The Broadcast Storm Problem in a Mobile Ad Hoc Network
Sze-Yao Ni, Yu-Chee Tseng, Yuh-Shyan Chen, and Jang-Ping Sheu
Department of Computer Science and Information Engineering
National Central University
Chung-Li, 32054, Taiwan
Tel: 886-3-4227151 ext. 4512, Fax: 886-3-4222681
Email: yctseng@csie.ncu.edu.tw
URL: http://www.csie.ncu.edu.tw/
yctseng/
Abstract
Broadcasting is a common operation in a network to resolve
many issues. In a mobile ad hoc network (MANET) in par-
ticular, due to host mobility, such operations are expected
to be executed more frequently (such as finding a route to
a particular host, paging a particular host, and sending an
alarm signal). Because radio signals are likely to overlap
with others in a geographical area, a straightforward broad-
casting by flooding is usually very costly and will result in
serious redundancy, contention, and collision, to which we
refer as the broadcast storm problem. In this paper, we iden-
tify this problem by showing how serious it is through anal-
yses and simulations. We propose several schemes to reduce
redundant rebroadcasts and differentiate timing of rebroad-
casts to alleviate this problem. Simulation results are pre-
sented, which show different levels of improvement over the
basic flooding approach.
Keywords: broadcast, communication, mobile ad hoc net-
work (MANET), mobile computing, wireless network.
1 Introduction
The advancement in wireless communication and economi-
cal, portable computing devices have made mobile comput-
ing possible. One research issue that has attracted a lot of
attention recently is the design of mobile ad hoc network
(MANET). A MANET is one consisting of a set of mo-
bile hosts that may communicate with one another and roam
around at their will. No base stations are supported in such
This work is supported by the National Science Council of the Republic of China
under Grants #NSC88-2213-E-008-013 and #NSC88-2213-E-008-014.
To appear in the fifth annual International Conference on
Mobile Computing and Networking (MobiCom’99), Seattle,
Washington, Aug. 15-20 1999.
an environment. Due to considerations such as radio power
limitation, channel utilization, and power-saving concerns, a
mobile host may not be able to communicate directly with
other hosts in a single-hop fashion. In this case, a multihop
scenario occurs, where the packets sent by the source host
are relayed by several intermediate hosts before reaching the
destination host.
Applications of MANETs occur in situations like battle-
fields or major disaster areas where networks need to be de-
ployed immediately but base stations or fixed network in-
frastructures are not available. Unicast routing in MANET
has been studied in several articles [6, 7, 14, 15, 23]. A work-
ing group called “manet” has been formed by the Internet
Engineering Task Force (IETF) to study the related issues
and stimulate research in MANET [21].
This paper studies the problem of sending a broadcast
message in a MANET. Broadcasting is a common operation
in many applications, e.g., graph-related problems and dis-
tributed computing problems. It is also widely used to re-
solve many network layer problems. In a MANET in partic-
ular, due to host mobility, broadcastings are expected to be
performed more frequently (e.g., for paging a particular host,
sending an alarm signal, and finding a route to a particular
host [6, 14, 15, 23]). Broadcasting may also be used in LAN
emulation [2] or serve as a last resort to provide multicast
services in networks with rapid changing topologies.
In this paper, we assume that mobile hosts in the MANET
shareasinglecommonchannelwithcarrier sense multiple
access (CSMA), but no collision detection (CD), capability.
Synchronization in such a network with mobility is unlikely,
and global network topology information is unavailable to
facilitate the scheduling of a broadcast. So one straight-
forward and obvious solution is broadcasting by flooding.
Unfortunately, in this paper we observe that serious redun-
dancy, contention, and collision could exist if flooding is
done blindly. First, because the radio propagation is omni-
directional and a physical location may be covered by the
transmission ranges of several hosts, many rebroadcasts are
considered to be redundant. Second, heavy contention could
exist because rebroadcasting hosts are probably close to each
1
other. Third, collisions are more likely to occur because the
RTS/CTS dialogue is inapplicable and the timing of rebroad-
casts is highly correlated.
Collectively, we refer to these problems associated with
flooding as the broadcast storm problem. Through analy-
ses and simulations, we demonstrate how serious the storm
is. Two directions to alleviate this problem is to reduce the
possibility of redundant rebroadcasts and differentiate the
timing of rebroadcasts. Following these directions, we de-
velop several schemes, called probabilistic, counter-based,
distance-based, location-based,andcluster-based schemes,
to facilitate MANET broadcasting. Simulation results are
presented to study the effectiveness of these schemes.
