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.