Peer-Based Collaborative Caching and Prefetching in Mobile Broadcast - Advanced Operating Systems and Kernel Applications - page 243

new neighbor who caches the data object, because
the mobile peers are moving. The second is after
certain time an existing neighbor has the required
data object, because it has, during the time, re-
quested and obtained the object from its neighbors.
By repeatedly sending out request messages, the
response time will improve as some queries may
be answered before broadcast hits . A mobile
peer should control the frequency of sending out
request message and avoid sending the request
message too frequently, because broadcasting
message consumes energy and increases chances
of wireless signal collision. KR is independent
of the cooperative cache management scheme;
therefore it can be applied on any cooperative
cache management scheme.
With Keep Requesting, the process of data ac-
cess becomes the one shown in Figure 2. Notice
that the states of local cache miss and neighbor
cache miss are omitted in the figure.

objects that are potentially useful to it, the mobile
peer is prefetching data from the broadcast chan-
nel and its data access is proactive. Prefetching
from broadcast channel improves response time,
but incurs more energy consumption (Acharya,
1996; Grassi, 2000; Hu, 2003). Generally speak-
ing, mobile peers with limited energy tend to
access the broadcast channel only when there are
pending requests, and mobile peers with enough
energy (such as devices on vehicles) may access
the broadcast channel proactively.
We assume that the mobile peers in a system
are with the same type and they have the same
d a t a a c c i.e. s s f a s h i o n , i . i.e. . t h i.e. y a r i.e. i.e. i t h i.e. r a l l d i.e. m a n d -
driven or all proactive.
In both demand-driven access and proactive
access, when a mobile peer gets a data object, it
faces the problem of deciding whether to store
the data object locally. A caching scheme helps a
mobile peer to make such decision under demand-
driven access, and a prefetching scheme helps
it to make such decision under proactive access.
Caching and prefetching schemes for indi-
vidual mobile peer in broadcast environments
have been studied extensively. Here “individual”
means that the caching (or prefetching) scheme
does not take the possible collaboration among
the mobile peers into account. With the advance
of short-range communication and mobile peer-
to-peer networks, researchers have begun to study

caching and prefetching

In a broadcast system, a mobile peer's data access
from broadcast channel can be demand-driven or
proactive. The data access is demand-driven if the
mobile peer retrieves data objects from broadcast
channel only when it has local cache miss. On the
contrary, if a mobile peer continuously monitors
broadcast channel and stores locally the data

Figure 2. Data access process

No

Monitor
Broadcast
Channel for
some time

Broadcast a
request to
neighbors

In local
cache?

Any
reply ?

Gotten
D k ?

To Access D k No No Yes Broadcast Hit

Yes

Yes

Local Cache Hit

Retrieve from a
neighbor

Neighbor Cache Hit