Wireless Network Design: Optimization Models and Solution ...

202 Khaldoun Al Agha and Steven Martin
Routing does not only consist of finding a route between two nodes, the goal
is to calculate an optimized route according to one or several constraints (number
of hops, maximum bandwidth, minimum end-to-end delay, etc.). Not designed
specifically for ad hoc networks, classical routing protocols (e.g. RIP and OSPF)
are inapplicable. They must be upgraded in order to be efficient and reliable, taking
into account the wireless and decentralized architecture, reducing control traffic and
managing mobility. Therefore, multi-hop ad hoc routing protocols present much
originality, since the mobile nodes in the network dynamically establish routing
among themselves to form their own network. Nevertheless, these protocols apply
properties of classical routing schemes. Indeed, we can distinguish two techniques
to calculate the best route:
• Distance-vector routing. Each node informs (periodically and after topology
changes) its neighbors of the estimated direction and distance to any other node.
The cost of reaching a destination is computed using various route metrics. The
Bellman-Ford algorithm is then used to calculate routes. Distance-vector routing
protocols have low computational complexity and message overhead. On the
other hand, the algorithm may struggle to converge on the new topology when
network conditions change.
• Link-state routing. Each node broadcasts its links’ states with its neighbors in
the network. Being acquainted with which nodes are connected to which other
nodes, every node attempts to construct a map of the connectivity of the network.
Then, the Dijkstra algorithm enables calculation of optimal routes. Instead of
sharing routing tables with the neighborhood, nodes applying a link-state routing
protocol flood the network only with their connectivity information.
These two techniques, well known in wired networks with RIP and OSPF, are
used in multi-hop ad hoc routing protocols (e.g., AODV and OLSR). Many other
differences appear between routing protocols. For instance, a packet may contain
in its header the complete path to follow in order to reach the destination (source
routing) or only the next node to its destination (hop-by-hop). We will see in Section
9.4 that a major distinction in multi-hop ad hoc routing protocols concerns the
time when a route is calculated. This defines two kinds of protocols: table driven
and on demand protocols.
9.3 Ad Hoc in the Link Layer
Several technologies enable ad hoc networks, both in Wireless Personal Area Networks
(WPAN) and Wireless Local Area Networks (WLAN). By definition, WPANs
are limited to a few meters, with data rates up to several hundred Kilobits per second.
As for WLANs, their communicating range is larger (e.g. a building) and they
can achieve data rates of tens of Megabits per second. A WLAN is obviously, and
above all, a LAN. Thus, in spite of wireless characteristics (e.g. bandwidth and en-