Why ICT professionals need to understand the OSI model.

Why ICT professionals need to understand the OSI model.

Posted by on Aug 16, 2012 in Enterprise, Front Gallery | 4 comments

Why ICT professionals need to understand the OSI model.

I have simply lost count of the amount of IT professional’s I have encountered that simply have no idea of what this is.

This, everyone is what sets you apart from the rest, this….. is your never wavering friend that helps you through almost every technical issue you encounter…..this….will simply set you free. Silly cliché’s aside,  this OSI model helps me every day, every single day.



To have a fundamental understanding of the frameworks involved in all that button poke-jiggery that you do everyday in your ZEN style IT guru-ness is crucial when random button pressing fails and nobody has really bothered documenting their trials and tribulations on Google in a step by step guide that doesn’t really require you to understand whats actually going on.

It is what you refer to when all else fails, well to be blunt it’s what you should refer to straight away. A very simple example, “My internet does not work” –  “is the cable plugged in?” = Layer 1 and so on. Here is a shameless rip off a website that explains what it is.

The OSI Model

The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, and proceeding to the

osi model

osi model

bottom layer, over the channel to the next station and back up the hierarchy.

Application (Layer 7)

This layer supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is application-specific. This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.

Presentation (Layer 6)

This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.

Session (Layer 5)

This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination.

Transport (Layer 4)

This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.

Network (Layer 3)

This layer provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling,congestion control and packet sequencing.

Data Link (Layer 2)

At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sub layers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sub layer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.

Physical (Layer 1)

This layer conveys the bit stream – electrical impulse, light or radio signal — through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components.


Now whilst this all seems rather boring and technical, I cannot stress enough how using the model to debug the next thorny issue you have at hand will make such an epic difference to the time it takes to rectify.

Which in turn will only add to your IT guru-ness. (if such a term actually exists)

By the way, here’s a hint, always start at Layer 1.




  1. Thank god there is someone else out there who actually “thinks” about how to attack a problem. The guys that surround me are sick of hearing me say “draw a picture”, “now draw another one” – it is ONLY by understanding the simple relationships between system components at EACH layer of the OSI model that you can have ANY hope of completely understanding the complexities of modern systems!!!!!

    Thanks Lance!

  2. I remember studying this in University, and thinking it was a mythical crock of sh*t and completely irrelevant to anything in reality. It seemed to be all theory, and nothing practical to actually acheiving anything.

    It wasn’t until my CCNA that I actually understood the relevance of the model.
    It’s crazy what starts being important when you progress from ‘break-fix level 1 support’ IT to the first port of call when things get hairy.

    • Yes, Its amazing how it does actually help, its just a shame more people do not believe that having a fundamental understanding of what they are touching is necessary.

  3. Good article,

    may I suggest including a real world example with round trip. eg. Application X at site B, sitting on Server C as used by person D.

    It also amazes me that vendors and software developers do not know the underlying topology on which they develop.




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