Information Technology Reference
In-Depth Information
r
o1
r
o
= r
o1
+ r
o2
D
D
D
ISS-ENT-2,
ISS-BP-1
D
ISS-ENT-1
ISS-ENT-2
D
D
ISS-ENT-1,
ISS-BP-2
D
D
Security Services
(ISS-ENT-1,
ISS-BP-1)
r
o2
D
D
D
ISS-BP-1
ISS-BP-2
ISS-ENT-2,
ISS-BP-2
D
Description of models
RE1 RE2 RE3 RE4
9
9
9
9
9
9
RB1
RB2
RB3
RB4
ISS-ENT-1
ISS-ENT-2
ISS-BP-1
ISS-BP-2
9
9
9
9
Fig. 4.
Example of combining two observable evolution rules
p
11
p
13
p
12
p
23
p
26
p
27
p
29
p
21
p
22
p
24
p
25
p
28
Fig. 5.
Multiple steps (phases) evolving requirement model
The multi-step evolution begins with an original model
RM
1
. This model can
evolve to one of the potential evolutions
RM
1
i
. In the second step, each
RM
1
i
then also evolves to one of many potential evolutions
RM
2
j
. The evolution stops
after
k
steps. If we represent a model as a node, and connect a model to its
potential evolutions as we have done as aforementioned, then we have a tree-like
graph, called
evolution tree
with
k
-depth.
Fig. 5 illustrates a two-step evolution, in which observable rules are denoted
as dotted boxes. The original model lays on top part of a box, and all potential
evolutions are in sub boxes laid at the bottom. There are directed edges connect-
ing the original model to potential evolutions. The label on each edge represents
the probability such that original model evolves to target model.
In Fig. 5, an initial requirement model
RM
1
evolves to either
RM
1
,
RM
2
or
RM
3
. Likewise,
RM
1
, where i=1..3 and j=1..9. Here, we
have a ternary complete tree of depth 2. Generally, the evolution tree of a
k
-step
consecutive evolution is a complete
k
-depth,
m
-ary tree.
We can always collapse a
k
-step evolution into an equivalent 1-step one in
terms of probability by letting the original model evolve directly to the very last
models with the probabilities that are multiplication of probabilities of interme-
diate steps. Therefore, any k-step evolution has an equivalent 1-step evolution.
Hence all analyses discussed in
evolves to
RM
2
j
i
ยง
4 are applicable without any modification.
