Mobility Aspects in WSN - Application and Multidisciplinary Aspects of Wireless Sensor Networks - page 115

The main weaknesses of random mobility models and predictable models are the
data transfer delays. Here, nodes collecting the information from the sensor field
must wait for the sink to pass in their vicinity in order to collect the data. This time
can have high value causing long delays in the data transfer and making this solu-
tion inappropriate for real-time applications. However, if controllable mobility is
used, then specific algorithms can guide the movement entities to collect informa-
tion from the sensor field in a desirable fashion and overcome the problem of delay.
The drawback of this approach is the cost of implementation in terms of hardware
resources and navigational needs. Moreover, a stronger model (such as global net-
work knowledge) is usually assumed, in contrast to the much weaker assumption
of random mobility.
The mobile sink approach has several disadvantages. All nodes must know the
position of the sink in order to route the information to it. Also, most scenarios have
the sink acting as a gateway to a backbone network and it is difficult to engineer a
system whereby a mobile sink is always connected to the backbone network. Mobile
sinks perform advantageous regarding network lifetime improvement compared to
other mobile entities. In such a way, moving the sink nodes demonstrates better
performances than the mobile relay approach, but relays are beneficial in application
scenarios where a mobile sink is not feasible (for example, in hostile terrains) [ 6 ].

6.2.2 Mobile Relay Approach

Another approach to enhance the connectivity and the network lifetime in WSNs is
the mobile relay approach. It can lead to elongating lifetime of bottleneck sensor
nodes . Mobile relay can follow a random, predictable, or deterministic mobility pat-
tern. Randomly moving Data MULEs are introduced as forwarding agents to help
gather the sensing data, thereby saving energy due to single-hop transmissions (i.e.,
from a sensor to a mule that is passing by) [ 13 ]. Controllably moving Message Ferry
is a mobility-assisted approach, which utilizes a set of special mobile nodes called
message ferries to provide communication service for nodes in the deployment area
[ 14, 15 ]. These two approaches are introduced for sparse sensor networks, while
deterministic mobility is suitable for dense sensor networks where the relay follows
a deterministic path for achieving network lifetime improvement [ 6 ].
Mobile relay approach has serious latency drawbacks leading to a decrease in the
achievable throughput. Therefore, this kind of solutions are mostly suitable where
mobile sinks cannot be implemented and are robust, flexible, and easier to design [ 6 ].

6.2.3 Mobile Sensor Approach

The movements of sinks and relays mainly improve network lifetime and provide
more efficient energy utilization. The mobile sensors , through network topology