What is OSI Reference Model

Over the years, communication has been continuously evolving. Few years back sending love letters through snail mail was a hit, text messaging rose to popularity, Yahoo Messenger was on the limelight, and then we fast-forward to this moment, whose everybody is communicating through various devices – handheld, portable, or even classic desktop computer.

Due to the high-speed interconnectivity of today and the explosion of devices, digital communication has never been on the same league of the past. Indeed, communication has never been easier. But with all these accomplishments, how does digital communication really communicate on those various devices and platforms?

The answer is simple, thanks to standards – particularly to the OSI Reference Model.

In sometime of 1970s, International Organization for Standardization (ISO) and International Telegraph and Telephone Consultative Committee (CCITT), independently to each other, developed documents with the goal to create a unifying standard for the architecture of networking systems. And on by 1983 the two documents were merged into one document standard as The Basic Reference Model for Open Systems Interconnection, or most commonly known today as Open Systems Interconnection Reference Model.

osi

Open Systems Interconnection (OSI) Reference Model is a conceptual guide used to define how communication works between devices and platforms. With this guide, different vendors are able to make their products interoperable to each other.

The OSI Reference Model is divided into 7 layers. Each layer performs specific functions and services to the layer above and below.

Layer 1: Physical – defines the physical and electrical medium for the data transfer
Components: cables, jacks, path panels, hubs
Concepts: topologies, baseband vs. broadband
Unit: bits

Layer 2: Data Link – establishes, maintains, and decides how transfer is accomplished over the physical layer, and ensure error-free transmission over the physical layer
Components: MAC address
Unit: bits

Layer 3: Network – controls the operation of routing and switching information to different networks
Components: IP addresses, subnets

Layer 4: Transport – ensures messages are delivered error-free, in sequence and with no losses or duplications
Components: ports, protocols
Unit: segments

Layer 5: Session – manages session establishment, maintenance and termination between network devices
Unit: Data

Layer 6: Presentation – translate the data format from sender to receiver in the various platforms that may be used
Concepts: data compression, data encryption
Unit: Data

Layer 7: Application – serves as a window for users and application processes to access network services
Unit: Data

Analogy: For you to better understand the OSI Reference Model, try to treat it as an ordinary guide with several parts. For now forget about its technicality, and imagine you’re about to get to your work.

First part – You have plenty of means of transportation to get to your work: bus, cab or use your own car.

Second part – You chose to use your own car, and it seems the city roads are in good quality – no potholes.

Third part – Although the roads are in good quality, you need to know what the traffic conditions along your journey are. So you check your GPS with traffic insight and chose the route with lease traffic.

Fourth part – You have reached the building where your office resides, but before getting to the elevator everyone is required to have a quick physical test – to know if a person is not work-ready for the day.

Fifth part – You are declared as work-ready! Now you just have to show your building badge, as your identity and access to the building facilities – like the elevator.

Sixth part – While you’re in the elevator, you comb your hair using your fingers because the company is strict about their employees’ appearance.

Seventh part – Now you are in the office and sitting on front of your desk and able to work with your boss and co-employees.

Seven layers and its corresponding words seem hard to remember, that’s why people choose to memorize this mnemonic phrase:

Please do not throw sausage pizza away!” – although personally speaking, it seems irrelevant to the OSI.

To learn more about fundamental concepts in networking, visit Microsoft Virtual Academy and search of Networking Fundamentals course.

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