Information Technology Reference
In-Depth Information
Local Cache Hit:
M
q
's local cache has
D
k
.
Local Cache Miss
:
M
q
's local cache does not
contain
D
k
.
Neighbor Cache Hit:
at least one of
M
q
's neigh-
bors has
D
k
in its local cache.
Neighbor Cache Miss:
none of
M
q
's neighbors
has
D
k
in its local cache.
Broadcast Hit:
M
q
gets
D
k
from the broadcast
channel.
t i o n s , m u l t i m e d i a i n f o r m a t i o n n e e d s t o b e d i s s e m i -
nated to (potentially) a large number of audiences.
Here we list two sample applications.
In an academic
conference
, people are often
interested in paper abstracts, presenters' profiles,
and short introduction (such as name, a small
photo, university, and research interests) of each
researcher who is attending the conference. Each
piece of these introduction (in the form of a web
page) information can be seen as a data object.
These data objects can be broadcast at the confer-
ence site. Notice that paper abstracts and present-
ers' profiles should be broadcast more frequently
than the attendees' information. Also notice that
a paper abstract, a presenter's profile, and an
attendee's profile are of similar sizes and they
normally do not change during the conference.
In
museums
, short video clips about the items
on display can be broadcast to the visitors. Each
video clip is a data object. The clips are of about
the same size and available throughout the exhibi-
tion. People are more interested in masterpieces;
therefore clips about the masterpieces should be
broadcast more frequently than the clips about
other art works. Each visitor to the museum gets
a mobile device that retrieves the information he/
she wants (the device obtains introduction clips
from the broadcast channel or a nearby device if
the neighboring device happens to have cached
the required clip locally).
When
M
q
a s k s f o r
D
k
from its neighbors, several
types of messages are exchanged. The mobile
peer first sends a
Request
message for
D
k
to its
neighbors.
M
q
's neighbors having
D
k
in their cache
send back a
Reply
message to
M
q
. Upon receiving
the
reply
message(s),
M
q
s e n d s a
Retrieve
message
to the neighbor who replied first, then the target
mobile peer transmits
D
k
to
M
q
.
Kr: Keep requesting
In some cooperative cache management schemes
for mobile peers in broadcast systems, when a
neighbor cache miss
happens, the mobile peer
simply tunes into the broadcast channel and waits
for the required data object to appear. In these
schemes, a
neighbor cache miss
always results in
a
broadcast hit
. However, a
neighbor cache miss
only means that the
current
neighbors do not have
the required data object, but not necessarily the
future
neighbors. Since a mobile peer's neighbors
may be changing all the time (especially in a
highly dynamic environment), a neighbor having
the required data object may come by before the
object appears on the broadcast channel.
In the schemes that we will discuss in detail,
an enhancement called Keep Request (KR) (Wu,
2006) is used. In the KR mode, a mobile peer
continues to send out
request
messages repeat-
edly to its neighborhood even though a
neighbor
cache miss
has happened. There are two situations
in which the query may be answered before the
required data object is broadcast by the server.
The first is after certain time the mobile peer has a
data access process
When a mobile peer wants to access a data object,
it first looks for it in its own local cache. If the data
object is not found in the local cache, the mobile
peer requests for it from its neighbors. If none of
its neighbors has the data object, the mobile peer
tunes into the broadcast channel and waits for the
data object to appear.
To facilitate the discussion, we define a few
terms to describe the situations of cache miss and
cache hit. Let
M
q
be a mobile peer and
D
k
be the
data object that
M
q
wants to access.
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