Page 644 - (ISC)² CISSP Certified Information Systems Security Professional Official Study Guide

sometimes explained in terms of upper and lower layers, where
security and privilege decrease when climbing up from lower to upper

layers). In discussions of OS architectures, the protected ring concept
is common, and it is not exclusive. There are other ways of
representing the same basic ideas with levels rather than rings. In such
a system, the highest level is the most privileged, while the lowest level
is the least privileged.

Levels Compared to Rings

Many of the features and restrictions of the protecting ring concept
apply also to a multilayer or multilevel system. Think about a high-
rise apartment building. The low-rent apartments are often found
in the lower floors. As you reach the middle floors, the apartments
are often larger and offer better views. Finally, the top floor (or
floors) is the most lavish and expensive (often deemed the

penthouse). Usually, if you are living in a low-rent apartment in
the building, you are unable to ride the elevators any higher than
the highest floor of the low-rent apartments. If you are a middle-
floor apartment resident, you can ride the elevators everywhere
except to the penthouse floor(s). And if you are a penthouse
resident, you can ride the elevators anywhere you want to go. You

may also find this floor restriction system in office buildings and
hotels. You may also have an elevator that operates directly
between the lowest level and the penthouse level, thus bypassing
all lower levels. However, if the direct elevator is breached, the
other layers of protection are of no value.

The top of a layered or multilevel system is the same as the center
ring of a protection ring scheme. Likewise, the bottom of a layered

or multilevel system is the same as the outer ring of a protection
ring scheme. In terms of protection and access concepts, levels,
layers, and rings are similar. The term domain (that is, a
collection of objects with a singular characteristic) might also be
used.

Communication between layers takes place only through the use of