A novel caching scheme for improving Internet-based
mobile ad hoc networks performance
q
Sunho Lim
*
, Wang-Chien Lee, Guohong Cao, Chita R. Das
Department of Computer Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA
Received 19 December 2003; accepted 14 April 2004
Available online 8 August 2004
Abstract
Internet-based mobile ad hoc network (
IMANETIMANET
) is an emerging technique that combines a wired network (e.g. Inter-
net) and a mobile ad hoc network (
MANETMANET
) for developing a ubiquitous communication infrastructure. To fulfill usersÕ
demand to access various kinds of information, however, an
IMANETIMANET
has several limitations such as limited accessibility
to the wired Internet, insufficient wireless bandwidth, and longer message latency. In this paper, we address the issues
involved in information search and access in
IMANETSIMANETS
.Anaggregate caching mechanism and a broadcast-based Simple
Search (SS) algorithm are proposed for improving the information accessibility and reducing average communication
latency in
IMANETSIMANETS
. As a part of the aggregate cache, a cache admission control policy and a cache replacement policy,
called Time and Distance Sensitive (TDS) replacement, are developed to reduce the cache miss ratio and improve the
information accessibility. We evaluate the impact of caching, cache management, and the number of access points that
are connected to the Internet, through extensive simulation. The simulation results indicate that the proposed aggregate
caching mechanism can significantly improve an
IMANETIMANET
performance in terms of throughput and average number of
hops to access data items.
2004 Elsevier B.V. All rights reserved.
Keywords: Aggregate cache; Cache admission control; Cache replacement algorithm; Internet-based mobile ad hoc network; Simple
search algorithm
1. Introduction
Over the past decade, Internet has changed our
daily life. With the recent advent in wireless tech-
nology and mobile devices, ubiquitous communi-
cation is touted to change our life further. It is
envisaged that in the near future, users will be able
1570-8705/$ - see front matter 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.adhoc.2004.04.013
q
This research has been supported in part by NSF grants
CCR-9900701, CCR-0098149, CCR-0208734, and EIA-
0202007.
*
Corresponding author. Tel.: +1 814 865 2729; fax: +1 814
865 3176.
E-mail addresses: slim@cse.psu.edu (S. Lim), wlee@cse.
psu.edu (W.-C. Lee), gcao@cse.psu.edu (G. Cao), das@cse.
psu.edu (C.R. Das).
Ad Hoc Networks 4 (2006) 225–239
www.elsevier.com/locate/adhoc
to access the Internet services and information
anytime and anywhere. To realize this vision, wire-
less carriers are developing state-of-the-art wireless
communication infrastructures. Nevertheless, a
mobile terminal (MT)
1
may still have difficulty
to connect to a wired network or Internet due to
limited wireless bandwidth and accessibility.
Under heavy traffic, an MT has to content for
bandwidth and may get blocked from a wireless
base station. Moreover, in some geographically re-
mote areas, an infrastructure may not be even
available. Thus, researchers are exploring an alter-
native technology, called Mobile Ad Hoc Network
(
MANETMANET
), for its low cost and ease of deployment.
A significant volume of research on
MANETSMANETS
has appeared in the literatu re in the past few years
[6,9,10,14–17,19]. Most of these efforts, however,
have focused on developing routing protocols to
increase connectivity among MTs in a constantly
varying topology. Due to the usersÕ grow ing inter-
est and falling cost in accessing the wireless Inter-
net, it has become imperative to consider the
integration of
MANETMANET
with the wired Internet.
Thus, to put the
MANETMANET
technology into the con-
text of real life, we consider an Internet-based
MANETMANET
, called
IMANETIMANET
[2], and investigate the
problem of information search and access under
this environment. Under
IMANETIMANET
, we assume that
some of the MTs are connected to the Internet or
wired private networks.
2
Thus, an MT may access
Internet information via a direct connection or via
relays from other MTs. Although there may exist
many potential applications, to the best of our
knowledge, none of the previous work has ad-
dressed the issues for information search and ac-
cess in
IMANETSIMANETS
. The followings are some of the
applicable scenarios for an
IMANETIMANE T
:
• Scenario 1: During special events such as Olym-
pic games or World Cup Soccer, the demand
from users to access the Internet and communi-
cate among themselves are exceedingly high.
