05-07-2012, 10:59 AM
PROTOCOLS AND ARCHITECTURE FOR WIRELESS AD HOC NETWORKS
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ABSTRACT
The subject of this dissertation is ad hoc wireless networks. In such networks packets are
relayed over multiple hops to reach their destination. In order to operate ad hoc networks
several protocols, for media access control, power control, routing, and transport are needed.
This dissertation is concerned with the development and evaluation of such protocols, and the
associated architecture of the protocol stack. We take a holistic approach to the problems in
ad hoc networks.
Transmission power control is important because of the fundamental nature of the wireless
network that it is interference limited. Transmission power control has the potential to increase
a network's tra±c carrying capacity, reduce energy consumption, and reduce the end-to-end
delay. We start by postulating general design principles for power control based on the e®ect
of transmit power on various performance metrics. These are used to design a set of protocols
which attempt to optimize di®erent performance metrics, as all the metrics cannot be simul-
taneously optimized in general.
INTRODUCTION
A wireless ad hoc network is a decentralized network of nodes with radios, possibly mobile,
sharing a wireless channel and asynchronously sending packets to each other, generally over
multiple hops. The most notable characteristics of an ad hoc network are a lack of infrastructure,
multihop communication by cooperative forwarding of packets, distributed coordination among
nodes, dynamic topology, and the use of a shared wireless channel.
The potential for deployment of ad hoc networks exists in many scenarios, for example, in
situations where infrastructure is infeasible or undesirable, like disaster relief, sensor networks,
etc. Ad hoc networks also have the potential of realizing a free, ubiquitous, omnipresent com-
munication network for the community. We provide a layer-by-layer summary of the progress
in ad hoc networks.
THE TRANSMIT POWER CONTROL PROBLEM
The wireless medium is a shared medium. Every transmission causes interference in the
surrounding area. Successful reception of packets is possible only if this interference is within
some limits. Thus, interference is a key feature of the wireless medium and fundamentally
a®ects the tra±c carrying capability of the wireless network. One of the e®ective mechanisms
of controlling this interference is by controlling the transmission power. This motivates the
transmit power control problem, which is the topic of this chapter.
The transmit power control problem in wireless ad hoc networks is that of choosing the
transmit power for each packet in a distributed fashion at each node. We begin by making the
case that power control is a challenging example of a design problem that cuts across several
layers. The problem is complex since the choice of the power level fundamentally a®ects many
aspects of the operation of the network.
Reducing the transmit power level reduces the average contention at the MAC layer
.
Changing the range changes the number of one-hop neighbors that each node has and
thus, the number of neighbors it has to contend with for media access. At the same time,
changing the range changes the number of hops in a route and thus, the relaying burden
that each node has to carry and consequently the amount of tra±c that each node has to
transmit. The following argument shows that the net radio tra±c in contention range is
proportional to r, which is minimized by reducing r [27].
The COMPOW Power Control Protocol
A ¯rst cut simple solution for power control is the COMPOW protocol [27]. In COMPOW,
the goal of the optimization for each node is to (a) choose a common power level, (b) set this
power level to the lowest value which keeps the network connected, and © keep the energy
consumption close to minimum. A common power throughout the network for all the packets
has the key property of ensuring bidirectionality of links due to reciprocity of electromagnetic
waves in space, assuming that other factors such as interference are relatively homogeneous.
Bidirectionality ensures that the MAC and network layers work properly and also enables use
of the standard OSI protocols like ARP, DHCP, etc. We ensure that using too low a power
level does not increase the energy consumption by restricting the lowest admissible power level
to the one corresponding to rcrit, in line with the argumentation in Section 2.1, point III©.