Power saving strategies generally attempt to maximize the time that nodes spend in a low power consumption sleep state. Such strategies often require the sender to notify the receiver about pending traffic using some form of traffic announcement. Although asynchronous traffic announcement mechanisms are particularly suitable for the ad hoc environment, they also provide relatively limited power savings. This paper proposes a mechanism that improves the efficiency of asynchronous traffic announcement mechanisms by reducing the proportion of time that nodes need to spend awake, while still maintaining good connectivity properties. The mechanism is based on allowing traffic announcements to be rebroadcast by neighbouring nodes

Braun, Torsten

Feeney, Laura Marie

English

Digitala Vetenskapliga Arkivet - Academic Archive On-line

Power saving in wireless ad hoc networks without synchronization

Swedish Institute of Computer Science Publications Database

Power Saving in Wireless Ad hoc Networks Without 
Synchronization
Torsten Braun 
Institute of Computer Science and Applied 
Mathematics, University of Bern 
Neubrückstrasse 10, CH 3012 Bern, Switzerland 
braun@iam.unibe.ch 
 
Laura Marie Feeney 
Computer and Network Architecture Laboratory 
Swedish Institute of Computer Science 
Box 1263, SE 164 29 Kista, Sweden 
lmfeeney@sics.se 
 
Abstract 
Power saving strategies generally attempt to maximize the 
time that nodes spend in a low power consumption sleep 
state.  Such strategies often require the sender to notify the 
receiver about pending traffic using some form of traffic 
announcement. Although asynchronous traffic 
announcement mechanisms are particularly suitable for the 
ad hoc environment, they also provide relatively limited 
power savings. This paper proposes a mechanism that 
improves the efficiency of asynchronous traffic 
announcement mechanisms by reducing the proportion of 
time that nodes need to spend awake, while still maintaining 
good connectivity properties.  The mechanism is based on 
allowing traffic announcements to be rebroadcast by 
neighbouring nodes. 
1 Introduction 
Power saving is an important and challenging issue in 
wireless networks, in particular for battery operated nodes 
such as mobile devices and sensor nodes. A popular 
approach to save energy is to periodically switch off a node 
or a few of its components for a certain time interval. In 
wireless ad hoc networks, switching off network nodes 
might not only have impact on the reachability of a single 
node but also on the connectivity of the whole network. 
Several approaches therefore propose to introduce 
synchronization mechanisms among the ad hoc network 
nodes. Nodes may wake up in a synchronized way, exchange 
data, and fall into sleep again after data exchange. 
Synchronization however is not easy to achieve and 
introduces also some overhead. We therefore propose a 
mechanism that avoids synchronization and try to take 
advantage of intermediate nodes that can relay traffic 
indication map messages between a sender and a receiver 
node.  
After discussing related work in Section 2, we motivate and 
present our mechanism in Section 3 for a single-hop mobile 
ad hoc network scenario. Section 4 extends the concept for a 
multi-hop scenario. Section 5 concludes the paper. 
2 Related Work 
IEEE 802.11 introduces ad hoc traffic indication map 
(ATIM) messages that can be used in an IBSS (Independent 
Basic Service Set) ad hoc network to indicate that a node has 
data for a certain destination node. The ATIM messages are 
broadcast at the beginning of a beacon interval during the so-
called ATIM window. All nodes must wake up at the 
beginning of the beacon interval and remain awake during 
the ATIM window.  A node goes back to sleep at the end of 
the ATIM window if it does not hear an ATIM for itself in 
the ATIM window. The mechanism therefore requires 
synchronization. Scalability issues of distributed beacon 
generation in the IEEE 802.11 timing synchronization 
function (TSF) are studied in [1] .  
In [2] quorum based systems have been proposed to reduce 
the wake period of a node. The authors propose that each 
node divides its beacon intervals into groups of n 
consecutive intervals. MTIM (Multi-hop TIM) messages are 
sent by the nodes at the beginning of an interval. Each group 
of n intervals is organized in n  * n  arrays with n  
columns and n  rows. Each node selects a column and a 
row and will be wake during the selected 2 n  – 1 intervals. 
This mechanism ensures that even if two hosts are not 
synchronized (i.e. they select the starting point of an interval 
in an asynchronous way), a node will receive a MTIM 
message not later than after n intervals. However, the 
number of n may be quite large and may result in a very high 
delay. Moreover, the wake ratio of (2 n  – 1)/n is rather 
high in this scheme. For n = 25 a node needs to be active for 
9/25 of the time, i.e. 36 %. Even for n = 100, the wake ratio 
is 19 %. This also means that we need many, but very small 
intervals to reach very low wake ratios. This increases the 
overhead of the approach, since the MTIM messages must 
be sent many times then.  
Another approach [3] proposes that nodes are active (0.5 + ε) 
I, with I as an interval time. This ensures that wake periods 
of any two nodes overlap by at least ε, providing a 
predictable traffic announcement.  
3 Power Saving for Single Hop Mobile 
Ad hoc Networks 
3.1 ATIM Forwarding with Synchronized 
Wake Periods 
Our power saving mechanism is inspired by quorum 
systems, but we try to reduce the wake ratio further by 
making use of the nodes’ forwarding capabilities. In contrast 
to [2], where connectivity is achieved by ensuring that two 
communicating nodes will both be wake during some time 
interval, we try to achieve connectivity by relaying nodes, 
that share a common wake period with both the sender and 
the destination node.  
Before presenting our mechanism in subsection 3.2 we 
describe a mechanism for the case that the wake periods of 
all nodes are synchronized. In this case, all wake periods 
have the same length and begin at discrete points of time. 
 describes when the different nodes are active. In Figure 1
total we have 2K = 10 periods grouped into two groups A 
and B of K = 5 periods each. The periods of group B 
alternate with the periods of group A. Each of the nodes 
selects one period out of group A and one out of group B. 
For example, node X is wake during the first period of group 
A (A1) and in the third period of group B (B3).  
If a node X wants to communicate with node Z, it broadcasts 
an ATIM message. This ATIM message is received by 
various other nodes that forward the ATIM message once 
during their next wake period. The ATIM message may then 
reach the desired node Y via such an intermediate node. In 
our example depicted in  this ATIM message may 
be relayed via a node Y to node Z. By carrying information 
about the wake periods of X in the ATIM message Z can 
contact X and coordinate the data transfer.  
Figure 1
Figure 1
Figure 1: ATIM Forwarding 
In our example, each node is only active during one out of K 
periods. In  with K=5 this results in a wake ratio of 
20 %. By ensuring that there is one node in each of the K2 
array fields we could guarantee full connectivity between 
arbitrary nodes.  
A1 A2 A3 A4 A5
B1
B2
B3
B4
B5
X
Z
Y
 