While a fixed infrastructure may be in place, it
is challenging to accommodate all the users
due to limited wireless bandwidth. With an
IMANETIMANET
, users can either access the required
information directly or indirectly (through
relays). Moreover, they can communicate
among themselves without going through a
wired infrastructure.
• Scenario 2: A visitor in a downtown, museum,
or shopping mall may need to access various
type of information (e.g. exhibition info, tour
info including maps, restaurants of choice,
hotels, theaters, and so on). A local service pro-
vider usually provides an electronic guide such
as an info-station [22] that contains the relevant
information. Although a visitor may lose con-
nection to the info-station because of mobility,
he/she can still access or share the information
through relays using
IMANETIMANET
.
• Scenario 3: In a battle fie ld or emergency site,
one MT may be connected to the Internet by
a satellite and serve as a proxy for other MTs.
The accessed information and services can be
shared by the other MTs via local ad hoc
communication.
An
IMANETIMANE T
has several constraints. First, not
all the MTs can access the Internet. Second, due
to mobility, a set of MTs can be separated from
the rest of the MTs and get disconnected from
the Internet. Finally, an MT requiring multi-hop
relay to access the Internet may incur a longer ac-
cess latency than those which have direct access to
the Internet.
To address these constraints, we propose an
aggregate caching mechanism for
IMANETSIMANETS
. The
basic idea is that by storing data items in the local
cache of the MTs, members of the
IMANETIMANET
can
efficiently access the required information. Thus,
the aggregated local cache of the MTs can be con-
sidered as an unified large cache for the
IMANETIMANET
.
The proposed aggregate cache can alleviate the
constraints of
IMANETSIMANETS
discussed above. When
an MT is blocked from direct access to the Inter-
net, it may access the requested data items from
the local cache of nearby MTs or via relays. If
an MT is isolated from the Internet, it can search
1
In this paper, we use the term mobile terminal (MT) to
refer to a portable device (e.g. a laptop computer, a personal
digital assistance (PDA), a mobile phone, a handheld computer,
etc) or a person who carries it.
2
Without loss of generality, we use Internet to refer to both
of Internet and wired private network for the rest of paper.
226 S. Lim et al. / Ad Hoc Networks 4 (2006) 225–239
other reachable MTs for the requested data item.
Finally, if an MT is located further from the Inter-
net, it may request the data items from other clo-
seby MTs to reduce access latency.
Here, two issues are addressed for implementa-
tion of an aggregate caching mechanism in
IMANETSIMANETS
:
• Efficient search: An efficient information search
algorithm is fundamental for locating the
requested data in
IMANETSIMANETS
.
• Cache management: To reduce the average
access latency as well as enhance the data acces-
sibility, efficient cache admission control and
replacement policies are critical. The cache
admission control policy determines whether a
data item should be cached, while the cache
replacement policy intelligently selects a victim
data item to be replaced when a cache becomes
full.
Information search in an
IMANETIMANE T
is different
from the search engine-based approach used in
the wired Internet. An MT needs to broadcast its
request to the possible data sources (including
the Internet and other MTs within the
IMANETIMANET
)
in order to retrieve the requested data efficiently.
Thus, we propose a broadcast-based approach,
called Simple Search (SS) algorithm, which can
be implemented on the top of existing routing pro-
tocols to locate the requested data. In addition, we
propose a cache admis sion control policy based on
the distance between MTs to reduce redundant
data caching, and a cache replac ement policy
based on time and distance, called Time and Dis-
tance Sensitive (TDS ) replacement, to reduce the
cache miss ratio and increase the accessibility of
the aggregate cache.
We conduct a simulation-based performance
evaluation to observe the impact of caching, cache
management, and access points (APs) (which are
directly connected to the Internet) upon the effec-
tiveness of
IMANETSIMANETS
. The overall results show that
the proposed methodology can relieve limitations
of
IMANETSIMANETS
and improve system performance
significantly.
This paper report s our initial study of Informa-
tion search and access on
IMANET SIMANETS
. The aggregate
cache idea is simple and can be used in practice
to enhance the communication performance of
IMANETSIMANE TS
. Focusing on the constraints of the
IMA-IMA-
NETNET
such as accessibility and latency, our contri-
bution is threefold:
• A simple search algorithm is developed to facil-
itate information search and access in an
IMANETIMANET
.
• An aggregate cache for
IMANETSIMANETS
is proposed to
address the issues of accessibility and latency.
