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OSI Model

Discussion in 'Network+' started by Clyde, Jan 11, 2006.

  1. Clyde

    Clyde Megabyte Poster

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    HI folks

    I recently read something on another posters personal site (advertised via their profile) that made me think a bit about our understanding of the OSI model. Particularly layer 6, the presentation layer.

    Many sources of information tell students that the presentation layer is all about data formats (true) and examples given are stuff like jpeg, mpeg etc.

    No, no, no! Jpeg and mpeg etc are FILE formats defined by an application. These formats are passed to the application layer as a stream of bits where a header is applied (header is dependent on function, eg email, web, ftp etc)
    The resulting chunk of data (a protocol data unit or PDU to be precise) is sent to the presentation layer.

    Now, the presentation layer is all about data presentation, or more accurately, about whether we do things like change the bit order from high to low or low to high (eg. send 1234 and store as 4321 or 1234 at the other end) or change character set (ASCII to EBCDIC for example) or change the byte order or chenge the file syntax (how the OS stores a file)

    It's nothing to do with JPEG, GIF, DOC or other file formats.


    Hope that helps.

    PS this could also be placed in networking but I thought this the best place..
     
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  2. simongrahamuk
    Honorary Member

    simongrahamuk Hmmmmmmm?

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    A google seems to suggest otherwise though clyde.

    Even the Wiki seems to suggest that JPEG, etc are at layer 6?

    I agree with what you're saying though, but general teaching seems to suggest otherwise?

    :blink

    EDIT: Link to Wiki on OSI: Here
     
  3. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    IMHO the presentation layer is one of the fuzziest of them all. :biggrin

    Harry.
     
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  4. Bluerinse
    Honorary Member

    Bluerinse Exabyte Poster

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    Okay firstly let me just say I hate the OSI model. I would rather not have a silly model, especially when it is so messy.

    However, Simon has contested your post Clyde, which prompted me to do a bit of research (googling) :biggrin

    As far as I can glean, the presentation layer is responsible for compression and encryption. As .jpg .mpg files are compressed file formats, I presume they are uncompressed at the so-called presentation layer :rolleyes:

    Here is what it says on Technet...

    http://www.microsoft.com/technet/archive/mail/appndix1.mspx
     
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  5. Clyde

    Clyde Megabyte Poster

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    think about it...

    what encodes jpeg ? the application.... by your logic bluerince, winzip operates at layer 6 - though the fact is winzip operates in the absence of a network. compression MAY BE a layer 6 funcion, but can be carried out by the application also, or indeed lower layers (ppp)

    the app then sends the bitstream to the application layer where a layer 7 protocol encodes it (eg email)

    then the layer 6 encoding takes place....

    jpeg etc is created by applications, NOT the networking subsystem.

    And as said earlier, compression can occur at the application or at differing layers of the osi model (eg .. ppp, a layer 2 protocol can compress datastreams )

    and wiki is definitely wrong here... (not surprising considering how wiki is created)

    to understand the presentation layer we need go to back to computer science classes and how data is represented (bid endian, little endian etc)
     
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  6. Clyde

    Clyde Megabyte Poster

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    agreed... and to complicate matters, many real world protocols don't even implement it!
     
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  7. Clyde

    Clyde Megabyte Poster

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    and simon, thats the reason for my post - general teaching is plain wrong on this one...

    for some reason, most texts talk bull*%it about this layer...

    come to think of it, OSI is generally misunderstood (as are real world protocols and their relationship to it)
     
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  8. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    Mostly, IMHO, because many real world protocol stacks don't align very well with the 'theoretical' OSI stack.

    I once saw a comment that so many academic people were arguing over the nitty-gritty of OSI they didn't notice the real world accelerating away from them!

    Harry.
     
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  9. Clyde

    Clyde Megabyte Poster

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    yup! thats true.. OSI is an idealised abstract reference. Useful as a construct for teaching and troubleshooting.. but not really real...
     
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  10. Bluerinse
    Honorary Member

    Bluerinse Exabyte Poster

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    And that is why I prefer to ignore it :dry

    Who needs an abstract model to confuse and complicate things more than they already are :rolleyes:

    True Clyde good point! Oh well I guess the silly model fails a logical examination once again :biggrin
     
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  11. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    We all do when trying to pass Network+! :biggrin

    Harry.
     
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  12. Bluerinse
    Honorary Member

    Bluerinse Exabyte Poster

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    I know and don't think it ends there because it will rear it's ugly head all over the show :eek: :biggrin
     
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  13. Clyde

    Clyde Megabyte Poster

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    ah bluerinse! OSI is not messy at all. Its a great way of explaining in a general way how 2 systems connect. What often happens though is its not explained properly and secondly, its not related to real protocols properly.
     
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  14. Boycie
    Honorary Member

    Boycie Senior Beer Tester

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    Just when i thought the OSI was sinking in...... :blink
     
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  15. arisen

    arisen Byte Poster

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    hehe :biggrin
     
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