To the best of our knowledge, the broadcast storm prob-
lem has not been addressed in depth for MANET before. It is
however worth of summarizing some results for broadcast-
ing that are for other environments. Works in [3, 4, 9, 10, 11,
20] assume a packet-radio network environment. Most of
these results rely on time division multiple access (TDMA,
which requires timing synchronization) and certain levels of
topology information. Their goal is to find a slot assign-
ment. Obtaining an optimal assignment has been shown to
be NP-hard [9]. The broadcast scheduling problem studied
in [8, 16, 24, 25], although carries a similar name, is not in-
tended to solve the problem addressed in this paper. Its goal
is to assign a contention-free time slot to each radio station.
The rest of this paper is organized as follows. Section 2
defines and analyzes the broadcast storm problem. Mecha-
nisms to alleviate the storm are proposed in Sections 3. Sim-
ulation results are in Section 4 and conclusions are drawn in
Section 5.
2 Preliminaries
2.1 Broadcasting in a MAN ET
A MANET consists of a set of mobile hosts that may com-
municate with one another from time to time. No base sta-
tions are supported. Each host is equipped with a CSMA/CA
(carrier sense multiple access with collision avoidance)[19]
transceiver. In such environment, a host may communicate
with another directly or indirectly. In the latter case, a mul-
tihop scenario occurs, where the packets originated from the
source host are relayed by several intermediate hosts before
reaching the destination.
The broadcast problem refers to the sending of a message
to other hosts in the network. The problem considered here
has the following characteristics.
The broadcast is spontaneous. Any mobile host can
issue a broadcast operation at any time. For reasons
such as the host mobility and the lack of synchroniza-
tion, preparing any kind of global topology knowledge
is prohibitive (in fact this is at least as hard as the
broadcast problem). Little or no local information may
be collected in advance.
The broadcast is unreliable.
1
No acknowledgement
mechanism will be used.
2
However, attempt should
be made to distribute a broadcast message to as many
hosts as possible without paying too much effort. The
motivations to make such an assumption are (i) a host
may miss a broadcast message because it is off-line,
it is temporarily isolated from the network, or it ex-
periences repetitive collisions, (ii) acknowledgements
may cause serious medium contention (and thus an-
other “storm”) surrounding the sender, and (iii) in many
applications (e.g., the route discovery in [6, 14, 15,
23]), a 100% reliable broadcast is unnecessary.
In addition, we assume that a host can detect duplicate
broadcast messages. This is essential to prevent endless flood-
ingofamessage.Onewaytodosoistoassociatewitheach
broadcast message a tuple (source ID, sequence number) as
that in [6, 23].
Finally, we comment that we do not confine ourselves to
the broadcasting of the same message.
3
In this paper we fo-
cus on the flooding behavior in MANET — the phenomenon
where the transmission of a packet will trigger other sur-
rounding hosts to transmit the same (or modified) packet.
We shall show that if flooding is used blindly, many redun-
dant messages will be sent and serious contention/collision
will be incurred. Our goal is to solve broadcast with effi-
ciency in mind.
2.2 Broadcast Storm Caused byFlo o ding
A straight-forwardapproach to perform broadcast is by flood-
ing. A host, 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 hosts. In a
CSMA/CA network, drawbacks of flooding include:
Redundant rebroadcasts: When a mobile host de-
cides to rebroadcast a broadcast message to its neigh-
bors, all its neighbors already have the message.
Contention: After a mobile host broadcasts a mes-
sage, if many of its neighbors decide to rebroadcast
the message, these transmissions (which are all from
nearby hosts) may severely contend with each other.
Collision: Because of the deficiency of backoff mech-
anism, the lack of RTS/CTS dialogue, and the absence
of CD, collisions are more likely to occur and cause
more damage.
1
A more strict one is reliable broadcast [1, 22], whose goal is to ensure all hosts re-
ceive the message. High-level acknowledgements between hosts are exchanged. Such
protocols are typically accomplished at the application layer and is out of the scope
of this paper. However, the result in this paper may serve as an underlying facility to
implement reliable broadcast.
2
The MAC specification in IEEE 802.11 [19] does not request acknowledgement
on receipt of broadcast packets.
