Wireless Sensor Network, 2010, 2, 755-767
doi:10.4236/wsn.2010.210091 Published Online October 2010 (http://www.SciRP.org/journal/wsn/).
Copyright © 2010 SciRes. WSN
Priority-Based Hybrid MAC for Energy Efficiency in
Wireless Sensor Networks
Ines Slama, Badii Jouaber, Djamal Zeghlache
Telecom and Management SudParis, Evry, France
E-mail: {Ines.slama, Badii.Jouaber, Djamal.Zeghlache}@it-sudparis.eu
Received June 30, 2010; revised August 9, 2010; accepted September 15, 2010
This paper introduces I-MAC, a new medium access control protocol for wireless sensor networks. I-MAC
targets at improving both channel utilization and energy efficiency while taking into account traffic load for
each sensor node according to its role in the network. I-MAC reaches its objectives through prioritized and
adaptive access to the channel. I-MAC performances obtained through simulations for different network to-
pologies, scenarios and traffic loads show significant improvements in energy efficiency, channel utilization,
loss ratio and delay compared to existing protocols.
Keywords: MAC Protocol, Prioritization, Energy Efficiency, Throughput, Channel Utilization, Wireless
Sensor Networks
1. Introduction
A critical design issue for wireless sensor networks (WSNs)
is the development of novel communication protocols
and algorithms that efficiently tackle the resource con-
straints and application requirements [1,2]. Therefore,
power aware and power efficient protocols at each layer
of the communication are required [2].
In WSNs, the MAC layer is considered as the main
responsible of energy consumption. Indeed, it has been
identified that the main reasons of energy waste in WSNs
are: collisions, overhearing, protocol overhead and fi-
nally transmission and reception operations [3].
At the medium access layer, both CSMA and TDMA
techniques have been widely used.
CSMA is a contention based access protocol consid-
ered as simple, flexible and robust. It is a distributed
mechanism that does not require clock synchronization
or any knowledge of the network topology. Moreover,
nodes can dynamically join or leave the network without
additional overhead or operations. However, all this
comes at the cost of collisions that occur when two or
more nodes (within the same radio range) transmit at the
same time. Collisions can cause serious energy waste,
high overhead and throughput degradation. CSMA per-
formance decreases under high contention in terms of
both energy consumption and channel utilization.
On the contrary, in TDMA, collisions are reduced by
scheduling the transmissions of neighboring nodes and
minimizing simultaneous transmissions [4]. Thus, TDMA
can deliver very good performance especially under high
contention. Moreover, it can provide higher energy saving
than CSMA since it allows avoiding wasted energy due
to collisions, idle listening and overhearing. However,
the challenge with TDMA is to setup an efficient and
scalable schedule that ensures collision free transmission
times for all nodes in the network. Additionally, since
sensor networks may undergo frequent topology changes
caused by battery outage and node failures, the TDMA
schedule should be able to gracefully handle these varia-
tions and dynamically adapt (frame length and/or time
slot assignment) with respect to energy conservation
aims. Moreover, TDMA gives much lower channel utili-
zation than CSMA under low contention as nodes can
only transmit during their scheduled slot. Finally, TDMA
requires clock synchronization, which incurs high-energy
These difficulties with TDMA and CSMA suggest that
a stand-alone TDMA or CSMA scheme cannot be effi-
On the other hand, in most of WSNs topologies, some
nodes are heavily responsible of forwarding operations
because of their central position or because they are
closer to the base station (sink) and therefore, most of the
routes go through them. Obviously, these critical nodes
require more bandwidth resources to achieve their for-
Copyright © 2010 SciRes. WSN
warding tasks. Besides, since these nodes have to trans-
mit much more packets than other nodes (relaying role),
they usually spend more energy and die earlier, which
can affect the entire network lifetime.
Controlling access to the channel in wireless sensor
networks plays then a key role in determining channel
utilization, energy consumption and fairness in channel
usage [4].
Optimizing channel and energy utilization have been
the main focus of significant researches over the last
years [4-11]. Different approaches dealing with this topic
have been proposed. In [4,6,7], it has been shown that
hybrid MAC schemes outperform contention based and
schedule based schemes. Indeed, hybrid approaches com-
bine the flexibility of contention-based schemes and the
efficiency of schedule based schemes. Unfortunately, most
of these hybrid mechanisms equally consider nodes and
do not consider the heterogeneity in their loads and roles
in the networks, which greatly motivated our work.
2. Prior Work
Prior research efforts in designing MAC protocols for
wireless sensor networks have resulted in a number of
interesting solutions. Some of the proposed schemes are
mainly designed to improve the channel utilization.
Some others focus on energy efficiency, which remains
the most important challenge in WSNs. The most popu-
lar and relevant to our work are the followings:
S-MAC (Sensor-MAC) protocol [10] uses the Carrier
Sense Multiple Access with Collision Detection (CSMA/
CD) algorithm. It is composed of two phases: a listen
period and a sleep period. During the sleep period the
nodes can switch off their transceiver. After the sleep
period, the nodes wake up and listen whether there is a
communication addressed to them or if they can address
themselves a communication to their neighbours. This
periodic sleep/listen schedule is set up within a virtual
cluster formed by neighbouring nodes. The listen and
sleep periods should be then locally synchronized be-
tween nodes. The listen period is divided into two sec-
tions. The first section is reserved for SYNC messages
used for synchronization and containing an identification
number of the sender and the next time slot where nodes
should go to sleep. The second section is reserved for
request to send RTS messages.
