Analysis and Design of a Modern SCADA System

by maintaining a hierarchical network. The second reason is that OPNET
package provides flat protocols only. The flat routing is further classified
into two main categories. These are source initiated on-demand and table
driven routing protocols [ 48 ].
On demand [reactive] routing protocols determine routes only
when a node has a data packet to send. A node with a packet to send is
referred to as source node. If the route to the destination is not known, the
source node initiates a search (route discovery) to find possible routes to
the destination [ 49 ]. The optimized route is then used and maintained,
establishing connection and communication until such a route is no longer
required or becomes invalid. The DSR, AODV and TORA are examples of
on demand routing protocols [ 48 ].
Table driven [proactive] routing protocols attempt to maintain
consistent and up-to-date information of all possible routes, to all
destinations, at all times, regardless of whether the routes are needed. To
support this consistency, the protocol sends messages by propagates
(broadcast) to gather update information and all possible connectivity
through the network [ 49 ]. Proactive protocols require each node to
maintain more than one table to store routing information regardless of the
need for such route information. They also share common features like,
background information exchange regardless of the communication request
strategy employed [49]. Examples of table driven routing protocols are:
Fisheye State Routing (FSR), Optimized Link State Routing (OLSR), and
Destination Sequenced Distance Vector (DSDV). Figure 2. 11 outlines the
classification of proactive and reactive routing protocols [47]. Routing
protocols namely OLSR, AODV and TORA are described briefly, refer to
Appendix- 2 , but DSR is further described, discussed, and chosen for
reasons listed in the sections below.