3
For instance, the routing protocols in [6, 14, 15, 23] rely on broadcasting a UDP
packet called route
request to search for a route from a source to a particular destina-
tion. When propagating such a request, a host generally appends its ID to the message
so that appropriate routing information can be collected.
2
Figure 1: Two optimal broadcasting schedules in MANETs.
Connectivity between hosts is represented by links. White
nodes are source hosts, and gray nodes are relay hosts.
Collectively, we refer to the above phenomena as the broad-
cast storm problem. The following discussion shows how
serious the storm is through analyses.
2.2.1 Analysis on Redundant Rebroadcasts
We first use two examples to demonstrate how much redun-
dancy could be generated. In Fig. 1(a), it only takes two
transmissions for the white node to broadcast a message,
whereas four transmissions will be carried out if no attempt
is made to reduce redundancy. Fig. 1(b) shows an even seri-
ous scenario: only two transmissions are sufficient to com-
plete a broadcast as opposed to seven transmissions caused
by flooding.
The following analysis shows that rebroadcasts are very
costly and should be used with caution. Consider the sim-
ple scenario in Fig. 2, where host A sends a broadcast mes-
sage, and host B decides to rebroadcast the message. Let
S
A
and S
B
denote the circle areas covered by A’s and B’s
transmissions, respectively. The additional area that can ben-
efit from B’s rebroadcast is the shaded region, denoted as
S
B
A
.Letr be the radii of S
A
and S
B
,andd the distance be-
tween A and B. We can derive that
j
S
B
A
j
=
j
S
B
jj
S
A
\
B
j
=
πr
2
INTC
(
d
)
;
where INTC
(
d
)
is the intersection area of
the two circles centered at two points distanced by d,
INTC
(
d
)=
4
Z
r
d
=
2
p
r
2
x
2
dx
:
When d
=
r, the coverage area
j
S
B
A
j
is the largest, which
equals πr
2
INTC
(
r
)=
r
2
(
π
3
+
p
3
2
)
0
:
61πr
2
. This shows
a surprising fact that a rebroadcast can provide only 0
61%
additional coverage over that already covered by the previ-
ous transmission.
Also,wewouldliketoknowtheaveragevalueofπr
2
INTC
(
d
)
. Supposing that B can randomly locate in any of
A’s transmission range, the average value can be obtained by
integrating the above value over the circle of radius x cen-
Figure 2: Analysis on the extra area that can benefit from
a rebroadcast: A sends a broadcast packet and B decides to
rebroadcasts the packet.
Figure 3: Analysis on redundancy: the expected additional
coverage EAC
(
k
)
(divided by πr
2
) after a host heard a broad-
cast message k times.
tered at A for x in
[
0
;
r
]
:
Z
r
0
2πx
πr
2
INTC
(
x
)
πr
2
dx
0
:
41πr
2
:
Thus, after the previous broadcast, a rebroadcast can cover
only additional 41% area in average.
Now consider the scenario of having received a broadcast
message twice: if host C decides to rebroadcast after it heard
A’s and B’s broadcasts. The area that can benefit from C’s
rebroadcast is S
C
(
A
[
B
)
. Through simulations (by randomly
generating A and B on C’s transmission range with grid es-
timation), we found that in average
j
S
C
(
A
[
B
)
j
0
:
19πr
2
.
This shows an even dimmer prospect of hoping rebroadcasts
to propagate the message to new hosts.
In general, we would like to know the benefit of a host re-
broadcasting a message after heard the message k times. The
result can be easily obtained from simulation by randomly
generating k hosts in a host X’s transmission range and cal-
culating the area covered by X excluding those already cov-
ered by the other k hosts. Denote this value by EAC
(
k
)
(EAC
stands for expected additional coverage). Fig. 3 shows our
simulation result. As can be seen, when k
4, the expected
additional coverage is below 0
:
05%.
3
Figure 4: Analysis on contention: the probabilities of having
k contention-free hosts among n receiving hosts.
2.2.2 Analysis on Contention
To address the contention problem, consider the situation
where host A transmits a broadcast message and there are n
hosts hearing this message. If all these hosts try to rebroad-
cast the message, contention may occur because two or more
hosts around A are likely to be close and thus contend with
each other on the wireless medium.
