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This is a draft version only.
Wireless Ad Hoc Networks
Zygmunt J. Haas, Jing Deng, Ben Liang, Panagiotis Papadimitratos, and S. Sajama
Cornell University
School of Electrical and Computer Engineering
323 Rhodes Hall
Ithaca, NY 14853
Tel: (607) 255-3454, Fax: (607) 255-9072
e-mail: {haas, jing, liang, papadp, sajama}@ece.cornell.edu
URL: http://www.ece.cornell.edu/~haas/wnl/html
Abstract
A mobile ad hoc network is a relatively new term for an old technology - a network that does
not rely on pre-existing infrastructure. Roots of this technology could be traced back to the
early 1970s with the DARPA PRNet and the SURAN projects. The new twitch is the application
of this technology in the non-military communication environments. Additionally, the research
community has also recently addressed some extended features of this technology, such as
multicasting and security. Also numerous new solutions to the "old" problems of routing and
medium access control have been proposed. This survey attempts to summarize the state-of-
the-art of the ad hoc networking technology in four areas: routing, medium access control,
multicasting, and security. Where possible, comparison between the proposed protocols is
also discussed.
Keywords: ad hoc networks, MANET, MAC protocols for ad hoc network, routing protocols for
ad hoc networks, proactive routing protocols, reactive routing protocols, hybrid routing
protocols, multicasting for ad hoc networks, security for ad hoc networks,
1. Introduction
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1.1 The Notion of the Ad Hoc Networks
A Mobile Ad Hoc Network (MANET) is a network architecture that can be rapidly deployed
without relying on pre-existing fixed network infrastructure. The nodes in a MANET can
dynamically join and leave the network, frequently, often without warning, and possibly without
disruption to other nodes’ communication. Finally, the nodes in the network can be highly
mobile, thus rapidly changing the node constellation and the presence or absence of links.
Examples of the use of the MANETs are:
•
tactical operation - for fast establishment of military communication during the
deployment of forces in unknown and hostile terrain;
•
rescue missions - for communication in areas without adequate wireless coverage;
•
national security - for communication in times of national crisis, where the existing
communication infrastructure is non-operational due to a natural disaster or a global
war;
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Perkins, Charles E., AD HOC NETWORKING, pp.221-225,
2001 Addison Wesley Longman, Inc.
Reprinted by permission of Pearson Education, Inc.
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•
law enforcement - for fast establishment of communication infrastructure during law
enforcement operations;
•
commercial use - for setting up communication in exhibitions, conferences, or sales
presentations;
•
education - for operation of wall-free (virtual) classrooms; and
•
sensor networks - for communication between intelligent sensors (e.g., MEMS
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)
mounted on mobile platforms.
Nodes in the MANET exhibit nomadic behavior by freely migrating within some area,
dynamically creating and tearing down associations with other nodes. Groups of nodes that
have a common goal can create formations (clusters) and migrate together, similarly to military
units on missions or to guided tours on excursions. Nodes can communicate with each other at
any time and without restrictions, except for connectivity limitations and subject to security
provisions. Examples of network nodes are pedestrians, soldiers, or unmanned robots.
Examples of mobile platforms on which the network nodes might reside are cars, trucks,
buses, tanks, trains, planes, helicopters or ships.
MANETs are intended to provide a data network that is immediately deployable in arbitrary
communication environments and is responsive to changes in network topology. Because ad-
hoc networks are intended to be deployable anywhere, existing infrastructure may not be
present. The mobile nodes are thus likely to be the sole elements of the network. Differing
mobility patterns and radio propagation conditions that vary with time and position can result in
intermittent and sporadic connectivity between adjacent nodes. The result is a time-varying
network topology.
MANETs are distinguished from other ad-hoc networks by rapidly changing network
topologies, influenced by the network size and node mobility. Such networks typically have a
large span and contain hundreds to thousands of nodes. The MANET nodes exist on top of
diverse platforms that exhibit quite different mobility patterns. Within a MANET, there can be
significant variations in nodal speed (from stationary nodes to high-speed aircraft), direction of
movement, acceleration/deceleration or restrictions on paths (e.g., a car must drive on a road,
but a tank does not). A pedestrian is restricted by built objects while airborne platforms can
exist anywhere in some range of altitudes. In spite of such volatility, the MANET is expected to
deliver diverse traffic types, ranging from pure voice to integrated voice and image, and even
possibly some limited video.
