OSI Networking Model

Exam: 640-822 - Interconnecting Cisco Networking Devices Part 1


OSI networking model does not exist any longer in the networking model due to the existence of TCP/IP though some of the original protocols referenced by the OSI model still exist. The terminology from the OSI model still remains today. We need to understand the OSI technology to understand the modern technology.

Comparison between OSI and TCP/IP

From the basic point of view, there are many similarities between OSI and TCP/IP. The OSI model constitutes of seven layers with each layer defining a set of typical networking functions. It refers to multiple protocols. New protocols or standards were not created in OSI instead referenced other protocols.

Comparison of OSI and TCP/IP Models

Comparison of OSI and TCP/IP Models


There are two ways in which OSI terminology is still used:

  • Describing other protocols
  • Describing the encapsulation process

OSI layers:

The bottom four layers of the OSI model maps closely coincide with the TCP/IP model.
Layer 7: Defines processes for user authentication along with providing a link between the software and applications that are used for communication.
Layer 6: This defines and negotiates various formats of data like - binary, BCD, JPEG, ASCII and EBCDIC text. OSI defines encryption as a presentation layer service.
Layer 5: This is a session layer that outlines starting, controlling and ending conversations. It permits the presentation layer to have an unhindered view of data stream that is coming in.
Layer 4: This makes available a variety of services with the area of focus on problems that are related to delivering data to another system.
Layer 3: This is a network layer that covers three primary features. The first is logical addressing, the second feature is routing and the third and the last feature is determination of path.
Layer 2: This is the data link layer that outlines the rules determining when a device can be used to send data over a specific medium.
Layer 1: This layer refers to the standards laid down by other organizations. These cover physical attributes of the medium being used for transmission and includes connectors, currents, pins, encoding and light modulation. It is inclusive of the rules of activation and deactivation of the physical medium.

Advantages of layered protocol specifications:

  • Few complexities
  • Standard interfaces
  • Easy to grab
  • Easy to develop
  • Multivendor interoperability
  • Modular engineering

OSI Terminology:

Describing protocols- often TCP/IP protocols and standards are described by referencing OSI layers by layer number and layer name. Even today, the protocols are defined using the OSI layers. For instance, we say that IP is a network layer protocol.
Encapsulation process: similar to TCP/IP layer, each OSI layer asks for service from the next layer. The lower layer encapsulates the higher layer's data behind a header. OSI uses a generic term: Protocol Data unit where as TCP/IP use terms such as segment, packet, and frame.


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