Let’s analyze the simpler case of n
=
2. Let hosts B and
C be the two receiving hosts. Let B randomly locate at A’s
transmission range. In order for C to contend with B,itmust
locate in the area S
A
\
B
. So the probability of contention is
j
S
A
\
B
j
=
πr
2
.Letx be the distance between A and B.Inte-
grating the above formula over the circle of radius x from 0
to r, the expected probability of contention is
Z
r
0
2πx
INTC
(
x
)
=
(
πr
2
)
πr
2
dx
59%
:
Clearly, the contention is expected to be higher as n in-
creases. We derived a simulation by randomly generating n
hosts in A’s transmission range. We observe the probabil-
ity cf
(
n
;
k
)
that k hosts among these n hosts experience no
contention in their rebroadcasting (cf stands for contention-
free). The results are shown in Fig. 4. We can see that
the probability of all n hosts experiencing contention (i.e.,
cf
(
n
;
0
)
) increases quickly over 0
:
8asn
6. So the more
crowded the area is, the more serious the contention is. On
the other hand, the probability of having one contention-free
host (i.e., cf
(
n
;
1
)
) drops sharply as n increases. Further,
it is very unlikely to have more contention-free hosts (i.e.,
cf
(
n
;
k
)
with k
2). Note that having k
=
n
1 contention-
free hosts implies having n such hosts, so cf
(
n
;
n
1
)=
0.
2.2.3 Analysis on Collision
In a MANET, there is no base station or access point. There-
fore, in this paper we exclude the use of the point coordinate
function (PCF) described in the IEEE 802.11 MAC speci-
fication [19], and study mainly the behavior under the dis-
tributed coordinate function (DCF).
The CSMA/CA mechanism requires a host to start a back-
off procedure right after the host transmitted a message, or
when a host wants to transmit but the medium is busy and
the previous backoff has been done. To perform a backoff, a
counter is first set to an integer randomly picked from its cur-
rent backoff window. If the channel clear assessment (CCA)
mechanism of the host detects no channel activity during the
past slot (a fixed period), the counter is decreased by one.
When the counter reaches zero, the backoff procedure is fin-
ished.
Now consider the scenario where several neighbor hosts
hear a broadcast from host X. There are several reasons for
collisions to occur. First, if the surrounding medium of X
has been quiet for enough long, all X’s neighbors may have
passed their backoff procedures. Thus, after hearing the
broadcast message (and having passed the DIFSperiod), they
may all start rebroadcasting at around the same time. This
is especially true if carriers can not be sensed immediately
due to such as RF delays and transmission latency. Second,
because the RTS/CTS forewarning dialogue is not used in
a broadcast transmission, the damage of collision is more
serious. Third, once collision occurs, without collision de-
tection (CD), a host will keep transmitting the packet even
if some of foregoing bits have been garbled. And the longer
the packet is, the more the waste.
The above problem is not addressed in the ordinary IEEE
802.11 MAC activities, possibly because the one-to-many
behavior is not considered therein. For all the above reasons,
we believe that the broadcast storm problem deserves serious
studies in a MANET environment.
3 Mechanisms to Reduce Redundancy, Cont ention, and
Collision
One approach to alleviate the broadcast storm problem is to
inhibit some hosts from rebroadcasting to reduce the redun-
dancy, and thus contention and collision. In the following,
we present five schemes to do so. These schemes differ in
how a mobile host estimates redundancy and how it accu-
mulates knowledge to assist its decision. Except the last
scheme, which relies on some local connectivity informa-
tion, all schemes operate in a fully distributed manner.
3.1 Probabilistic Scheme
An intuitive way to reduce rebroadcasts is to use probabilis-
tic rebroadcasting. On receiving a broadcast message for
the first time, a host will rebroadcast it with probability P.
Clearly, when P
=
1, this scheme is equivalent to flooding.
4
Note that to respond to the the contention and collision
problems addressedin Section2.2.3, we should inserta small
random delay (a number of slots) before rebroadcasting the
message. So the timing of rebroadcasting can be differenti-
ated.
3.2 Counter-Based Scheme
When a host tries to rebroadcast a message, the rebroadcast
message may be blocked by busy medium, backoff proce-
dure, and other queued messages. There is a chance for the
host to hear the same message again and again from other
rebroadcasting hosts before the host actually starts transmit-
ting the message.