The design goal of S-MAC is essentially energy effi-
ciency. The energy waste caused by idle listening is re-
duced by sleep schedules. Moreover, time synchroniza-
tion overhead may be prevented by sleep schedule an-
nouncements. However, since S-MAC employs RTS/CTS,
the overhead is quiet high because most data packets in
sensor networks are usually small. Moreover, when send-
ing broadcast messages, RTS/CTS is not used which will
increase the probability of collisions. Finally, the lis-
ten/sleep period size is predefined and fix which limits
the efficiency of the protocol under variable traffic.
T-MAC (Time-out MAC) [11] is very similar to S-
MAC. It is proposed to improve the energy efficiency
and throughput performances of S-MAC by introducing
an “adaptive” duty-cycle that dynamically adjusts the
length of the active periods according to the load varia-
tions. A node can save energy by switching off its radio
when not concerned with any communication and then
avoid overhearing and idle listening energy waste.
B-MAC (Berkley-MAC) [12] is a carrier sense media
access protocol for wireless sensor networks that pro-
vides a flexible interface to ensure ultra low power op-
eration, high channel utilization and collision avoidance.
To improve the channel utilization, B-MAC uses the
clear channel sensing technique (CCA). Moreover, B-
MAC supports Low Power Listening (LPL), which aims
to surmount the idle listening problem by requiring po-
tential receivers to wake up periodically and briefly to
listen for activity on the channel. The implementation of
LPL is integrated with the hardware, and is not currently
available for most platforms. B-MAC is shown to have
better performances than S-MAC and T-MAC in terms
of energy efficiency and throughput.
Z-MAC (Zebra-MAC) [4] is a hybrid MAC protocol
designed for wireless sensor networks and combines the
strengths of CSMA and TDMA while offsetting their
weaknesses. It operates in two modes: the set-up mode
and the transmission mode. During the setup mode, each
node is assigned a time slot. For a given slot, we denote
by “owner”, the node to which that slot is allocated. All
other nodes are denoted by “non-owners”. Z-MAC uses
DRAND [13] to assign slots to nodes. DRAND is run
during setup phase and guarantees that no two nodes
within a two-hop neighbouring be assigned the same
time slot. In the transmission mode, time is divided into
slots. Z-MAC uses CSMA as the baseline MAC scheme
and uses a TDMA schedule to enhance contention reso-
lution. In Z-MAC, a node may transmit during any time
slot. Before a node transmits, it senses the channel and
only transmits a packet if it is sensed clear. However, the
owner of the slot always has higher priority than other
nodes in accessing the channel. Hence, by combining
CSMA and TDMA approaches, Z-MAC delivers a ro-
bust scheme, which, in worst case, performs as well as
TH-MAC [9] is a traffic pattern aware hybrid MAC
protocol inspired from Z-MAC. It uses A-DRAND as
slot assignment algorithm. A-DRAND is an improved
version of DRAND for clustered wireless sensor net-
works where cluster heads require more slots to relay
packets. To allot more slots to cluster heads and coordi-
nators, the time frame is divided into two parts: a re-
served part and a non-reserved part. If a node is a normal
sensor, it can only select a slot from the non-reserved
part and not yet selected by none of its one-hop or
two-hop neighbours. However, if the node is a cluster
head or a coordinator, it can get a slot, not only from the
non-reserved part but also from the reserved part. TH-
MAC can assign then more slots to cluster heads and
backbone nodes to improve channel utilization and th-
BAZ-MAC (Bandwidth Aware Z-MAC) [8] is another
hybrid MAC protocol inspired from Z-MAC and pro-
posed for Ad Hoc networks. Like TH-MAC, BAZ-MAC
uses a bandwidth aware slot allocation algorithm during
the set-up phase, to assign slots to the nodes according to
their bandwidth requirements.
S-MAC and T-MAC mainly consider the energy effi-
ciency problem in WSNs, whereas TH-MAC and BAZ-
MAC address resource allocation according to nodes’
bandwidth needs. Moreover, bandwidth resource alloca-
tion in these two latter mechanisms is rather static since
it is performed during the setup phase. If changes in
bandwidth requirements occur during the transmission
phase, the allocation does not auto-adapt. The unique
solution is to re-run at least a part of the initial slot allo-
cation algorithm.
In the following, we propose a dynamic mechanism
able to automatically adapt to bandwidth requirement
changes at each node while maintaining energy saving
objectives and improving resource utilization.
3. I-MAC Protocol Design
I-MAC is a new hybrid MAC protocol designed for wire-
less sensor networks. It not only combines the strengths
of CSMA and TDMA techniques while obviating their
weaknesses but also introduces an adaptive priority scheme
for channel access.
I-MAC operates in two phases: a set-up phase and a
transmission phase. During the set-up phase, several op-
erations are successively run:
Neighbours discovery
TDMA Slot assignment
Local framing
Global synchronization
These operations run only once during the setup phase
and do not run until a signicant change in the network
topology (such as sensor’s position changes, joining or
leaving node) occurs.
Neighbours discovery operation is run by each node
and mainly consists in determining the list of its one-hop
and two-hop neighbours. This list is then used as an input
for the slot assignment operation.
