The OSI Model

Purpose of the layered model

Designing a complete communication system requires electrical signals, addressing, error checking, data formatting, application interfaces, and many other functions. Describing all of these functions at once makes design difficult. The OSI model organizes them into layers.

The Open Systems Interconnection (OSI) Model is an abstract reference model that organizes the functions of a telecommunication or computing system into seven logical layers. It is not an implementation specification. Its primary purpose is to help systems from different manufacturers interoperate through a common reference point for designing network protocols.

Principles of layered communication

Communication in a layered model follows several principles:

• Abstraction: Each layer performs a specific, well-defined set of functions. Its operation does not require knowledge of the details of the layers above or below.
• Peer-to-peer communication: In the model, Layer N on the sending machine communicates logically with Layer N on the receiving machine. Peer layers use the same protocol.
• Services and interfaces: In an implementation, data passes vertically. Each layer provides services to the layer directly above it and uses the services of the layer directly below it through a well-defined interface.

The Process: Encapsulation and Decapsulation

As data moves down the stack on the sending device, a layer may add control information in a header or trailer. This process is called encapsulation. It resembles putting a letter into an envelope and then putting the envelope into a larger courier pouch. The unit of data at each layer has a specific name, known as a Protocol Data Unit (PDU).

The receiver performs the reverse process, decapsulation, removing information added during encapsulation as the data moves up towards the application.

Protocol Data Units (PDUs) by Layer

• Layers 7, 6, 5 (Application, Presentation, Session): The PDU is generally referred to as Data.
• Layer 4 (Transport): The PDU is a Segment (for TCP) or Datagram (for UDP).
• Layer 3 (Network): The PDU is a Packet.
• Layer 2 (Data Link): The PDU is a Frame.
• Layer 1 (Physical): The PDU consists of Bits. In some transmission techniques, bits are represented by Symbols.

An Analogy: Sending an Office Memo

The roles of the layers can be shown through an office message sent by a project manager named Erik to a manager named Louis in another building.

1. (Layer 7) Application - Erik, the Project Manager: Creates the original message or request (e.g., "Request for Q4 sales data").
2. (Layer 6) Presentation - The Translator: Translates the message into a standard company format and language that Louis' office will understand. It may encrypt the message for confidentiality.
3. (Layer 5) Session - The Cryptographer/Session Clerk: Establishes and manages the formal communication "session" with Louis' office, ensuring both sides are ready to communicate.
4. (Layer 4) Transport - The Secretary: Takes the long message and splits it into numbered pages so it can be reassembled in the correct order. In this example, it represents reliable transport and handles acknowledgements.
5. (Layer 3) Network - The Postmaster: Puts the pages into an envelope with Louis' full mailing address (street, city, zip code). Chooses an overall route for the mail truck.
6. (Layer 2) Data Link - The Mailroom Clerk: Takes the envelope from the postmaster and puts it in the correct local courier bag for the next stop (e.g., the bag going to the downtown postal hub). Adds a local delivery tag.
7. (Layer 1) Physical - The Courier (Van, Plane): The physical means of transport. The courier is responsible for moving the item from point A to point B.

The Seven Layers of the OSI Model

The seven OSI layers perform the following functions, from top to bottom.