routing: Routers or gateways route packets to their destination across interconnected networks.

4. Transport Layer

This layer ensures end-to-end delivery, guaranteeing message integrity and order. It monitors error and flow control from origin to destination. It can also create a logical connection between two ports, involving connection establishment, data transfer, and release.

Responsibilities:

  • Service Point Addressing: Includes a service point address or port in the header, sending the message to the appropriate process.
  • Segmentation and Reassembly: Divides messages into segments and replaces lost packets.
  • Connection Control: Can be connection-oriented or connectionless.
  • Flow Control: Implements end-to-end flow control.
  • Error Control: Provides end-to-end error control.

5. Session Layer

This layer manages the dialogue of the network, establishing, maintaining, and synchronizing interaction between two communication systems.

Responsibilities:

  • Dialog Control: Allows two systems to communicate in half-duplex or full-duplex.
  • Synchronization: Allows adding checkpoints in a data stream.

6. Presentation Layer

This layer deals with the semantics and syntax of exchanged information.

Responsibilities:

  • Translation: Translates the bit stream to ensure interoperability between different coding methods.
  • Encryption: Transforms data to ensure privacy.
  • Compression: Reduces the number of bits for transmission, especially for multimedia data.

7. Application Layer

This layer provides user interfaces and supports services like email, file transfer, and remote access.

Specific Services:

  • Network Virtual Terminal: Access to a remote machine.
  • FTAM: Access files on a remote machine.
  • Mail Services: Sending and storing emails.
  • Directory Services: Access to distributed databases.