Hello all you ninjas fanatics,

The Network+ exam from CompTIA is one exam every network technician and administrator should have, and believe it or not this is very easy to achieve. Today we are going to be talking about the OSI model and how it applies to you. The OSI is not a specific way for how data is transferred; it is all theoretical. It is the idea for how data is being transferred from one spot to another. There are seven layers to this model and they are as follows:

  • Application
  • Presentation
  • Session
  • Transport
  • Network
  • Data Link
  • Physical

Each layer has it’s own specific part for how the data is transferred. Physical, Data Link, and Network are all part of the Media Layer, while Transport Session Presentation and Application are all part of the host layer. The media layer deals with the hardware and how it is transferred through the cabling, routers, hubs, and interfaces and such while the host layer deals with anything that happens in the host, such as how the information is packaged to be sent in protocols. Data is sent “Down mine and Up yours”. This means that it starts at my application layer and goes down through my physical layer, and goes up starting at your physical layer all the way to your application layer. Here is an explanation of each of the seven layers.

Application- This is the layer closest to the user. You operate on this layer and it is what you see, or sometimes don’t see. This layer contains the most applications such as Telnet, FTP, HTTP, and SMTP.

Presentation- On this layer, encryption takes place. This is mainly the layer that converts information from the application layer into data that can be sent through the session layer.

Session- This is where the connections actually take place. It deals with the creation, maintaining, and closing of connections. There are three ways in which data is sent through this layer: full-duplex, half-duplex, and simplex. Full-duplex is where both ends can talk and receive at the same time. An example of this would be a phone conversation. While you might not want to, you can talk and listen at the same time and the phones will send the data. The next is half-duplex. This one is used for only sending in one direction at a time. An example of this would be walkie-talkies because you can only talk OR listen; you can not do both. And the last way to send data is simplex, and this is used when you only need to send data in one direction and no reply is needed.

Transport- This is where TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) take place. Data is sent through one of these. TCP is where there is a handshake and it is a connection-oriented protocol. If something goes wrong, it make sure the proper packets gets to the recieving end and they aren’t corrupt. UDP is connectionless-oriented. It just sends the data and it could care less if the data gets there or not.

Network- Yay we are at the layer where routing takes place! This is where your routers and multi-layer switches are. Instead of making a decision based off of the MAC Address, we use IP addresses now. The data would come in through one interface, and then the router will try to look up its destination inside its database called a routing table. If it can not find it’s path, it forwards the packet to it’s default gateway who then does the same look up. It repeats this process until it figures out where to be routed to.

Data Link- This is the layer where switches are located at. Switches do not route, rather they quiet the network down by cutting the network into segments. They send data based off of the MAC Address on the NIC (Network interface card) of the computer. If the MAC Address is on the left side of the network, instead of sending it everywhere and broadcasting the whole network it will only send the data to the left side of the network. Switches are becoming more sophisticated though and multi-layer switches are starting to come out with routing capabilities that also operate on layer three. Switches communicate with each other using the Spanning Tree Algorithm to prevent broadcast storms. Broadcasts are used to communicate with everyone on the network, or to find a certain node for communication such as the DHCP server to get an IP address. Let’s say there are two switches on the same network. The broadcast would get to one switch, and that switch would repeat it to every interface. Then the data would get to the next switch, and it would also repeat it to every interface. What would happen if they both kept repeating the same data back and forth to each other? The amazing broadcast storm until your network fails. My point; they are slowly becoming more sophisticated to avoid problems.

Physical- The physical layer holds all the physical media for which data is transferred. There is no computing done on this layer at all. It is solely used for the transferring of data through this layer. It consists of the cabling, hubs, repeaters and network adapters. Notice how all of those do no computing on their own and they are used for transferring the media or repeating it. There is no routing that takes place on this layer.

If you plan on getting this certification which is highly recommended, you must know the OSI concept and how it operates because it is highly tested on the exam. One way to remember this model is to use acronyms. A great one is Please Don’t Ninja The Stupid Pirates Apples. There are many places on the internet to find information about it, but why not ask some people who have passed the test? Head over to the forums and ask any questions you might have!

Have a great ninja day!

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2 Responses to “Network+: The OSI Model”
  1. matthew_ottoson says:

    Great summary. This actually will help me alot. I happen to have an interview tomarrow for a Infosec/network job and have been looking through different breakdowns of the OSI model so I have it fresh in my head going in. Thanks.

  2. Venom says:

    Good luck!

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