Information Technology Reference
In-Depth Information
example, Shakespeare's plays are logical entities, which can have multiple
physical existences — as topics, on the stage, as audio tapes, etc. There
would be no confusion between the logical and the physical in such cases.
People usually relate to everything logically — using names. Data and
information on computers are referred to logically, through a file name,
and not the sectors on a disk. Naming introduces a logical concept to
things that are physical.
Logical entities can also be grouped differently from the physical reality
underneath. For example, an automobile dealership application screen
may contain the customer's name, the VIN (vehicle identification number),
the vehicle miles, problem details, and an initial estimate for a repair job.
The physical organization of the same could be in four different database
tables or flat files, cross-referenced by the customer ID and the part ID.
These tables, in turn, may themselves be viewed as logical entities that
group certain kinds of information together going all the way to the
physical bits stored on the disk.
In some cases, the mapping is not specific. One's bank statement is
just a logical view of the money that one has in one's name in the bank.
It is not mapped to any particular bundle of notes, which the bank has
physical possession of in its vaults.
Another example is your LAN (local area network). The logical view
is a bus that runs through the entire network, and various devices
(computers, printers) are connected to it. However, the physical topology
is very different; it is a star, where each of these devices on the network
connects to a central hub or router.
Thus, logical is a way of compressing information, an abstraction for
human beings to comprehend better, instead of getting into details.
In many systems, we use a logical representation to describe the
role
.
A traffic signal at each intersection has a red light, which is logical,
indicating a physical stop for all vehicles. The light by itself does not
directly act to physically stop traffic by lowering a gate or some such
thing. The conversion from logical to physical is achieved by drivers
following driving rules. Traffic lights are therefore logical implementations
from a systems point of view.
In software, the purpose of the logical model is to show how the data
is captured and used by the application to satisfy business requirements.
It shows how this data is related (identifiers, attributes, types, and rela-
tionships), and explores any integration requirements with business areas
outside the scope of the development project. It is created without any
specific computer environment in mind. The purpose of the physical
model, on the other hand, is to show how this data capture and usage
will be implemented and stored (tables, indices, foreign keys, columns).
of an entity, whereas the physical nature describes its actual
appearance