• A distance-based admission control policy and
three cache replacement policies (TDS_D,
TDS_T, and TDS_N) are proposed as a part
of the aggregate caching scheme. These policies
are capable of providing better performance
than the well known LRU replacement policy.
The rest of this paper is organized as fol-
lows. Work related to the research is reviewed in
Section 2. The system model and simple search
algorithm, and the aggregate cache management
mechanism are presented in Sections 3 and 4,
respectively. Section 5 is devoted to performance
evaluation and comparisons of various policies.
Finally, we concludes the paper with future direc-
tions in Section 6.
2. Related work
Research on
MANETMANET
has mainly focused on
developing routing protocols such as Destina-
tion-Sequenced Distance Vector (DSDV) [16],
Dynamic Source Routing (DSR) [10] , Ad hoc On
Demand Distance Vector (AODV) [17], Tempo-
rally-Ordered Routing Algorithm (TORA) [15],
and their variations. These algorithm s assume that
a sender MT knows the location of receiver MT
based on the route information, which is accu mu-
lated and analyzed by a route discovery or route
maintenance algorithm. Although a route discov-
ery operation captures the current network topol-
ogy and related information, it has to be executed
whenever an MT needs to transmit a data item. To
avoid repetitive route discovery, the MTs can
cache the previous route information. Hu et al.
[9] compared the performance of two caching
S. Lim et al. / Ad Hoc Networks 4 (2006) 225–239 227
strategies based on the DSR routing protocol: a
path cache and a link cache. In the path cache, a
complete path from a sou rce to the destination is
stored. In the link cache, a group of paths, which
are collected from previous route discovery or
other operations, is constructed to generate a
graph style data structure. In our work, instead
of addressing the issue of route discovery and its
caching, we emphasize on efficient information
search and da ta caching to enhance data
accessibility.
Caching is an important technique to enhance
the performance of wired or wireless network. A
number of studies has been con ducted to reduce
the Web traffic and overall network congestion
by deploying various caching schemes in the Inter-
net [3,4,21]. A cooperative caching scheme is sug-
gested in [3], in which a couple of individ ual
caches are treated as a unified cache and they inter-
act among themselves to eliminate the duplicate
copies, and increase cache utilization. Fan et al.
[4] proposed a summary cache, where proxies
share their summary of cache contents represented
by bloom filters. When a proxy has a cache miss
for a request, it sends the request to other proxies
based on a periodically upda ted summary of cache
contents in other proxies. A proxy cache reloca-
tion scheme is proposed based on the prediction
of userÕs mobility to reduce delay during a handoff,
a mechanism of trans ferring an on-going call from
the current cell to the next cell to which a user
moves, in a cellular network [5]. However, no such
work has been conducted in a
MANETMANET
, in which a
network topology frequently changes.
Ren et al. [18] employed a semantic caching
scheme to manage location-dependent data (e.g.
weather, traffic, and hotel information), in which
an MT maintains semantic description of data
in a mobile environment. When an MT needs to
generate a query, it processes the query, analyzes
the descriptions, and finds out results (or partial
results) from the appropriate cache. Based on
the results, the MT tailors or reduces the query
and requests the server to get the rest of results
to reduce communication. In contrast to the tra-
ditional cache replacement policies, the Furthest
Away Replacement (FAR) is used in this study.
With this policy, a victim is selected such that it
is not on the way in which the MT might move,
but is located far away from the current location
of the MT.
In particular in
MANETSMANETS
, it is important to
cache frequently accessed data not only to reduce
the average latency, but also to save wireless
bandwidth in a mobile environment. Hara [6] pro-
posed a replica allocation method to increase data
accessibility in
MANETSMANETS
. In this scheme, an MT
maintains a limited number of duplicated data
items if they are frequently requested. Replicated
date items are relocated periodically at every relo-
cation period based on the followings: each MTÕs
access frequency, the neighbor MTsÕ access fre-
quency or overall ne twork topology. Update of
the replicated data is further considered in [7].
Since an MT cannot access data when it is iso-
lated from others, replication is an effective means
to improve data accessibility. Due to the limited
size of information that an MT can maintain,
however, simply replicating data items and access-
ing them in
MANETMANET
s cannot fulfill usersÕ require-
ments to access a wide variety of information,
available over the Internet.
