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entity means network management has to be distributed across different nodes, which
brings added difficulty in fault detection and management.

1.3.3.2. Dynamically Changing Network Topologies. In mobile ad hoc networks,
since nodes can move arbitrarily, the network topology, which is typically multihop, can
change frequently and unpredictably, resulting in route changes, frequent network parti-
tions, and, possibly, packet losses [12, 36].

1.3.3.3. Physical Layer Limitation. The radio interface at each node uses broadcast-
ing for transmitting traffic and usually has limited wireless transmission range, resulting
in specific mobile ad hoc network problems like hidden terminal problems, exposed ter-
minal problem, and so on. Collisions are inherent to the medium, and there is a higher
probability of packet losses due to transmission errors compared to wireline systems.

1.3.3.4. Limited Link Bandwidth and Quality. Because mobile nodes communi-
cate with each other via bandwidth-constrained, variable capacity, error-prone, and inse-
cure wireless channels, wireless links will continue to have significantly lower capacity
than wired links and, hence, congestion is more problematic.

1.3.3.5. Variation in Link and Node Capabilities. Each node may be equipped
with one or more radio interfaces that have varying transmission/receiving capabilities
16 MOBILE AD HOC NETWORKING WITH A VIEW OF 4G WIRELESS: IMPERATIVES AND CHALLENGES


Table 1.2. Mobile Ad hoc Network Applications
Applications Descriptions/Services
Tactical networks Military communication, operations
Automated Battlefields
Sensor networks [25] Collection of embedded sensor devices used to collect real-time data to
automate everyday functions. Data highly correlated in time and space,
e.g., remote sensors for weather, earth activities; sensors for manufacturing
equipment.
Can have between 1000“100,000 nodes, each node collecting sample
data, then forwarding data to centralized host for processing using low
homogeneous rates.
Emergency services Search-and-rescue operations as well as disaster recovery; e.g., early re-
trieval and transmission of patient data (record, status, diagnosis) from/to
the hospital.
Replacement of a fixed infrastructure in case of earthquakes, hurricanes,
fire, etc.
Commercial E-Commerce, e.g., electronic payments from anywhere (i.e., in a taxi).
environments
Business:
dynamic access to customer files stored in a central location on the fly
provide consistent databases for all agents
mobile office
Vehicular Services:
transmission of news, road conditions, weather, music
local ad hoc network with nearby vehicles for road/accident guidance
Home and enterprise Home/office wireless networking (WLAN), e.g., shared whiteboard
networking application, use PDA to print anywhere, trade shows
Personal area network (PAN)
Educational Set up virtual classrooms or conference rooms
applications
Set up ad hoc communication during conferences, meetings, or lectures
Entertainment Multiuser games
Robotic pets
Outdoor Internet access
Location-aware Follow-on services, e.g., automatic call forwarding, transmission of the
services actual workspace to the current location
Information services
push, e.g., advertise location-specific service, like gas stations
pull, e.g., location-dependent travel guide; services (printer, fax, phone,
server, gas stations) availability information; caches, intermediate
results, state information, etc.
17
1.3. MOBILE AD HOC NETWORKS


and operate across different frequency bands [130, 137]. This heterogeneity in node radio
capabilities can result in possibly asymmetric links. In addition, each mobile node might
have a different software/hardware configuration, resulting in variability in processing ca-
pabilities. Designing network protocols and algorithms for this heterogeneous network
can be complex, requiring dynamic adaptation to the changing power and channel condi-
tions, traffic load/distribution variations, load balancing, congestion, and service environ-
ments.

1.3.3.6. Energy Constrained Operation. Because batteries carried by each mobile
node have limited power, processing power is limited, which in turn limits services and
applications that can be supported by each node. This becomes a bigger issue in mobile ad
hoc networks because as each node is acting as both an end system and a router at the
same time, additional energy is required to forward packets from other nodes [23].

1.3.3.7. Network Robustness and Reliability. In MANET, network connectivity is
obtained by routing and forwarding among multiple nodes. Although this replaces the
constraints of fixed infrastructure connectivity, it also brings design challenges. Due to
various conditions like overload, acting selfishly, or having broken links, a node may fail
to forward the packet. Misbehaving nodes and unreliable links can have a severe impact
on overall network performance. Lack of centralized monitoring and management points
means these types of misbehaviors cannot be detected and isolated quickly and easily,
adding significant complexity to protocol design.

