osi-model

OSI - Open Systems Interconnection Reference Model

The OSI is a guide that simplifies how we think about how data travels through a network. This is not a standard or a protocol. It is, for all intents and purposes, a mental model used for understanding and communicating network principles.

Pneumonic Devices

Top Down: A Priest Slapped The Nun During Prayer

Bottom Up: Please Do Not Touch Steve’s Pet Alligator

The Layers

1 - Physical Layer

The cables or radiowaves that carry signals. A problem here means that a cable is broken or disconnected

2 - Data Link

Most fundamental communication that happens across the network

3 - Network

AKA the Routing Layer. IP addresses will deal with fragmenting data in the frames so that it can transverse different networks.

4 - Transport

The “Post Office” layer. Adds the port to the data packet.

5 - Session

How applications talk across the network. Responsible for establishing, managing, and terminating the session

6 - Presentation

Formats the information so that it is understood by the receiving system. Shares several responsibilities with layer 7.

7 - Application

What the user sees, e.g. a browser. Responsible for initiating or services the request.

Asynchronous Transfer Mode (ATM)

This is an old replacement layer for the OSI Datalink layer. It is the basis for SONET/SDH, on which PSTN and ISDN are based.

IP based networks have largely replaced ATM.