OSI Layer: Foundations & Models (reference model for every layer below)

Definition

The OSI (Open Systems Interconnection) model is a 7-layer conceptual framework standardised by ISO (1984) that divides network communication into functionally independent layers, each serving the layer above and using services of the layer below.

Why Layering?

• Modularity — a layer can be redesigned without disturbing others
• Interoperability — vendors implement layers to a common standard
• Abstraction — each layer hides its internal complexity from neighbours
• Troubleshooting — faults isolated to a specific layer
• Peer-to-peer communication — layer n on the sender logically talks to layer n on the receiver

The Seven Layers (bottom-up)

1. Physical Layer

• Transmits raw bits over the medium
• Defines voltage levels, pin layout, connectors, data rate, transmission mode (simplex/half-duplex/full-duplex)
• Data unit: bit | Devices: repeater, hub, cables

2. Data Link Layer

• Node-to-node delivery on the same network
• Framing, physical (MAC) addressing, error detection (CRC), flow control, access control (CSMA/CD, CSMA/CA)
• Sublayers: LLC (logical link control) and MAC (media access control)
• Data unit: frame | Devices: switch, bridge, NIC

3. Network Layer