Table of Content

• Overview
• The Seven Layers of the OSI Model
  • Application Layer
  • Presentation Layer
  • Session Layer
  • Transport Layer
  • Network Layer
  • Data Link Layer
  • Physical Layer
• Mnemonic for Remembering the Seven Layers
• Protocol Data Unit (PDU)
• Summary Table

Overview

Introduced in 1977 by the International Organization for Standardization (ISO), the OSI model serves as a reference that describes how data from an application on one computer is sent to/receive from another application on another computer. The OSI (Open System Interconnection) reference model consists of seven layers, where each performs a specific communication function to support reliable inter-computer data transfers.

The purpose of dividing the framework into seven layers is to help the development, management, and troubleshooting of network applications and hardware be more defined and simpler. Each layer operates independently from each other where each can perform its own specific communication task without dependency from other layers. The top most layer of the OSI model is the application layer, whereas the physical layer is the lowest.

If your job is or will be a network engineer, network security, router technician, CCNA, CISSP, or even a system administrator, having an understanding of the OSI model is crucial to effectively perform the job. This article is intended for novices with the aim of providing basic information without too much technical details as a high level overview of the OSI 7-layer reference model.

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OSI 7-Layer Reference Model
Layer 7	Application
Layer 6	Presentation
Layer 5	Session
Layer 4	Transport
Layer 3	Network
Layer 2	Data Link
Layer 1	Physical

The Seven Layers of the OSI Model

Application Layer

The Application layer, the top most layer of the OSI model, serves as the interface to the user's application. This Application layer, despite its name, is not referring to 'Microsoft Word' or any other user software. It refers to application services on a device used for file transfers, emails, network printing, telnet, network file browsing, and other network-related services.

Presentation Layer

The sixth layer, Presentation layer, is responsible for ensuring data is encoded in a fashion that can be understood at the other end. It converts data from one data format into another so that two devices can communication. This is analogous to a person translating a book written in English into Russian - the story (information ) is the same, but it is 'presented' in a format that a Russian-speaking person can understand. In computer, such a translation would be ASCII to EBCDIC, and visa versa.

Session Layer

The Session layer, layer five of the OSI model, handles the creation, management, and termination of connection between applications. Sessions ensure data for one application is isolated from data for another.

Transport Layer

This fourth layer is responsible for data flow control and error-handling/re-transmission to ensure reliable delivery of segments. In other words, the Transport layers makes sure data are delivered intact - no errors, no losses or duplicates, and in sequence.

TCP/IP port numbers are used at the Transport layer to ensure data is delivered to the correct application for handling. For example, port 21 is used by FTP (file transfer protocol) and port 53 is used by DNS (domain name system).

Network Layer

This third layer, Network layer, is where routing is handled. Routing takes place when data for a device is on a network that is different than the sending device. This is where network routers operate - handling the transmission of data between different network. Because of this, routers are often called layer 3 devices. If data is being sent from and received by devices that are on the same network (subnet), no routing needs to take place.

Data Link Layer

Data link is the second layer of the OSI model and is responsible for ensuring successful transmission of the data from one device to another on the same network. It accomplishes this by managing flow control and data errors. This layer is divided into two sub-layers:

• Media Access Control (MAC) Layer - Determines the physical (aka hardware) address (the 6-byte 'burned-in' MAC address of a network device) of hosts.
• Logical Link Control (LLC) Layer - Controls frame synchronization, flow control, and error-checking (such as using Common Redundancy Check (CRC)).

This layer makes use of a network device's MAC address to receive and forward data. Because most network switches uses MAC addresses, to move data to the correct destination, they are often called layer-2 switches.

Physical Layer

This first layer of the OSI model is where physical circuits are made. This layer involves the network components such as network cards, repeaters, hubs, and any component that takes data (in the form of zeros and ones) and converts them into electrical, optical, or radio signal, and visa versa.

The network cables (e.g., twisted pair, coaxial cables, fiber optic cables) or any other transmission medium used to transport the signal (data) is considered to be at layer 0, which is included in the 7-layer OSI model. However, many include the transmission medium as part of the Physical layer, layer 1, of the OSI model.

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Mnemonic for Remembering the Seven Layers

There are a few popular mnemonics used to help professionals and students remember the seven layers of the OSI layer in the right order. These mnemonics take the first letter each layer's name to then form an easy to remember phrase. For the OSI model, we have the following letters: A, P, S, T, N, D, P. These letters are used to form a memorable phrase.

For example, starting with the application layer (layer 7), the following mnemonics are commonly used to help with memorizing the OSI layers in order from layer 7 to layer 1:

• All People Seem To Need Data Processing.
• All Pros Search Top Notch Donut Places

Alternatively, starting with the physical layers (layer 1), there are the following mnemonics:

• Please Do Not Throw Sausage Pizza Away
• People Don't Need Those Stupid Packets Anyway

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Protocol Data Unit (PDU)

The information handled by each layer is called a protocol data unit (PDU). It consists of protocol-specific control information along with the data itself. As a PDU passes down (or up) each layer, each will add (or remove) it's own protocol information. At each layer, the PDU has a different name to reflect its function. The adjacent table shows the name of the PDU at each layer.

PDU Name	OSI Layer
Data	Application
Data	Presentation
Data	Session
Segments	Transport
Packets	Network
Frames	Data Link
Bits	Physical

Summary Table

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