7 Layers Of The OSI Model – How To Peel Back The Layers

7 Layers Of The OSI Model

7 Layers Of The OSI Model

Let’s take a closer look at the 7 Layers of the OSI Model…

The OSI (Open Systems Interconnection) model is a method that was developed in 1983 by database design professionals to depict the interaction of various protocols and applications on NAD’s. It has since become the X200 recommendation of the ITU-TS and an international standard adopted by the ISO.

This universally recognized systematic drawing of message transmissions assists product implementers by establishing a consistency in design ensuring compatibility in use. The Open Systems Interconnection model is the standard reference of the IT industry describing 7 functional layers which take place on each communication end. Although not always adhered to in all layers, this common model for telecommunication serves as a valuable single reference in discussions by certified IT Pros. In education, the understanding of this universal model of telecommunication is essential for CCNA IT certificates.

7 Layers Of The OSI Model – Two Important Sidebars…

  • First – The most commonly used everyday protocols, such as the TCP/IP, employ a modified system of the 7-layer model and uses only 4-6 layers. However, the standard 7 layers of the OSI model used in learning IT principles of intercommunication is still referenced in discussions and in the exchange of ideas.
  • Second – Some refer to the 7-Layer model as the ISO model due to the Open Systems Interconnection group initially working within the ISO. Both models are identical.

Traditional diagrams of the Open Systems Interconnection model are from the bottom-up, Layer 7 on top and Layer 1 on bottom, and is useful in explaining the location of protocols and devices found in various data center operations. The 7 layers can be considered in 2 groups according to messaging protocol. The top 4 (Layers 7-4) are utilized in message transmission to or from a user. The lower 3 (Layers 3-1) are used in transmission via the host server. Transmission to the top layers are only allowed for the intended host.

 

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Lesson #4 of the Cisco CCNA training course covers the OSI Model in great detail. In addition, the TCP/IP Model and the Encapsulation Process are also covered.

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7 Layers Of The OSI Model

7 Layers Of The OSI Model

The 7 Layers Of The OSI Model are as follows:

  • Layer 7 – The Application

This layer is the identifying and validating level of communication. 3 main areas of functioning of the application service provider are naming desired communication and users, determining the availability of resources and synchronizing the applications communication. End-user communication protocols like SMTP, FTP, TFTP, HTTP and telnet live here.

  • Layer 6 –  The Presentation

This layer functions as a dual-traffic conversion and coding of data for the packets sent and received from Layer 7. It also ensures data bundle commonality between each end. Examples that live here include JPEG, GIF, and Quicktime.

  • Layer 5 –  The Session

This could be considered the control layer of the model. The session establishment, maintenance and termination amongst top layer applications happens here. A vital layer for E commerce due to the user purchasing products via a “shopping cart”. Layer 5 prevents the interference of user shopping and load-balancing across other servers. SCP, RPC and Apple Talk’s ZIP are found here.

  • Layer 4 – The Transport

The responsibility for the transmission of data, both reliable and connectionless (wireless) is the role of layer 4. The transport layer does this by end to end flow control, data stream organization of upper layers (5-7), error checking, recovery and management of virtual circuits. Powerful communication protocols such as TCP operate on this layer.

  • Layer 3 – The Network 

This web access layer defines the hierarchy of the data flows for connections, grouping devices together by common web addresses. This is the IP home. All network routers operate on this level.

  • Layer 2 –  The Data Link 

Data transmission over a physical medium is defined by this layer. Devices such as switches and bridges on the Ethernet depend on this layer. LAN’s using MAC addresses and WAN’s live here.

  • Layer 1 – The Physical layer

The electrical and physical connections of the web characterize this layer. NIC cards and router interfaces operate on this level.

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