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comparisons in terms of protocol characteristics, functionalities, advantages, drawbacks;
and to provide required performance-optimization solutions. Many studies and simula-
tions have been performed to compare and contrast the large number of routing protocols
developed for MANETs; see, for example, [56, 175“177]. A theoretical framework is pre-
sented in [178] that compares ad hoc network routing protocols (in an implementation-in-
dependent manner) by measuring each protocol™s performance relative to a theoretical op-
timum.
Most MANET simulations are carried out using network simulators such as OPNET
[179], NS-2 [180], and Glomosim [181] and its commercial version QualNet [182]. These
simulators provide advanced simulation environments to test and debug different net-
working protocols, including collision-detection modules, radio propagation, and MAC
protocol. These tools include libraries containing predefined models for most communi-
cation protocols (e.g., 802.11, Ethernet, TCP, etc.). In addition, they often provide graphi-
cal interfaces that can be used both during the model development phase and during sim-
ulation runs to simplify the following dynamic protocol and network behaviors.
Some recent results, however, question the validity of simulations based on these tools.
Specifically, [183] presents results of the flooding algorithm using OPNET, NS-2, and
Glomosim, as significant disparities between the simulators have been measured recently.
The observed differences are not only quantitative (not the same absolute value), but also
32 MOBILE AD HOC NETWORKING WITH A VIEW OF 4G WIRELESS: IMPERATIVES AND CHALLENGES


qualitative (not the same general behavior), making some past observations of MANET
simulation studies an open issue.

1.4.8. Quality of Service and Optimization
In mobile ad hoc networks, the presence of additional bandwidth, link and medium con-
straints, as well as the constant change in network topology, make supporting Quality of
Service more difficult than in fixed wireline networks, which only need to deal with static
constraints such as bandwidth, memory, or processing power. Due to the lack of suffi-
ciently accurate knowledge of the network states, both instantaneous and predictive, even
statistical QoS guarantees may be impossible if the nodes are highly mobile [148]. Conse-
quently, many existing QoS solutions developed for Internet are not suitable for MANET
environments and need to be adapted.
In general, Quality of Service is not related to any dedicated network layer, but, rather,
requires coordinated efforts from all layers. Important QoS components in the MANET
domain include QoS models, QoS Medium Access Control (MAC), QoS routing and re-
source reservation signaling, and so on [132]. A QoS model outlines the overall Quality of
Service goals and architecture for implementing a given application or service. These ob-
jectives may include link capacity, latency, link utilization percentage, throughput, band-
width and energy consumption, and so on. QoS routing refers to the discovery and main-
tenance of routes that can satisfy QoS objectives under given the resource constraints,
whereas QoS signaling is responsible for actual admission control, and scheduling, as
well as resource reservation along the route determined by QoS routing or other routing
protocols. Both QoS routing and QoS signaling coordinate with the QoS MAC protocol to
deliver the QoS service required.
Much research has been done in each of these component areas [130“133]. A first at-
tempt at QoS modeling has been presented in [135], where a Flexible QoS Model for
MANET (FQMM) based on both IntServ and Diffserv is proposed. It is a hybrid scheme
in that it tries to preserve the per-flow granularity for a small portion of traffic in
MANET, while using DiffServ-based per-class granularity for the rest of the traffic.
INSIGNIA is one of the early QoS signaling protocols specifically designed for resource
reservation in ad hoc environments [136, 138]. It supports in-band signaling by adding a
new option field in the IP header called INSIGNIA to carry the signaling control informa-
tion. Like RSVP, the service granularity supported by INSIGNIA is per-flow management.
The INSIGNIA module is responsible for establishing, restoring, adapting, and tearing
down real-time flows. It includes fast-flow reservation, restoration, and adaptation algo-
rithms that are specifically designed to deliver adaptive real-time service in MANETs
[136]. If the required resource is unavailable, the flow will be degraded to best-effort ser-
vice. QoS reports are sent to the source node periodically to report network topology
change as well as QoS statistics results such as loss rate, delay, and throughput etc. Other
QoS signaling protocols proposed for MANET include dynamic RSVP (dRSVP) [145].
QoS routing helps establish the route for successful resource reservation by QoS sig-
naling in MANETs [132]. This is a difficult task. In order to make an optimal routing de-
cision, QoS routing requires constant updates on link state information such as delay,
bandwidth, cost, loss rate, and error rate to make policy decisions, resulting in large
amounts of control overhead, which can be prohibitive for a bandwidth-constrained ad
hoc environment. In addition, the dynamic nature of MANETs makes maintaining the
precise link state information extremely difficult, if not impossible [132, 147, 148]. Final-
33
1.5. FUTURE RESEARCH DIRECTIONS


