Energy Efficient Medium Access Protocol for Wireless Medical Body Area Sensor Networks-t6W.pdf

IEEE TRANSACTIONS ON BIOMEDICAL CIRCUITS AND SYSTEMS, VOL. 2, NO. 4, DECEMBER 2008

251

Energy Efcient Medium Access Protocol for

Wireless Medical Body Area Sensor Networks

Okundu Omeni , Member, IEEE , Alan Chi Wai Wong, Alison J. Burdett , Senior Member, IEEE , and

Christofer Toumazou , Fellow, IEEE

ABSTRACT— This paper presents a novel energy-efcient MAC

Protocol designed specically for wireless body area sensor

networks (WBASN) focused towards pervasive healthcare appli-

cations. Wireless body area networks consist of wireless sensor

nodes attached to the human body to monitor vital signs such as

body temperature, activity or heart-rate. The network adopts a

master-slave architecture, where the body-worn slave node peri-

odically sends sensor readings to a central master node. Unlike

traditional peer-to-peer wireless sensor networks, the nodes in

this biomedical WBASN are not deployed in an ad hoc fashion.

Joining a network is centrally managed and all communications

are single-hop. To reduce energy consumption, all the sensor nodes

are in standby or sleep mode until the centrally assigned time

slot. Once a node has joined a network, there is no possibility of

collision within a cluster as all communication is initiated by the

central node and is addressed uniquely to a slave node. To avoid

collisions with nearby transmitters, a clear channel assessment

algorithm based on standard listen-before-transmit (LBT) is

used. To handle time slot overlaps, the novel concept of a wakeup

fallback time is introduced. Using single-hop communication

and centrally controlled sleep/wakeup times leads to signicant

energy reductions for this application compared to more “exible”

network MAC protocols such as 802.11 or Zigbee. As duty cycle is

reduced, the overall power consumption approaches the standby

power. The protocol is implemented in hardware as part of the

Sensium™ system-on-chip WBASN ASIC, in a 0.13- m CMOS

process.

INDEX TERMS— Hardware MAC, MAC Protocol, wireless body

area sensor network, wireless sensor networks.

chronic conditions that require ongoing clinical management

[ 1 ] , [ 2 ] . Sensium™ is a trademark of Toumaz Technology Ltd,

UK.

Vital signs monitoring using wireless sensor network tech-

nologies have previously been described, but these systems are

typically bulky and power hungry and rely on MAC protocols

such as Bluetooth and 802.11 which are inefcient for such

WBASN applications [ 3 ] – [ 6 ] . More general Wireless Sensor

Network (WSN) MAC protocols, which have been the focus

of fairly intensive research [ 6 ] , [ 7 ] , [ 9 ] , [ 10 ] , are also not well

suited to these specic biomedical WBASN applications either.

Zigbee/IEEE 802.15.4 [ 6 ] which is designed for similar net-

works does not have sufcient ’network device’ exibility in

non-beacon mode. It also lacks the cross-layer optimization fea-

tures which the proposed protocol brings to this particular area.

This paper describes a novel MAC Protocol designed specif-

ically for wireless body area sensor networks focused on perva-

sive healthcare applications. Like other wireless sensor network

MAC protocols, a primary design goal was low power consump-

tion. This is achieved through a focus on collision avoidance (a

primary source of energy wastage [ 6 ] , [ 7 ] , [ 9 ] , [ 10 ] ), and the use

of centrally controlled time slotting for sensor nodes. The com-

plete hardware MAC also incorporates cross-layer optimization,

performing some ISO/OSI upper layer functions (from session

layer down to PHY) at the hardware MAC layer to reduce the

power overhead of software implementations.

As a result of the network topology adopted in the MAC

protocol, many of the traditional problems that plague wireless

sensor networks have been either eliminated or signicantly re-

duced. Specically, idle listening and over-hearing are not an

issue in this protocol as trafc is managed centrally. Table I

highlights some of the key features of traditional ad hoc wire-

less sensor networks (WSN) and emerging wireless body area

sensor networks (WBASN). In the following sections, the pro-

posed MAC Protocol is presented in more detail, from concep-

tion to design, implementation together with measured results.

the convergence of all media and data services appears to

be gaining wide acceptance. The healthcare sector is becoming

increasingly interested in using this new technology to more ef-

fectively administer healthcare delivery. In particular, wireless

vital signs monitoring is an area of modern healthcare that is

growing very fast. This is due to its potential for slowing down

the unsustainable growth of healthcare spending due to an in-

creasing number of people living for years or even decades with

II. R ELATED W ORK

Manuscript received December 31, 2007; revised March 12, 2008. Current

version published November 19, 2008. This work was supported in part by

Toumaz Technology Ltd, Oxfordshire, U.K. This paper was recommended by

Associate Editor E. MacPherson.

