The OSI (Open Systems Interconnection) model is a conceptual framework used to understand how different networking protocols interact in layers to create a network. It was developed by the International Organization for Standardization (ISO) in the late 1970s and early 1980s. The OSI model divides the networking process into seven separate layers, each with a specific function:

Layer 7: Application Layer

  • Closest to the end-user.
  • Provides network services directly to applications such as web browsers and email clients.
  • Protocols & Services: HTTP, FTP, SMTP, POP3, DNS.

Layer 6: Presentation Layer

  • Transforms data into a format that can be consumed by the application layer above it.
  • Handles data compression, encryption, and translation services.
  • Examples: SSL/TLS, ASCII, EBCDIC, MPEG.

Layer 5: Session Layer

  • Establishes, maintains, and terminates user connections.
  • Manages sessions or connections between applications on different devices.
  • Protocols: RPC, PPTP, SMB.

Layer 4: Transport Layer

  • Ensures end-to-end communication, flow control, and error recovery.
  • Segments data from the session layer and adds source and destination port numbers.
  • Protocols: TCP, UDP, SCTP.

Layer 3: Network Layer

  • Responsible for determining the best path to route data packets from the source to the destination.
  • Deals with logical addressing (like IP addresses) and routing.
  • Protocols: IP (both IPv4 and IPv6), ICMP, OSPF, BGP.

Layer 2: Data Link Layer

  • Provides a means for data to be transferred from one device to another on a local network or a point-to-point connection.
  • Concerned with physical addressing (MAC addresses), error detection, and framing.
  • Sub-layers: Logical Link Control (LLC) and Media Access Control (MAC).
  • Protocols & Technologies: Ethernet, Wi-Fi, PPP, ARP.

Layer 1: Physical Layer

  • Defines the physical medium (like cables) and the characteristics of the transmission (like voltage levels).
  • Concerned with bit-level transmission, signaling, and the physical connectors.
  • Technologies: Ethernet cables (Cat5e, Cat6, etc.), optical fiber, USB, Bluetooth.

It’s important to note that the OSI model is largely theoretical. The TCP/IP model, which is a more practical framework and is the foundation of the internet, condenses the OSI model into four layers. However, the OSI model remains a valuable tool for understanding and teaching network operations.