Information Technology Reference
In-Depth Information
The hybrid method also has several drawbacks. Querying the database to pull
data interrupts the server and consumes bandwidth and power that could otherwise
be used for pushing data on the broadcast. As a result, one can conclude that the
backchannel should not be used excessively.
3 . 2 I n d e x i n g O b j e c t s o n t h e B r o a d c a s t C h a n n e l
Retrieving information from the air channel has two requirements:
Minimizing the overall response time at the mobile unit; and
Minimizing the amount of power consumed in the retrieval process at the mobile
unit.
The concept of index has been extensively studied and applied in traditional file
and database systems. It is a function that takes a key value and provides an address
referring to the location of the associated data. Its main advantage lies in the fact
that it eliminates the need for an exhaustive search through the pages of data on the
storage medium. B-trees, B+-trees, and hash tables are some of the most common
index structures that have been used in practice. An index is also used as a tool
to facilitate the query optimization. Similarly, within the scope of broadcasting, an
index can be used to point to the location or possible availability of a data item on
the broadcast and allowing the mobile unit to predict the arrival time of the data
item requested. This enables the mobile unit to switch its operational mode into
an energy-saving mode and minimize energy consumption. It should be noted that
the advantages of indexing comes at the expense of computational overhead and
increased length of the broadcast, which increases the response time.
3.2.1 Signature-Based Indexing
The basic idea behind the application and use of signatures in a broadcast channel
is to add a control part to the contents of an information frame
[27-30,60]
—a hash
function is applied to the contents of the information frame to generating a bit vector
that is superimposed on the data frame. As a result, a signature partially reflects the
data content of a frame. In short, this technique creates a set of signatures for data
frames on a broadcast and interleaves them with their associated data frame. Differ-
ent allocations of signatures on a broadcast channel have been studied, including:
single signature
,
integrated signature
, and
multi-level signature
[28-30]
.
In the single signature scheme, the signature frame is broadcast before the corre-
sponding data frame. In the integrated signature scheme, a signature is constructed
for a group of consecutive frames called a frame group. The multi-level signature
scheme is a combination of the simple and integrated signature methods in which
