Before i begin I would love to appreciate these two great minds @greenrun and @abigail-dantes for supporting and encouraging me just when i needed it.

No matter how complex a chemical compound is, it definitely has root in the periodic table. The same is applicable to OSI model. The internet is growing in complexity everyday but it has its root in the OSI model. It is a guide to the development of internet technology. The OSI (Open System interconnect) model was developed by the ISO (International Standards Organization) with the sole aim of standardizing various functions performed by the telecommunications system irrespective of the technology applied in developing and designing the system. The model is in the form of layers (abstract) which are seven (7) in number. The layers inter operates by having a lower layer serve the layer directly above it.

A network is the interconnection of two or more computers (not just PC) while an inter-network or internet for short is the connection of two or more networks using a network device observing certain protocol(s) with a standard addressing scheme (IPv4 or IPv6). The model was introduced in 1983 and the concept of seven layers was put forward by Charles Bachman. To really appreciate the concept of the model, imagine a situation whereby you open a web page and a pop up comes saying "this is a Macintosh server you cant view its content cause you're in a windows environment", or you type an address on your search bar and you get a pop up saying "your internet gateway is a Cisco equipment, you can't go through a Juniper router".

When you set up a hotspot or Xender connection, you've already created a network and any device that has a functional wireless fidelity can connect to your network and will be able to not only communicate with you, but also the internet if you have internet access. In the second paragraph, three words are in highlight which are network, device and protocol (s). Initially devices can "talk" to each other only if they're of the same manufacturer but the introduction of the model not only enforced interoperability, but also ensured specialization ease of understanding of the whole internet technology.

credit: Fossbytes
As shown in the figure above, the model comprises of seven layers which are; the Application layer,Presentation layer, Session layer, Transport layer, Network layer, Data link layer and Physical layer. This abstract classification is so important that every network engineer must know and recognize them faster than their own names. There is a popular mnemonic code for remembering them and this is " All People Seem To Need Data Processing" ( just like the periodic table has its own). I'll take a top-down approaching to explaining these layers.

The Application layer or simply layer 7, is the layer that interact with computer programs that interact with us. It is the closest to the internet user. For instance, when you are on your steemit.com blog try and off every internet related connection or even go as far as removing your network interface card, then come back to the page and click on "Post". The steemit site performs a lot of caching functions in order to combat data loss. The post button functions because the page is already cached to your computer memory, hence no need making request to the network layers. But the moment you click on "Post" button that submits your posts without network in place, an error message is returned because there is no layer interaction which helps retrieve or submit requests to the steemit server(s). I'm not saying the browsers exist in the application layer, they communicate with the program that communicate with application layer.

Hence, the application layer performs two major functions which are;

Searches and confirms connectivity between parties that wants to establish communication

Ensuring that there are enough resources needed to carry out the communication process.

It is obvious that for the application layer to perform the above functions, it needs information from the layers below it. Typical examples of the applications that reside in this layer are the Domain Name System (DNS), the Simple Mail Transfer Protocol (SMTP), the Trivial File Transfer Protocol (TFTP), the File Transfer Protocol (FTP) and many more.

The Presentation layer performs the function of presenting standard formatted data to and from the application layer.

Within an environment say, IOS, images can take different format but when that image is being transferred outside the native environment, it must be formatted using standard format for instance standard image formats are "jpg", "png", etc. Since application layer monitors resources during communication, the presentation layer could be asked to perform functions like data compression and decompression, data protection functions like encryption and decryption.

The Session layer is charged with the function of creating and tearing down sessions between peer layers (presentation layers of another computer). When the need to create different connections between hosts arises, the session layer is called upon. This "communication" is can be in three different forms; full-duplex, half- duplex and simplex.

When data is exchanged in full-duplex mode, both computers can send and receive information at the same time without losses or collision. This is the best and the fastest method of communication. In half-duplex mode, information flow can be in both direction but on a turn-by-turn basis. When both devices speak at the same time, a collision is said to occur. This is not an optimal form of communication. This form of communication is usually found on devices like the obsolete hub. An illustration for the half-duplex communication is the communication done using a walkie-talkie. The Simplex mode is the communication form whereby information or data flow is in only one direction. A typical example of simplex mode of communication is the communication radio broadcasting services and the communication between Idi Amin and his subjects, (that was a joke) lol.

The Transport layer perform roles such as breaking up lumps of data and resembling them at the desired destination. The most popular protocol available in this layer are the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP). The UDP is used mostly for voice related traffic since it has low overhead. When UDP is used for transfer of information, there is no means of acknowledgement of reception of the data sent, hence any data lost during the course of transfer is lost for good.

credit: Firewall
When making calls, when the voice of the person at the other end is not "clear" or you didn't hear a particular sentence very well, you urge the person to repeat, this happens because the voice packets are just delivered to and fro the telephone channel without ensuring data integrity. The TCP ensures that data transmitted is received by the receiving device by requesting for acknowledgment. This form of data transfer makes network equipment perform more task (more overhead). The image above summarizes this point.

The Network layer just as the name implies is responsible tracking devices on the network. This include routing functions. The network layers selects the best path to a remote device. Devices found in this layer are called Routers. To perform these functions, the router maintains what is called routing table. This table contains the detailed list of all the networks known to the router. Network addressing is also done at this layer. I have a post that explains IP addresses, you can check it here.

The Data Link layer makes provision for the physical transfer of data. Devices that occupy this layer are called Switches. Switches does not and can not leave a network. Hence networks created using only switches are called Local Area Network (LAN). Switches formats data in what is called frames. Frames contains information concerning where information should be sent, details of this information, and ensures that this information is not modified. Data link layer is subdivided into two; the Media Access Control (MAC) and the Logical Link Control (LLC). Heard about MAC address before?

The Physical layer does the function of sending and receiving information in from the upper layers in the form of ones and zeroes, in bits. Devices that exist in this layer include connecting equipment like Cables, Assess Points, Fiber Optic technologies, etc.

REFERENCES

1. Layers of the OSI -lifewire
2. OSI model-wikipedia