Presentation Layer Data Conversion

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Hey

Just a quick question regarding the presentation layer of the OSI model.
I understand that part of the work done by this layer is to format data
into recognised formats (i.e. JPEG, TIFF, Quicktime etc..). I would have
thought that the majority of this sort of work would already be done by
applications (for example Paint saving an image in .GIF format), so in
what sort of instances would additional encoding be required?

How does this affect the data transfer of very proprietary formats?
Would the data just be reverted to something very basic like ASCII for
the transfer and dealt with at the other end?

I initially thought that files would be recognisable by their file
extensions, until I realised that they're not cross platform, so there
has to be a method by which data types are sorted, but I don't really
understand how the mechanism works.

Can anybody explain this to me in relative laymans terms, give me an
example of the sort of conversion work that the presentation layer might
do, or point me at a link that might explain this?

Thanks for any help.

Cheers.

Alex.

Re: Presentation Layer Data Conversion
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Ever get an email with a picture in it?  Look at the raw text of the email.
You will see something like this (below) in it.  This layer is a little
unclear to me as well, but I believe this is the spirit of it.  Same idea
when you download pictures in a web page.  I imagine that any mime type thus
qualifies.

Jim


Content-Type: image/jpeg;
 name="picture.jpg"
Content-Transfer-Encoding: base64

/9j/4AAQSkZJRgABAQEAYABgAAD/2wBDAAoHBwgHBgoICAgLCgoLDhgQDg0NDh0VFhEYIx8lJCIf
IiEmKzcvJik0KSEiMEExNDk7Pj4+JS5ESUM8SDc9Pjv/2wBDAQoLCw4NDhwQEBw7KCIoOzs7Ozs7




Re: Presentation Layer Data Conversion
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The applications that create the data are saving it in a presentation
layer format.  MS paint is not the gif file, it uses the gif file.
Remember, this distinction was created after the applications, and is
really just a concept of how to look at what's going on.  The TCP/IP
model makes no distinction at all between the two.  If someone had
come up with a 20 layer OSI model we might be talking about layer 16
here, it's just an academic distinction to help narrow the focus
between the application and the data the application creates.


Re: Presentation Layer Data Conversion

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Try not to confuse local use pc applications with network applications.
Keep in mind that the osi model refers to networking.  So, when you are
speaking of such, the program that created and saved the image has nothing
to do with in.  In this particular example, the layer 7 application is your
email client, not paint.  The presentation encapsulation would be as I layed
out before, not the actual image headers that reside in the file. An image
contains many characters that are not valid in an email file.  So, an image
would need to be converted to a format that this application can understand,
thus the mime type.  And, that is your layer 6 in action.

Another example would be converting data sets or objects to xml format.

The idea is how can we send this data across the network so that the other
side can understand it - not how can we save it on the pc so that other
applications can use it.

Does that help clear things up?




Re: Presentation Layer Data Conversion
Scooby wrote:
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Quite nicely I think, especially with regards to the layers being
conceptual, which I'd forgotten a little.

Thanks for taking the time to reply everybody.


Alex.

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