To overcome the limited information availabil-
ity in
MANETMANET
s, Sailhan et al. [19] proposed a
cooperative caching scheme to increase data acces-
sibility by peer-to-peer communication among
MTs, when they are out of bound of a fixed
infrastructure. It is implemented on top of a well-
known ad hoc routing protocol, called Zone Rout-
ing Protocol (ZRP). Papadopouli et al. [14]
suggested the 7DS architecture, in which a couple
of protocols are defined to share and disseminate
information among users. It operates either on a
prefetch mode, based on the information and
userÕs future needs or on an on-demand mode,
which searches for data items in a single-hop
multicast basis. Depending on the collaborative
behavior, a peer-to-peer and server-to-client model
are used. Unli ke our approach, this strategy fo-
cuses on data dissemination, and thus, the cache
management including a cache admission control
and replacement policy is not well explored.
To the best of our knowledge, none of previous
work has explored an aggregated caching scheme
along with an efficient information search algo-
rithm in the realm of
IMANETSIMANE TS
.
228 S. Lim et al. / Ad Hoc Networks 4 (2006) 225–239
3. Information search in
IMANETSIMANETS
3.1. System model
In this subsection, we describe a generic system
model of
IMANETSIMANE TS
. We assume that an MT can
not only connect to the Internet but also can for-
ward a message for communication with other
MTs via a wireless LAN (e.g. IEEE 802.11), as
used in most prior study [19,14,13]. As illustrated
in Fig. 1,an
IMANETIMANET
consists of a set of MTs that
can communicate with each other using an ad hoc
communication protocols (illustrated by dashed-
line). Among the MTs, some of them can directly
connect to the Internet, and thus serve as access
points
3
(AP) for the rest of MTs in the
IMANETIMANET
.
Thus, an AP is a gateway for the Internet and is
assumed to have access to any information. An
MT located out of the communication bound of
an AP has to access the Internet via relays through
one of the access points. An MT can move in any
direction and make information search and access
requests from anywhere in the covered area.
When an MT is located near by an AP (e.g.
within one-hop), it makes a connection to the AP
directly. When an MT is located far away from
an AP, however, information access has to go
through several hops in the ad hoc network before
reaching the AP.
3.2. Information search algorithm
As mentio ned in the introduction, the main fo-
cus of this paper is to support information access
in
IMANETSIMANETS
. Unlike a routing protocol, which
establishes a path between a known source and
destination, any MT can be an information source
in the
IMANETIMANET
. Thus, without knowing the desti-
nation address for any requested information, a
search algorithm is needed for
IMANETSIMANETS
as is done
in the Internet. In the following, we describe the
basic idea of an information search algorithm
employed in the paper. This algorithm can be
implemented on top of an existing routing proto-
col for
MANETSMANETS
.
Since an aggregate cache is supported in an
IMANETIMANE T
design, requested data data items can be
received from the local cache of an MT as well
as via an AP connected to the Internet. When an
MT needs a data item, it does not know exactly
where to retrieve the data item from, so it broad-
casts a request to all of the adjacent MTs. When
an MT receives the request and has the data item
in its local cache, it will send a reply to the reques-
ter to acknowledge that it has the data item; other-
wise, it will forward the request to its neighbors.
Thus, as illustrated in Fig. 2,
4
a request may be
flooded in the network and eventually acknow-
ledged by an AP and/or some MTs with cached
copies of the requested data item.
Based on the idea described above, we propose
an information search algorithm, called Simple
Search (SS), to determine an information access
path to the MTs with cached data of the request
or to appropriate APs. The decision is based on
the arriving order of acknowledgments from the
MTs or APs. Let us assume an MT (n
i
) sends a re-
quest for a data item (d ) and an MT (n
k
) is located
along the path in which the request travels to an
AP, where k 2 {a,b,c, j}. The SS algorithm is de-
scribed as follows:
satellite
LEO or GEO
AP
Fixed network
Fixed router
MT
Imanet
Fig. 1. A generic system model of
IMANETIMANET
.
3
The AP here is a logical notation. An AP equipped with
appropriate antennas can directly communicate with the
Internet through wireless infrastructures including cellular base
stations, and Low Earth Orbit (LEO) or geostationary (GEO)
satellites.
4
A dotted circle represents the communication range of an
MT or an AP. For the sake of simplicity, we assume that both
an MT and an AP have the same diameter of communication.
S. Lim et al. / Ad Hoc Networks 4 (2006) 225–239 229
