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a r X i v : 1 2 0 7 . 3 5 9 9 v 1 [ c s . N I ] 1 6 J u l 2 0 1
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Adaptive-Reliable Medium Access ControlProtocol for Wireless
Body Area Networks
A. Rahim, N. Javaid, M. Aslam, U. Qasim ‡, Z. A. Khan §‡
University of Alberta, Alberta, CanadaDepartment of Electrical
Engineering, COMSATSInstitute of Information Technology, Islamabad,
Pakistan.
§Faculty of Engineering, Dalhousie University, Halifax,
Canada.
I. M OTIVATION
Extensive energy is consumed by Transceiver communica-tion
operation [1]. Existing research on MAC layer focusesto maximize
battery-powered sensor node’s life. Bottleneck of MAC layer
protocol design for WBAN is to achieve highreliability and energy
minimization. Majority of MAC proto-cols designed for WBANs are
based upon TDMA approach.
However, a new protocol needs to be dened to achieve highenergy
efciency, fairness and avoid extra energy consumptiondue to
synchronization.
I I . P ROTOCOL DES IGN
Proposed MAC protocol, AR-MAC is based upon TDMAapproach to
minimize energy consumption. AR-MAC assignsGuaranteed Times Slot
(GTS) to each sensor node for com-munication based upon the
requirements of sensor node . Toreduce overhearing and idle
listening, proposed system usesperiodic sleep and wakeup according
to node requirements.We assume a star topology; a Central Node (CN)
collectsdata from sensor nodes and communicates with a
Monitoring
Station (MS), direct or through an Access Point (AP).CN is
usually equipped with larger batteries and higher
computational power. One or two transceivers may be usedwithin a
single CN. In case of two transceivers total timeframe T Frame is
allocated for communication with sensornodes. We assume CN with
single transceiver where T Frame
is divided into three parts: Contention Free Period (CFP)
forcommunication with sensors, Contention Access Period (CAP)to
accommodate emergency or on-demand trafc and timeT MS for
communicating sensor nodes’ data to MS.
A. Channel Selection
Initially, CN starts scanning for available free Radio
Fre-quency (RF) channels. If the current RF Channel is busy,
CNswitches to another RF Channel. CN selects a free RF channelfor
communication. After successful selection of RF Channel,CN
broadcasts the Channel Packet with address and channelinformation
to sensor nodes. On the other side end nodes scanRF channels for
Channel Packet from CN. Sensor node scansthe RF channel if it is
free it switches to another RF channel.If the channel is busy it
waits for time T CP to listen ChannelPacket. If sensor node does
not receive the Channel Packet,it switches again to next channel.
After successful reception
of Channel Packet, node starts transmission and sends
anacknowledgment (ACK) packet to CN.
B. Time Slot Assignment
Once sensor node selects a proper RF Channel after re-ceiving
Channel Packet from CN, sensor node sends out aTime Slot Request
(TSR) packet to CN. TSR packet includes
sensor node’s data rate and required time slot
information.Authors in [6], propose time slots of xed length with
xedguard band time, T GB . In-body and on-body biomedicalsensors
have different data rates and sampling intervals withdifferent
clock drifts. Assigning time slots of equal length forunequal
requirements is wastage of resources. AR-MAC usesan adaptive scheme
for Time Slot (TS) and GBT time. Basedon trafc pattern of nodes, CN
assigns time slot and sendsTime Slot Request Reply (TSRR). These
time slots are of variable length depend upon the requirements of
sensor nodes.Assigned time slot can easily accommodate the
transmissionof data packet, reception of ACK packet and some
acceptabledelay, based on the communication model. T GB is
inserted
between the two successive time slots to avoid the
interferencedue to clock drift of node and CN as shown in
Fig.3.
TS1 TSn------------------ CAP T MS
Guard Time Contention Access Period
Communication with MSGuaranteed Time Slot
Fig. 1. Time Slots Assignment with Guard-band Time
Value of T GB depends upon length of the successivetime slots.
Adaptive GB avoids possibilities of collision andinterference due
to clock drift. We calculate T GB as follows:
T GBn,n +1 = F 100
× 12
[T S n + T S n +1 ] (1)
T GB1 = F × T S 1
100 (2)
T GBn = F × T S n
100 (3)
http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1http://arxiv.org/abs/1207.3599v1
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Where F is guard band factor, depends upon the average
driftvalue. However guard band time T GB1 is inserted before
rsttime slot and similarly guard band time T GBn is placed afterN
time slot. After successful time slot assignment, sensornodes enter
into sleep mode and wakeup only to send datato CN in allocated time
slots. Periodic sleep reduces energyconsumption due to idle
listening and Overhearing. Allocatedtime slots in CFP are
completely collision free thus reduce theenergy consumption and
make the communication reliable.