O. Omeni, A. C. W. Wong, and A. J. Burdette are with the Toumaz Tech-

nology Ltd, Oxfordshire OX14 4RZ, U.K. (e-mail: okundu.omeni@toumaz.

com; alan.wong@toumaz.com; Alison.burdett@toumaz.com).

C. Toumazou is with the Toumaz Technology Ltd, Oxfordshire OX14 4RZ,

U.K., and also the Institute of Biomedical Engineering, Imperial College

London, London SW7 2AZ, U.K. (e-mail: c.toumazou@ic.ac.uk).

Color versions of one or more of the gures in this paper are available online

at http://ieeexplore.ieee.org.

Digital Object Identier 10.1109/TBCAS.2008.2003431

A. Review Stage

MAC Protocol design is a very broad research area, and a lot

of recent work has focused on the area of wireless sensor net-

works [ 6 ] , [ 7 ] , [ 9 ] , [ 10 ] . As widely reported [ 6 ] , [ 7 ] , [ 9 ], [ 10 ]

major causes of energy wastage in wireless sensor networks

are collisions, idle listening, overhearing, trafc uctuations and

protocol overhead.

In the more specic area of wireless body area networks,

the rst three sources of wastage can be eliminated by using

AUTHORIZED LICENSED USE LIMITED TO: IMPERIAL COLLEGE LONDON. DOWNLOADED ON JUNE 07,2010 AT 19:36:21 UTC FROM IEEE XPLORE. RESTRICTIONS APPLY.

I. I NTRODUCTION

T HE wireless communications revolution which is leading

252

IEEE TRANSACTIONS ON BIOMEDICAL CIRCUITS AND SYSTEMS, VOL. 2, NO. 4, DECEMBER 2008

TABLE I

C OMPARING T RADITIONAL AND B ODY A REA W IRELESS S ENSOR N ETWORKS

6) Sensor nodes are resource constrained, i.e., they have low

processing power and limited memory.

7) Data from the wireless sensor nodes is forwarded to a cen-

tral master node for processing; this central node is signif-

icantly less resource and power constrained relative to the

wireless sensor nodes.

These listed attributes are the main inuences leading to the

specic MAC Protocol implementation described in this paper.

These attributes also differentiate the particular application from

more generic wireless sensor network protocols, and other pro-

tocols which have been deployed in biomedical applications

such as Bluetooth, IEEE 802.11 and 802.15.4.

a master-slave architecture with time division multiple access

with clear channel assessment (TDMA/CCA) [ 6 ] network ac-

cess scheme. In a recent paper, Lamprinos et al. [ 11 ] proposed a

MAC protocol for Patient Personal Area Networks (essentially a

wireless body area network application) in which a master-slave

architecture is employed, whereby, to avoid idle listening, all the

slaves have to lock onto the Rx slot of the master and go into

standby at the same time. This approach imposes a limitation

on the duty cycles of the slaves on the network. Some would

have low duty cycle because they are serviced rst while others

would have a higher duty cycle since they are serviced later in

the Rx slot.

B. Network Architecture

As a result of the attributes in the previous section, a point

to multi-point (star) network architecture is proposed. In this

architecture, the central node acts as the master while the other

nodes are slaves. The slave nodes are the actual WBASN nodes

which acquire sensor data and transmit to the central node for

processing. Each individual master-slaves network is refereed to

as a cluster. For ease of management, the maximum number of

slaves connected to a master in one cluster is 8 (many more can

be connected, but the time-slotting would have to be managed

outside the protocol). Although it is possible to form complex

networks of a “central master” with other masters, this paper

concentrates on the protocol as it relates to one cluster.

