Understanding the Open Systems Interconnection Model

The OSI model is a visual representation of how network functions work

Abstract details of computer servers
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The OSI model defines networking in terms of a vertical stack of seven layers. The upper layers of the OSI model represent software that implements network services like encryption and connection management. The lower layers of the OSI model implement hardware-oriented functions such as routing, addressing and flow control. All data that goes over a network connection passes through each of the seven layers.

 

The OSI model was introduced in 1984. Designed to be an abstract model and teaching tool, the OSI model remains a useful tool for learning about today's network technologies such as Ethernet and protocols like IP. The OSI is maintained as a standard by the International Standards Organization. 

The Flow of the OSI Model 

Data communication in the OSI model starts with the top layer of the stack at the sending side, travels down the stack to the sender's lowest (bottom) layer, then traverses the physical network connection to the bottom layer on the receiving side, and up its OSI model stack.

For example, Internet Protocol (IP) corresponds to the Network layer of the OSI model, layer 3 (counting from the bottom). TCP and UDP correspond to OSI model layer 4, the Transport layer. Lower layers of the OSI model are represented by technologies such as Ethernet. Higher layers of the OSI model are represented by application protocols like TCP and UDP.

The Seven Layers of the OSI Model

The bottom three layers of the OSI Model are referred to as the Media Layers, while the top four layers are the Host layers. The layers are numbered from 1 through 7 beginning at the bottom. The layers are:

  • Application layer (layer 7) - Network Process to Application. This end-user layer packages the data received from the Presentation Layer in the format needed by the application or end-user process that receives it. Examples include browsers, SMTP, HTTP, and FTP. This layer also creates what is to be sent back to the Presentation Layer.
  • The presentation layer (layer 6) - Data Representation and Encryption, including format conversions. Think of this layer as the translator. Examples include ASCII, TIFF, JPEG, MIDI, and MPEG.
  • Session layer (layer 5) - Interhost Communication. This layer manages multiple types of communications and sends data to logical ports, including those using NFS and SQL.
  • Transport layer (layer 4) - End-to-End Connections and Reliability. As the name implies, this layer moves data across network connections, usually using TCP. It also handles error recovery and re-transmissions.
  • Network layer (layer 3) - Path Determination, IP, and Routing. Layer 3 formats data as packets. Directs the data to the correct physical path.
  • Data Link layer (layer 2) - This is the most complex layer in the OSI model, and it is sometimes divided into two parts: one for media access control and one for logical link control.
  • The physical layer (layer 1) - Media, Signal and Binary Transmission. Examples include hubs, repeaters, and Ethernet cables. Data is transmitted by an electric voltage, radio frequencies, infrared or ordinary light.

Having trouble remembering the layer order? Just keep the phrase "All People Seem To Need Data Processing" in mind.