Overview of the OSI Model
Think of a scenario:
- You open your PC
- You open the Mozilla Firefox browser
- You write an URL address of a website in the address bar of browser
- You can now see the website on your computer screen
Have you ever think how does this thing happen? You write some words after "http:" and "//" and your computer finds the exact website and present it right before your eye.Do you think your PC hard disk/memory has whole world’s websites stored in it and you just make a search in your personal hard disk and pick one of them? Definitely not.
We use a technology called 'internet' to view a website on our PC screen. Internet is nothing but billions of interconnected computers. When you typed the url, your browser connects with a remote computer called server that stores that particular website in its memory.When your PC is with internet, then it is connected to billions of PCs around the world! Wala....Ya! No need to be so happy at all. Because there are things you need to worry about. That is security; otherwise all the personal data stored in your computer will simply leak all over the world.
That’s why International Organization of Standardization (ISO) set a standard model of communication from PC to PC over internet in 1978. This is called Open Systems Interconnection (OSI) Model. All the software and products running on a computer need to meet the OSI model’s design specifications.
OSI model sets 7 layers or steps for data transmission over network; from computer to computer. These Layers are:
- Application Layer
- Presentation Layer
- Session Layer
- Transport Layer
- Network Layer
- Data-link Layer
- Physical Layer
Data traveling between systems pass through these seven layers on each system or computer. For example if two PC is communicating over internet, then data transmission in one PC may starts from Application Layer and ends at Physical Layer. While the data reach the other end, it must follow the reverse path from Physical Layer to Application Layer in another PC.
Let’s make an analogy to describe this layer to make these layers more understandable. Think, of the first scenario, you are typing an address of website (http://www.domain.com) in your Mozilla Browser.What is the activity you just performed? You are actually requesting your computer to show that particular website by retrieving it from internet. This Firefox or any software installed on your computer works in the application layer
The application layer points your typed URL address to be moved into presentation layer which deals with serialization, encryption or decryption. In serialization process your text (http://www.domain.com) is converted into sequence of 0s and 1s (bits) so that it can be transmitted across network.
Encryption happens in this layer as well. Encryption means transforming information with algorithm to make it unreadable expect the requested server. For example, you are typing your username and password in Gmail login page. Gmail will encrypt the request in presentation layer so that your password can only be readable at Google server; otherwise there is a risk of your password being stolen by someone.
The next layer of data transmission path is session layer. This layer talks with the remote PC or server to establish, manage and terminate the connections between your local PC and remote server. For example, if you close your Mozilla browser then the connection between your PC and remote server of that website (http://www.domain.com) will be terminated by session layer.
The data will enter the transport layer next. Remember, your url request is converted into bits in presentation layer. In transport layer, those bits (0s and 1s) will be packatized for further forwarding to next layer. This layer works as a post Office or courier service who transfer your packet (envelop) to it’s destination.This layer is also responsible to send acknowledgements to confirm that data was properly received, like most often you might see information in a browser like “server not found” or “your request can’t be processed further”.
The next layer of OSI model is Network Layer. Network layer writes address on each packet sent by transport layer. We write addresses on a letter envelop before posting it. But for internet communication this address need to be written in machine recognizable format which is port number and IP address Network layer put port numbers and IP in the header of each Transport Layer data packet. This process is called logical addressing.
Network Layer also determines the path of travel for packets sent by transport layer. This function is called routing. Fragmentation and reassembly is another function in this layer. If the data packet sent by transport layer is too large, the network layer splits those packets before sending to Data Link Layer.
In the Data link layer, the data packets are transformed into frames. Frames are the units of data link layer protocol. This layer use to stamp each frames with source and destination MAC address to ensure that data intended for a specific machine gets to it properly.
MAC address is unique serial number provided in the network card (hardware) of each computer so that the device could be uniquely identifiable amongst all other network devices in the world.
Data link layer also ensure error-free transfer of data frames from one system to another.
For each data frame the Data-link layer sends, it seeks an acknowledgement. If it doesn’t receive one, it sends the data frame again.
Physical layer is the only hardware layer in whole OSI model. The physical wires like fiber optic cable, twisted-pair cable, coaxial cable, etc. through which a device or a computer is connected to internet is in Physical layer. At this layer, raw, binary data frames transmit over internet; from one system to another.
Now, let’s think about the reverse side. When your browser request has received by the server it sends the website information through the physical layer of your computer. Data link layer receives the frames and match the MAC address with requested frames before sending it to Network layer. Network layer checks the IP address and send it to transport layer for packatization. The continuous transmission of these packets is maintained by session layer. If the sent packets need any decryption, then presentation layer performs it. At the end, the application layer i. e. the Mozilla browser visualizes the website on your computer screen.