ly, even after resource reservation, QoS still cannot be guaranteed due to the frequent dis-
connection and topology change. Many QoS routing algorithms were published recently.
These work with a variety of QoS requirements and resource constraints, for example,
CEDAR [140], ticket-based probing [139], predictive location-based QoS routing [149],
localized QoS routing [146], and QoS routing based on bandwidth calculation [141].
QoS MAC protocols solve the problems of medium contention, support of reliable uni-
cast communication, and resource reservation for real-time traffic in a distributed wireless
environment [132]. Among numerous MAC protocols and improvements that have been
proposed, protocols that can provide QoS guarantees to real-time traffic in a distributed
wireless environment include the GAMA/PR protocol [142] and the Black-Burst (BB)
contention mechanism [144].


1.5. FUTURE RESEARCH DIRECTIONS

Despite the large volume of research activities and recent progress made in the mobile ad
hoc networking area both in the research community and industry, there are still many in-
teresting and important research problems to be solved in order to enable the large-scale
commercialization of the technology. Future challenges for ad hoc wireless networks in-
clude, but are certainly not limited to:

Routing Protocol Optimization. As stated, ad hoc routing has been the most active
research area in MANET, and many routing protocols have been proposed, each fo-
cusing on solving specific problems in the routing domain and suitable for a specif-
ic ad hoc environment. Future research is required to provide a scalable, adaptive,
and robust solution that can support commercialization of large ad hoc networks, as
well as optimize performance of these protocols for given cost objective, such as en-
ergy consumption, throughput, delay, or control overhead. Some promising initia-
tives in this area include power-aware routing and location-aided routing. Location-
aided routing aims at using the mobile node™s positioning information provided by
GPS or other mechanisms to define associated regions for nodal communication,
thereby reducing routing control overhead, saving energy, and improving routing
performance. Further work is also needed to investigate the feasibility and perfor-
mance of hybrid ad hoc routing approaches to allow the integration of hierarchical,
table-driven routing protocols with on-demand routing to create scalable routing
strategies that can adapt well to various ad hoc environments. Finally, multicasting,
which is essential for supporting multiparty multimedia communications, has not
been studied as extensively, and significant research effort is needed to come up
with efficient multicasting algorithms to cope with multicast group dynamics.
QoS Support. Quality of Service support is inherently difficult in an ad hoc environ-
ment. In order to support real-time multimedia applications, effort must be made to
control network QoS factors such as end-to-end delay, packet loss, and jitter. It has
been recognized that hard QoS guarantees will be difficult to achieve in a dynamic
environment. Consequently, there is a trend toward an adaptive QoS approach in-
stead of the “plain™™ resource reservation method with hard QoS guarantees. Since
end-to-end QoS guarantee requires coordinated effort from all layers, more research
effort is need to come up with coherent mechanisms that present an “all layer QoS”
solution instead of individual optimization within each layer.
34 MOBILE AD HOC NETWORKING WITH A VIEW OF 4G WIRELESS: IMPERATIVES AND CHALLENGES


Simulation. Modeling and simulation are at present incomplete and inadequately
supported tasks. Existing tools do not realistically and cleanly blend the physical
and MAC layers with data link, network, and higher layers. More research is needed
to address this issue. Most simulation tools in use today do not exceed networks of a
hundred nodes in size, and simulation environments that scale to larger networks are
only being developed.
Security. The security area has not been addressed adequately in existing research.
So far, solutions based on encryption, digital signatures, timestamps, and similar
methods, do help in achieving authentication, integrity, nonrepudiation, and privacy
to a certain degree, but more work need to be done to identify the possible network
trust relationships and physical security requirements as well as to improve pro-
posed algorithms to work in a distributed dynamic environment.
Standardization and Interoperability. Standardization is critical for lowering net-
work and development costs and for ensuring adoption and interoperability. With
the ever-increasing number of protocols and algorithms being proposed, guidance is
needed in developing solutions that target achieving overall scalability and high sys-
tem performance, as well as interoperability among the technologies.

