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What is the OSI Model?

The OSI model (Open Systems Interconnection model) is a conceptual framework used to understand and design how data is transmitted over a network. It divides the networking process into seven distinct layers, each with specific roles and responsibilities. Think of it as a universal language for computer networking.


Application Layer (Layer 7)

  • Purpose: Interface for end-user processes and applications.
  • Examples: HTTP, FTP, SMTP, DNS
  • Analogy: The app you’re using (like a web browser).

Presentation Layer (Layer 6)

  • Purpose: Translates data formats, encryption/decryption, compression.
  • Examples: SSL/TLS, JPEG, MPEG
  • Analogy: The translator that makes sure both ends understand the data (e.g., translating from English to French).

Session Layer (Layer 5)

  • Purpose: Manages connections (sessions) between applications.
  • Examples: NetBIOS, RPC
  • Analogy: A moderator keeping a conversation on track and orderly.

Transport Layer (Layer 4)

  • Purpose: Reliable data transfer with error checking, flow control.
  • Examples: TCP, UDP
  • Analogy: A delivery truck ensuring your packages are sent and received in order.

Network Layer (Layer 3)

  • Purpose: Routing, addressing, and delivering packets between networks.
  • Examples: IP, ICMP, IPsec
  • Analogy: GPS or map used by the delivery truck to find the right address.

  • Purpose: Provides reliable data transfer across the physical link. Handles MAC addressing.
  • Examples: Ethernet, PPP, MAC addresses
  • Analogy: The street address or apartment number on the envelope.

Physical Layer (Layer 1)

  • Purpose: Transmits raw bits over physical medium (electrical, light, radio).
  • Examples: Cables, switches, radio signals
  • Analogy: The actual wires or Wi-Fi signals that carry the envelope.

Mnemonic to Remember:

“Please Do Not Throw Sausage Pizza Away”
(Physical, Data Link, Network, Transport, Session, Presentation, Application)