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

4-6 Industrial Communication Systems
4.3.2 reactive Routing Protocols
Reactive or on-demand* routing protocols were designed to reduce the overhead of the proactive routing
protocols; they maintain information on active routes only. When one node wants to communicate with
another node, one route to the destination is demanded. Although network resources such as energy and
bandwidth are used more efficiently than in the proactive protocols, the delay in the routing discovery
procedure increases. The route discovery procedure is usually made by flooding a route request packet
through the network. If a node with a route to the destination (or the destination itself) receives the route
request packet, it sends back a route reply packet to the source node using the link reversal if the packet
has traveled through bidirectional links. The route replay packet contains the desired route. The routing
algorithms found among the reactive routing protocols are distance vector and source routing.
Reactive routing protocols are divided into two groups, source-based and hop-by-hop or point-to-point.
• In source-based on-demand protocols, each data packet transports in its header the complete source
to the destination, that is, the information of every neighboring node from the source to the destination.
Every intermediate packet consults the header packet to know where to forward it to. Therefore,
there is no need for the intermediate nodes to keep the routing information continuously up to date
by means of forwarding routing messages periodically as in proactive routing protocols. In contrast,
in large ad hoc networks, the probability of link failures occurring increases with the number of
nodes. As the number of intermediate nodes increases, the size of the header packet increases too.
This kind of protocol is not recommended in large networks with a lot of hops and high mobility
because they do not scale well.
• In the hop-by-hop routing protocols, the packet only carries the destination address and the next
hop address; in this way, every intermediate node that participates in the route would consult its
routing table to decide which way to send the packet. The advantage is that every intermediate
node keeps its routing table up to date continuously and independently, so that when a packet
reaches an intermediate node, it can decide how to route it according to the current state of the
network; the routes can therefore adapt to the dynamic topology of this kind of route more easily.
The drawback is that every intermediate node along the route must constantly update its routing
table by means of beaconing messages.
The route discovery overhead can grow O(N + M) if a link reversal is possible, where N represents the
number of nodes in the network and M the number of nodes in the reply path; for unidirectional links,
the discovery overhead is O(2N).
4.3.2.1 Hybrid Routing Protocols
Hybrid routing protocols are a new generation of protocols that use the characteristics of reactive and
proactive protocols. These are designed to increase scalability and reduce the overload involved in route
discovery. This is possible by means of proactive route maintenance for the closer nodes and a determination
of routes for the more distant nodes by means of a route discovery strategy.
Most of the hybrid protocols are zone based, that is, the network is divided into a number of zones,
each containing one node that is selected to be in charge of all the mobile nodes within their transmission
range, or a subset of them. Other hybrid routing protocols group the nodes into trees or clusters.
4.4 routing Protocol Families for Wireless Sensor Networks
Routing in sensor networks is very challenging due to a number of characteristics that distinguish them
from contemporary communication and wireless ad hoc networks.
* Note that the terms reactive and on demand are used interchangeably.
© 2011 by Taylor and Francis Group, LLC