wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Ad Hoc Networks 7-9
7.4 Challenges and Issues
7.4.1 Quality-of-Service
With the increase in portable computing devices and demand for multimedia applications, the importance
of providing QoS in ad hoc networks will certainly grow. There are two key challenges in providing
QoS over ad hoc networks: the fickle nature of the radio channel and the mobility of the nodes.
The varying quality of the transmission channel due to interference and fading results in dynamically
changing BERs and bandwidth throughput as well as problems for the network layer in distinguishing
between congestion or link-layer loss. Furthermore, it is difficult to apply conventional techniques that
estimate the effective bandwidth at a node for providing QoS in order to ensure bandwidth reservation,
admission control, policing, and flow control in ad hoc networks. The survey papers by [KRD06] and
[RKMM06] provide an excellent overview to the issues and challenges by providing a layer-wise classification
and QoS frameworks for ad hoc networks. QoS over multihop networks poses further challenges
as each node may be forwarding multiple flows of varying demands [MRV09]. Recent advances in
multiple input multiple output (MIMO) technology have led to a growing interest in multiantenna wireless
ad hoc networks where each node has more than one antenna. The advances in MIMO technology
result in significant gains in link capacity while combating multiple path interference and multiple-user
interference, making cross-layer approaches to providing QoS attractive [GLB09].
7.4.2 Energy Management
Since ad hoc wireless networks are mostly collection of battery-operated portable devices, efficient
energy management is critical. These networks offer communication using intermediate mobile hosts to
route information, severely straining their already limited power. Additionally, in wireless sensor networks,
many devices are operated by batteries that cannot be replaced at all. Therefore, energy-efficient
designs are required for these devices to enable them to operate for prolonged period of time without
replacing batteries. The power-conservative schemes are classified into transmitter power-control
mechanisms and power-management algorithms.
Power control involves controlling the host transmission powers in accordance with distances
between the terminals so that any required criterion for the received power can be satisfied. In [RR00],
two centralized algorithms are proposed which output the transmission power of each host by constructing
either a uni-connected network or a bi-connected network. Although these algorithms are
designed for static networks where location of all hosts is known, they allow a continuous adjustment
of the host transmission powers to maintain a certain QoS. These algorithms, however, require a guaranteed
knowledge of global connectivity without which they may lead to a disconnected network. This
global network connectivity is ensured in [WLBW01], which assumes that each receiver is able to determine
the direction of the sender when receiving a message.
Power-management strategies are applied to MAC or higher layers of OSI model. The power consumption
of hosts in ad hoc networks strongly depends on the MAC policy [WESW98]. In [BCD99,BCD01],
the authors propose a distributed contention control (DCC) protocol, which preemptively estimates the
probability of successful transmission before transmitting each frame and defers transmission if
the probability of success is too low. Thus, battery energy is saved by avoiding unnecessary retransmissions,
which consume significant power. In [RS98], a power-aware multiaccess protocol with signaling
(PAMAS) is introduced for power conservation in ad hoc networks, which proposes separate data and
signaling channels. The signaling channel is used for exchanging RTS/CTS packets which enables a host
to determine when and for how long to power its antenna off. Similarly, several power-saving techniques
are proposed [CT00,SL01] for the network layer for ad hoc networks. For route selection, these protocols
not only take into account the traditional routing protocol metrics such as hop-count, link quality, and
congestion but also consider battery power levels. Power-management protocols are also implemented
© 2011 by Taylor and Francis Group, LLC