Also in this architecture, the network access is clear channel

assessment [ 3 ] , [ 6 ] and collision avoidance with time division

multiplexing (CCA/TDMA). This network access scheme sig-

nicantly reduces the likelihood of collision and idle listening,

leading to signicant power savings. In addition time-slot al-

location is dynamically controlled by the master, so a slave

time slot could be changed every time it communicates with the

master. This enables the system to better cope with uctuating

trafc.

The penalty is increased complexity of the central node. How-

ever, this is not a major problem because the central node is

expected to have signicantly more power and processing re-

sources. The key idea used in this network architecture is to

move much of the network and protocol complexity away from

the power constrained wireless sensor nodes and into the much

more capable central node.

This network topology is shown in Fig. 1. To accommodate

for intercommunication between clusters, access to an IP net-

work may be used. This way complex network structures can

still be built which extend wide areas.

III. M AC P ROTOCOL D ESIGN

The main goal of the proposed MAC Protocol is to reduce

power consumption from sources like idle listening, overhearing

and collision.

The closest existing MAC Protocol to the one presented is

IEEE 802.15.4 [ 6 ], however it had 3 differences which were not

well suited to this specic application.

1) Data reliability isn’t handled in the MAC layer.

2) Multiple communication modes increase the complexity of

implementation. Hence, this new scheme is easily imple-

mented in hardware.

3) Time-slotting is limited (16 slots in a super frame) and

must all be equally spaced

Before describing the MAC Protocol, assumptions about

wireless body area networks are outlined.

A. Attributes of Wireless Body Area Sensor Networks

In specifying this MAC Protocol, the following attributes can

be inferred about the wireless body area sensor network.

1) All wireless sensor nodes are attached to the body.

2) The data being monitored is of low frequency

3) The network does not need to respond immediately to

changes (can be inferred from 2).

4) Sensors monitor a range of vital signs which are typically

at a low data rate kB e.g., Temperature, pressure or

heart-rate reading. However some higher data rate applica-

tions must also be catered for, such as streaming of elec-

trocardiogram (ECG) signals.

5) The nodes are miniature, battery powered and need to

run ideally for days from very low capacity batteries such

as exible printed battery technologies or miniature coin

cells.

C. Basic Operation

The proposed MAC protocol operations are based on three

main communication processes. The rst is when a wireless

sensor node wants to join a cluster. This is called the Link

establishment process. The second is when a slave and master

wake-up after an assigned sleep period. This is called the

wakeup service process. The last process is an exception

process which occurs when a slave urgently wants to send

information to the cluster master. This is called an Alarm

process. In all three processes, communication can only be

AUTHORIZED LICENSED USE LIMITED TO: IMPERIAL COLLEGE LONDON. DOWNLOADED ON JUNE 07,2010 AT 19:36:21 UTC FROM IEEE XPLORE. RESTRICTIONS APPLY.

OMENI et al. : ENERGY EFFICIENT MEDIUM ACCESS PROTOCOL FOR WIRELESS MEDICAL BODY AREA SENSOR NETWORKS

253

master without waiting for its next wake-up. This alarm con-

dition may be due to an out of bounds measurement (of say

body temperature or blood glucose level) or a “sensor memory

overow” alert. When this mode is enabled, the master contin-

uously sends out a request to all the slave addresses on the net-

work sequentially. The slave listens for its address and commu-

nicates the alarm condition when it receives it. This only hap-

pens when the master is not busy servicing a scheduled wakeup,

and would be terminated when a slave wake-up needs to be ser-

viced. [ Fig. 2(c) ].

All three communication processes described above are illus-

trated in Fig. 2(a) – (c) .

Fig. 1. Proposed MAC Protocol Network topology ( Slave Node,

Master node).

D. Wakeup Fallback Time

The central management of time slotting can be a complex

task for the master especially when complicated by the occur-

rence of sporadic alarm conditions. To ensure that every sensor

slave node maintains a guaranteed time slot [ 6 ] even if another

slave ags an alarm condition, the novel concept of wakeup

fallback time (WFT) is proposed. If a slave wakes up and fails

to communicate with the master (either because it is busy ser-

vicing an alarm, or the channel is temporarily occupied by an

interferer), it goes back to sleep with a sleep time set by the

WFT. During this time it continues to buffer the sensor data.

After the WFT, it wakes up and searches for the master again.

