What are the functions of OSI layer?

It is said that upper layers function at a larger time scale and the opposite for the lower layers, what is the explanation of this? What are the features of designing at the upper layer or at the lower layer?

Re: OSI layer functions?

In my view, the whole ISO OSI thingy is bit academic, and to try to find those layers in a pratical system is a no-gain affair. It should be taken as a description to understand what processesing is required to establish and run a comm network, but dont expect to find that strict partitioning in any real system.

To come to your question, imagine the comm line gets physically broken. The lowest physical layer by itself might not notice, but it will pass junk recieved data to the higher layers. It works on a shorttime scale ie; at bit level or at best at frame level. It has no idea bout the content of the message. The higher level reads the message and finds out it is junk, and then might ask the lower layer to try something else, or even try another channel, if avialable. Thus the higher level is a slow moving process that might deal in ethernet packets or such bigger data structures.

Again, this doesnt apply exactly to all systems, it is only a generalisation.
-b

OSI layer functions?

any one have more details about the physical layer