C. Synchronization
TDMA schemes require extra energy cost for
periodicsynchronization [8]. Synchronization of nodes after N
numberof cycles is energy consuming process. AR-MAC uses a
novelsynchronization mechanism to avoid collision and
energyconsumption. After successful assignment of time slots
tonodes, CN listens to data packet within expected time slot.Upon
arrival of data packet, CN compares current arrivaltime of the
packet and expected arrival time with acceptabledelay (D ). Based
on the difference of current arrival timeand expected arrival time
a Drift Value (DV ) is calculated.This DV is transmitted to node
within ACKnowledgment(ACK) packet to adjust time slot for future
communication.However, this value depends upon acceptable delay and
F . If the difference between the expected arrival time and
currentarrival is greater than D , CN sends DV with in
SYNChro-nization ACKnowledgment (SYNC-ACK) packet for
futuresynchronization to sensor nodes otherwise, CN sends simpleACK
packet for received data packet. For communication of data in
future, sensor node adjusts its wakeup time scheduleaccording to DV
. Using this scheme of synchronization a nodecan go into sleep mode
without loosing synchronization for Nnumber of cycles. Acceptable
delay D is linked with guard
band factor F as under:
D = M in (T S 1 ......TS n ) × F 100
(4)
For future synchronization, decision of sending DV to endnodes
is based upon the difference of current arrival timeand expected
arrival time of data packet. △T represents thisdifference.
△ T = ExpectedArrivalT ime − CurrentArrivalT ime (5)
DV = 0 if | △T |< D△T if | △T |> D (6)
D. Frame Formate
Proposed AR-MAC uses two types of packets: Data Packetsand
Control Packets. In data packet sensor node sends itsperiodic data
in allocated time slot. For emergency data, nodeuses CAP. Control
Packets are:
1) Channel Packet: After channel selection central
nodeadvertises channel information and its unique addressin Channel
Packet.
2) Time Slot Request (RSR) Packet: Sensor node sendsinformation
to Central Node for Guaranteed Time Slot Assignment in Time Slot
Request packet.
3) Time Slot Request Reply (TSRR) Packet: Central nodesends
Guaranteed Time Slot information with CAP in- formation to node in
Time Slot Request Reply packet.
4) Synchronization-Acknowledgment (SYNC-ACK)Packet: For
synchronization, Central node sends therequired Drift Value to end
node with ACK of previouslyreceived data packet in Synchronization
Packet.
5) Data Request (DR) Packet: For on demand traf- c/information,
Central Node sends Data Request Packet to end node.
6) Acknowledgment (ACK) Packet Each data packet isacknowledged
using Acknowledgment Packet
Preamble Sync Frame Len MPDU
Control Address Other Payload (Variable ) CRC
Packet y!e AC" OD
Fig. 2. MAC Layer Frame Formate
The MAC Protocol Data Unit (MPDU) stars with 3 octetsof
overhead. First octet carries information about packet
type.Preceding two octets carry address information. In case of
emergency trafc sensor node waits for CAP. Upon successfulClear
Channel Assessment (CCA), sensor node starts commu-nication with
CN. However, in case of on-demand trafc CNsets On-Demand Trafc
(ODT) eld in ACK or SYNC-ACKpacket to 1. After receiving this
packet sensor node wakes upin CAP to listen data request from CN.
After receiving datarequest from CN, sensor node sends requested
data packetsand waits for ACK packet. Sensor node enters into sleep
modeafter successful reception of ACK.
III. E NERGY C ONSUMPTION A NALYSIS
In order to model energy consumption, we consider theenergy
consumption related to transceiver. In this study, weassume energy
consumption of sensing and processing unitsto be constant. We
assume periodic trafc pattern, i.e., sensornodes send periodic data
to CN in assigned time slots. Most of the time sensor nodes remain
in sleep mode. During allocatedtime slots, they wakeup to send
data. We use the followingequation to measure the energy
consumption for N numberof cycles.
E Total =N
k =1
E Sleep k +N
k =1
E Active k (7)
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Energy consumption is a function of time and currentdrawing from
voltage source for a specic task. When nodesenter into sleep mode
they still consume energy. The sleepmode duration can be calculated
from total time frame lengthand time for which the node is in
active mode.
