OSI Model

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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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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?



EDIT: Link to Wiki on OSI: Here

 

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

Harry.

 

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)

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

Here is what it says on Technet...

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

 

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)

 

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

 

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)

 

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.

 

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

 

And that is why I prefer to ignore it

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

True Clyde good point! Oh well I guess the silly model fails a logical examination once again

 

We all do when trying to pass Network+!

Harry.

 

I know and don't think it ends there because it will rear it's ugly head all over the show

 

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.

 

Just when i thought the OSI was sinking in......

 

hehe