The slot assignment problem can be defined as allo-
cating a time slot for each node such that two nodes
within two-hop distance should not be allocated the same
time slot. To this end, DNIB (Distributed Neighborhood
Information Based algorithm), an energy efficient and
scalable TDMA channel scheduling algorithm is pro-
posed. DNIB achieves distributed and parallel TDMA
scheduling based on 2-hop neighborhood information,
which allows handling the dynamic topology changes
with low complexity and small energy waste.
After the slot assignment phase, each node reuses its
assigned slot periodically in every predetermined period
called Local Frame. As in Z-MAC [4], we denote a node
assigned a given time slot the owner of that slot and the
other nodes the non-owners of that slot. Once each node
computes its Local Frame, Global Synchronization is
set-up and the Transmission phase can then be started.
During the transmission phase, time is divided into
slots. All nodes are allowed to transmit during any slot.
Before transmitting, a node performs carrier sensing and
transmits a packet when the channel is clear. The channel
access by the different nodes is controlled by a priority
scheme. Indeed, the idea behind I-MAC is to assign dif-
ferent levels of priority to nodes according to their roles.
For each node, the priority is implemented by adjusting
the initial contention window size according to its prior-
ity level. If it has something to transmit, the owner of the
slot is guaranteed to get the access first. Whenever the
owner has no data to transmit, or when the slot is not
owned, non-owners can compete to use the slot. The
chance a non-owner node gets a slot depends on its pri-
ority level.
Assigning a higher priority to forwarding nodes (that
have more packets to transmit) ensures that these nodes
will be able to transmit ahead of low priority nodes (in
most of the cases) and hence improve the channel utili-
zation. At the same time, and since shorter back off pe-
riods are given to higher priority nodes, their energy
consumption is reduced [14]. In addition, since competi-
tion to access the channel (to get a slot) occurs between
nodes within the same priority group, probability of col-
lisions is lowered because the number of competing
nodes within each group is lower than the total number
of nodes [14]. This energy saving meets the main re-
quirement of WSNs that is lifetime maximization, since
it is known that highly loaded nodes usually run out of
energy first.
In I-MAC, priorities are dynamically attributed during
the transmission phase according to the dynamic topol-
ogy changes and hence to the requirements of the nodes
in terms of bandwidth and energy. Resource allocation is
statically performed by DNIB during the set up phase
and do not consider each node requirements. However,
the priority scheme, used in the transmission phase, comes
Copyright © 2010 SciRes. WSN
Copyright © 2010 SciRes. WSN
to dynamically and fairly distribute the resources over
nodes according to their requirements.
In the following, we first describe how we implement
the different set-up phase operations mentioned above
and then discuss how they are integrated with transmis-
sion control in I-MAC.
3.1. Set-up Phase of I-MAC
3.1.1. TDMA Slot Assignment Algorithm: DNIB
TDMA scheduling can be generally defined as assigning
transmitting and receiving slots to each node so that col-
lision free transmissions can be achieved [15].
Transmissions may collide in two ways typically re-
ferred to as primary conflict and secondary conflict [16].
Primary conflict occurs when a node must do more than
one thing at the same time, for instance, transmitting and
receiving at the same time or receiving from two nodes
at the same time. This latter problem is also referred to as
hidden terminal problem. Secondary conflict occurs when
a receiver u, tuned to a particular transmitter v, is within
the transmission range of another transmitter whose trans-
mission, though not intended to u, interferes with the
transmission of v.
Combining these two definitions, we can say that two
nodes are in conflict if and only if the simultaneous trans-
missions from these two nodes cause radio interference
at some node.
There are two kinds of TDMA scheduling: Broadcast
and link [16]. In the broadcast scheduling, the entities
scheduled are the nodes whereas, in the link scheduling,
they are the links between the nodes. Hence, in the broad-
cast scheduling, conflict can happen among all the nodes
within a two-hop neighbourhood. In the link scheduling,
conflict can happen among all the nodes within a onehop
neighbourhood of a transmitter and a receiver. In this work,
we focus on broadcast scheduling.
From above definitions, we can define the slot assign-
ment problem as finding a time slot for each node, such
that no two nodes within two-hop distance should be allo-
cated the same transmitting slot.
An appropriate slot assignment algorithm should allo-
cate as few slots as possible to ensure more spatial reuse
and shorter delay. It should also be as simple as possible
with low running time (the maximum time taken for all
the nodes in the network to be assigned a slot for all
executions of the algorithm) and message complexity
(the number of messages exchanged between all the
nodes in the network to decide on the time slots). Other-
wise, high-energy waste can be expected due to mes-
sages transmission and reception operations and over-
In I-MAC, we use DNIB (Distributed Neighborhood
Information Based algorithm) a new time slot assignment
algorithm that we proposed in a previous work.
The idea behind the DNIB algorithm is to let each
node decide its own slot according to the information
collected from its one and two-hop neighbors. This in-
formation includes neighbors IDs and whether they are
or not yet scheduled.
The DNIB algorithm, running in parallel at each node,
is composed of the following procedures:
Slot assignment procedure:
During the slot assignment procedure, each non-sche-
duled node selects the minimum unassigned slot within
its one and two-hop neighbors.
Update procedure:
Once a node gets assigned a slot, it sends a “one-hop
broadcast” message to inform its one-hop neighbors. Those
one-hop neighbors send in their turn a “two-hop broad-
cast” message to update two-hop neighbors.
