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Presentation Layer Data Conversion

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.

Reply to
Alex Walker
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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 Content-ID:

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

Reply to
Scooby

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.

Reply to
Matt nickerson

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?

Reply to
Scooby

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.

Reply to
Alex Walker

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