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Info: The OSI Model

post #1 of 28
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Open Systems Interconnect (OSI)

To standardize communication within networks the International Standards Organization developed the OSI reference model in 1978. OSI serves as a conceptual framework for designing network environments. It details the working of the network hardware and software for facilitating communication (Simmons et Causey 2003).

OSI has an architecture that distributes the tasks involved in network communication among seven layers. The layers of the OSI model are mapped to every computer on the network. This mapping is done so that each layer can communicate with its associated layer on the other computer. Such communication is called virtual communication, though actual communication occurs between the adjacent OSI layers of both the source and destination computers (Simmons et Causey 2003).

In the source computer data passes through all seven layers and at each layer the software adds some additional information to the data. From the lowest layer of the source computer data is transferred through the network media to the lowest layer of the destination computer. In the destination computer the data moves from the lowest layer to the highest. The software at each layer reads the relevant information to it and passes it to the next higher layer (Simmons et Causey 2003).


The Seven Layers of the OSI Model

Application Layer – This is the top layer of the model which provides the services that directly support user applications such as software for file transfer, database access and email. It also manages network access and flow control because the data to be sent across the network is generated at this layer. In addition it manages the error recovery because at the destination computer the data is changed back to it's original form (Simmons et Causey 2003).

Presentation Layer - This determines the format of data exchange between networked computers. At the source computer it converts the data received from the application layer into an intermediary form. At the destination computer it converts the received data into the original form. Formatting the data also includes translating and encrypting the data, changing or converting the character set and expanding graphics commands. It also manages data compression which reduces the number of bits of data to be transmitted (Simmons et Causey 2003).

Session Layer - This layer functions to allows two applications on different computers to establish use and end a connection. In addition it provides synchronization between user tasks by placing checkpoints in the network data stream. This ensures that only the data after the last checkpoint has to be retransmitted if the network fails. It also implements dialog control between communication processes by regulating the transmission of data (Simmons et Causey 2003).

Transport Layer - This layer transmits error-free packets in sequence over the network. On the destination computer this layer reassembles the packets to form the original message and sends an acknowledgement of receipt. This ensures that there is no loss or duplication of data. When incorrect transmission occurs this layer initiates the retransmission of data (Simmons et Causey 2003).

Network Layer - This layer is responsible for addressing the messages. It converts logical addresses and NetBIOS names into physical addresses. In addition it determines the route from the source to the destination computer over which the data will be transmitted. This route is determined by certain criteria such as network conditions and the priority of the service. It also manages network traffic problems such as controlling the congestion of data. At times the network adapter on the router is unable to transmit a large data block sent by the source computer. In such a situation the network layer on the router breaks the data into smaller units (Simmons et Causey 2003).

Data-Link Layer - This layer sends data frames from the network layer to the physical layer on the source computer. On the destination computer it packages raw bits of data into data frames. A data frame is an organised logical structure in which data can be placed. It also provides an error-free transfer of data frames across computers. At times this layer detects a problem with a data frame in the destination computer. In such a situation the layer does not send an acknowledgement for that frame. This data frame then needs to be resent (Simmons et Causey 2003).

Physical Layer - This layer transmits the raw bit stream of data via a variety of methods, both analogue and digital, over the medium connecting the network. The transmitted bits of data do not have any defining meaning at this layer, only the data encoding and the bit synchronisation is defined. In addition, the time that each bit will last and how each bit will be translated into the appropriate electrical or optical impulses is also defined. It also carries the signals that transmit the data generated by all the higher layers of the OSI model (Simmons et Causey 2003).

References

Simmons, C. et Causey, J., 2003. Windows XP Networking Inside Out
Washington: Microsoft Press
Edited by t4ct1c47 - 12/13/08 at 12:08pm
post #2 of 28
basic knowledge but for some noob users good job

lol i took the software portion of the A+ today, NONE of it was networking... i was like ***FFFFF
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post #3 of 28
One way to remember the order is "APS Transport Network Data Physically" Works for me
 
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post #4 of 28
Quote:
Originally Posted by Krunk_Kracker View Post
One way to remember the order is "APS Transport Network Data Physically" Works for me
My way is this:

All
People
Seem
To
Need
Data
Processing

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post #5 of 28
Quote:
Originally Posted by bigvaL View Post
My way is this:

All
People
Seem
To
Need
Data
Processing

My way is:

"damn what order was it again." And then i use Google.
    
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post #6 of 28
or....

People
Dont
Need
To
See
Pamela
Anderson (it's backwards though)
 
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post #7 of 28
Quote:
Originally Posted by Krunk_Kracker View Post
or....

People
Dont
Need
To
See
Pamela
Anderson (it's backwards though)
They don't?? Hmmm...
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post #8 of 28
too bad we don't use that model.

i like those sayings though, maybe now i'll remember it
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post #9 of 28
Thread Starter 
Quote:
Originally Posted by TrAncE XD
lol i took the software portion of the A+ today, NONE of it was networking... i was like ***FFFFF
There is very minimal networking learning involved in A+ Certification.

Quote:
Originally Posted by stanrc View Post
too bad we don't use that model.
Could you give an explanation as to what you mean?
post #10 of 28
Quote:
Originally Posted by stanrc View Post
too bad we don't use that model.

i like those sayings though, maybe now i'll remember it
yes indeed we use that model. it's how networking is done.

and the A+ doesn't involve networking, the Net+ does

Please
Do
Not
Throw
Sausage
Pizza
Away

^ another backwards one

it should be noted also that data transmission begins at the application layer, moves through the other layers, and goes back up and ends on the receiving computer's application layer.

and the data link layer is split into two sub layers, the LLC (Logical Link Control) and MAC (Media Access Control)
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