The “one-hop broadcast” message contains:
- the scheduled node id,
- the assigned slot id and
- a “two-hop broadcast schedule” that defines for
each one-hop neighbor the slot in which it will transmit
the two-hop broadcast message.
The “two-hop broadcast” message contains all already
known scheduling information about one-hop neighbors.
During this procedure, two types of collisions may
- collisions between “two-hop broadcast” mes-
- collisions between “two-hop broadcast” mes-
sages and “one-hop broadcast” messages.
The “two-hop broadcast schedule” defined above (sent
within the “one-hop broadcast” message) is used to avoid
the first type of collisions.
Collisions of the second type can be reduced by de-
laying one-hop broadcast messages. In DNIB, we pro-
pose that, before sending the “one-hop broadcast” mes-
sage, each scheduled node should wait a predetermined
number of slots.
Recovery procedure:
This procedure is activated when a node does not suc-
ceed to get scheduled for a predetermined period of time.
This may happen for instance because it misses informa-
tion about some of its one and/or two-hop neighbors. In
this case, the node can send a “request” message to its
one-hop neighbors. This request message triggers an up-
date procedure forcing its one-hop neighbors to send a
“two-hop broadcast” message with all already known
scheduling information.
The performances of DNIB were evaluated and results
have shown very good performances in terms of running
time, message complexity and energy efficiency com-
pared to existing slot assignment algorithms.
An example of slot assignment allocation using DNIB
is illustrated in Figure 1.
Readers interested in more details, analysis and simu-
lation results of DNIB are referred to [17].
3.1.2. Local Framing
Once slot allocation is achieved, the next issue concerns
the TDMA frame size of each node. Indeed, after being
assigned a slot, a node needs to know in how many slots
it will be able to reuse this slot to transmit data in a colli-
sion free manner. We call this period time frame and we
use the Time Frame rule (TF rule) proposed in Z-MAC
[4] to determine its size. According to this rule, each
node maintains its proper local time frame that depends
on its two-hop neighbourhood size and that avoids any
collision with its neighbours. Local frame approach
makes the slot assignment scheme adaptive to dynamic
topology changes and improves the channel utilization in
non uniform density networks. Readers interested in
these issues are referred to [4] for more details.
The TF rule is defined as follows:
Let ASNmax be the maximum allocated slot number
within two-hop neighbourhood of a node. The local
frame size of this node is equal to 2a, such that:
a-1 a
2 ASN 2
Thus, a node can reuse its assigned slot each 2a slot
with the guarantee that no other node within its two-hop
neighbourhood uses any slot that it uses [4].
3.1.3. Global Synchronization
Once each node computes its Local Time Frame, a glo-
bal synchronization should be set-up i.e., each node starts
Figure 1. A DNIB slot assignment example. The top figure
shows the network topology and the numbers are the id of
the nodes. The bottom figure shows the slot schedule of all
its time slot 0 at the same time. Like in Z-MAC, this is
achieved by setting the beginning of the real time (when
the synchronized clock value is 0) to be the beginning of
time slot 0.
The global synchronization is performed only once and
during the set-up phase. However, each node needs to
run a local synchronization scheme during the transmis-
sion phase.
At the end of the setup phase, each node forwards its
local frame size and its assigned slot number to its two-
hop neighbourhood. Thus, before starting the transmis-
sion phase, a node knows at which slots each of its one-
hop and two-hop neighbours will transmit. After this phase,
all nodes are synchronized to slot 0 and ready to transmit.
During the transmission phase, a local synchronization
will be run at each node.
3.2. Transmission Control of I-MAC
In I-MAC, we propose to use three levels of priority. The
priority of each node is set according to its role within
the network and to its traffic load. Three priority groups
are then formed. We discuss in a future section how pri-
orities can dynamically adapt to network changes. Let’s
denote by gk (k = 0, 1, 2) the group with priority level k.
After the slot allocation phase, each node can reuse its
assigned slot periodically after a predetermined number
of slots (the node’s local frame).
We remind that during the transmission phase, all nodes
can compete to get the slot. The owner of the slot is al-
ways given earlier chance to access the channel. If the
slot is not used because not owned or because its owner
has no data to transmit, non-owners can compete to use
this slot. The chance a non-owner node gets a slot de-
pends on its priority level as well as its priority group
The competition is achieved by using a parameterized
carrier sensing before transmitting data. Nodes can only
transmit if the channel is sensed clear. Priority is imple-
mented by adjusting the initial contention window size
according to the owner/not-owner status and to the prior-
ity group of each node. Owners are always guaranteed to
get the access first. Within non-owners, those belonging
to a higher priority group are able (in most of the cases)
to transmit ahead of those belonging to lower priority
ones. As a consequence, owners always transmit in a
collision free manner since they have early access to
their allocated slots. For non-owners, collisions are re-
duced since competitions mostly occur within reduced
number of nodes (i.e. same priority group). This has the
effect of significantly increasing resource usage ratio and
decreasing energy consumption. Energy saving is also
achieved at high priority nodes while trying to access the
channel since they are given shorter back off periods.
Copyright © 2010 SciRes. WSN
Copyright © 2010 SciRes. WSN
Finally, I-MAC allows loaded nodes to which are attrib-
uted a high priority to get more frequently access to the
channel (during non-owned/free slots) in order to deliver
data and then improve the channel utilization and in-
crease the throughput.