1.2. The Communication Environment and the
MANET
Model
The following are a number of assumptions about the communication parameters, the network
architecture, and the network traffic in a MANET.
•
Nodes are equipped with portable communication devices. Lightweight batteries may
power these devices. Limited battery life can impose restrictions on the transmission
range, communication activity (both transmitting and receiving) and computational power
of these devices.
•
Connectivity between nodes is
not
a transitive relation; i.e., if node A can communicate
directly with node B and node B can communicate directly with node C, then node A
may
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M
icro-
E
lectro-
M
echanical-
S
ystems
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not,
necessarily, be able to communicate directly with node C. This leads to the hidden
terminal problem [Tob75].
•
A hierarchy in the network routing and mobility management procedures could improve
network performance measures, such as the latency in locating a mobile. However, a
physical hierarchy may lead to areas of congestion and is very vulnerable to frequent
topological reconfigurations.
•
We assume that nodes are identified by fixed IDs (based on IP [Pos81] addresses, for
example).
•
All the network nodes have equal capabilities. This means that all nodes are equipped with
identical communication devices and are capable of performing functions from a common
set of networking services. However, all nodes do not necessarily perform the same
functions at the same time. In particular, nodes may be assigned specific functions in the
network, and these roles may change over time.
•
Although the network should allow communication between
any
two nodes, it is envisioned
that a large portion of the traffic will be between geographically close nodes. This
assumption is clearly justified in a hierarchical organization. For example, it is much more
likely that communication will take place between two soldiers in the same unit, rather than
between two soldiers in two different brigades.
A MANET is a peer-to-peer
network that allows direct
communication between any two nodes,
when adequate radio propagation conditions exist between these two nodes and subject to
transmission power limitations of the nodes. If there is no direct link between the source and
the destination nodes, multi-hop
routing is used. In multi-hop routing, a packet is forwarded
from one node to another, until it reaches the destination. Of course, appropriate routing
protocols are necessary to discover routes between the source and the destination, or even to
determine the presence or absence of a path to the destination node. Because of the lack of
central elements, distributed protocols
have to be used.
The main challenges in the design and operation of the MANETs, compared to more traditional
wireless networks, stem from the lack of a centralized entity, the potential for rapid node
movement, and the fact that all communication is carried over the wireless medium. In
standard cellular wireless networks, there are a number of centralized entities (e.g., the base-
stations, the Mobile Switching Centers (MSCs), the Home Location Register (HLR), and the
Visitor Location Register (VLR)). In ad-hoc networks, there is no preexisting infrastructure, and
these centralized entities do not exist. The centralized entities in the cellular networks perform
the function of coordination. The lack of these entities in the MANETs requires distributed
algorithms to perform these functions. In particular, the traditional algorithms for mobility
management, which rely on a centralized HLR/VLR, and the medium access control schemes,
which rely on the base-station/MSC support, become inappropriate.
All communications between all network entities in ad-hoc networks are carried over the
wireless medium. Due to the radio communications being vulnerable to propagation
impairments, connectivity between network nodes is not guaranteed. In fact, intermittent and
sporadic connectivity may be quite common. Additionally, as the wireless bandwidth is limited,
its use should be minimized. Finally, as some of the mobile devices are expected to be hand-
held with limited power sources, the required transmission power should be minimized as well.
Therefore, the transmission radius of each mobile is limited, and channels assigned to mobiles
are typically spatially reused. Consequently, since the transmission radius is much smaller
than the network span, communication between two nodes often needs to be relayed through
intermediate nodes; i.e., multi-hop routing is used.
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β
γ
α
Fig. 1: An example of the hidden terminal problem
Because of the possibly rapid movement of the nodes and variable propagation conditions,
network information, such as a route table, becomes obsolete quickly. Frequent network
reconfiguration may trigger frequent exchanges of control information to reflect the current
state of the network. However, the short lifetime of this information means that a large portion
of this information may never be used. Thus, the bandwidth used for distribution of the routing
update information is wasted. In spite of these attributes, the design of the MANETs still needs
to allow for a high degree of reliability, survivability, availability, and manageability of the
network.
