The LogicalPhysical Distinction

The Logical/Physical Distinction

One concept of fundamental importance when considering the core and common models is the distinction between physical and logical entities. This may seem to be an obvious distinction but consider the question: "Is the network adaptor in my desktop computer physical or logical?"

You may answer "physical" without needing to take a great deal of thoughtyou may remember holding a network adaptor in your hand at some time. But the test to apply is, "Can you attach a label to it?" Certainly you can stick a label onto the printed circuit board which implements the network adaptor, but the network adaptor itself is a logical and abstract concept. What you have held in your hand is actually not a network adaptor but a printed circuit board which, among other things, implements a network adaptor. If the printed circuit board also implements a modem and a video interface, then the physical network adaptor is even harder to find.

The distinction which the DMTF makes in its Core Model white paper (DSP0111) is as follows :

Physical Elements, occupying space and conforming to the elementary laws of Physics. The Physical Element class represents any component of a System that has a physical identityit can be touched or seen.

Logical Elements, representing abstractions used to manage, configure and co-ordinate aspects of the physical or software environment. Logical Elements typically represent Systems themselves , System components , System capabilities and software.

The distinction between Logical and Physical Elements is fundamental to the structure of the Core Model. The principal distinguishing feature of a Physical Element is that it cannot have a "realization" (per Webster's ^[2] definition of realization, it cannot be "brought into being"). It can be composed of parts , but there is no sense in which, for example, a system enclosure is "realized" (or "brought into being") by a piece of molded plastic; it simply is a piece of molded plastic.

Logical Elements ( especially Logical Devices) can be "realized" by installing Physical Elements and/or software. For example, it is not possible to attach a label to a modem. It is only possible to attach a label to the Card that "realizes" the modem. The same card could also "realize" a LAN adapter. These tangible Managed System Elements have a physical manifestation of some sort . However, the physical manifestation is very different than its management aspects and attributes. The latter are addressed by the Logical Elements, realized from the physical, usually accessed via software.

^[2] A dictionary of American English.