In Section 2.2.1 we have shown that EAC
(
k
)
,theex-
pected additional coverage after heard the message k times,
is expected to be lower when k increases. We can prevent a
host from rebroadcasting when the expected additional cov-
erage of the host’s rebroadcast becomes too low. This is what
the counter-based scheme is based on. Specifically, a counter
c is used to keep track of the number of times the broad-
cast message is received. A counter threshold C is chosen.
Whenever c
C, the rebroadcast is inhibited. The scheme is
formally derived below.
S1. Initialize counter c
=
1 when a broadcast message msg
is heard for the first time. In S2, if msg is heard again,
interrupt the waiting and perform S4.
S2. Wait for a random number of slots. Then submit msg
for transmission and wait until the transmission actu-
ally starts.
S3. The message is on the air. The procedure exits.
S4. Increase c by one. If c
<
C, resume the interrupted
waiting in S2. Otherwise c
=
C, proceed to S5.
S5. Cancel the transmission of msg if it was submitted in
S2. The host is prohibited from rebroadcasting msg.
Then exits.
3.3 Distance-Based Scheme
In the previous scheme, a counter is used to decide whether
to drop a rebroadcast or not. In this scheme, we will use the
relative distance between hosts to make the decision.
For instance, suppose host H heard a broadcast message
from S for the first time. If the distance, say d,betweenH
and S is very small, there is little additional coverage H’s
rebroadcast can provide. If d is larger, the additional cov-
erage will be larger. In the extreme case, if d
=
0, the ad-
ditional coverage is 0 too. Earlier, we have analyzed the
relationship between the distance d and the additional cov-
erage πr
2
INTC
(
d
)
. So this can be used as a metric by H
to determine whether to rebroadcast or not.
Now, suppose that before a rebroadcast message is actu-
ally sent, host H has heard the same message several times.
Let d
min
be the distance to the nearest host from which the
same message is heard. Then H’s rebroadcast will provide
additional coverage no more than πr
2
INTC
(
d
min
)
.Inour
distance-based scheme, we will use d
min
as the metric to
evaluate whether to rebroadcast or not. If d
min
is smaller than
some distance threshold D, the rebroadcast transmission of
H is cancelled. The scheme is formally derived below. In
Section 4, we will test several possible values of D.
S1. When a broadcast message msg is heard for the first
time, initialize d
min
to the distance to the broadcasting
host. If d
min
<
D, proceed to S5. In S2, if msg is heard
again, interrupt the waiting and perform S4.
S2. Wait for a random number of slots. Then submit msg
for transmission and wait until the transmission actu-
ally starts.
S3. The message is on the air. The procedure exits.
S4. Update d
min
if the distance to the host from which msg
is heard is smaller. If d
min
<
D, proceed to S5. Other-
wise, resume the interrupted waiting in S2.
S5. Cancel the transmission of msg if it was submitted in
S2. The host is inhibited from rebroadcasting msg.
Then exits.
Below, we comment on how to obtain the distance in-
formation. One possibility is to estimate from the signal
strength on which a message is received. Specifically, let
P
t
and P
r
be the power levels on which a message is sent and
received, respectively. According to [26], P
r
=
P
t
(
c
1
d
)
n
c
2
,
where n
;
c
1
;
and c
2
are constants related to physical environ-
ment, the carrier’s wavelength, and antenna gains, respec-
tively. Since P
r
and P
t
can be measured, the distance d can
be estimated from this formula.
Having understood the relationship between the distance
and the power, we can even directly replace the role of dis-
tances by signal strengths by establishing a signal-strength
threshold. As a comment, we note that signal strength in-
formation was also used in [12] to facilitate routing in a
MANET.
3.4 Location- Based Scheme
Earlier we have used the number of times that a broadcast
message has been heard or the distances to sending hosts as
our rebroadcasting metrics. If we can acquire the locations
of those broadcasting hosts, it is even possible to estimate the
additional coverage more precisely. Such an approach may
be supported by positioning devices such as GPS (Global
Positioning System) receivers [17]. We note that location
information was also used to facilitate route discovery in a
MANET [5, 18].
Without loss of generality, let a host’s location be
(
0
;
0
)
(hereweusexy-coordinate to facilitate our presentation; in
fact, devices such as GPS receivers can provide 3-D loca-
tions in longitude, latitude, and altitude). Suppose a host has
received the same broadcast message from k hosts located at
5