The OSI (Open Systems Interconnect) Model is a concept used to describe the functions of a network. It is seperated into 7 layers, which each function independently. The seperation into layers helps two things: learning about networking, and creating hardware or software that interfaces with a network. It helps you to learn networking because it seperates a complex system into logical parts. It helps to create hardware or software because your device or program only has to work at one layer of the model. This also helps with scalability. The layers use peer-to-peer communication to pass data through the model.

The seven layers are:

• Layer 7: Application - Interfaces between an application and the rest of the network. For example: connecting an e-mail client to a network.
• Layer 6: Presentation - Handles data syntax, format, compression, structure, and encryption.
• Layer 5: Session - Responsible for communication between two hosts, applications, devices, etc. Initiates, maintains, and terminates the connection.
• Layer 4: Transport - Responsible for data reliability. Requires confirmation that data was recieved (ie: handshakes in TCP). End to end communication.
• Layer 3: Network - Best path determination (route selection). Uses a logical address, like IP.
• Layer 2: Data Link - Access to the media, flow control, logical topology. Uses the physical address (MAC).
• Layer 1: Physical - Charicteristics for whatever networking media (twisted pair, fiber optic, coaxial) you're using.

At each layer, the data is encapsulated in a protocol data unit (PDU):

• Layer 7: Data
• Layer 6: Data
• Layer 5: Data
• Layer 4: Segment
• Layer 3: Packet
• Layer 2: Frame
• Layer 1: Bit

Some mnemonics to help remember the layers:

• All People Seem To Need Data Processing
• Programmers Do Not Throw Sausage Pizza Away (reverse mnemonic)
• ...and my personal favorite: All People Studying This Need Daily Prayer