Re: Relational vs network vs hierarchic databases

tonyisyourpal_at_netscape.net (Tony Douglas) wrote in message news:<bcb8c360.0411100351.b6cee45_at_posting.google.com>...
> "Dan" <guntermann_at_verizon.com> wrote in message news:<MW5kd.951$2h7.843@trnddc03>...
>
> <snip>
>
> > Why do we have an OSI model that divides the requirements of communication
> > into seven layers, but rarely does a protocol stack consist of 7 layers when
> > implemented?
>
> Because the OSI 7-layer model was futzed with to accommodate TCP/IP ?
> ;)

:-) True.

Well, we have ATM that is a connection-oriented network layer packet switching alternative to IP, and other combinations as well. So it is not restricted to just an OSI vs. TCP/IP argument.

>
> Don't you end up with the full 7 layers anyway ?

TCP/IP, if I recall correctly, can accomodate and corresponds to five layers of the OSI reference model, with a fraction that bleeds into the data link layer. TCP/IP's correspondent network layer is actually divided into an Internet layer and a network interface layer. The data link and physical layers are actually independent of the TCP/IP protocol stack.

The interfaces and layer interactions don't seem to be as clean as the reference model, but the functions/services within the protcol stack implementation are virtually the same, just not necessarily in the same places or called the same things. Again, its a model vs. implementation issue, I think.

An interesting aspect for me in terms of model vs. implementation in the context of the OSI architecture is that each layer has clearly defined functions and clearly defined interfaces; however, function at some layer is how most people learn the OSI model. For me, I'd like to know the relevance and importance of interfaces in the model versus implementation argument.  

Some of the layers
> are specific to particular applications rather than the general
> protocol stack, no ?

Yes. For example, an application layer protocol might bypass or incorporate functions of the session, presentation, and transport layer (TCP/UDP) and interface directly with any lower layer down to correspondent network layer. There is a lot of flexiblity based on application, as you point out.
>
> - Tony

Regards,

Dan Received on Wed Nov 10 2004 - 19:58:58 CET