Similarly, if the master is unable to communicate with the slave

at the wakeup time, it also defaults to the WFT. Hence, both

master and slave wakeup at the common WFT and commu-

nicate, restoring the schedule. The WFT is a programmable

parameter and is a fraction of the shortest sleep time on the

network to mitigate continuous time-slot collisions. Also it is

global to the network and originally set by the master during

the link establishment process. This scheme ensures that time

slot overlaps are seamlessly managed and do not degrade the

network in the long run. Also it allows a slave with a long sleep

time more opportunities to communicate its data to the master

without having to wait for the whole sleep-time again.

initiated by the master. In addition only one slave can join the

network at a time as the network is non- ad hoc.

1) Link Establishment: When a master node is rst enabled,

it continuously tries to establish a link with unattached slave

nodes. It does this by rst scanning for a vacant RF channel.

When it nds one, it remains on that channel and starts sending

out a beacon containing a unique address and conguration for

a slave and then listening for a xed time for a response. The

sum of the master’s beacon transmit and listen time is termed

. Alternatively, when a slave node is enabled, it also scans

the available RF channels to nd the master beacon. If a channel

is vacant for , it hops to the next one. If it is occupied, it

listens for xed period for preamble from the master

beacon and if it doesn’t receive it, it moves on again to the next

channel. Once the beacon is received, it responds with an ac-

knowledgement to the master. The master node then assigns a

sleep time and ends the transaction. At the end of the link estab-

lishment process, the slave has a unique address, conguration

information and sleep time [ Fig. 2(a) ]. Subsequent additions to

the network have to be specically initiated (e.g., by software)

on the master. After Link establishment, the RF channel of com-

munication is xed and can only be changed by higher layer in-

tervention.

2) Wakeup Service: After link establishment, both master

and slave sleep timers start to count up to the sleep time. Hence,

they both wake-up at about the same time, the difference in

wake-up times determined by the offsets between both timers

and the length of the sleep time. On wake-up, the master in-

terrogates the slave which alternatively listens. It (master) may

simply request for its (slave’s) sensor data, or request status in-

formation. Whatever the communication, a new sleep time is as-

signed to the slave, setting the next wakeup time-slot [ Fig. 2(b)]

To mitigate long-term time-slot drift between the master and

slaves in a cluster, there is an optional synchronization phase

during every communication when the slave can synchronize

its timer to that of the master. The master’s timers never change.

This dynamic time-slotting does not in any way preclude the use

of the protocol in a xed time-slotting application. It just offers

this added functionality which may be used if required.

3) Alarm: If the slave detects an “alarm condition” while per-

forming some local processing, it may communicate with the

E. Cross Layer Functionality

When a data packet transmission fails, the MAC automati-

cally retries a programmable number of times before dropping

the packet. In addition large packets can be automatically broken

in to smaller frames and transmitted one at a time. The protocol

also provides for the receiver to reassemble the fragmented data

packets as they are received. One additional function provided

is the control of the frequency and rate of sensor data acquisi-

tion depending on the application.

These functions are usually handled by higher layers in the

ISO/OSI protocol stack. In this protocol, hardware implemen-

tation directly at the MAC layer is preferred as signicant power

savings over software implementations is achieved. This is be-

cause the processor would normally need to run continuously

(signicantly increasing standby power) to perform these func-

tions like determining when to take the next sensor reading, how

many should be taken and when to switch to another sensor.

Also the delay involved in communicating through the protocol

stack layers is eliminated [ 12 ].

AUTHORIZED LICENSED USE LIMITED TO: IMPERIAL COLLEGE LONDON. DOWNLOADED ON JUNE 07,2010 AT 19:36:21 UTC FROM IEEE XPLORE. RESTRICTIONS APPLY.

254

IEEE TRANSACTIONS ON BIOMEDICAL CIRCUITS AND SYSTEMS, VOL. 2, NO. 4, DECEMBER 2008

Fig. 2. Stages in the 3 processes. (a) Link establishment. (b) Wakeup servicing. (c) Alarm processing.

IV. M AC P ROTOCOL I MPLEMENTATION

A. Implementation Platform—Sensium™

Following detailed system modeling, the MAC Protocol was

implemented as a key part of a custom system-on-chip (SoC)

ASIC for biomedical WBASN applications. This mixed-signal

SoC, known as Sensium™, integrates a half-duplex transceiver,

programmable sensor interface circuitry and a digital block con-

taining the hardware MAC plus a low power 8051 microcon-

troller integrated with 32 kB of code and 32 kB of data memory.