1.3.3.8. Network Security. Mobile wireless networks are generally more vulnerable
to information and physical security threats than fixed-wireline networks. The use of open
and shared broadcast wireless channels means nodes with inadequate physical protection
are prone to security threats. In addition, because a mobile ad hoc network is a distributed
infrastructureless network, it mainly relies on individual security solution from each mo-
bile node, as centralized security control is hard to implement. Some key security require-
ments in ad hoc networking include:

Confidentiality: preventing passive eavesdropping
Access control: protecting access to wireless network infrastructure
Data integrity: preventing tampering with traffic (i.e., accessing, modifying or in-
jecting traffic)
Denial of service attacks by malicious nodes

1.3.3.9. Network Scalability. Current popular network management algorithms were
mostly designed to work on fixed or relatively small wireless networks. Many mobile ad
hoc network applications involve large networks with tens of thousands of nodes, as
found, for example, in sensor networks and tactical networks [16]. Scalability is critical to
the successful deployment of such networks. The evolution toward a large network con-
sisting of nodes with limited resources is not straightforward and presents many chal-
lenges that are still to be solved in areas such as addressing, routing, location manage-
ment, configuration management, interoperability, security, high-capacity wireless
technologies, and so on.
18 MOBILE AD HOC NETWORKING WITH A VIEW OF 4G WIRELESS: IMPERATIVES AND CHALLENGES


1.3.3.10. Quality of Service. A quality of service (QoS) guarantee is essential for
successful delivery of multimedia network traffic. QoS requirements typically refer to a
wide set of metrics including throughput, packet loss, delay, jitter, error rate, and so on
[150]. Wireless and mobile ad hoc specific network characteristics and constraints de-
scribed above, such as dynamically changing network topologies, limited link bandwidth
and quality, variation in link and node capabilities, pose extra difficulty in achieving the
required QoS guarantee in a mobile ad hoc network.


1.4. TECHNICAL CHALLENGES AND RESEARCH OVERVIEW

The specific MANET issues and constraints described in the previous section present a host
of challenges in ad hoc network design. A significant body of research has been accumu-
lated to address these specific issues and constraints. In this section, we describe some of
the main research areas within the mobile ad hoc network domain. Figure 1.3 shows the
MANET network layers and the corresponding research issues associated with each layer.

1.4.1. Media Access Control and Optimization
In MANET, use of broadcasting and shared transmission media introduces a nonnegligi-
ble probability of packet collisions and media contention. In addition, with half-duplex ra-
dio, collision detection is not possible, which severely reduces channel utilization as well


Network Layers Challenges in each layer

New/Killer Applications; All Layers:
L7: Application Layer
Network Auto-configuration
L6: Presentation
Location Services Energy
Layer Security (authentication, Conservation;
L5: Session Layer encryption) QoS,
Reliability,
Scalability,
Network
Tcp Adaptation,
L4: Transport Layer
Simulation,
Backoff Window
Performance
Optimization,
IP Routing,
L3: Network Layer H/W,S/W tools
Addressing, support
Optimization,
Multicasting

Media access control
L2: Data Link Layer
Error Correction
Optimization


Spectrum usage/allocation
L1: Physical Layer



Figure 1.3. MANET network layers and research challenges.
19
1.4. TECHNICAL CHALLENGES AND RESEARCH OVERVIEW




A B C

Figure 1.4. Hidden-terminal problem.


as throughput, and brings new challenges to conventional CSMA/CD-based and MAC
protocols in general. Among the top issues are the hidden-terminal and exposed-terminal
problems.
The hidden-terminal problem occurs when two (or more) terminals, say, A and C, can-
not detect each other™s transmissions (due to being outside of each other transmission
range) but their transmission ranges are not disjoint [38, 152]. As shown in Figure 1.4, a
collision may occur, for example, when terminal A and C start transmitting toward the
same receiver, terminal B in the figure.
The exposed-terminal problem results from situations in which a permissible transmis-
sion from a mobile station (sender) to another station has to be delayed due to the irrelevant
transmission activity between two other mobile stations within sender™s transmission range.
Figure 1.5 depicts a typical scenario in which the exposed-terminal problem may oc-
cur. Let us assume that terminals A and C can hear transmissions from B, but terminal A
cannot hear transmissions from C. Let us also assume that terminal B is transmitting to
terminal A, and terminal C has a frame to be transmitted to D. According to the CSMA
scheme, C senses the medium and finds it busy because of B™s transmission, and, there-
fore, refrains from transmitting to D, although this transmission would not cause a colli-
sion at A. The exposed-terminal problem may thus result in loss of throughput.




