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

Ad Hoc Networks 7-5
link failure notification (ELFN) is a technique similar to TCP-f where route failures are notified to
downstream nodes in ELFN message sent by upstream nodes of the failed link. Another TCP variant is
ad hoc TCP [LS01] which requires network layer feedback and split TCP [KKFT02] to deal with congestion
control and end-to-end reliability separately.
In the non-TCP variant approach, the transport layer is built from scratch while considering the
limitations of wireless ad hoc networks. Although this approach outperforms TCP variants approach
in a stand-alone environment [MK04, p. 130], it poses many challenges when the machines with such
transport layer have to talk to the global Internet. Ad hoc transport protocol (ATP), one of the non-TCP
variants, is specifically built for ad hoc networks. ATP exploits synergies between different layers to
enhance performance. As mentioned earlier, one of the disadvantages of ATP is its incompatibility with
systems that are running plain TCP.
A reliable transport protocol for ad hoc networks is still needed. While designing a transport layer
protocol, the key trade-off is between compatibility and performance of the network.
7.2.2 Network Layer
Routing is used to send the data from source to destination. The easiest way is to broadcast the data in
the network that is wasteful of the bandwidth resource. A more efficient way is to compute route from
source to destination and send data in a multihop fashion. There are different ways to classify routing
protocols. The routing protocols can be classified on the basis of (a) the way the routing information is
updated, (b) network structure (flat and hierarchical), and (c) position and nonposition-based routing
protocols. The different categories are discussed in the following sections.
7.2.2.1 Proactive, Reactive, and Hybrid Routing Protocols
Proactive routing protocols are also known as table-driven protocols. The consistent and up-to-date
routing tables are computed ahead of transmission time. Whenever topology changes, update messages
are triggered to assure consistency of network map being maintained at each node. Destinationsequenced
distance vector (DSDV) [PP94] is a proactive routing protocol, which uses Bellman Ford
algorithm to compute the shortest path and ensures loop-free routing tables. Wireless routing protocols
(WRP) [MSGJ96], based on improved Bellman Ford, restrict route updates to immediate neighbors
only. The cluster gateway switch routing (CGSR) reduces routing table size and routing information
exchange by employing a clustering hierarchy where only the cluster heads communicate among themselves.
Optimized link state routing (OLSR) is a distributed, table-driven, and proactive routing protocol
for mobile ad hoc networks that work on the principle of “multipoint relays,” which means selecting
a set of nodes from within one-hop neighbor to provide ability to reach all the two-hop neighbors. OLSR
works well in large-scale and high-density network but incurs large routing overhead.
Overhead of periodic updates, slow route repair, and maintenance of unused routing information
are few of the problems of the proactive routing protocols. To overcome these issues, reactive routing
protocols have been designed.
Unlike the proactive routing protocols, the reactive routing protocols do not maintain routing tables
and compute routes whenever required. These are also known as “on-demand” routing protocols. Route
discovery is accomplished by flooding request in the network. Although, less control overhead makes
them scalable, larger delays are incurred while discovering route whenever needed. Dynamic source
routing (DSR) [DD96] and ad hoc on-demand distance vector (AODV) [PR99] are two examples of
reactive protocols. AODV is based on DSDV but is reactive and loop free. Temporally ordered routing
algorithm (TORA) [PC97] is a reactive scheme and is uniquely featured by maintaining multiple paths
between given pair of source and destination nodes.
There are different routing protocols which exhibit a hybrid approach for routing, for example, zonebased
routing (ZBR) [DHY03]. ZBR is more suitable for large networks where the network is divided into
clusters. Intracluster routing is proactive while intercluster routing is reactive in ZBR. Other protocols
© 2011 by Taylor and Francis Group, LLC