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  • In this section I will look at the OSI model. To understand networking it payees to have

  • an understanding of the OSI model as parts of it are used in modern networking.

  • The OSI model, or Open System interconnection model, is a network model designed back in

  • the late 1970’s. The model itself did not get adopted, however references are still

  • made to it in modern networking especially to the names and the numbers in the OSI model.

  • You will find that most network technologies do not match exactly to the OSI model and

  • do not provide an exact fit, however it is useful when discussing networking to reference

  • it against the osi model. The OSI model has 7 layers. Each layer communicates

  • with the layer above and below it only. By doing this you can change a layer with effecting

  • other layers as long as the new layer is compatible with the adjacent layers.

  • To understand the OSI model better and how it works, consider a manufacturing plant.

  • Image the first step of the manufacturing plan is to make a part. The next step put

  • a protective coating on the part. After this the part is put in a box. The box than has

  • a shipping label placed on it. The important thing to consider is that each process of

  • the manufacturing process works on the previous. For example you could not apply the coating

  • on the part if you had already place the part in the box. The OSI models works the same,

  • each process or layer of the OSI model adds to the previous.

  • The OSI model consists of seven layers. Each layer handles a different part of network

  • communication. The first is the application layer. This is where the application programming

  • interface or API’s are found. Applications call functions in theAPI to sent network traffic.

  • Next you have the presentation layer. This layer coverts the data inputted from

  • the application into a form suitable for transmission over the network. The session layer controls

  • communication between two applications. For example the session layer may decide on the

  • port used for communication. The transport layer ensure the data arrives in the same

  • order it was sent and is not duplicated. The network layer determines the path used

  • to transfer data across the network. The Data link layer provides error checking and creates

  • the packet to be sent over the network. Lastly the physical layer sends the data over the

  • network media to the other party. To remember the seven layers of the OSI model

  • you may want to consider using a mnemonics. Two popular ones are all people seem to need

  • data processing and please do not tell sales people anything. Take the first letter of

  • each word to helps you remember each layer name. For A is for Application, P is for presentation,

  • S is for session and so on. The second mnemonics works from the bottom up, for example P is

  • for physical, D is for Data Link, N is for network and so on.

  • To better understand how the OSI model works, consider this. If a computer want to sent

  • a message to anther computer on the network it communicates with the application layer.

  • The application layer communicates with the presentation layer. The communication goes

  • all the way down to the physical layer. As the data go down the layers more information

  • is added like packet headers and checksums. The data is than communicate across the network

  • over the physical layer. The physical layer will check the checksum

  • and pass the data up to the data link layer. As the data goes up the layers more data is

  • removed like packet headers. When the data gets to the top, all the headers and check

  • sums are removed and the data is presented to the application in a way that is can understand.

  • As stated before the OSI model in it’s entirety was not adopted however the layers names and

  • layer numbers are still used today. TCP IP is divided into 4 layers and can be roughly

  • mapped to the OSI model. To better understand why TCP is divided into 4 layers it so a good

  • idea to look at what happens at each layer. At the application layer you have protocols

  • like http, smtp, d n s and RIP. At the next layer you have t c p and U D P. Using T C

  • P you have reliable transmission, that is packets are guarantee to be delivered and

  • delivered in order. U D P uses unreliable transmission and does not guarantee delivery

  • or order of delivery. At the internet layer you have the choice

  • of ip version 4 or ip version 6. The final layer provides a link to your network media.

  • As you can see using layers allows you to add and remove protocols and services very

  • easily. For example you could add a new network card, for example a token ring network card

  • to your computer and as long as the new network card can communicate correctly to the layer

  • above nothing else needs to be changed. The new network card will work with any protocols

  • at the application layer and any transports at the transport layer without any changes.

  • As you can see, using layers makes adding and subject hardware, protocols and services

  • easy. To put the OSI model in prospective, you will

  • see in a lot of literature that refers to a device as a layer 2 device or layer 3 device.

  • Switches work on layer 2 of the OSI model. If you consider this small network. The switch

  • is able to decide based on the destination packet which device to sent it to. For example,

  • packets coming from the computer to the printer would be directed straight to the printer.

  • Packets going to the server from the computer will not be seen by the printer. The switches

  • looks at the destination address or mac address to determine where to sent the packet.

  • If you add anther network separated by a wide area network, the layer 2 switch will not

  • be able to router traffic to this network. To router traffic between different networks

  • you need a layer 3 device for example a router. The router will be able to router traffic

  • between the two network as routing is a function on layer 3 of the OSI model. You may see devices

  • on the market called a layer 3 switches. Don’t get confused by this, remember layer 3 devices

  • support routing. Simply put, a layer 3 switch supports routing and switching. Layer 3 switches

  • performing routing in hardware rather than software so tend to be a faster than routers

  • at routing traffic between networks. Since Layer 3 switches logic is build into hardware

  • they lack some features of routers. Keep this in mind if you decide to deploy one on your

  • network. Just remember this, layer 2 devices perform

  • local switching in a local network. Layer 3 devices supports routing between networks.

  • Devices like firewalls can work on much high levels of the OSI model. For example a good

  • firewall can work all the way up to the application level and detect threats in protocol’s like

  • http. If you see a device that says it works on

  • a particular level of the OSI model, think what that level does and that will tell you

  • what the device does. For the exam, make sure understand the basics of how the OSI model

  • works.

In this section I will look at the OSI model. To understand networking it payees to have

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B1 US osi layer network model data application

The OSI Model

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