A B C D



Figure 1.5. Exposed-terminal problem.
20 MOBILE AD HOC NETWORKING WITH A VIEW OF 4G WIRELESS: IMPERATIVES AND CHALLENGES


The very early access protocols such as Aloha, CSMA, Bram and, TDMA introduced
in the 1970s [7] were primarily intended as solutions to multiaccess channels, such as any
broadcast media, similar to early LANs, and quickly proved inadequate to effectively deal
with the needs of current-day ad hoc network applications. The first protocols designed
specifically for mobile and multihop mobile networks [37, 130, 137] were designed with
tactical communication in mind and were based on slotted channels requiring rigid syn-
chronization. As recent ad hoc technologies started to take shape, a very large number of
new-generation ad hoc protocols such as MACA (multiple access with collision avoid-
ance protocol), MACAW (MACA with CW optimization), FAMA (floor acquisition mul-
tiple access), MACA/PR and MACA-BI (multiple access with collision avoidance by in-
vitation protocol) [39“44] have been proposed to resolve the various hidden-terminal,
exposed-terminal and similar problems, and improve channel performance in MANET.
The key ideas behind these protocols involve sending RTS (request to send) and CTS
(clear to send) packets before the data transmission has actually taken place [38]. When a
node wishes to transmit a packet to a neighbor, it first transmits a RTS packet. The receiv-
er then consents to the communication by replying with a CTS packet. On hearing the
CTS, the sender can transmit its data packet.
For example, a virtual carrier sensing mechanism based on the RTS/CTS mechanism
has been included in the 802.11 standard to alleviate the hidden-terminal problem that
may occur by using physical carrier sensing only. Virtual carrier sensing is achieved by
using two control frames, Request To Send (RTS) and Clear To Send (CTS), before the
data transmission actually takes place. Specifically, before transmitting a data frame, the
source station sends a short control frame, named RTS, to the receiving station, announc-
ing the upcoming frame transmission. Upon receiving the RTS frame, the destination sta-
tion replies by a CTS frame to indicate that it is ready to receive the data frame. Both the
RTS and CTS frames contain the total duration of the transmission, that is, the overall
time interval needed to transmit the data frame and the related ACK. This information can
be read by any station within the transmission range of either the source or the destination
station. Hence, stations become aware of transmissions from hidden stations, and the
length of time the channel will be used for these transmissions.
However, studies [45, 46] show that when traffic is heavy, a data packet can still expe-
rience collision due to loss/collision of RTS or CTS packets. To alleviate this problem,
comprehensive collision-avoidance mechanisms have been introduced via a backoff
mechanism. In principle, once a transmitting node senses an idle channel, it waits for a
random backoff duration (determined by a contention window, and increasing exponen-
tially with each reattempt) before attempting to transmit the packet, and congestion con-
trol is achieved by dynamically choosing the contention window based on the traffic con-
gestion situation in the network. Besides backoff methods, other mechanisms have also
been proposed to address this problem. DBTMA (dual busy tone multiple access) [46]
provides a scheme whereby special signals called busy tones (BTt/BTr) are used to pre-
vent other mobile hosts unaware of the earlier RTS/CTS dialogues from destroying the
ongoing transmission. The distributed collision resolution protocol EMMCRR [50] uses
power control and energy measurement techniques to achieve efficient collision avoid-
ance; and in [38], a combination of RTS/CTS, power control and busy-tone techniques are
used to further increase channel utilization.
In IEEE 802.11, CSMA/CA (Carrier Sense Multiple Access with Collision Avoid-
ance), a variation of the MACA protocol, is used for the MAC layer, and DCF is used to
provide collision avoidance and congestion control [47].
21
1.4. TECHNICAL CHALLENGES AND RESEARCH OVERVIEW


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