T Sleep = T Frame − T Active (8)
E Sleep = T Sleep × I Sleep × V (9)
I Sleep is the current drawing from voltage source V duringsleep
mode. In T Active the nodes receive, transmit and waitfor
Acknowledgment. Energy is also consumed in switching,from sleep to
active and active to sleep mode. The energyconsumed for all these
tasks will be considered as energy inT Active .
E Active = 2 × E Sw + E Trans + E Rec + E TOut (10)
Where E Sw is Switching energy, E Trans Transmissionenergy, E
Rec is Receiving energy and E TOut is Time-Out
energy. We describe these terms in details in the
followingsubsections.
A. Switching Energy
Most of the time, sensor nodes remain in sleep mode.Sensor nodes
turn on its transceiver in wakeup mode forcommunication. Switching
energy is the consumed energy forswitching transceiver between
states; sleep mode and wakeupmode. Frequently switching of
transceiver between states leadsto high energy consumption. Energy
consumed for switchingthe transceiver is determined by the
following equation.
E Sw = T Switch × I Switch × V (11)
Where T Switch is the required for the transceiver to
switchbetween sleep and wakup mode and I Switch is the
requiredcurrent.
B. Transmission Energy
Transmission energy is the energy consumed for transmis-sion of
Data or Control packet of length P . Following equationlinks the
transmission energy with length of packet P , timerequired for
transmission of single byte T Byte , current drawduring
transmission I Trans and a voltage source V.
E Trans = P × T Byte × I Trans × V (12)
C. Receiving EnergyReceiving Energy is the consumed energy while
receiving
packets and their associated overhead. Receiving energy
isexpressed as:
E Rec = P × T Byte × I Rec × V (13)
Where I Rec is the Current during reception, T Byte is thetime
for single byte, P is the length of packet and V is thevoltage
source.
D. Time-Out Energy
The energy consumed after transmission and before recep-tion of
an ACK packet is termed as Time-Out energy. Fortime T TOut ,
current I TOut and voltage source V , we usedthe equation given
below to calculate the energy consumptionduring Time-Out.
E T − Out = T TOut × I TOut × V (14)
IV. S IMULATION R ESULTS
We use MATLAB to measure and compare the energy ef-ciency of the
AR-MAC with that of IEEE 802.15.4. In energyconsumption comparison,
we consider the energy consumptionof RF transceiver. We use the
energy consumption model fromCrossbow MICAz data sheet as shown in
Table II. Packets aredropped randomly with average Packet Error
Rate probabilityfrom 1% to 20%. Time frame size used in simulations
isT Frame = 1 Second. We used packets format as shown inFig.4.
Simulation has been carried out for 10 Sensor Nodes.
We used Eq. 7 to calculate the energy consumption for N Table.1.
Simulation Parameters Valu
Parameter Value
Time frame( T F rame ) 1 SecondVoltage Source 3 voltsCurrent
Draw in Receive Mode 19.7 mACurrent Draw in Transmit Mode 17.4
mACurrent Draw in Idle Mode 20.0 mACurrent Draw in Sleep Mode 1
micro-ANumber of Sensor Nodes 10Number of Cycles N 1000
2 4 6 8 10 12 14 16 18 209.25
9.3
9.35
9.4
9.45
9.5
9.55
9.6
Packet Error Rate[%]
E n e r g y
C o n s u m p
t i o n
[ J o u
l e ]
802.15.4
AR−MAC
Fig. 3. Energy consumption of AR-MAC and IEEE 802.15.4 for N =
1000
= 1000. Figure 5 shows the energy comparison of the AR-MAC with
IEEE 802.15.4. The graph in Figure 5 shows thatenergy consumption
of IEEE 802.15.4 increases with increasein probability of Packet
Error Rate. This increase in energyconsumption is due to extra
energy requirement of CSMA/CA
operation in IEEE 802.15.4. The energy consumption of AR-MAC
increases with a minor variation due its adaptive timeallocation
and adaptive guard band mechanism. AR-MACassignees guaranteed time
slots to sensor nodes for communi-cation, to overcome the packet
collision and overhearing.
REFERENCES[1] N. F. Timmons et al. , “An adaptive energy efcient
MAC protocol for
the medical body area networks,” IEEE Wireless VITAE , 2009.[2]
H. Li and J. Tan, “Heartbeat-Driven Medium-Access Control for
Body
Sensor Networks,” IEEE TITB , JANUARY, 2010.
