2.3
Planning to Satisfy Requirements
The exercise serves to
enumerate all the potential attacks and exposes the set of potential threats against which no
appropriate plans exist.
The techniques described in
the previous sections can be used to determine the different attributes of a computer system
plan that shows the set of component sand
their interconnection.
The problem of finding a
computer system plan that is the best one to meet a given set of requirements is harder than the
reverse problem. In some cases, it can
even be shown that determining the best plan is NP complete. To make matters worse, one set of requirements may
be conflicting with another set (e.g.,cost,
performance, and security requirements often conflict with each other).
In many cases, some of the steps may be obvious
from the requirement specifications
while other steps (or part of a step) may require significant investigation. When a wide area network is
designed, the locations of access routers
are known, but one needs to determine the locations of internal network routers (or switches) and the capacity of
links between them.
2.3.1 Systems Planning Process
The systems planning process can be viewed as
developing a system design or plan
by following a four-step process as shown in Fig. 2.9. In the first step of the process, an architecture for the system is
developed. The architecture is mapped to
a logical design in the next step. The logical design is then translated to a concrete design. Finally, the concrete
design is translated into the layout of the computer
system.
A more detailed description of the
activities and contents of these steps is provided
below:
Architecture: In the architecture step, decisions are
made regarding the different
layers or zones that will be present within the system. An architecture describes the higher level
decomposition of the system into well-defined
areas, where each area provides a specific function required for the functioning of the computer system, and
specific protocol choices are made
for systems communicating with each other.
Logical plan: In the logical plan of the system, the
types of physical machines that
are to be located in each architectural zone are specified. The logical plan may specify that the security zone of a
web-hosted site will consist ofa
set of packet filtering firewalls, the caching zone will consist of a load balance followed by three web caches, and
the web serving zone will consist
of a load balance followed by two web servers, and that each zone will be connected by a local area network.
Concrete plan: In the concrete plan of the system, the
exact hardware and software of
each machine are specified. The vendor and machine type for each of the devices are determined.
Physical layout: In the physical layout of the system, the
location of the hardware assets
is determined. This includes specifying which of the different rack locations a server would be
located in, and the closest in which
the different communications equipment will be placed in.
During each of the stages, the plan or design that is produced needs to be analyzed
to validate that it satisfies all the requirements desired from the system. The set of requirements that are
evaluated may not be the same a teach
layer of the design, e.g., the layout may only look at thermal requirements, the logical design may look at performance
and availability, and the architecture
ensures that manageability and security requirements have been satisfied.
And it is also worth remembering that
the architecture definition from a system’s
planning perspective is very different than the architecture of a software or hardware system. When a system
planner is creating the architecture for the
system, the goal is to reuse existing pieces of hardware and software that are commercially available or developed
previously.
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