3.2.1. Priority Scheme
The priority scheme that we propose in I-MAC is in-
spired from IEEE802.11e.
In I-MAC, like in IEEE802.11e, if a node that has data
to transmit listens to the medium and find it idle for a
time interval greater than an Arbitration Interframe Space
(AIFS) then it contends to access to the medium. A back-
off time counter is then randomly selected between 0 and
Contention Window (CW). At the first transmitting at-
tempt, CW is assigned the value CWmin and will increase
up to the maximum value CWmax = 2mCWmin due to suc-
cessive collisions. Once the backoff timer expires and if
the channel is idle, the node can begin transmitting the
The AIFS values as well as the CW limits depend on
the priority level. Smaller values of AIFS and CW yield
higher priority. Therefore, a node with a small AIFS has
a higher priority than a node with a greater one. More-
over, assigning a short CW to a high priority node en-
sures that in most cases, this node is able to transmit
ahead of low priority nodes.
The values of CW limits and AIFS used in I-MAC for
different priority levels are given in Table 1. For owners,
CWmin = CWmax = CWo. This is because collisions be-
tween owners almost never happen since their slots are
scheduled a priori by the slot allocation algorithm to
avoid collision. For non-owners belonging to any priority
group, AIFS = CWo. This is to ensure that owners, if
they have something to transmit, be guaranteed to access
the channel (get the slot). Figure 2 gives an illustration
of how the priority is implemented in I-MAC.
3. 2. 2. Transmission Rule
When a node i has data to transmit, it checks whether it
is the owner of the current slot. If it is the case, then it
Table 1. Priority parameters.
Non owner
category Group0 Group1 Group2
level 0 1 2 3
CWmin CWno,min (CWno,min + 1)/2-1 (CWno,min + 1)/4-1CWo
CWmax CWno,max CWno,min (CWno,min + 1)/2-1CWo
Figure 2. Priority based channel access in I-MAC.
selects a random back off within [0, CWo]. When the
back off timer expires, it runs CCA (clear channel as-
sessment) to check whether the channel is clear. If it is
the case, the node transmits its data. However, if the
channel is busy, the node waits until it becomes clear to
repeat the process. If the node i is a non-owner of the
current slot, then it waits for CWo (AIFS) and then takes
a random back off within [CWo, CWnok], k = 0, 1, 2, the
contention window corresponding to the priority group
gk it belongs to. When the back off timer expires, the
non-owner runs CCA and if the channel is clear, it trans-
mits. However, if the channel is busy, the node waits
until it becomes clear and repeats the process.
We can see from Table 1 that non-owners with higher
priority have shorter contention window than those with
lower priority. This lets them in most cases get the slot
ahead those with low priority.
3.2.3. TDMA Slot Size
The TDMA slot size is kept constant for a given scenario.
It is calculated as the sum of CWo, CWno,max correspond-
ing to the lowest priority, the CCA period, one packet
propagation time and time taken to receive an acknowl-
edgement if the transmission succeed.
3.2.4. Receiving Schedule of I-MAC
In I-MAC, the transmission schedule is statically defined
during the set-up phase by DNIB. However, since in
I-MAC a node can transmit at any time, a reception sche-
dule must be defined. An appropriate solution, as men-
tioned in Z-MAC, is to rely on the LPL (Low Power
Listening) mechanism proposed in [12] and which uses
the preamble sampling method to reduce the energy
consumed in idle listening. In LPL, each node remains
asleep and periodically wakes up to check whether the
channel is busy. When a node transmits data, a long
preamble is attached to the packet. Once the receivers
listen this preamble, they keep their radio on until they
receive the real data. The preamble is generally longer
than the listening interval also called check period. This
mechanism was basically proposed to guarantee reliable
data transmission, as well as low power consumption.
3.2.5. Dynamic Priority Attribution
As explained previously, attributing a high priority to a
node gives it more chances to access the channel than
other low priority nodes and allows it to save some en-
ergy. The priority of each node is then set at the begin-
ning of the network deployment according to the re-
quirements of each node and the constraints it may face.
However, this priority should not be statically set. Indeed,
many changes may occur in the network. Some of the
reasons behind these network changes are addition or
lost of new nodes, permutation of node’s roles as in
clustered networks where the cluster head responsibility
is shared by the cluster members, moving nodes or vary-
ing interference which may alter the connectivity of the
network, change the routes from the sources to the base
station and hence modify the traffic load of some nodes,
etc. Therefore, the priority scheme must be able to dy-
namically adapt to the network changes. An appropriate
solution is to make nodes auto attribute their own priority
level. This priority can then be set according to some
statistics collected by these nodes during their lifetime
and which may concern the forwarding load, the re-
maining battery charge, the distance to the base station,
4. Performance Evaluation
This section is dedicated to the evaluation of the per-
formances of the overall I-MAC protocol in terms of
energy efficiency as well as channel utilization.
4.1. Simulations Setup
The aim of these simulations is to assess I-MAC through-
put and energy efficiency performances. Comparative
simulations of I-MAC and Z-MAC are run on NS2
simulator. Z-MAC implementation over NS2 provided in
[18] is used. Default parameter settings are summarized
in Tables 2-4 T0 and Tno, (respectively the owner and
non-owner contention windows of Z-MAC as defined in
[4]), are equal to 8 and 32 slots respectively.
The following one-hop and multi-hop scenarios (simi-
lar to the ones used in [4] and [8]) have been investigated.
