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OSI Model: Seven Layers of Network Communication

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OSI Model (Open Systems Interconnection)

The OSI model is a theoretical model that describes how data travels through a network in seven layers, from the physical transmission of data to the application.

The layers of the OSI model describe the different functions required to transfer data across a network:

  1. Application layer: Provides services to applications (e.g., HTTP, FTP) and is similar to the application layer of the TCP/IP model.
  2. Presentation layer: Responsible for formatting and encrypting data so that it is understandable to the recipient.
  3. Session layer: Manages sessions between users and maintains the order of communication (e.g., control tokens).
  4. Transport layer: Ensures reliable communication and controls errors and packet order (e.g., TCP, UDP).
  5. Network layer: Responsible for routing data between networks and adding network addresses (e.g., IP).
  6. Data link layer: Ensures the transfer of data in frames within the same network, with error control.
  7. Physical layer: Transmits bits over a physical medium (wires, radio waves).

1. Application Layer

  • Function: Provides an interface for communication between the user and the network (e.g., web browsing, email, file transfer).
    Protocols:
    • HTTP/HTTPS: Transfer of web data (secured via HTTPS).
    • FTP: File transfer.
    • SMTP, POP, IMAP: Sending and retrieving email.
    • DNS: Translation of domain names into IP addresses.
    • DHCP: Automatic assignment of IP addresses.
    • SSH/Telnet: Remote device management (SSH is secure, Telnet is not).
    • BitTorrent/Gnutella: Peer-to-peer file transfer.

2. Presentation Layer

  • Function:
    • Ensures correct encoding, encryption and formatting of data for transmission, and enables interpretation of data at the destination.
  • Protocols: SSL/TLS: Data encryption and protection against eavesdropping or manipulation.

3. Session Layer

  • Function:
    • Manages sessions (connections) between applications. Maintains sessions and enables resumption after interruptions.
  • Protocols:
    • ADSP, ASP: Session management in AppleTalk networks.
    • NetBIOS: Communication in local area networks (LAN).
    • PAP: Authentication of clients and servers.
    • SMPP: Transfer of SMS messages.

4. Transport Layer

  • Function:
    • Divides data into segments and ensures their reliable transmission.
    • Controls data flow and reassembles segments at the destination.
  • Protocols:
    • TCP: Reliable transmission (e.g., web, email).
    • UDP: Faster but unreliable transmission (e.g., VoIP, streaming).

5. Network Layer

  • Function:
    • Addresses and routes packets across the network using IP addresses. Manages routes for data transfer between networks.
  • Protocols:
    • IP: Addressing and routing of packets.
    • ICMP: Checking destination availability (ping).
    • IPsec: Security encryption of data at the network layer.

6. Data Link Layer

  • Function:
    • Manages physical and logical addressing of devices (e.g., MAC addresses). Controls flow and detects errors in data transmission. Establishes and terminates connections between physically connected devices.
  • Protocols:
    • Ethernet: Standard for data transmission in local area networks (LAN).
    • Wi-Fi (802.11): Standard for wireless data transmission.
    • PPP (Point-to-Point Protocol): Used for data transfer between two points, e.g., via a modem.
    • Frame Relay: Data transmission in wide area networks.

7. Physical Layer

  • Function:
    • Transmits raw bits of data over physical media (cables, optical fiber, radio waves).
    • Defines the electrical, optical or radio characteristics of the signal.
  • Protocols and standards:
    • Ethernet (physical aspect): Defines cable characteristics and transmission speeds (e.g., Cat5, Cat6).
    • USB (Universal Serial Bus): Protocol for physical data transfer between devices.
    • DSL (Digital Subscriber Line): Data transmission over telephone lines.
    • Bluetooth: Standard for short-range wireless communication.

IEEE 802 - Division of the Data Link Layer

  1. MAC (Media Access Control): Controls network access and the method of data transmission (addressing and routing).
  2. LLC (Logical Link Control): Identifies network layer protocols, performs encapsulation and frame synchronization.

Data Link Layer Protocols

  • ARP (Address Resolution Protocol): Maps an IP address to a MAC address.
  • RARP (Reverse Address Resolution Protocol): Maps a MAC address to an IP address.

General Frame Format

  1. Frame start: A sequence of bits marking the beginning of the frame.
  2. Address field: Contains the MAC addresses of the source and destination devices.
  3. Type: Indicates the type of data.
  4. Control field: Contains data for managing the link.

Frame End

  1. Check field: Performs error detection.
  2. Frame end: Marks the end of the frame and the start of the next one, if any.

Topology

  • Physical topology: The layout of devices and their connections.
  • Logical topology: The path data takes through the network.

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