2.1.1. Layer 1-3: The Infrastructure Layers
These layers handle physical transmission and logical delivery:
| Layer | Name | Function | PDU | Key Devices | Key Protocols |
|---|---|---|---|---|---|
| 1 | Physical | Bit transmission | Bits | Hubs, cables, NICs | Ethernet (physical), 802.11 (RF) |
| 2 | Data Link | Frame delivery, error detection | Frames | Switches, bridges | Ethernet, 802.11, PPP |
| 3 | Network | Logical addressing, routing | Packets | Routers, L3 switches | IP, ICMP, ARP, OSPF |
Layer 1 (Physical): Deals with electrical signals, light pulses, or radio waves. Everything here is about the medium—cable types, connector types, frequencies, encoding. A hub operates here: it receives signals and repeats them to all ports (no intelligence, just amplification).
Layer 2 (Data Link): Handles reliable delivery across a single link using MAC addresses. Switches operate here: they learn which MAC addresses connect to which ports (building a CAM table) and forward frames only where needed. This creates separate collision domains per port.
Layer 3 (Network): Handles logical addressing and routing between networks. Routers operate here: they examine destination IPs, consult routing tables, and forward packets toward the destination. This creates network boundaries—devices on different router interfaces are on different networks.
⚠️ Exam Trap: A Layer 3 switch performs routing but uses hardware (ASICs) for speed. The exam may ask whether a Layer 3 switch is a "switch" or "router"—it's both. It switches at Layer 2 within VLANs and routes at Layer 3 between VLANs.