4.1.1. One Hop Scenario
This scenario is inspired from [4] where n nodes and a
base station are placed at one-hop distance from each
other so that there are no hidden terminals (see Figure 3).
Each node belongs to a priority group, which corresponds
Table 2. Energy parameters.
Power Value(J)
etx 1
erx 0.67
eidle 0.82 erx = 0.5494
Table 3. Default settings of various parameters for I-MAC
and Z-MAC.
Parameters Default values
Communication bandwidth 19.2 kbps
Communication range 40 m
Contention slot size 400 µs
Transmission slot size 60 ms
Table 4. Priority parameters of I-MAC.
Non owner
Node cate-
gory Group0Group1 Group2 Owner
Priority level0 1 2 3
AIFS (slot)8 8 8 0
CWmin (slot)32 16 8 8
CWmax (slot)64 32 16 8
Figure 3. One-hop scenario topology.
to a priority level equal to 2, 1 or 0. All the nodes trans-
mit data to the base station, they always have something
to transmit and they transmit with the full transmission
This scenario is used to:
First evaluate the achievable throughput or channel
utilization of I-MAC for different levels of contention
within a one-hop neighbourhood and compare it to Z-
Second, to show the influence of the prioritization
scheme between the three different groups of nodes (dif-
ferent priority groups) on the channel utilization as well
as the energy consumption per transmitted packets.
Copyright © 2010 SciRes. WSN
Copyright © 2010 SciRes. WSN
4.1.2. Multi-Hop Scenario In this scenario, the number of nodes is set to 10 and
the number of senders varies from 1 to 10. The different
priority levels are alternatively distributed over the send-
ing nodes. The senders send data to the base station and
are considered as having always something to transmit.
According to the local frame rule, the frame size of I-
MAC and Z-MAC is 16 slots (0.96 s).
For the multi-hop scenario, we adopted the topology used
in [8] to demonstrate the effectiveness of the proposed
approach. The topology consists in two clusters separated
by a three-hop distance (see Figure 4). Each cluster is
composed of 8 transmitting nodes. The second cluster
also contains a base station to which all the transmissions
are destined. The nodes in the cluster 1 deliver the data
to the base station through node 1 and node 0. These two
nodes have an important role in the network since they
relay the traffic coming from all nodes in cluster 1. Such
nodes can then experience bottleneck in case of high
traffic and consume a lot of energy for forwarding pur-
poses. To evaluate the efficiency of our scheme, each
node is attributed a priority. All the nodes in the clusters
have a priority level equal to 0. Nodes 1 and 0 have a
higher priority equal to 1. When n users are said to be
senders, the n highest id nodes (not including the base
station) are transmitting.
Moreover, in the original version of Z-MAC provided
in [18], a node can only transmit one packet per slot. The
slot size is equal to the sum of the maximum contention
back off duration and the necessary time to transmit a
packet and receive the acknowledgment. When the con-
tention back off is smaller than this maximum, the re-
maining time is wasted. We decided as in [8] to allow the
nodes to transmit more than one packet at a time when-
ever it is possible. It has been proved in [8] that this sig-
nificantly improves Z-MAC throughput.
Figure 5 depicts the channel utilization of the three
different priority groups independently. It shows that the
channel utilization obviously depends on the priority
level of the senders. The higher the priority is the more
important is the corresponding channel utilization. In fact
and to be fairer, let’s consider the 3, 6 and 9 senders
points in Figure 5 where the number of nodes in each
priority group is the same. We can see that in the three
cases, the channel utilization value of priority 2 (0.31 for
6 senders point) is higher than the one of priority 1 (0.18)
which is itself higher than the one of priority 0 (0.13).
This happens because high priority nodes use smaller
congestion back off window size than nodes with low
priority. Therefore, during empty/non-owned slots, nodes
who belong to a high priority group are in most cases
able to transmit ahead of those belonging to low priority
4.2. Simulations Results
We present in the following the results and analysis of
the simulations run for the scenarios described above.
Both channel utilization and energy efficiency are inves-
tigated. Figures illustrating the comparison between our
proposed scheme and Z-MAC are then provided.
4.2.1. Channel Utilization
The following sub-sections describe the observations con-
cerning the channel utilization.
Channel utilization is calculated here as the fraction of
the channel capacity which was utilized to successfully
transmit data.
a) One-hop scenario
Figure 4. Multi-hop scenario topology.
Figure 5. Channel utilization of nodes with different priori-
ties in I-MAC one-hop scenario.
When the number of senders increases, the number of
slots where nodes transmit as owners increases. Therefore,
the channel utilization corresponding to priority 2 decreases
from 0.7 to 0.3 and the ones of priority 1 and 0 slowly go
up. This is because most slots are used by their owners
and therefore the number of owned slots used by the three
different priority groups becomes almost the same. More-
over, the number of empty/non-owned slots becomes sma-
ller and the percentage of non-owned slots used by the
highest priority nodes decreases.
Figure 6 presents both Z-MAC and I-MAC channel
utilizations for the one-hop scenario. It can be seen that
the channel utilization of Z-MAC varies between 0.39
and 0.68. This is because, when the number of senders
increases, the number of nodes transmitting in their own
slot increases and then the average back off duration is
reduced. Therefore nodes can in most cases transmit more
than one packet per owned slot. Moreover, when the
number of senders increases, collisions decrease since
most of the slots of the frame are owned and then Z-MAC
mostly behaves like a schedule based scheme.