In addition to the above areas, further research is needed in the areas of media access con-
trol, service discovery, addressing and autoconfiguration, resource management (band-
width, energy, etc.), location management, billing models, internet protocol operability,
and applications for mobile networks, to mention only a few of the most challenging, as
MANETs and 4G networking environments in general are taking shape.


1.6. CONCLUSIONS AND INTRODUCTION TO REMAINING CHAPTERS

In coming years, it seems inevitable that mobile computing will flourish and evolve toward
integrated, converged fourth generation wireless technology. Ad hoc networking will play
an important role in this evolution. Its intrinsic flexibility, ease of maintenance, lack of
needed infrastructure, autoconfiguration, self-administration capabilities, and significant
cost advantages make it a prime candidate for becoming the stalwart technology for per-
sonal pervasive communication. The opportunities for and importance of ad hoc networks
are being increasingly recognized by both the research and industry community, as evi-
denced by the flood of research activities, strong industry interest, and almost exponential
growth of the Wireless LAN and Bluetooth sectors. In moving forward and successfully ful-
filling this opportunity, developing and seamlessly integrating MANET with other wireless
networks and fixed internet infrastructures, the successful addressing of many of the open
research and development issues discussed in this article will play a critical role.
The rest of the chapters in this book cover many important areas and design issues in
mobile ad hoc networks that, due to space limitations, have been only touched upon in this
overview chapter. Specifically, subsequent chapters focus on the following areas:

Chapter 2, entitled “Off-the-Shelf Enablers of Ad Hoc Networks,” by Gergely V Záru-
.
ba and Sajal K. Das, discusses the WPAN and WLAN technologies as a basis for ad
hoc networks. Specifically, the chapter analyzes the IEEE 802.11 family, Hiper-
LAN, and Bluetooth.
35
1.6. CONCLUSIONS AND INTRODUCTION TO REMAINING CHAPTERS


Chapter 3, entitled “IEEE 802.11 in Ad Hoc Networks: Protocols, Performance and
Open Issues,” by Giuseppe Anastasi, Marco Conti, and Enrico Gregori, presents the
IEEE 801.11 technology and discusses its utilization for constructing ad hoc net-
works. Special attention is devoted to the interaction between the TCP protocol and
IEEE 802.11-based ad hoc networks. The aim is to analyze the performance of In-
ternet applications such as Web browsing and file transfer in such environments.
Chapter 4, entitled “Bluetooth Scatternet Formation in Bluetooth Networks,” by Ste-
fano Basagni, Raffaele Bruno, and Chiara Petrioli, describes the state of the art in
scatternet formation using Bluetooth technology, that is, formation of multihop ad
hoc nets of Bluetooth devices.
Chapter 5, entitled “Antenna Beamforming and Power Control for Ad Hoc Networks,”
by Ram Ramanathan, discusses the techniques to guarantee an efficient utilization
of channel capacity. These techniques include: (1) utilizing directional antennas in
ad hoc networks to increase effective capacity, increase connectivity, and lower
probability of detection/interference, and (2) controlling the topology of an ad hoc
network by changing the transmitting power.
Chapter 6, entitled “Topology Control in Wireless Ad Hoc Networks,” by Xiang-Yang
Li, discusses methods for designing and maintaining network topology to enable
network scalability, such as how to decide transmission radius to reduce interfer-
ence and conserve energy while enabling good network connectivity, topology up-
dates, and neighbor discovery.
Chapter 7, entitled “Broadcasting and Activity-Scheduling in Ad Hoc Networks,” by
Ivan Stojmenovic and Jie Wu, surveys existing methods for broadcasting in a wire-
less network intelligently (using omnidirectional or directional antennas, with equal
or adjusted transmission radii) and for scheduling node activities to ensure both re-
liability and power and bandwidth efficiency.
Chapter 8, entitled “Location Discovery,” by Andreas Savvides and Mani B. Srivastava,
surveys the requirements and broad applications that can supported by location dis-
covery, as well as technologies and algorithms that have been developed in this do-
main, with an emphasis on the application and usage in wireless systems for routing
calls to mobile users.
Chapter 9, entitled “Mobile Ad hoc Networks (MANETs): Routing technology for dy-
namic, wireless networking,” by Joseph P. Macker and M. Scott Corson, provides
specific insights into standardization activities and efforts involved in mobile ad
hoc networking.
Chapter 10, entitled “Routing Approaches in Mobile Ad Hoc Networks,” by Elizabeth
M. Belding-Royer, presents a comprehensive set of techniques used for routing in
ad hoc networks.
Chapter 11, entitled “Energy Efficient Communication in Ad Hoc Wireless Networks,”
by Laura M. Feeney, summarizes the evaluation of energy consumption in medium-
access control, routing, and transport protocols, including metrics and protocols
used to prolong network life, and design of localized algorithms that avoid commu-
nication overhead for updating network information. Power-efficient medium ac-
cess and the use of geographic position for power optimization are also discussed.
Chapter 12, entitled “Ad Hoc Network Security,” by Pietro Michiardi and Refik Molva,
presents recent research in the security area, including recent advances in providing
36 MOBILE AD HOC NETWORKING WITH A VIEW OF 4G WIRELESS: IMPERATIVES AND CHALLENGES


