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OSI Reference Model

Posted on 2010-01-12
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I need to explain the process that occurs between a client (browser) and Web server by describing the functionality of the OSI reference model.

What does it me, Diagram the interaction between the client and the server and illustrate the data flow.

How do I Discuss the importance of the OSI reference model for those who has responsible for network administration and network protocol development.
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Question by:CubanRican_NY
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AnilKumarSharma earned 2000 total points
ID: 26301279
Something like you have to explain each and every layer for OSI and how the top layer data is bundled and then transmitted down to each layer by appending its own specific header before it transmitted to physical medium and how the other side is listening and accept the data and know that it is meant for it. Then how the each layer unpack and decode its header and then pass it to upper layer.
This will be good to demonstrat with the help of diagram.
Also to mimic the things just like two high level delegates that have different languages are discussion with help of their secretary that actually take input of its higer level and translate to secretary of other delegate that in turn transmit the message to his delegate.
Hope it help you a bit.
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by:Mysidia
ID: 26320351
This is kind of tricky since  internet protocols aren't designed around the OSI reference model   the  "OSI"  protocol suite  was a dismal failure,   IP took off instead,   and OSI is basically died before it ever started,  other than IS-IS and some academic folks still referring to the reference model  that was to be used for OSI,  as if it applied to IP  (the complex model may well be part of what doomed OSI), anyways:

So something you ought to realize is there's not actually such thing as a "Presentation layer protocol"  or a  "Session layer protocol"  involved in browsing the web.

You will need to construct some sort of  'fiction'  to  describe some function of the web browsing process that  takes the place of the 'presentation'  and 'session' layer.     I suppose, for example, you could describe  the process of   converting  hypertext  HTML  into  on-screen layout instructions,   or   translating to human-readable fonts as presentation:  that is,  whatever  fits   the   OSI model's  description of that layer.


Otherwise, you will probably want to start  thinking of  'mapping'  existing protocols and parts of application logic involved in browsers to OSI layers.
With the exception of   Presentation Layer and Session layer,  it should be pretty obvious...

E.g.   map       Browser Application,  TCP,  IP,   Ethernet ("MAC protocol"),  
Cabling,   to  their respective OSI layers.

Once you've mapped them,  then an explanation should probably trace the path from peer A to peer B.

If you need a primary to understand OSI,   I would suggest
Radia Perlman's book,  Interconnections: Bridges, Routers, Switches, and Internetworking Protocols  2nd Ed.


Which discusses  the OSI  model,  in slightly more detail than most introductory books on networking.


Key characteristics of OSI to remember are:

*It is a model comprised of  7  layers,  that exist  on  communication peers.

* The  Xth layer on one peer  communicates with the Xth layer  on the other peer,  by sending   PDUs  (Protocol Data Units)  through the other layers.

* The Xth layer's  message on one peer  to  the next layer is called a  *SDU  Service Data Unit  The *  is replaced  with the name of the Layer communicating.

An additional protocol header  and layer of encapsulation is added in each *SDU  when travelling -down- the stack one one of the hosts towards the physical layer.
The header is a message to the same layer on the other host,  and the data encapsulated,  is the message to the next layer  on the other host.
 
After the  message has crossed the physical layer, it is now travelling UP  away from Layer 1.     When  Service Data Units are sent up,  each layer  strips off  its own header.



For example, the "Application Layer"  on one peer,  encapsulates the PDU to the other peer's   "Application Layer" in a     PSDU    (Presentation Layer Service Data Unit),  and this process repeats at each layer,  until the physical layer:
Where  peer A's  physical layer is in contact with peer B's physical layer.

When the  datalink layer on host B  receives  the  Service Data Unit  from host B's physical layer,  it strips off the  datalink header  (removes the 'datalink encapsulation'),  and  transmits   the PDU  to host B's  network layer,  with the network layer encapsulation intact.


Certain elements of a 'web browsing' session do fit nicely into the OSI model.
For example,   IP  packets  really do  get  Encapsulated in Ethernet packets,
and you can  map that association  to the OSI model's  Service Data Units at the proper layer..






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by:Glen5750
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Thank you for your prompt answer. It saved my day.  
Glen
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