What is the OSI Model?
The open systems interconnection (OSI) model is a conceptual model created by the International Organization for Standardization which enables diverse communication systems to communicate using standard protocols. In plain English, the OSI provides a standard for different computer systems to be able to communicate with each other.
The OSI Model can be seen as a universal language for computer networking. It’s based on the concept of splitting up a communication system into seven abstract layers, each one stacked upon the last.
Why does the OSI model matter?
Although the modern Internet doesn’t strictly follow the OSI Model (it more closely follows the simpler Internet protocol suite), the OSI Model is still very useful for troubleshooting network problems. Whether it’s one person who can’t get their laptop on the Internet, or a web site being down for thousands of users, the OSI Model can help to break down the problem and isolate the source of the trouble. If the problem can be narrowed down to one specific layer of the model, a lot of unnecessary work can be avoided.
The OSI model defines 7 layers:
- Application Layer
- Presentation Layer (Operation System)
- Session Layer
- Transport Layer
- Network Layer
- Data Link Layer
- Physical Layer
Application Layer is the layer that the user interacts with, like firefox, internet explorer, outlook, etc. It’s the actual applications that is used to send information back and forth. Possible problems with this layer include mis-configurations or corruption of applications ie. a plugin breaks Firefox.
Underneath the Application Layer is the Presentation Layer, or Operating System (OS) in other words.
The Presentation Layer (OS) is the software that serves as the connecting platform between hardware & software resources and provides common services needed for computer programs to function off of.
Underneath the the Presentation Layer is the Session Layer.
The Session Layer deals with the communication between 2 computers. You can say that the 2 computer needs open up a “session” to communicate with each other. ie. if you go to a website, your computer has to make a session with the web server you are trying to access. Another example is if you go to a bank account website from your computer; your computer and the banking website server makes a session between each other to communicate. Possible problems within the session layer include website server mis-configurations. For example if the PHP or Apache configurations changed or became corrupted.
Underneath the Session Layer is the Transport Layer.
The Transport Layer deals with the back & forth communication of data. The Transport Layer decides how much information should be sent at one time. So when your communicating with a website, this is the layer that decides the rate of communication packets your computer will send and receive.
Note that the Transport Layer implements a procedure called Windowing. Windowing is a procedure where computers decides how large a block of information is to be sent and how long the receiving computer should wait until it receives an acknowledgment from the other server/computer.
Underneath the Transport Layer is the Network Layer.
The Network Layer operates the router. In other words, the Router resides on Network Layer. Also included on the Network Layer is the IP Address, Default Gateway, Subnet Masks and DNS (Domain Name Server, which keeps the names of websites). Possible problems dealing with the Network Layer include entering in the wrong IP Address so that you don’t go where you want. Another example is if your router dies, you have a Network Layer problem.
Underneath the Network Layer is the Data Link Layer.
The Data Link Layer comprises of Switches that deal with changing operators of data. The MAC address can be found on the switch of a computer. Every single networking device has a unique MAC address. Also note that the Address Resolution Protocol (ARP) is on the Data Link Layer.
Under the Data Link Layer is the Physical Layer.
The Physical Layer includes all the physical aspects of the computer. It’s the Layer that we interact with physically with the computer hardware; like with mouse, keyboard, monitor. Possible problems include a cut network cable, mis-wiring, power cable unplugged, broken hardware etc.
95 percent of Network Layer problems are physical layer problems; basically someone unplugged the Ethernet cable. Another example scenario is if people can’t connect to a server, you have to the back of the server to make sure that the little connection light is on. If it is, that means you most likely don’t have a Physical Layer problem. But if there the connection light is off, then you know that the wire got cut somewhere along the way.
To re-iterate, OSI modle is used as a map to troubleshoot IT computer problems.
Is the 7 Layer OSI Outdated?
It is obsolete. It teaches young children that the world should fit into a very particular mold, as conceived by stuffed shirt pie-in-the-sky dreamers. It has nothing to do with reality, as the real world decided that what they came up with was MUCH more complicated than necessary.
TCP/IP Five Layer Software Model Terminology Reference
How data flows through the OSI Model
In order for human-readable information to be transferred over a network from one device to another, the data must travel down the seven layers of the OSI Model on the sending device and then travel up the seven layers on the receiving end.
For example: Mr. Cooper wants to send Ms. Palmer an email. Mr. Cooper composes his message in an email application on his laptop and then hits ‘send’. His email application will pass his email message over to the application layer, which will pick a protocol (SMTP) and pass the data along to the presentation layer. The presentation layer will then compress the data and then it will hit the session layer, which will initialize the communication session.
The data will then hit the sender’s transportation layer where it will be segmented, then those segments will be broken up into packets at the network layer, which will be broken down even further into frames at the data link layer. The data link layer will then deliver those frames to the physical layer, which will convert the data into a bitstream of 1s and 0s and send it through a physical medium, such as a cable.
Once Ms. Palmer’s computer receives the bit stream through a physical medium (such as her wifi), the data will flow through the same series of layers on her device, but in the opposite order. First the physical layer will convert the bitstream from 1s and 0s into frames that get passed to the data link layer. The data link layer will then reassemble the frames into packets for the network layer. The network layer will then make segments out of the packets for the transport layer, which will reassemble the segments into one piece of data.
The data will then flow into the receiver’s session layer, which will pass the data along to the presentation layer and then end the communication session. The presentation layer will then remove the compression and pass the raw data up to the application layer. The application layer will then feed the human-readable data along to Ms. Palmer’s email software, which will allow her to read Mr. Cooper’s email on her laptop screen.
- Narrowing down the issue, based on the ping status, the faulty layer can be traced and start working from the end layer that is convenient to reach the faulty layer.
- Based on the Ping trace understand the sequence of layers to troubleshoot the issue- most of the time NAS issues can be tracked from layer 7.
- Putting different IT infra teams together to work on respective layers that require fixing or changes on the NAS connectivity or data access.
- Resolving the issue, glancing the OSI model and proper sequential process carry out with a collaborative approach aligning IT infrastructure respective teams to get the services with minimal SLA and compare against working configurations through the post validation process concludes the issue resolution with an incident update. Check the performance in the speed of data access from NAS.
- Problem management to carry out to ensure there is no repetition of the issue by addressing the cause and fixing it.
I hope the above information helps the freshers in IT infrastructure support team or IT professionals to understand the OSI models and troubleshooting steps.
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