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

4-4 Industrial Communication Systems
4.2 routing Protocols and Classification
The purpose of routing protocols is to determine the best path from source to destination. The routing
protocols have to enable routers to communicate with each other exchanging topology and state information.
There is a diverse range of routing protocols, which may be classified according to several different
characteristics.
Depending on the type of routing algorithms used, routing protocols can be divided into three families:
distance vector, link state, and source routing:
• Protocols that use the distance vector technique maintain a table for communication and employ
diffusion (not flooding) for information exchanged between their neighbors.
• Link state technique maintains a table with the entire topology network. The topology is built
by finding the shortest path in terms of link cost. This cost is periodically exchanged among all
the nodes through a flooding technique. Each node updates its routing table using the link cost
information. This technique can cause loops in networks that change the topology quickly [M95].
• In the source routing technique, all the data packets have the routing information on the headers.
The source node makes the route decision. With this technique, loops can be avoided, but the protocol
overhead is quite significant. This technique is not efficient for fast moving topologies due to
route invalidation around the path of a packet.
However, there are other classifications that take into account other characteristics.
• Dynamic or static. A dynamic or adaptative routing protocol changes its behavior according to the
network state, while a static one does not.
• Centralized or distributed. Dynamic routing protocols can also be classified as centralized or
distributed. The classification is based on which node makes the routing decisions. In a distributed
routing protocol, all nodes are responsible for making their own routing decisions. In a centralized
routing protocol, the decisions take place at a central node. One example of centralized
routing protocols is the cluster-based routing protocols, where the central node is the so-called
clusterhead. It is responsible for the routing process.
Routing in wireless networks is of growing importance as currently more and more applications
involve client mobility. This means the implementation of routing mechanisms that facilitate communication
between nodes and which can adapt to the particular characteristics of this type of
medium. It is because of this that wireless environments cannot use the classic routing protocols.
These classic protocols must be adapted to meet the requirements of the new wireless environment.
There follows a description of some of the problems and characteristics to be found in wireless
environments [CM99]:
• Firstly, as the nodes are mobile elements, the network topology changes continuously, and therefore
the links between nodes are created and broken dynamically.
• The bandwidth available in a wireless interface is less than that with a wired interface. Sometimes,
the maximum available bandwidth is in the order of tens and hundreds of Kbps and is underutilized
due to reduction and interference in the electromagnetic signals. As the distance between
the user and its peer link entity grows, the available bandwidth decreases due to the need for a
robust coding due to a smaller signal-to-noise and interference (SNIR) ratio.
• The link latency is high due principally to the extensive processing required at the physical layer of
these networks and due to the transmission delays in the radio access network.
• In wireless links, losses due to corruption may occur, even in static conditions, due to several
phenomena such as fading, multipath, and attenuation.
© 2011 by Taylor and Francis Group, LLC