Information Technology Reference
In-Depth Information
6.1.2 Single-Class Method
In this scheme, we assume that the first page of every index contains informa-
tion indicating the location of each index class. This structure can be implemented
by including a vector of pairs [
class_id, offset
]. Assuming that the size of the
offset
and the
class_id
is size 4 bytes each, the size of this structure would be 8
c
, where
c
is the number of class indexes on the broadcast. The protocol below consists of
accessing the first page of an index to get the offset to all the required indexes. The
indices and their corresponding data pages are accessed sequentially, as in the previ-
ous scheme.
Single-Class Protocol
1)
Probe onto channel and get offset to the next index
active
2)
Reach the index
doze
3)
Retrieve offsets to the indexes of required classes
active
4)
for every required class
5)
Reach the index
doze
6)
Retrieve the required index pages
active
7)
Reach the required data
doze
8)
Retrieve required data pages
active
6.1.3 Performance Evaluation
The simulator reported in Section
5
was extended to study both the response time
and energy consumption for the aforementioned allocation schemes. The overall
structure of the schema graph determines the navigational paths among the classes
within the graph. The relationships of the navigational paths within the graph influ-
ence the number and structure of indexes to be used. Realizing this fact, we take a
closer look at the effects of the inheritance and aggregation relationships.
Inheritance relationship
: Within an inheritance hierarchy, classes at the lower
level of the hierarchy inherit attributes of the classes at the upper level. There-
fore, objects belonging to the lower-level classes tend to be larger than those
within the upper levels. The distribution of the number of objects is application
dependent. In our analysis, without loss of generality, we assumed the objects
to be equally distributed among the classes of the hierarchy.
Aggregation relationship
: In an aggregation hierarchy, objects belonging to
lower classes are considered “part of” objects and those at the higher ends are
the “collection” of such parts. In other words, objects at the upper classes are
composed of objects of the lower ends. Therefore, objects belonging to higher
classes are generally larger than those belonging to the lower ones. In addi-
tion, the cardinality (number of objects per class) of a class at the upper end is
