All 7 OSI layers with PDU, device/protocol examples, and role.
| # | Layer | PDU | Devices / Protocols | Role |
|---|---|---|---|---|
| 7 | Application | Data | HTTP, DNS, SMTP, FTP | User-facing services and APIs. |
| 6 | Presentation | Data | TLS/SSL, JPEG, ASCII | Encryption, compression, data format. |
| 5 | Session | Data | NetBIOS, RPC, SIP | Opens, manages, and closes sessions. |
| 4 | Transport | Segment | TCP, UDP | End-to-end delivery and ports. |
| 3 | Network | Packet | IP, ICMP · routers | Logical addressing and routing. |
| 2 | Data Link | Frame | Ethernet, ARP · switches | Framing, MAC addressing, error check. |
| 1 | Physical | Bits | Cables, hubs, repeaters | Raw bit transmission over media. |
Read top-down 7→1: upper layers serve apps, lower layers move bits. PDU names: Data, Segment, Packet, Frame, Bits.
L4 Transport → Segment → TCP/UDP → end-to-end delivery.Layers 7–5: Data, Layer 4: Segment, Layer 3: Packet, Layer 2: Frame, Layer 1: Bits.
Routers work at Layer 3 (Network); switches at Layer 2 (Data Link); hubs at Layer 1.