Open Systems

44 The growth in standards for computing must be one of the most significant devel- opments of the last few years. The term `Open Systems" has come to refer not just to the original idea of being able to inter- connect different makes of computer, but also to a whole range of products mainly centred on Unix and X-Windows. Sun Microsystems" NFS is often included, although this is really a proprietary system that has become generally accepted. At the heart of all the osi applications lies the standard itself, which is what allows them to interchange information, typically over a thin-wire Ethernet. The standard was one of the first systems to be based on a layered model. The layered model A layered model is really a form of struc- tured programming, in that high-level operations are separated from low-level operations. Rather than being simply top- down, however, it is split into a vertical stack, so that the higher levels are cut off from the lower levels at certain points. An accu...