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📘 01-03: The OSI Model

Network Systems

Module 01: Introduction to Networking

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❓ What Is the OSI Model?

The OSI (Open Systems Interconnection) model is a 7-layer conceptual framework that standardizes how data moves through a network. Each layer has a specific job and talks to the layers above and below it.

📌 The Seven Layers

# Layer Job PDU Examples
7 Application Interface to user apps Data HTTP, DNS, SMTP
6 Presentation Format, encrypt, compress Data TLS, JPEG, ASCII
5 Session Establish/manage sessions Data NetBIOS, RPC
4 Transport Reliable/unreliable delivery, ports Segment TCP, UDP
3 Network Logical addressing, routing Packet IP, ICMP, routers
2 Data Link Physical addressing, framing Frame Ethernet, MAC, switches
1 Physical Transmit raw bits on the medium Bits Cables, NICs, hubs

Mnemonics: Layer 7→1 "All People Seem To Need Data Processing"; Layer 1→7 "Please Do Not Throw Sausage Pizza Away."

📌 Protocol Data Units (PDUs)

Data is named differently at each layer as headers are added (encapsulation):

Data  →  Segment  →  Packet  →  Frame  →  Bits
(L5-7)   (L4)        (L3)       (L2)      (L1)
  • Encapsulation: each layer adds its own header as data moves down the sending host.
  • Decapsulation: each layer strips its header as data moves up the receiving host.

📌 Which Devices Live Where

Device Layer
Hub, cable, NIC (physical) 1
Switch, bridge 2
Router, Layer 3 switch 3
Firewall (stateful) 3–4 (NGFW up to 7)

💡 Why It Matters

  • Troubleshooting: work bottom-up (Physical first) or top-down. A dead cable is Layer 1; a routing issue is Layer 3.
  • Design: knowing the layer of a device/protocol clarifies its role.

Tip

Key idea — The OSI model splits communication into 7 independent layers. Remember the order, the PDU names (segment → packet → frame → bits), and which device operates at which layer.

See also: Network Hardware Devices, The TCP/IP Suite & Encapsulation