Information Technology Reference
In-Depth Information
the upper level signatures are integrated signatures and the lower level signatures are
simple signatures
[29]
.
During the retrieval process, a query is resolved by generating a signature based
on the user's request. The query signature is then compared against the signatures
of the data frames in the broadcast. A successful match indicates a potential hit.
Consequently, the content of the corresponding information frame is checked against
the query to verify that it corresponds to the user's demands. If the data of the frame
corresponds to the user's request, the data is recovered; otherwise, the corresponding
information frame is ignored. This reduces the access time and the tune-in time.
As part of the studies done on signature-based indexing schemes, the three afore-
mentioned signature-based schemes have been analyzed with respect to each other
using the access time and tune-in time as the performance metrics
[28-30]
.There-
sults showed that, with fixed signature size, the multi-level scheme has the best tune-
in time performance at the expense of a longer access time. The integrated scheme,
on the other hand, has the best average access time, but its tune-in time depends
on the similarity among the information frames. And finally, the simple scheme has
an adequate access time and tune-in time. This study concluded that in comparison
to broadcasting without using indexing, all three signature-based schemes improved
tune-in time performance significantly with a reasonable access time overhead.
3.2.2 Tree-Based Indexing
An index is a meta data representing one or several data attributes pointing to
the location of data collection (i.e., information frames) sharing the same common
attribute value(s). This auxiliary information is usually organized as a tree in which
the lowest level of the tree points to the location of the information frames on the
broadcast channel. With this concept in mind, the frames on the broadcast are of two
kinds: data frames and index frames.
A broadcast channel is a sequential medium and hence, to reduce the mobile unit
active and tune-in time, and consequently to reduce the power consumption, the in-
dex frames are usually replicated and interleaved with the data frames. Otherwise,
the request would have to wait for the beginning of the next broadcast cycle—an
increase in the query response time. Two index replication schemes (namely,
distrib-
uted indexing
and (1,
m
)
1
indexing
) have been studied
[38]
. In distributed indexing,
the index is partitioned and interleaved in the broadcast cycle. Each part of the index
in the broadcast is followed by its corresponding data frame(s). In
(
1
,m)
indexing,
the entire index is interleaved
m
times during the broadcast cycle—the whole index
is broadcast before every 1
/m
fraction of the cycle. A series of analyses for both
methods were carried out and compared with the scenarios that provided the best
1
m
represents the number of times the index is replicated during one broadcast cycle.