an automated key management scheme that does not require the presence of an ex-
ternal infrastructure or bootstrap phase in which keys are distributed, as well as cur-
rently available security mechanisms implemented in the data-link layer.
Chapter 13, entitled “Self-Organized and Cooperative Ad Hoc Networking by Silvia
Giordano and Alessandro Urpi, presents methods for exploiting certain characteris-
tics of ad hoc networks (e.g., cooperation and the relationship among nodes) based
on community and social network concepts.
Chapter 14, entitled “Simulation and Modeling of Wireless, Mobile, and Ad Hoc Net-
works,” by Azzedine Boukerche and Luciano Bononi, focuses on the use of simula-
tion methods and tools used in the performance analysis of ad hoc network architec-
ture and protocols, including synthetic models for describing the users™ mobility
and the pros and cons of various available simulation tools (NS-2, Glomosim, etc.).
Chapter 15, entitled “Modeling Cross-Layering Interaction Using Inverse Optimiza-
tion,” by Violet R. Syrotiuk and Amaresh Bikki, discusses modeling protocol inter-
action at different layers of a networking system.
Chapter 16, entitled “Algorithmic Challanges in Ad Hoc Networks,” by Andras Farago,
describes solved and open algorithmic problems that form the basis for many of the
fundamental solutions and protocols in ad hoc networking.


REFERENCES

1. S. Hara and R. Prasad, “Overview of Multicarrier CDMA,” IEEE Communications Magazine,
Dec. 1997, pp. 126“133.
2. Y. Bing Lin and I. Chlamtac, Wireless and Mobile Network Architectures, Wiley, 2000.
3. D-Link WLAN Access Point User Manaul and On-line Help, 2002.
4. H. Harada and R. Prasad, “A New Multi-carrier CDMA/TDD Transmission Scheme Based on
Cyclic Extended Spread Code for 4th Generation Mobile Communication System,” in Pro-
ceedings of the IEEE International Conference on Personal Wireless Communication Confer-
ence, pp. 319“323, 1997.
5. R. G. T. Anderson, B. Bershad, and D. Wetherall, “A System Architecture for Pervasive Com-
puting,” In Proceedings of the 9th ACM SIGOPS European Workshop, pp. 177“182, Kolding,
Denmark, September 2000.
6. Wireless World Research Forum (WWRF): http://www.ist-wsi.org.
7. I. Chlamtac and M. El-Zarki, “Introduction to Computer Networks,” in Encyclopedia of
Telecommunications, Vol. 9, 1994, Marcel Dekker.
8. S. Giordano, “Mobile Ad-hoc Networks,” in XXX XXX, Wiley, 2000.
9. C. E. Perkins (Ed.), Ad Hoc Networking, Addison-Wesley Longman, 2000.
10. The official Bluetooth web site: http://www.bluetooth.com/.
11. IBM Zurich Research Laboratory web site: http://www.zurich.ibm.com/cs/wireless/blue-
tooth.html.
12. I. Chlamtac and J. Redi, “Mobile Computing: Challenges and Opportunities,” in Encyclopedia
of Computer Science, 4th Edition, edited by D. Hemmendinger, A. Ralston, and E. Reilly, In-
ternational Thomson Publishing, 1998.
13. D. L. Lough, T. K. Blankenship, K. J. Krizman, Tutorial on Wireless LANs and IEEE 802. 11,

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