3.2 ATIM Forwarding Without 
Synchronization 
Full synchronization as required for the mechanism proposed 
in subsection 3.1 is hard to achieve and we aim to avoid it. 
The power saving mechanism introduced in the following, 
therefore, avoids any synchronization among nodes.  
As depicted in  nodes select a certain basic interval 
length and wake up twice during that interval for a fixed 
period (wake periods). During the other part of the interval 
the nodes are sleeping (sleep period). We distinguish 
between primary and secondary wake periods. A node 
strictly alternates between a primary (A) and secondary (B) 
wake period. For a primary wake period, a node wakes up 
always at the same point of time relative to the start of the 
interval. In a secondary wake period a node wakes up 
randomly, but avoiding overlaps of primary and secondary 
wake periods.  
Figure 2
Figure 2
Figure 2: Operation of Power Saving Mechanism 
We do not assume that two communicating nodes have 
intervals that are synchronized with each other or with a 
relaying node, but the intervals and wake periods are 
expected to be similar at all nodes. Due to low drifts in 
clocks this should be easily achievable in practice. Small 
differences as they might happen due to clock drifts do not 
matter.  
 illustrates the operation of our mechanism. Node X 
sends two ATIM messages, one in the primary wake period 
and another in its secondary wake period. We send two 
ATIMs in order to increase the probability to reach a node. 
Each node receiving an ATIM message forwards it during its 
next wake period. Forwarding of ATIM messages is limited 
to one intermediate node, because we have to limit 
uncontrolled flooding of those messages. This can easily be 
done using a time-to-live bit in the ATIM message set to 1 
by the origin. 
In our example, the first of the two ATIM messages sent by 
X is received by node Y and forwarded to node Z. However, 
it might happen that an ATIM message does not reach its 
target destination node, if two nodes X and Z do not have 
overlapping wake periods and if there is also no intermediate 
node Y that overlaps with wake periods of both X and Z. 
This problem can be solved by ATIM retransmissions during 
other wake periods and by changing the wake periods of the 
intermediate nodes.  
This also motivates our choice for a fixed primary wake 
period pattern and the random secondary wake period 
pattern. The fixed primary wake period pattern simplifies to 
contact a node if its periodically occurring primary wake 
period pattern is known. The random selection of the 
secondary wake period ensures that ATIM messages are 
retransmitted with different delays after the start of the 
interval. It makes also sure that not always the same set of 
intermediate nodes forward retransmitted ATIM messages.  
A B
ATIM
X
Y
Z
 