The data memory is directly accessible via a DMA controller

by both the Sensor Interface ADC (to write sensor readings)

and by the hardware MAC (to read/write sensor readings for

direct transmission/reception). Having direct access to system

memory allows the slave devices to operate entirely without pro-

cessor intervention. The processor can therefore be switched to a

low clock frequency and used to service irregular events like link

errors. On the master, processor intervention is also minimal,

Fig. 3. Sensium™ System on chip block diagram.

and so it is freed up to handle higher layer functions or transfer-

ring acquired data to a PC for further processing. Which blocks

are active in a given mode is controlled by the power man-

agement unit. The Sensium™ system block diagram is shown

below in Fig. 3 .

AUTHORIZED LICENSED USE LIMITED TO: IMPERIAL COLLEGE LONDON. DOWNLOADED ON JUNE 07,2010 AT 19:36:21 UTC FROM IEEE XPLORE. RESTRICTIONS APPLY.

OMENI et al. : ENERGY EFFICIENT MEDIUM ACCESS PROTOCOL FOR WIRELESS MEDICAL BODY AREA SENSOR NETWORKS

255

TABLE II

T YPICAL A PPLICATION R EQUIREMENTS

From the above analysis, it has been shown that the duty

cycle in continuous monitoring applications like ECG is af-

fected mainly by the communication symbol rate.

Table II illustrates this using typical numbers for 3 important

applications. For spot measurement applications, we can reduce

duty cycle by increasing the sleep time because more payload

data means that the overhead time becomes less signicant and

(3) approaches (4). This is however not the case for continuous

monitoring applications like ECG as the amount of sensor data

must increase with sleep time. For applications like this, the

sleep time is usually limited by the system memory resources

available for storing the sensor data. Fig. 4 shows the graphs of

obtained by plotting (1) and (2) for a temperature sensing ap-

plication. The payload size was kept xed; while the sleep time

was changed (which means that sampling interval was spread

evenly over the chosen sleep time which is acceptable since the

data is signicantly more than required as shown in Table II) . It

can be concluded from the plot that the power is dependent on

the sleep time (4a) as well as the number of retransmissions (4b).

Fig. 4(b) also shows that the power consumption approaches the

standby power as sleep time increases

The physical layer for the radio operates in the 870/900 MHz

SRD/ISM bands, employing FSK modulation with 50 KHz de-

viation to give an over air bit rate of 50 kbps [ 13 ] . The sensor in-

terface block features sensor driving and interface circuitry for a

range of biomedical sensors, and includes a 10-bit, 50–500 Hz

sampling rate DSM-ADC [ 14 ]. For error control, an (11, 15)

hamming code is implemented in the MAC hardware together

with CRC frame checking. This provides 2 levels of error cor-

rection and detection.

B. Mac Complexity

The entire Hardware MAC protocol, including the error

control and framing block, was under 12 K-gates for the slave

and K-gates for the master. The gate count of the hard-

ware implementations points to the simplicity of the protocol.

Since no hardware implementations of 802.15.4 were found,

we compared the software implementations of both protocols.

The proposed protocol can be implemented in around 16 kB of

code (including application code) while 15.4 would require at

least 32 kB. The power consumption for this implementation

is around 500 W while it is 15.4 is 10 mW [ 15 ] , [ 16 ] . This

is because of the difference in clock frequency required to run

both protocols. This protocol can be run on an

(1)

The general equation for average power is

C. System Power and Duty Cycle Analysis

(2)

The average power consumption is dependent on the duty

cycle of operation. So even though a sensor node has a very

long sleep time, but also has a long active time, the duty cycle

would be high and hence average power. This can be computed

for spot measurement applications like for temperature and glu-

cose and also for continuous monitoring applications like ECG.

Table II gives compares common applications.

A detailed analysis of the relationships between the parame-

ters that affect duty cycle and average power computation fol-

lows.

Expading the DC equation further, we have

(3)

Fir spot measurement applications, is signicant because

of the small data payload and hence cannot be ignored.

AUTHORIZED LICENSED USE LIMITED TO: IMPERIAL COLLEGE LONDON. DOWNLOADED ON JUNE 07,2010 AT 19:36:21 UTC FROM IEEE XPLORE. RESTRICTIONS APPLY.

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: