Energy Efficient MAC Protocols

Abstract

This paper presents a survey of energy efficiency of Medium Access Control (MAC) protocols for Wireless Body Area Sensor Networks (WBASNs). We highlight the features of MAC protocols along with their advantages and limitations in context of WBASNs. Comparison of Low Power Listening (LPL), Scheduled Contention and Time Division Multiple Access (TDMA) is also elaborated. MAC protocols with respect to different approaches and techniques which are used for energy minimization, traffic control mechanisms for collision avoidance are discussed.We also present a survey of path loss models for In-body, On-body and Off-body communications in WBASNs and analytically discuss that path loss is maximum in In-body communication because of low energy levels to take care of tissues and organs located inside the body. Survey of Power model for WBANs of CSMA/CA and beacon mode is also presented.

Full text

arXiv:1207.2567v1 [cs.NI] 11 Jul 2012
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Energy Efficient MAC Protocols
S. Hayat, N. Javaid, Z. A. Khan§, A. Shareef, A. Mahmood, S. H. Bouk
Department of Electrical Engineering, COMSATS
Institute of Information Technology, Islamabad, Pakistan
§Faculty of Engineering, Dalhousie University, Halifax, Canada
Abstract—This paper presents a survey of energy efficiency of
Medium Access Control (MAC) protocols for Wireless Body Area
Sensor Networks (WBASNs). We highlight the features of MAC
protocols along with their advantages and limitations in context of
WBASNs. Comparison of Low Power Listening (LPL), Scheduled
Contention and Time Division Multiple Access (TDMA) is also
elaborated. MAC protocols with respect to different approaches
and techniques which are used for energy minimization, traffic
control mechanisms for collision avoidance are discussed.We also
present a survey of path loss models for In-body, On-body and
Off-body communications in WBASNs and analytically discuss
that path loss is maximum in In-body communication because of
low energy levels to take care of tissues and organs located inside
the body. Survey of Power model for WBANs of CSMA/CA and
beacon mode is also presented.
Index Terms—Medium Access Control protocol; Wireless Body
Area Networks; Energy-Efficiency.
I. INTRODUCTION
EVOLUTION of wireless, medical and computer network-
ing technology has merged into an emerging horizon of
science and technology called Wireless Body Area Networks
(WBANs). However, applications of WBANs are not limited
to medical field only. Miniaturization and connectivity are
notable parameters of this field. WBANs consist of three
levels; first level is low power sensors or nodes which are
battery powered and need to be operated for a long time
without repairing and maintenance. These nodes may be
placed on the body, around the body or implanted in the body.
Second level is called master node, gateway or coordinator
which controls its child nodes; its power requirements may
be less strengthened than nodes due to its applications and
flexibility. Third level is the local or metropolitan or internet
network that serves for monitoring purposes.
Energy efficiency or effective power consumption of a
system is one of the basic requirements for WBANs because
of limited power of batteries. The most suitable layer for
discussing energy and power issues is MAC Layer. The basic
way of saving power or enhancing energy efficiency is to
minimize the energy wastage. There are several sources of
energy wastage including packet collisions, over hearing, idle
listening, control packet overhead, etc. Major source of energy
inefficiency among the above listed sources is packet collision
for WBANs. Fig. 1. best explains that how a node’s battery is
consumed, in the process of communication.
Collision avoidance for energy efficiency, minimum latency,
high throughput, and communication reliability, are basic
requirements in the design of MAC protocol. The fundamental
way of saving power or enhancing energy efficiency is to
minimize the energy wastage. Simulations are performed in
MATLAB for different scenarios to compute path loss. Results
show that path loss is maximum in In-body communication, as
compared to On-body and Off-body communication because
human body is composed of tissues and organs in which
communication is difficult and thus results in high path loss.
On-body and Off-body also results some variations when the
source and destination sensors or nodes are placed Line of
Sight (LoS) and Non Line of Sight (NLoS).
In this paper, we therefore, provide a survey of energy
efficient MAC protocols for WBANs. First, we elaborate the
protocol features and then their advantages and limitations are
discussed. Sources that contribute to the energy inefficiency in
a particular protocol is also identified. Moreover, comparisons
of MAC protocols in the context of WBANs are tabulated in
detail.
II. RELATED WORK
Gopalan et al. [1] survey MAC protocols for WBANs along
with the comparison of four protocols i.e., Energy Efficient
MAC, MedMac, Low Duty Cycle MAC, and Body MAC.
Some key requirements and sources of energy wastage are also
discussed. They also discussed some open research issues in
this survey. Still a lot of work has to be done in data link
layer, network layer and cross layer design.
In [2], Shahjahan kutty et al. discuss the design challenges
for MAC protocols for WBANs. They classify data traffic
for WBANs into three categories: energy minimization tech-
niques, frame structures and network architecture. However,
the comparison of protocols is not provided by them.
Sana Ullah et al. in [3] provide relatively a comprehensive
study of MAC protocols for WBANs. Comparison of the
low power listening, scheduled contention and Time Division
Multiple Access (TDMA) is provided. MAC requirements,
frame structures and comparison of different protocols and
their trade-offs are discussed in detail.
III. ENERGY MINIMIZATION TECHNIQUES IN MAC
PROTOCOLS FOR WBANS
Low power mechanisms play an important role in per-
formance enhancement of MAC protocol for WBANs. In
this section, different approaches and techniques that provide
energy efficiency in MAC protocols for WBANs are discussed
and compared.

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Energy efficiency is an important issue because the power
of nodes in WBANs is limited and long duration of operation
is expected. The key concept for low power consumption is
to minimize the energy consumption in the following sources:
sensing, data processing and communication.
Most of the energy wastage is caused during communication
process because of the collision of packets, idle listening,
over hearing, over-emitting, control packet overhead and traf-
fic fluctuations. Idle listening can be reduced through duty
cycling. To reduce energy waste in order to increase network’s
life time and to enhance the performance of MAC protocol,
different wake-up mechanisms are used.
There are three main approaches adopted for the energy
saving mechanisms in MAC protocols for WBANs, which
are: Low Power Listening (LPL), Scheduled Contention, and
TDMA.
MAJOR SOURCES OF ENERGY WASTE
CONTROL
OVERHEAD
POWER
CONSUMPTION IN
SENSING
POWER
CONSUMPTION IN
DATA PROCESSING
POWER CONSUMPTION IN
COMMUNICATION
OVER
EMITTING
IDLE
LISTENING
OVER
HEARING
PACKET
COLLISIONS
NODE POWER
SOURCE
(BATTERY)
Fig. 1. Sources of Power Consumption
A. Low Power Listening
LPL procedure is that “node awakes for a very short period
to check activity of channel”. If the channel is not idle then the
node remains in active state to receive data and other nodes
go back to sleeping mode. This is also termed as channel
polling [3]. This procedure is performed regularly without any
synchronization among the nodes. A long preamble is used by
the sender to check polling of the receiver. LPL is sensitive
to traffic rates which results in degradation of performance in
the scenario of highly varying traffic rates. However, it can be
optimized effectively for already known periodic traffic rates.
Wise-MAC [3] is one of the MAC protocols which is based on
LPL. This protocol reduce Idle listening using non-persistent
CSMA and preamble sampling technique.
B. Scheduled Contention
Scheduled Contention is the combination of the scheduling
and contention based mechanisms to effectively cope with
the scalability and collision problems. In contention based
protocols, contending nodes try to access the channel for data
transmission therefore, ability of collision of packet is greatly
increased. Example of contention based MAC protocol is Car-
rier Sense Multiple Access/Collision Avoidance (CSMA/CA)
in which Clear Channel Assessment (CCA) is performed by
the nodes before transmitting data.
Scheduling or contention free means that each node has the
schedule of transmission in the form of bandwidth or time slot
assignment. TDMA, CDMA and FDMA schemes are some
examples of scheduling mechanisms. However, CDMA and
FDMA are not suitable for WBANs because of high compu-
tational overhead and frequency limitations, respectively.
TDMA is the most suitable scheduling scheme, even though
it requires extra power consumption due to its sensitivity for
synchronization. The scheduled contention is the combination
of scheduling and contention based mechanisms. In scheduled
contention, a common schedule is adopted by all the nodes
to transmit data. This schedule is exchanged periodically
among the nodes to make communication adaptive, flexible
and scalable.
Sensor MAC (S-MAC) is one of a MAC protocol based on
the scheduled contention. In this protocol, low duty mode is set
as default mode for all the nodes which assures the coordinated
sleeping among neighboring nodes. The energy wastage due to
collision, overhearing, idle listening etc. is minimized because
the node is turned on only for transmission of data and remains
in sleep mode, otherwise.
C. Time Division Multiple Access
In TDMA mechanism, a super frame consists of a fixed
number of time slots is used. Time slots are allocated to
the sensor nodes by a central node and is known as Master
Node (MN), Cluster Head (CH), coordinator or Base Station
Transceiver (BST). Traffic rate is one of the key parameter
used by the coordinator to allocate time for each contending
node. The scheme is power efficient because a node gets time
slot for transmission of data and remains in sleep mode for
rest of the time. However, the synchronization requirements
may degrade performance in terms of power consumption.
Therefore, it is highly sensitive to clock drift, which may result
in limited throughput. Preamble-Based TDMA (PB-TDMA)
protocol is one of the TDMA based protocol. Other examples
include Body-MAC (B-MAC) [5], MedMAC [3] etc.
These techniques are briefly compared in Table.I.
IV. ENERGY EFFICIENT MAC PROTOCOLS
In this section, we briefly discuss the energy efficient MAC
protocols for WBAN.
A. Okundu MAC Protocol
An energy efficient MAC protocol for single hop WBANs
is proposed by Okundu et al. in [4]. This protocol consists

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Table.1. Comparison between LPL, Schedule Contention, and TDMA
Energy Saving
Mechanisms LPL Scheduled Contention TDMA
Adaptability to
traffic and delay Scalable and adaptive to traffic load and low
delay Better delay performance due to
sleep schedules Better end-to-end reliability, smaller delays,
high reliability
Transmission
latency and
throughput
Flexible, high throughput, tolerable latency,
and low power consumption High transmission latency, loosely
synchronized, low throughput Good for energy efficiency, prolonged net-
work’s lifetime, load balancing
Synchronous/
Asynchronous Asynchronous Synchronous Synchronous-Fine grained time synchro-
nization
Traffic
heterogeneity
requirements
Low duty cycle nodes do not accommodate
aperiodic traffic. Very hard to satisfy the
WBANs traffic heterogeneity requirements
Low duty cycle nodes do not re-
quire frequent synchronization of
schedules. Hard to satisfy the
WBANs traffic heterogeneity re-
quirements
Low duty cycle nodes do not require
frequent synchronization at the beginning
of each superframe. Easy to satisfy the
WBANs traffic heterogeneity requirements
Sensitivity Sensitive to tuning for neighborhood size
and traffic rate Sensitive to clock drift Very sensitive to clock drift
Performance
with respect to
traffic rates Poor performance when traffic rates changes With the increase in traffic, perfor-
mance is improved Throughput and number of active nodes are
limited
Cost incurred by
sender and re-
ceiver
Receiver and polling efficiency is gained at
much greater cost of senders Similar cost incurred by sender and
receiver Require clustering
Extravagant It does not listen for full contention period
as a result it is less expensive Listening for full contention period Low duty cycle
Scalability and
adaptability Challenging to adapt LPL directly to new
radios like IEEE 802.15.4 Scalable, adaptive, and flexible Limited scalability and adaptability to
changes on number of nodes
of three main processes: link establishment, wakeup service,
and alarm process. Basic energy saving mechanism of this
protocol consists of central control of wakeup/sleep time and
Wakeup Fall-back Time (WFT) processes. WFT mechanism
is used to avoid collision due to continuous time slot. This
mechanism states that, if a slave node wants to communicate
with a MN and it fails in its task due to MN’s other activities,
then it goes back to sleep mode for a specific time computed
by WFT. However, data is continuously being buffered during
the sleep time.
To minimize time slot collision, the concept of WFT has
been introduced. This concept helps every slave node to
maintain a guaranteed time slot even if it fails to communicate
with the MN. In this protocol, problems like idle listening and
over-hearing can be reduced because of central management
of traffic.
In one cluster, only 8slave nodes can be connected to MN
which restricts inclusion of other slave nodes. In link establish-
ment, wakeup service, and alarm processes, communication is
initiated by the MN. Another main problem is that, only one
slave node can join network at a time.
B. MedMac Protocol
N. F. Timmons et al. in [5] propose a TDMA-Based
MAC protocol for WBANs called MedMAC. This protocol
consists of two schemes for the power saving: Adaptive Guard
Band Algorithm (AGBA) and Drift Adjustment Factor (DAF).
AGBA along with time stamp is used for synchronization
among coordinator and other nodes. This synchronization is
introduced using Guard Band (GB) between time slots to
allow the node to sleep for many beacon periods. DAF is
used to minimize bandwidth. GB is calculated by AGBA
and shows the worst cases. However, practically gaps may
be different between time slots depending upon application
scenarios. DAF adjusts GB according to practical situation
and avoids overlapping between consecutive slots.
MedMac outperforms IEEE 802.15.4 for Class 0 (lower
data rate applications such as health monitoring and fitness)
and Class 1 (medium data rate medical applications such as
EEG). Energy waste due to collision is reduced by introducing
Guaranteed Time Slot (GTS). Each device has exclusive use
of a channel for a fixed time slot, therefore, synchronization
overhead is also reduced.
This protocol works efficiently for low data rate applica-
tions, and work inefficiently for high data rate applications.
However, In-body and On-body applications of WBAN are
usually of higher data rate.
C. Low Duty Cycle MAC Protocol
Low Duty Cycle MAC protocol for WBANs is designed in
[6]. In this protocol, analog to digital conversion is performed
by slave nodes while the other complex tasks such as digital
signal processing is carried out at MN. MNs are supposed to
be less power than slave nodes.
This protocol introduces the concept of Guard Time (T g )
to avoid overlapping between consecutive time slots. After T
frames a Network Control (NC) packet is used for general net-
work information. Power saving is achieved by using effective
TDMA strategy.
This protocol is energy efficient because it sends data in
short bursts. By using TDMA strategy, this protocol effectively
overcomes the collision problem. It allows monitoring patient’s
condition and can reduce the work load on medical staff, while
keeping minimum power usage.
TDMA strategy is used in WBANs, and it is found that
TDMA is more suitable for static type of networks with a
limited number of sensors generating data at a fixed rate
therefore, this protocol may not respond well in a dynamic
topology.

4
D. B-MAC Protocol
B-MAC protocol achieves energy efficiency by using three
bandwidth management schemes: Burst, Periodic and Adjust
Bandwidth.
Burst bandwidth consists of temporary period of the band-
width, which includes several MAC frames and recycled by the
gateway (coordinator). Bandwidth is reduced to half if it does
not fully utilized by the nodes, which is also informed about
reduction of bandwidth. Periodic bandwidth is a provision for a
node to have access to the channel exclusively within a portion
of each MAC frame or few MAC frames. It is also allocated
by the gateway based on node’s QoS requirements and current
availability of the bandwidth [7]. Adjust bandwidth defines the
amount of bandwidth to be added to or reduced from previous
Periodic Bandwidth [7].
Nodes can enter into sleep mode and wake up only when
they have to receive and transmit any data to the gateway,
because the nodes and the gateway are synchronized in time.
The time slot allocation in Contention Free Period (CFP) is
collision free, which improves packet transmission and thus,
saves energy.
The protocol uses CSMA/CA in the uplink frame of Con-
tention Access Period (CAP) period, which is not reliable
scheme due to its unreliable CCA and collision issues.
E. Ta-MAC Protocol
Traffic aware MAC (Ta-MAC) protocol utilizes traffic infor-
mation to enable low-power communication. It introduces two
wakeup mechanisms: a traffic-based wakeup mechanism, and a
wakeup radio mechanism. Former mechanism accommodates
normal traffic by exploiting traffic patterns of nodes whereas,
later mechanism accommodates emergency and on-demand
traffic by using a wakeup radio signal.
In the traffic-based wakeup mechanism, the operation of
each node is based on traffic patterns. The initial traffic pattern
is defined by the coordinator and can be changed later. The
traffic patterns of all nodes are organized into a table called
traffic-based wakeup table. In wakeup radio mechanism, a
separate control channel is used to send a wakeup radio signal.
The coordinator and the member node send wakeup radio
signal in on-demand and emergency case.
In Ta-MAC, a node wakes up, whenever it has a packet
to send/receive. Since the traffic patterns are pre-defined and
known to the coordinator, it does not have to wait for resource
allocation information/beacon. As a result, delay is minimized
comparitive to other MAC protocols. This protocol accommo-
dates normal, emergency and on-demand traffic in a reliable
manner. To achieve energy efficiency in MAC protocol, the
central coordination and resource allocation is based upon the
traffic patterns of the nodes.
As, in this protocol, the traffic pattern are defined by the
coordinator, in a static topology. Therefore, it does not work
efficient in dynamic topology (in dynamic topology, traffic
patterns are changed frequently).
F. S-MAC Protocol
S-MAC [8] protocol is proposed for WBASNs. This pro-
tocol uses fixed duty cycles to solve idle listening problem.
Table. 2. Qualitative Comparison of MAC Protocols
Protocols Advantages Disadvantages
Okundu MAC
Minimize time slot
collision, reduce idle
listening and over-
hearing
Only 8 slave nodes
can be communicated
to MN
MedMAC Energy waste due to
collision is reduced Do not support high
data rate applications
Low Duty Cycle Collision problem is
reduced, allows pa-
tients’ monitoring
Not suitable for dy-
namic type of net-
works
B-MAC Improves packet
transmission hence
saves energy
Uses CSMA/CA in
the uplink frame of
CAP period, which is
not a reliable scheme
Ta-MAC
Accommodates
normal, emergency
and on-demand
traffic, energy
efficient, reasonable
delay
Not suitable for dy-
namic topologies
S-MAC
High latency and time
synchronization over-
head may be pre-
vented due to sleep
schedules
Low throughput,
overhearing and
collision may cause if
packet is not destined
to listening node
T-MAC
Packets are sent in
burst and with low
latency which collec-
tively gives better re-
sult under variable
load
Suffers from sleeping
problems
H-MAC
Improves BSN’s en-
ergy efficiency and
reduces extra energy
cost
Does not support
sporadic events
and posseseslow
spectral/bandwidth
efficiency
DTDMA Reduce packet drop-
ping rate, less energy
consumption
Does not support
emergency and
on-demand traffic
Nodes wakeup after a specific time, as assigned by coordinator,
sends data and goes back to sleep mode again. As, all
the nodes are synchronized, therefore, collision can also be
easily avoided. S-MAC gives considerably low latency. In this
protocol, time synchronization overhead may be prevented due
to sleep schedules.
Fluctuating traffics are not supported and no priority is given
to the emergency traffic scenarios by S-MAC. Therefore, it
is not a reliable for WBANs. Overhearing and collision may
occur if the packet is not destined to the listening node.
G. T-Mac Protocol
Mihai et al. [9] suggested Time-out MAC (T-MAC) for
WBASNs. It uses flexible duty cycles for increasing energy
efficiency. In T-MAC, the node wakes up after time slot
assignment, sends pending messages. If there is no activation
event for Time Interval (TA), the node goes back to sleep
mode again. If a node sends Route To Send (RTS) and does
not receive Clear To Send (CTS), then sends RTS two more
times before going to sleep. To solve early sleep problem, it
uses future RTS for taking priority on full buffer.
In T-MAC, packets are sent in burst, as a result delay is
minimized. It also outperforms other MAC protocols under
variable load. The main disadvantage in this protocol is that
it suffers from sleeping problems.

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Table. 3. Energy Minimization Techniques and Mechanisms
Protocol Energy Efficiency Mechanism
Okundu MAC Wake up Fall back Time (WFT)
MedMAC TDMA, Adaptive Guard Band Al-
gorithm (AGBA) and Drift Adjust-
ment Factor (DAF)
Low Duty Cycle TDMA, concept of Guard Time
(T g)
B-MAC TDMA, Bandwidth mechanism
Ta-MAC Central coordination according to
traffic patterns of the nodes
S-MAC Scheduled based, organized in slots
and operation based on schedules
T-MAC Have slots and operation is based
on schedules
H-MAC Heartbeat Rhythm information is
used for synchronization
DTDMA TDMA based, use of slotted aloha
in CAP field
H. H-MAC Protocol
Heartbeat Driven MAC (H-MAC) uses heart beat rhythm
information for synchronization of nodes. This avoids the use
of external clock and thus reducing the power consumption.
Also guaranteed time slot (GTS) provision to each node helps
to avoid collision.
H-MAC aims to improve BSNs energy efficiency by exploit-
ing heartbeat rhythm information, instead of using periodic
synchronization beacons to perform time synchronization [3].
Although, H-MAC protocol reduces extra energy cost of
synchronization, however, it does not support sporadic events.
Since TDMA slots are dedicated and are not traffic adaptive,
H-MAC protocol encounters low spectral/bandwidth efficiency
in case of low traffic. The heartbeat rhythm information varies
depending on patient’s condition. It may not reveal valid
information for synchronization all the time [3].
I. DTDMA Protocol
Reservation based dynamic TDMA (DTDMA) protocol
uses slotted ALOHA in CAP field of super frame to reduce
collisions and to enhance power efficiency.
Through the adaptive allocation of the slots in a DTDMA
frame, WBAN’s coordinator adjusts the duty cycle adaptively
with traffic load. Comparing with IEEE 802.15.4 MAC proto-
col, DTDMA provides more dependability in terms of lower
packet dropping rate and low energy consumption especially
for an end device of WBAN [3]. It does not support emergency
and on-demand traffic. Furthermore, DTDMA protocol has
several limitations when considered for the Medical Implant
Communication Service (MICS) band. The MICS band has ten
sub-channels and each sub-channel has 300 Kbps bandwidth.
DTDMA protocol can operate on one sub-channel, however,
cannot operate on ten sub-channels simultaneously [3].
The main purposes of a MAC protocol are to provide energy
efficiency, network stability, bandwidth utilization and reduce
packet collision.
The energy minimization techniques and mechanism in
MAC protocols are summarized in Table. 3.
V. PERFORMANCE TRADE-OFFS MADE BY MAC
PROTOCOLS
In this section, we discuss the performance of the MAC
protocols they achieve and price they pay. In other words,
trade-offs, the MAC protocols have to make.
A. Okundu MAC Protocol
Network’s scalability is mainly application dependent, e.g.,
ECG can support upto maximum of 8 slave nodes because of
8 percent duty cycle. However, in practice this is 6 to allow for
possible retransmissions. Therefore, we have a trade-off, for
retransmission, slave nodes attached to the MN are reduced to
attain scalability of network.
B. MedMac Protocol
The low data rate applications of Class 0medical devices in-
clude monitoring of respiration system, temperature of human
body, pulse monitoring etc. Power consumed by respiration
transceiver is slightly high in MedMAC protocol with respect
to other protocols, while temperature and pulse node show
much low power consumption, as compared to other protocols.
MedMAC trade-offs power consumption of respiration for less
power of other two applications.
C. Low Duty Cycle MAC Protocol
The number of extra slots needed for protocol robustness is
dependent on Packet Error Rate (PER) and Packet Loss Ratio
(PLR). When PER is high, it will increase PLR. However,
PLR, may be reduced by using extra slots in the time frame.
Therefore, this protocol can trade-offs extra slots for less PLR.
D. B-MAC Protocol
B-MAC trade-offs idle listening for a reduced time to
transmit and reception of data. As, we know that reducing duty
cycle increases sleep time which in turn reduces idle listening.
Another trade-off is between idle listening and packet length,
because this overhead dominates the energy consumption.
E. Ta-MAC Protocol
Ta-MAC uses two wakeup mechanisms one for handling
data traffic and other for emergency traffic. By using these
two mechanisms this protocol outperforms all other protocols
in terms of power consumption because problems like idle
listening, collision and overhearing are reduced. However,
by sending frequent control messages to the nodes increases
node’s overhead, which is a trade-off. The initial traffic pat-
terns of all the nodes are defined by the coordinator, as a result
delay is also slightly increased.
F. S-MAC Protocol
For transmission and reception of data in S-MAC, an
extremely low duty cycle is used. When throughput increases
SAC’s duty cycle also increases , which further increases the
overhead of SYNChronization (SYNC) period, as a result,
power consumption is increase linearly. S-MAC can trade-offs
throughput for energy, also it can trade-offs energy for latency.

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Table. 4. Trade-offs Made by MAC Protocols
Protocol Trade-Offs
Okundu MAC Trade-offs number of slave nodes
attached to the MN are reduced for
scalability of network
MedMAC Trade-offs idle listening for a re-
duced time to transmit and recep-
tion of data
Low Duty Cycle Can trade-off extra slots for less
PLR
B-MAC Trade-off is between idle listening
and packet length
Ta-MAC Trade-off delay for low power con-
sumption
S-MAC Can trade-off energy for latency.
T-MAC Can trade-off latency for high
throughput
H-MAC Trade-offs between energy effi-
ciency and bandwidth utilization
efficiency
DTDMA Trade-offs overhead for a low
power consumption
G. T-Mac Protocol
T-MAC uses adaptive duty cycle, implemented as a time out
after the last event. At lower transmission rates, throughput
increases because probability of packet loss is much less than
received packet, however, the latency is increased between
source and destination node.
H. H-MAC Protocol
In H-MAC a Guard Band is introduced in time slots to avoid
collision by overlapping of data, however, when time slots are
completely aligned then there will be no data transmission in
Guard Band, therefore, it reduces bandwidth utilization. The
coordinator of BSN then uses this GB for synchronization, by
sending re-synchronization control packets, hence achieving
energy efficiency. Thus making a trade-offs between energy
efficiency and bandwidth utilization efficiency.
I. DTDMA Protocol
DTDMA is a TDMA based protocol which uses time slots
for data transmission and as a result low power is consumed.
However, TDMA requires synchronization between nodes
and the coordinator, as a result, overhead is increased. This
overhead is a trade-off for energy.
The trade-offs of each selected protocol are summarized and
given in Table. IV.
VI. MAC FRAME STRUCTURE
MAC frame structure consists of control portion or control
packet and data portion. Control portion is responsible for
the management and control messages (beacon period, request
period, topology management period) to control and manage
dynamic topology and varying data rate traffic. Data portion
consist of two sub parts: CAP and Contention Free Period
(CFP). CAP consists of CSMA/CA while the nodes contend
in CAP transmit MAC control packets. Similarly, small size
data packets can also be transmitted in CAP.
In [7], the allocation of time slots is controlled by the
coordinator. The coordinator arrange the duration of control
Table. 5. Comparison between IEEE 802.15.4 MAC and Original IEEE 802.15.4
IEEE 802.15.4 MAC Original IEEE 802.15.4
Low power consumption High power consumption
Higher data rate Low data rate
Higher flexibility Low flexibility
TDMA based Contention based
Collision Free Greater collisions
Sleep mode Idle listening
and data packet on the basis of current traffic of topology that
is why the slots are allocated to CFP and are collision free.
In each frame, bandwidth allocation in CFP can be changed.
In [5], the GB is used to maintain synchronization among
devices even if a node is sleeping for many beacon periods.
In [4][9][10][11], MAC protocols use slotted ALOHA in its
frame structure to divide a slot into 4 equal mini slots. In [6],
T g is introduced in its frame structure to reduce overlapping
between the two following nodes.
VII. TECHNIQUE FOR COLLISION AVOIDANCE FOR
TRAFFIC CONTROL
The main schemes of MAC protocol for WBANs are divided
into two groups: contention based i.e., CSMA and contention
free i.e., TDMA. Most of the traffic is interrelated in WBANs,
therefore, contention based solutions are not suitable for them.
For example, if a patient is suffering from fever, the body
temperature increases which increases blood pressure, hence,
the sensor sensing temperature variation and the sensor that
senses blood pressure variation, both become active. Along
with them other respiration sensors also become active at the
same time and try to access the channel/coordinator. However,
in this situation, collision occurs in CSMA. In TDMA, each
node communicate to MN according to the assigned pattern
by the coordinator. As a result, collision in data traffic is low,
as compared to CSMA.
A. Okundu MAC
This protocol controls traffic using centrally controlled
wakeup/sleep time. Slots are assigned to sensors change every
time when coordinator detects any change in traffic pattern.
Assignment of different time slots, decreases collision between
the nodes . It makes the system to handle fluctuating traffic.
The sensor nodes establishe link with the coordinator after
listening to the Radio Frequency (RF)-channel for a fixed time
period. MN sends request to the sensor node for information
by setting and communicating the next wakeup time after
establishing the link.
B. MedMAC
MedMac reduces the collision by using AGBA. AGBA
allows the sensor nodes to sleep for a GB time period between
each time slot. Each node has specific time slot to commu-
nicate with master node/coordinator, which means there is no
collision. Thus minimizes the synchronization overhead.