It can also be seen that for I-MAC, the channel utiliza-
tion remains more or less stable at 0.65 when the number
of senders varies. Moreover, when comparing the channel
utilization of Z-MAC and I-MAC in Figure 6, we can see
that I-MAC outperforms Z-MAC especially when the
number of senders is small. This is because, when the
number of senders is low, most of the nodes transmit in
non-owned slots. In I-MAC, non-owned slots are mostly
used by highest priority nodes. These nodes have small
contention windows (8, 16) and then succeed in most
cases to transmit more than one packet in a non-owned
slot. Whereas, in Z-MAC, non-owners have a larger con-
tention window (8, 32) and then can only transmit one
packet per slot. Besides, in these conditions, the risks of
collision are considerable in Z-MAC since all non-owner
nodes have the same CW size (8, 32). Whereas in I-MAC,
the different CWs sizes relative to the different levels of
priority reduces collisions since non-owners only compete
1 2 3 4 5 6 7 8 9 10
umber of Senders
Figure 6. Comparison of channel utilization in one-hop sce-
nario: Z-MAC vs I-MAC.
with the ones of the same priority group, the number of
which is obviously less than the total number of senders.
However, the two protocol performances are almost the
same when the number of senders is high. In fact, in such
case, the number of nodes transmitting in their own slot
increases in both I-MAC and Z-MAC and then priority
scheme influence becomes less significant.
Figure 7 here shows the average channel utilization for
different priority nodes for I-MAC in one-hop scenario
when packet transmission rate is varied. For rates lower
than 2 packets per frame, the channel utilization of the
different priority groups is almost the same. In fact, with
low transmission packet rate, nodes don’t have a lot of
data to transmit. Therefore, no more than one packet is
transmitted in owned slots as well as in non-owned slots
used by high priority nodes. Hence, all the nodes, with
high or low priority, transmit almost the same amount of
data. However, when the packet transmission rate in-
creases, a channel utilization differentiation can be ob-
served due to smaller contention windows of higher pri-
ority nodes. Highest priority nodes can achieve a gain of
almost 50% in channel utilization compared to lower pri-
ority nodes.
b) Multi-hop scenario
In this scenario, the total number of nodes is 16 (8
nodes in each cluster). The frame size for each node of
I-MAC and Z-MAC according to the local frame rule is
16 slots (0.96 s).
Figure 8 depicts the channel utilization of both I-MAC
and Z-MAC for the multi-hop scenario when the number
of senders is varied. The packet transmission rate is fixed
to 2 packets/frame. It can be observed from this figure
that the performance of I-MAC is globally better than that
of Z-MAC. When the number of sender is lower than 4
and under a packet rate equal to 2 packet/frame, both pro-
tocols deliver all the packets and achieves about the same
utilization. However, we can see from Figure 4 that the
first 8 senders are located in cluster 1. Since the slots have
been allocated equally during the set-up phase, nodes 1
and 0 will form a bottle neck when the number of senders
in cluster 1 increases. Therefore, the channel utilization of
both I-MAC and Z-MAC falls down till the number of
opyright © 2010 SciRes. WSN
Copyright © 2010 SciRes. WSN
Figure 7. Channel utilization of nodes with different priori-
ties in I-MAC one-hop scenario.
Figure 8. Channel utilization comparison in multi-hop sce-
transmitting nodes is 8. However, the channel utilization
of Z-MAC falls down much earlier and faster than that of
I-MAC. This can be explained by the higher priority of
the forwarding nodes (nodes 1 and 0) compared to that of
the senders. In fact, with I-MAC the empty/non-owned
slots are mostly used by the forwarding nodes thanks to
their smaller CWs. This lets them get more access to the
channel and then relay more packets to the base station
compared to Z-MAC.
When the number of senders increases beyond 8, send-
ers of cluster 2 also start transmitting data. Since senders
of cluster 2 are one hop away from the base station, utili-
zation goes on increasing for I-MAC and Z-MAC but the
performance of I-MAC remains better than that of
Figure 9 shows channel utilization of Z-MAC and I-
MAC under the multi-hop scenario as we vary the packet
rate of the senders. Under packet rates lower than 1.5
packet/frame, both protocols deliver all the packets and
achieve the same utilization. When the rate increases
(more than 1.5 packet/frame), forwarding nodes with high
priority get access to most of empty/non-owned slots which
is not the case for Z-MAC. Hence, I-MAC utilization
remains nearly constant whereas Z-MAC utilization falls
down. When the packet rate goes beyond 4 packets/ frame,
empty slots are no more sufficient to deliver all the traffic
and then channel utilization remains constant. We observe
from the figure that, under such conditions, I-MAC achi-
eves about 65% higher utilization than Z-MAC.
Figure 9. Channel utilization comparison in multi-hop sce-
nario (only the nodes of cluster 1 are sending).
4.2.2. Energy Efficiency
a) One-hop scenario
Figure 10 represents a comparison of the energy con-
sumption of Z-MAC as well as the three different priority
groups of I-MAC in the one-hop scenario when the num-
ber of senders is varied. The curves presented in this fig-
ure represent the mean energy required by a sender of
group gk, k = 0, 1, 2 (as well as Z-MAC) to successfully
deliver a packet to the base station. This energy includes:
the energy to transmit the packet, to receive the acknowl-
edgment, to retransmit the packet in case of collision, to
sense the channel and finally the energy spent in idle state
during the back off duration.
