Module-III
Routing Protocols for Wireless Sensor Networks
DATA DISSEMINATION AND GATHERING
The way that data and queries are forwarded between the base station
and the location where the target phenomena are observed is an important
aspect and a basic feature of WSNs.
A simple approach to accomplishing this task is for each sensor node to
exchange data directly with the base station.
A single-hop-based approach, however, is costly, as nodes that are farther
away from the base station may deplete their energy reserves quickly,
thereby severely limiting the lifetime of the network.
DATA DISSEMINATION AND GATHERING
Data exchange between the sensors and the base stations is usually
carried out using multi-hop packet transmission over a short
communication radius.
leads to significant energy savings and reduces considerably
communication interference between sensor nodes competing to access
the channel.
Depending on the nature of the application, sensor nodes can aggregate
data correlated on their way to the base station.
In a multi-hop WSN, intermediate nodes must participate in forwarding
DATA DISSEMINATION AND GATHERING
ROUTING CHALLENGES AND DESIGN ISSUES
in WSN
Although WSNs share many commonalities with wired and ad hoc networks, they
also exhibit several unique characteristics that set them apart from existing
networks.
Challenges:
Energy constraints, limited computing and communication capabilities, the
dynamically changing environment within which sensors are deployed, and unique
data traffic models and application-level quality of service requirements.
Network Scale and Time-Varying
Characteristics
Sensor nodes operate with limited computing, storage, and
communication capabilities under severe energy constraints
The densities of the WSNs may vary widely, ranging from very sparse to
very dense.
In Many applications, the sensor nodes, numbering in the hundreds if not
thousands, are deployed in an ad hoc and often unsupervised manner
over wide coverage areas.
In these networks, the behavior of sensor nodes is dynamic and highly
adaptive
As the need to self-organize and conserve energy forces sensor nodes to
adjust their behavior constantly in response to their current level of activity
or the lack thereof.
Sensor nodes may be required to adjust their behavior in response to the
erratic and unpredictable behavior of wireless connections
Resource Constraints
Sensor nodes are designed with minimal complexity for large-scale
deployment at a reduced cost.
Energy is a key concern in WSNs, which must achieve a long lifetime
while operating on limited battery reserves.
Multihop packet transmission over wireless networks is a major source of
power consumption.
Reducing energy consumption can be achieved by dynamically controlling
the duty cycle of the wireless sensors.
Sensor Applications Data Models
The data model describes the flow of information between the sensor
nodes and the data sink.
A class of sensor applications requires data collection models that are
based on periodic sampling.
In other applications, data can be captured and stored, possibly processed
and aggregated by a sensor node, before they are forwarded to the data
sink.
Third class of sensor applications requires bidirectional data models in
which two-way interaction between sensors and data sinks is required
ROUTING STRATEGIES IN WIRELESS SENSOR
NETWORKS
The WSN routing problem presents a very difficult challenge that can be
posed as a classic trade-off between responsiveness and efficiency.
In a WSN, overhead is measured primarily in terms of bandwidth
utilization, power consumption, and the processing requirements on the
mobile nodes.
Finding a strategy to balance these competing needs efficiently forms the
basis of the routing challenge.
Routing algorithms for ad hoc networks can be classified according to the
way information is acquired and maintained and the way this information is
Proactive Routing
Flat proactive routing strategies have the potential to compute optimal
paths.
The overhead required to compute these paths may be prohibitive in a
dynamically changing environment.
Hierarchical routing is better suited to meet the routing demands of large
ad hoc networks.
Reactive routing
Establish routes to a limited set of destinations on demand.
These strategies do not typically maintain global information across all
nodes of the network.
Rely on a dynamic route search to establish paths between a source and a
destination.
It involves flooding a route discovery query, with the replies traveling back
along the reverse path.
Reactive routing strategies vary in the way they control the flooding
process to reduce communication overhead
Hybrid strategies
Rely on the existence of network structure to achieve stability and
scalability in large networks.
The network is organized into mutually adjacent clusters, which are
maintained dynamically as nodes join and leave their assigned clusters.
A hybrid routing strategy can be adopted whereby proactive routing is
used within a cluster and reactive routing is used across clusters.
The main challenge is to reduce the overhead required to maintain the
clusters.
WSN Routing Techniques
The design of routing protocols for WSNs must consider the power and
resource limitations of the network nodes, the time-varying quality of the
wireless channel, and the possibility for packet loss and delay.
One class of routing protocols adopts a flat network architecture in which
all nodes are considered peers.
Advantage is, minimal overhead to maintain the infrastructure and the
potential for the discovery of multiple routes between communicating
nodes for fault tolerance.
WSN Routing Techniques
A second class of routing protocols imposes a structure on the network to
achieve energy efficiency, stability, and scalability.
Network nodes are organized into clusters in which a node with higher
residual energy, for example, assumes the role of a cluster head.
Clustering has the potential to reduce energy consumption and extend the
lifetime of the network.
WSN Routing Techniques
A third class of routing protocols uses a data-centric approach to
disseminate interest within the network.
The approach uses attribute-based naming, whereby a source node
queries an attribute for the phenomenon rather than an individual sensor
node.
Interest dissemination is achieved by assigning tasks to sensor nodes and
expressing queries to relative to specific attributes.
Different strategies can be used to communicate interests to the sensor
nodes, including broadcasting, attribute-based multicasting, geo-casting,
WSN Routing Techniques
A fourth class of routing protocols uses location to address a sensor
node.
It is useful where the position of the node within the geographical
coverage of the network is relevant to the query issued by the source
node.
Such a query may specify a specific area where a phenomenon of interest
may occur or the vicinity to a specific point in the network environment.
