The OSI model

Why a model?

Sending a message from your phone to a server involves electricity or radio, addresses, routes, error checks, and the app itself. That is too much to think about at once. A model breaks the job into layers. Each layer does one part and trusts the layers below to do theirs. This lets different companies build parts that work together: a Wi-Fi chip from one maker, an operating system from another and a browser from a third.

The OSI model (Open Systems Interconnection), published by the ISO in the 1980s, has 7 layers. Real networks run the TCP/IP protocols, covered in the next topic, but everyone still uses OSI layer numbers when they talk. "It is a Layer 1 problem" means "check the cable".

The seven layers

PDU means Protocol Data Unit: the name for a chunk of data at a given layer. Learn the PDUs by heart: data at the top, segment at Layer 4, packet at Layer 3, frame at Layer 2, bits at Layer 1.

Remembering the order

From Layer 7 down: All People Seem To Need Data Processing (Application, Presentation, Session, Transport, Network, Data link, Physical). From Layer 1 up: Please Do Not Throw Sausage Pizza Away. Pick one and practise writing the list from memory.

A closer look at each layer

• Layer 1, Physical: voltage on copper, light in fibre, radio waves. Cable types, connector pins and signal timing live here.
• Layer 2, Data link: frames on one local link. Uses MAC addresses to say which NIC on this link should take the frame, and a checksum (FCS) to detect damage.
• Layer 3, Network: IP addresses that work across the whole internet, and routing, which picks the path through many networks.
• Layer 4, Transport: delivers data to the right program using port numbers. TCP adds reliability; UDP is fast and simple.
• Layers 5 to 7: in practice, apps and protocols like HTTP, TLS and DNS handle these jobs together. That is why the TCP/IP model merges them into one Application layer.

Troubleshooting with layers

When "the network is down", work through the layers in order. The bottom-up method starts at Layer 1: is the cable plugged in and is the port light on? Then Layer 2: does the switch see the MAC address? Then Layer 3: does the PC have the right IP address and gateway, and does ping work? Then Layer 4 and above: is the service listening on its port, is a firewall blocking it?

Layer 1  Link light off?            -> reseat or swap the cable
Layer 2  MAC not in switch table?   -> check port, VLAN, port security
Layer 3  Wrong IP or gateway?       -> fix addressing, test with ping
Layer 4  Port blocked or closed?    -> check firewall rules, the service
Layer 7  App error (HTTP 500)?      -> it is the application, not the network