7
Multi-superframe m beacon period
Control Packet Data Portion
CAP CFP
Beacon Beacon
slot
Mini
Slot
GTMS
Guard time GTS
CFP : Contention free period
CAP : Contention access period
GTS : Granted time slots
GTMS : Granted time mini slots
Beacon period with n time slots
Guard time
CAP : Contention Free Period
CAP : Contention Acess Period
GTS : Guranteed Time Slots
GTMS : Guranteed Time Mini Slots
Fig. 2. MAC FRAME STRUCTURE
C. Ta-MAC
Ta-MAC protocol uses two channel access mechanisms
for traffic control i.e., traffic based wakeup mechanism for
normal traffic, and wakeup radio mechanism for on-demand
and emergency traffic. In traffic-based wakeup mechanism, all
nodes have traffic patterns that are assigned by the coordinator.
The initial patterns are defined and updated by the coordinator.
The traffic patterns of all nodes are synchronized and arranged
in a specific table, known as T raf f ic B ased W a keup T abl e.
Node’s ID and its respective traffic patterns are stored in this
table.
Normally, all the nodes become active/wakeup according to
their traffic patterns. If two or more nodes have same wakeup
pattern then the node with high priority is treated first by the
coordinator, as shown in Fig. 3. By assigning these patterns,
load at the coordinator is minimized, and chances of collision
is also reduced.
D. B-MAC
B-MAC uses downlink and uplink schemes along with
sleeping mode for data traffic control. Downlink is only used
by MN, therefore, traffic and data load on downlink are
reduced. Uplink is divided into CAP and CFP. MN allocates
time slots to CFP according to data traffic which makes CFP
collision free. In case, when nodes have no data to transmit
or receive then they go to sleeping mode.
E. Low Duty Cycle
Low duty cycle MAC protocol is based on TDMA. In
TDMA, time slots are assigned to the sensor nodes by the
coordinator. To avoid collision between the data traffic, the
concept of T g is introduced. Use of T g between every con-
secutive slots prevents the transmission overlaps and controls
data traffic.
F. IEEE 802.15.4 MAC
The basic requirement of QoS is to minimize delay and
maximize the probability of successful transmission. CFP
scheme is used to control data traffic to guarantee the QoS. If a
node wants to send data, first it listens for the network beacon.
After node finds the beacon that is sent by the coordinator, the
node synchronizes to the super frame structure.
IEEE 802.15.4 supports up to 250 Kbps data rate with
possible coverage of 10 meters. This data rate is not enough
to support the required rates of WBANs that is up to 10
Mbps. According to IEEE 802.15.4, packets are transmitted
in the contention period, which may result longer delays in
real time critical applications. When traffic is increased, the
nodes compete for the contention based slots, resulting in long
delays and the actual size of the network is almost doubled
[7].
In [10], to satisfy the requirements of WBANs including
QoS, network scalability, support for multiple PHY’s and mul-
tiple application traffics, IEEE 802.15.4 MAC is proposed. It
is the modified version of original IEEE 802.15.4. QoS means

8
Fig. 3. Data Traffic Control
to decrease the packet latency and increase the probability of
successful transmission of data packets without collision and
loss of data. In original IEEE 802.15.4, GTS mechanism is
provided to support the emergency data. GTS is very effective
for data transfer, however, inherently the limit of GTS in a
super frames is seven. As a result, it cannot support more
than seven devices simultaneously in CFP. Whereas, in IEEE
802.15.4 MAC, the coordinator may allocate more than seven
GTS simultaneously to the sensor devices.
IEEE 802.15.4 MAC and 802.15.4 original is compared
briefly in Table. 5.
VIII. CONCLUSION
We present a survey of different MAC protocols with respect
to energy efficiency and their advantages and disadvantages
in WBANs. Low power listening, scheduled contention and
TDMA are also compared. It is observed that TDMA is
more power efficient, however, suffers with synchronization
sensitivity. Techniques for collision avoidance of different
MAC protocols are also comparatively analyzed. Path loss
model for In-body, On-body and Off-body communication in
WBANs is also described. Because human body is composed
of tissues and organs in which communication is difficult and
thus results in high path loss. On-body and Off-body also show
some variations in results when the source and destination
sensors or nodes are LoS and NLoS.
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