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

Routing in Wireless Networks 4-5
• On some occasions, a mobile user may experience a link outage in some situations, such as driving
through a tunnel or being in an elevator. During these periods, the user has no radio connectivity,
and if such periods have a duration greater than application timers, ongoing communications
may be aborted.
• Delay jitter may be caused by a number of reasons when using wireless technologies.
• Some or all of the nodes will be powered by batteries, which means that energy saving is an important
factor when dealing with this type of network. As energy consumption is directly proportional
to the distance between hosts (proportional to radius squared), single-hop transmissions between
two hosts require a lot of power, causing interference with other hosts. To avoid this problem in routing,
two hosts can use a multi-hop transmission to communicate with each other via other nodes
in the network.
• Problems related to security are accentuated in wireless networks. As it is a shared environment to
which anybody can have access, confidentiality of data is an important consideration.
• When we work with wireless networks, we must consider some problems such as hidden terminals,
which occur when there are two nodes within communication range of a third node but not
of each other. Both may try to communicate with the third node simultaneously and might not
detect any interference in the wireless medium. Thus, the signals collide at the third node, which
will not be able to receive the transmissions from either node [P99]. The typical solution is that the
terminals coordinate their transmissions, by means of request to send/clear to send (RTS/CTS).
• Supposing node B communicates with node A, and node C wants to transmit a packet to node D.
During the transmission between node B and node A, node C perceives the channel as busy.
Node C falsely concludes that it may not send to node D, even though both transmissions
would succeed. Bad reception would only occur in the zone between node B and node C,
where neither of the receivers are located. That is the problem of the exposed terminals, and
this together with the hidden terminal cause significant reduction of network throughput
when the traffic load is high.
4.3 routing Protocol Families for Ad Hoc Networks
MANET routing protocols are traditionally divided into three main categories [AWD04] depending on
the type of information being exchanged and the frequency with which it is done: proactive or global
protocols, reactive or on-demand protocols, and hybrid protocols. The first group aims to maintain upto-date
routing information in the nodes through periodic control message exchange, while the second
attempts to find routes on demand. There are also hybrid routing protocols, which combine features
from both proactive and reactive approaches.
4.3.1 Proactive Routing Protocols
In proactive routing protocols, each node keeps the routing tables constantly up-to-date. The routing
tables contain the routing information for every other node in the network, which is updated periodically
and when there are changes in the topology of the network. The routing information can be kept
in different routing tables depending on the routing protocol. Proactive techniques typically use
algorithms such as distance vector and link state.
This type of protocol will operate in networks in which it is necessary for the route discovery procedure
to not have excessive latency, and this type of protocol is appropriate in terms of consumption of
resources such as bandwidth and energy. In most of the proactive routing protocols, the control message
overhead grows as O(N 2 ), where N represents the number of nodes in the network.
The difference between these protocols is in the manner that the routing information is updated,
detected, and the type of information that is stored in each routing table.
© 2011 by Taylor and Francis Group, LLC