3.3 Analytical Evaluation 
First, we assume a wireless ad hoc scenario in which each 
node can be reached by any other node. If p is the wake ratio 
(= length of wake period / interval length) of a node, q = 1 – 
p, and N = total number of nodes that can reach other, we 
can calculate the failure probability that the sender does not 
reach the destination by two ATIM messages neither directly 
nor via intermediate nodes. The number of nodes M that 
receive the first TIM message of the sender node is given by  
M = p (N-1) 
The sender node transmits an ATIM message twice in its 
interval, once in the primary and once in the secondary wake 
period. An intermediate node can forward each of the two 
messages only once. P’ is then the probability that a single 
ATIM message does not reach the destination either directly 
from the source or indirectly via one of the M intermediate 
nodes. It can be calculated by 
P’ = q qM.  
Both ATIM message transmissions are independent. So, P’’ 
is the probability that both ATIM messages sent by a sender 
do not reach the destination.  
P’’ = (P’)2 = q2(M+1) = q2((p(N-1)+1) = (1-p)2(p(N-1)+1)  (1) 
This failure probability decreases for large wake periods, but 
also decreases for a large number of nodes in a given area. 
While we need a wake ratio of 50 % for 10 nodes to achieve 
a failure probability below 5 %, this can be achieved with a 
wake ratio of 20 % for 50 nodes and 10 % for 100 nodes. 
 shows the results of equation (1). Each line 
represents the failure probability for a single wake ratio 
value.  
Figure 3
Figure 3: Analysis of failure probability 
0.0
0.2
0.4
0.6
0.8
1.0
10 50 100 500 1000
# of nodes
fa
ilu
re
 p
ro
ba
bi
lit
y 0.5
0.2
0.1
0.05
0.02
0.01
wake ratio
 
3.4 Simulation 
In order to prove our analysis and to investigate the 
behaviour in a network under load we performed simulations 
using Omnet++. The diagram in  shows the 
simulated failure probability dependent on number of nodes 
and wake ratio. As simulation parameters, we have chosen a 
50 % loaded 10 Mbps network, data packet size of 1000 bits 
and ATIM message sizes of 100 bits. Potential collisions 
among ATIM messages and data have been simulated. The 
basic interval length was set to 2 s. The simulation results 
nicely confirm the result given by the equation (1) above. 
Figure 4
Figure 4: Simulation of failure probability 
0.0
0.2
0.4
0.6
0.8
1.0
10 50 100 500 1000
# of nodes
fa
ilu
re
 p
ro
ba
bi
lit
y 0.5
0.2
0.1
0.05
0.02
0.01
wake ratio
 
As already mentioned our mechanism does not guarantee 
that an ATIM message does arrive at a destination. In case of 
a failure, which can be detected by missing 
acknowledgements, the sender of an ATIM message shall 
retransmit ATIM transmissions until receiving an 
acknowledgement. In order to evaluate the efficiency of this 
mechanism we performed simulations with the same 
parameters as above. We repeated ATIM transmissions until 
the sender has received an acknowledgement from the 
destination.  
In our evaluation we used wake periods with durations that 
depend on the number of nodes. Intuitively, wake periods 
should be shorter for high node densitites and longer for 
lower node densities.   
As mentioned in subsection 3.1 a full reachability can be 
achieved, if there is at least one node in each of the K2 array 
fields representing the fully synchronized scenario depicted 
in Figure , i.e.  1
N ≥ K2. 
With p = 1/K this results in  
N
p
p
N 112 ≥⇔≥ . 
As a first approach, we select  
1, ≥= αα
N
p   
Figure 5
Figure 5: Retransmissions of ATIM pairs per packet  
 shows the results of an experiment, in which we 
repeat ATIM pairs in scenarios with different numbers of 
nodes and wake periods. We set α = 2 for all simulations. 
For all scenarios we have between one and two additional 
ATIM pair transmissions in average. Note that we achieve 
full reachability in that case.  
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
0 200 400 600 800 1000
# of nodes# 
of
 A
TI
M
 re
tra
ns
m
is
si
on
s 
pe
r p
ac
ke
t
 