Based on the above discussion, we require the following features for the MANETs:
•
Robust routing and mobility management algorithms
to increase the network’s
reliability and availability; e.g., to reduce the chances that any network component is
isolated from the rest of the network;
•
Adaptive algorithms and protocols
to adjust to frequently changing radio propagation,
network, and traffic conditions;
•
Low-overhead algorithms and protocols
to preserve the radio communication resource
•
Multiple (distinct) routes
between a source and a destination - to reduce congestion in
the vicinity of certain nodes, and to increase reliability and survivability;
•
Robust network architecture
to avoid susceptibility to network failures, congestion
around high-level nodes, and the penalty due to inefficient routing.
In this paper, we present a survey of techniques used to establish communications in
MANETs. In particular, we concentrate on four areas: the medium access control (MAC)
schemes, the routing protocols, the multicasting protocols, and the security schemes.
2. MAC-Layer Protocols for Ad Hoc Networks
Applicability of the existing MAC-layer protocol, in particular the family of the Carrier Sense
Multiple Access (CSMA), to the radio environment is
limited by the following two interference mechanisms:
the hidden terminal and the exposed terminal problems.
The hidden terminal problem occurs because the radio
network, as opposed to other networks, such as a LAN,
for instance, does not guarantee high degree of
connectivity. Thus, two nodes, which maintain
connectivity to a third node, do not, necessarily, can
hear each other. Consider the situation in Figure 1.
Node
α
is in communication with node
β
. Node
α
is currently transmitting. Node
γ
wishes to
communicate with node
β
as well. Following
the CSMA protocol, node
γ
listens to the
medium, but since there is an obstruction
between node
α
and node
γ
, node
γ
does not
detect node' s
α
transmission, declaring the
medium is free. Consequently,
γ
accesses the
medium, causing collisions at
β
.
β
γ
α
Fig. 2: An example of the exposed terminal problem
δ
STOP
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The second problem, the exposed terminal problem, is depicted in Figure 2. In the figure, node
α
is transmitting to node
β
, while node
γ
wants to transmit to node
δ
. Following the CSMA
protocol, node
γ
listens to the medium, hears that node
α
transmits and defers from accessing
the medium. However, there is no reason why node
γ
cannot transmit concurrently with the
transmission of node
α
, as the transmission of node
γ
would not interfere with the reception at
node
β
due to the distance between the two. The culprit here is, again, the fact that the
collisions occur at the receiver, while the CSMA protocol checks the status of the medium at
the transmitter.
In general, the hidden terminal problem reduces the capacity of a network due to increasing
the number of collisions, while the exposed terminal problem reduces the network capacity due
to the unnecessarily deferring nodes from transmitting.
Several attempts have
been made in the
literature to reduce the ill
effect of these two
problems. The necessity
of a dialogue between
the transmitting and the
receiving nodes that
preempts the actual
transmission and that is
referred to as the
RTS/CTS dialogue, has
been generally
accepted.
The RTS/CTS dialogue is depicted in Figure 3. A node ready to transmit a packet, send a short
control packet, the Request To Send (RTS), with all nodes that hear the RTS defer from
accessing the channel for the duration of the RTS/CTS dialogue. The destination, upon
reception of the RTS responds with another short control packet, the Clear To Send (CTS). All
nodes that hear the CTS packet defer from accessing the channel for the duration of the DATA
packet transmission. The reception of the CTS packet at the transmitting node acknowledges
that the RTS/CTS dialogue has been successful and the node starts the transmission of the
actual data packet. Although the RTS/CTS dialogue does not eliminate the hidden and the
expose terminal problems, it does provide some degree of improvement over the traditional
CSMA schemes.
In what follows, we present a number of attempts to further improve the performance of the
MAC-layer protocols for ad hoc networks.
2.1 The
Multiple Access Collision Avoidance (MACA)
scheme
In Multiple Access Collision Avoidance (MACA) [Kar90], Karn proposed the use of RTS/CTS
dialogue for collision avoidance on the shared channel. Through the use of the RTS/CTS
dialogue, the MACA scheme reduces the probability of data packet collisions caused by hidden
terminals.
β
γ
α
Fig. 3: The RTS/CTS dialogue reduces the chances of collisions
δ
RTS
RTS
CTS
CTS
DATA
DATA
DATA