It can be observed from the figure that the higher is the
priority the less is the amount of energy consumed per
delivered packet. Hence, a node with high priority con-
sumes less energy to successfully transmit a packet to the
base station than a node with lower priority. This is
mainly because high priority nodes have smaller CW and
then spend less energy during the back off duration than
lower priority nodes. It can also be seen that I-MAC
mostly outperforms Z-MAC in terms of energy efficiency.
This is due, as explained in Section 4. B.1.a, to reduced
collision in I-MAC due to prioritization and to smaller
CW size of high priority groups compared to the one of
In sensor networks, some nodes are more solicited in
forwarding than others, notably due to their central posi-
tion on the routes. They thus consume more energy to do
the relaying task and drain out of energy before the others
which can cause trouble in the network connectivity.
From the results below, we can conclude that a node with
higher priority consumes les energy to successfully
transmit a packet than a lower priority node. Hence, to
solve such problems, these critical nodes should be attrib-
uted higher priority than other nodes in the network,
which can make them save more energy when transmit-
ting packets and then have longer lifetime duration.
b) Multi-hop scenario
Figure 11 shows the energy consumption of Z-MAC
Figure 10. Comparison of average energy consumption per
delivered packet per sender in one-hop scenario.
and I-MAC under the multi-hop scenario as we vary the
number of senders. The curves represent the energy con-
sumed by the network to successfully deliver one packet
to the base station. As stated in the last section, this en-
ergy includes the energy for transmission, reception, re-
transmission and back off duration listening.
It can be observed from the figure that the energy con-
sumption in Z-MAC is worse than that in I-MAC.
When the number of nodes rises from 3 to 8, the energy
consumption in Z-MAC will increase while it remains as
constant in I-MAC. This is because in Z-MAC, when the
number of senders in clusters 1 increase, a bottle-neck is
formed on the forwarding nodes which cause the loss of
some packets and then their retransmissions. This is not
the case with I-MAC thanks to the higher priority of these
two nodes that gives them more chances to access the
channel (with shorter back-off duration).
When the number of nodes increases above 8, nodes in
the second cluster begin transmitting. These nodes can
transmit without collision or forwarding problems since
they are at one-hop distance from the base station. This
explains the decrease of the energy consumption curves.
The energy consumption variation of Z-MAC and
I-MAC is also plot when varying the packet rate. The
results are presented in Figure 12. Obviously, we can see
that I-MAC still outperforms Z-MAC for the same rea-
sons explained above.
4.2.3. Loss Rate
In this section, we present a comparison between I-MAC
and Z-MAC in terms of the ratio of average lost packets
to average successfully transmitted packets. It can be seen
from Figure 13 that it reaches for both protocols the ma-
ximum value when the number of senders is 8 i.e., all the
nodes of cluster 1 are transmitting. However, the loss
ratio of I-MAC still remains better than that of Z-MAC.
These results confirm the results and analysis given about
the channel utilization of the two protocols in the sections
Figure 11. Energy consumption comparison as we vary the
number of senders.
Figure 12. Energy consumption comparison for 8 senders of
cluster 1 sending only.
Figure 13. Average lost packets to average successfully trans-
mitted packets ratio (by senders).
4.2.4. Transmission Delay
We finally plot the average delay results of the two pro-
tocols under the multi-hop scenario as we vary the num-
ber of senders. We can observe from Figure 14 that the
performance of Z-MAC is worse than that of I-MAC,
which corresponds to our expectations. When the number
of senders is less than 3, the two protocols deliver all the
packets without any delay. As the number of senders var-
ies from 3 to 8, more and more packets are lost due to
bottleneck problem at forwarding nodes. I-MAC over-
comes this weakness thanks to the prioritization scheme.
When the senders in the cluster 2 start transmitting
(above 8 senders), the mean delay decreases since these
nodes are only at one-hop distance from the base station
and then rarely experience collision or forwarding prob-
Copyright © 2010 SciRes. WSN
Copyright © 2010 SciRes. WSN
Figure 14. Comparison of average delay (in seconds).
5. Conclusions
In this paper, we introduced I-MAC, an adaptive hybrid
medium access protocol for wireless sensor networks.
I-MAC allows sensor nodes that have higher load, for
instance because of their forwarding role, to get more
chances to access the radio resources. This is achieved
through an adaptive prioritization mechanism embedded
within the transmission mechanism. Moreover, the life-
time of loaded sensors (that usually run out of energy first
with usual MAC mechanisms) is prolonged since the pri-
oritization mechanism not only shortens their listening
period (backoff), but also reduces the collision when they
access to the channel.
Hence, by combining CSMA and TDMA techniques
and introducing prioritization to access the channel, I-
MAC becomes more energy efficient, more robust to
topology changes and fairer in resource allocation. It
gracefully adapts to the level of contention in the net-
work and improves the channel utilization as well as the
The performances of the overall MAC scheme were
evaluated through simulations over NS2 and the pre-
sented results have shown a significant improvement,
compared to Z-MAC, mainly in energy efficiency, chan-
nel utilization, loss ratio and delay.
We aim, in a future work, to achieve further enhance-
ments over I-MAC through the proposal of a more dy-
namic and efficient mechanism for priority adaptation.
We also look forward to implement it over TinyOS to
validate the simulation results provided above.
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