3.5 Estimating the Number of Nodes 
Based on our results, we propose adapting the wake period 
dependent on the number of nodes: For larger number of 
nodes, shorter wake periods should be chosen, while a node 
should be wake for a longer time if there are fewer nodes in 
its environment. The number of redundantly received ATIM 
messages can serve as an indication whether a node should 
reduce or increase its wake period. 
Shortening the wake periods has another advantageous side 
effect, because for high node densities the number of 
duplicated ATIM messages becomes lower. If the wake 
periods were not shortened, the number of duplicated ATIM 
messages would increase for large node densities and 
become a problem if they consume a significant fraction of 
the network capacity. Therefore, it would be beneficial if the 
length of the wake periods could be controlled dependent on 
the number of nodes.  
We propose to use the number of received ATIM duplicates 
as a parameter to estimate the node number of nodes and to 
control the wake period. Assume that a sender generates  
ATIM messages during the primary and the secondary wake 
period. We further assume that all nodes have wake ratios of 
p. The two ATIM messages are then duplicated by p N 
nodes. This results in 2 p N ATIM copies in addition to the 2 
ATIM messages generated by the sender. In total 2 + 2 p N 
ATIM copies are generated. If the receiver has a wake ratio 
of p, it will receive p (2 + 2 p N) ATIM messages in average. 
Let us now assume that a node has received M copies of a 
single ATIM message. Then we have  
M = p (2 + 2 p N) = 2 p + 2 p2 N 
N = (M – 2 p) / (2 p2) (2) 
This result means that if all nodes have the same wake ratio 
and this ratio is known to all nodes, each node can estimate 
the number of nodes in the single hop ad hoc network based 
on the received copies of ATIM messages.  
In our simulation scenario described in Section 3.4 we have 
tried to estimate the given numbers of nodes based on the 
received ATIM copies according to equation (2).  
shows the results. We have averaged all estimated values of 
all involved nodes in the simulation. The results suggest a 
nice match between estimation and real number of nodes.  
Figure 6
Figure 6: Estimation of node numbers 
10 50
100
500
1000
9 47
96
499
991
0
200
400
600
800
1000
1200
number of  nodes est imated number of nodes
 
It is important to note that the estimation requires equal 
wake ratios at all nodes. This can only be achieved by some 
control mechanism. For example, a dedicated node could 
estimate the node number, calculate the resulting wake ratio 
and broadcast that value to its environment.  
Another possibility is that nodes choose individual wake 
ratios dependent on the number of received ATIM 
duplicates. A node might increase its wake ratio if the 
number of received ATIM duplicates falls below a certain 
threshold, and decrease the wake ratio if the number of 
received ATIM duplicates exceeds another threshold. Such 
an algorithm has the risk of oscillations and need to be 
carefully designed and evaluated in future work.  
4 Power Saving for Multi Hop Mobile Ad 
hoc Networks 
The concept of forwarding ATIM messages can also be 
applied in a scenario, where not all nodes are directly 
connected to each other. We propose to adapt our 
mechanism to reactive mobile ad hoc network routing 
protocols. Route request messages are triggered by the 
source node and forwarded via intermediate nodes that 
receive the route request during their wake period. Each 
node forwards a received route request only once during the 
next wake period.  
We simulated a scenario of 1000 x 1000 m with 200 nodes 
and 100 m transmission range. Route requests have been 
randomly generated between random pairs of nodes. We 
increased the wake ratio from 0.1 to 1. Beyond a wake ratio 
of 0.2 all route requests reached the target destinations, while 
1 % of the route requests did not reach the destination for a 
wake ratio of 0.1. Smaller wake ratio values result in a larger 
number of hops, because due to the sleep periods of some 
nodes the optimal path may not be found. F  shows 
that the average number of discovered routes decreases from 
6 to below 4 for increasing the wake ratio from 0.1 to 1.  
igure 7
0
1
2
3
4
5
6
7
0.1 0.2 0.4 0.6 0.8 1
wake ratio
av
er
ag
e 
# 
of
 h
op
s
 
Figure 7: Length of established routes 
5 Conclusions and Outlook 
In this paper, we have proposed a mechanism for ATIM 
message exchange supported by intermediate nodes that 
share wake periods with both the sender and the destination 
of an ATIM message. Our analysis and simulations ina 
single hop scenario showed that the proposed mechanism 
works if there is a sufficiently large node number and the 
wake ratios are appropriately chosen.  
The mechanism can also be adapted for multi-hop scenarios. 
Initial simulation results show that routes across a multi-hop 
network can be established for very low wake ratios, but the 
chosen routes may then not be optimal in terms of hop 
counts. Future work will focus on the improvement and 
detailed evaluation of the multi-hop scenario.  
6 References 
[1] Lifei Huang and Ten-Hwang Lai: On the scalability of 
IEEE 802.11 ad hoc networks, in Proceedings of the 
ACM International Symposium on Mobile Ad Hoc 
Networks, 2002 
[2] Y.C. Tseng, C.S. Hsu and T.Y. Hsieh: Power-saving 
protocols for IEEE 802.11-based multi-hop ad hoc 
networks, in Proceedings of IEEE INFOCOM, New 
York, June 2002 
[3] Laura Marie Feeney: A QoS aware power save protocol 
for wireless ad hoc networks, in Proceedings of the First 
Mediterranean Workshop on Ad Hoc Networks (Med-
Hoc Net ), Sardegna, Italy, September 2002. 


Power saving in wireless ad hoc networks without synchronization

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