Protocols a 100 reliable broadcast is unnecessary in

protocols), a 100% reliable broadcast is unnecessary.
In addition, we assume that a MH can detect duplicate broadcast
messages, as this is essential to prevent endless flooding of a message.
One way to do so is to associate a tuple (source ID, sequence number)
with each broadcast message as in the case of DSR and AODV.
Here, we focus on the flooding behavior in a MANET - the
phenomenon where the transmission of a packet will trigger other
surrounding MHs to transmit the same (or modified) packet. We shall
show that if flooding is used blindly, many redundant messages will be
sent and serious contention/collision will be incurred.
3.2.2 Flooding-Generated Broadcast Storm
A straightforward approach to perform broadcast is by flooding. A
MH, on receiving a broadcast message for the first time, has the
obligation to rebroadcast the message. Clearly, this costs n transmissions
in a network of n MHs. In a CSMA/CA network, drawbacks of flooding
include:
• Redundant rebroadcasts: When a MH decides to rebroadcast a
broadcast message to its neighbors, all its neighbors already have the
message;
• Contention: After a MH broadcasts a message, if many of its
neighbors decide to rebroadcast the message, these transmissions
(which are all from nearby MHs) may severely contend with each
other;
• Collision: Because of the deficiency of backoff mechanism, the lack
of RTS/CTS handshake in broadcasts, and the absence of collision
detection (CD), collisions are more likely to occur and cause more
damage.

Chapter 3: Broadcasting, Multicasting and Geocasting 85
Figure 3.1 - The broadcast storm problem in a MANET with 13 nodes
(average degree is 2.6)
As we have mentioned before, the collection of these drawbacks is
referred to as the broadcast storm problem. Figure 3.1 exemplifies the
broadcast storm problem, where node S initiates a route request to node
D through a flooding. As we can see, flooding is highly redundant. Each
node receives the route request degree times, and the route request
propagates far beyond node D. Because nearby nodes will receive and
rebroadcast the route request at nearly the same time, contention (when
senders can hear each other) and collision (when senders cannot hear
each other) will be common.
3.2.3 Redundancy Analysis
The main reason for redundancy is that radio signals from different
transceivers may negatively overlap with each other. Let us consider two
examples to illustrate the effects of redundancy, where we denote node S
is the source of the broadcast and node D as the "last" node to receive the
broadcast. In Figure 3.2(a), it only takes two transmissions for node D to
broadcast a message whereas four transmissions will be carried out if no
attempt is made to reduce this redundancy. Figure 3.2(b) presents a more

86
AD HOC & SENSOR NETWORKS
(a) (b)
Figure 3.2 - Two optimal broadcasts in a MANET. The links represent the connectivity
among the nodes. Node S is the source and node D is the "last" network node
[Taken from Nil999]
serious scenario where only two transmissions are sufficient to complete
a broadcast as opposed to a total of seven transmissions generated if