Zigbee Body Sensor

8/3/2019 Zigbee Body Sensor

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Abstract

Recent advances in Wireless Sensor Network (WSN) have facilitated the realization of pervasive healthmonitoring for both homecare and hospitalenvironments. I 802.15.4/ZigBee sensor networksupport small power consumption and nodeexpansion compared to other network stndards forWSN. Body sensor networks (BSN) require a largenumber of sensors for sensing the medical

information om human body, and low powerconsumption for monitoring a patient's status forlong time. nfortunately, ZigBee has limited bandwidth and hard to support real time data transmission because of the adoption of CSMA-CAas its medium access control (MAC) protocol.Depending on the varying trac loads, there aredierent back-o times for packet transmission. Thisaects the data loss rate and latency for data packetsgenerated by a network node. Proper conguration isimportant in the successl operation of a ZigBeenetwork. This paper analyzes the eect of dierent back-o parameters on the performance of

beaconless operation of ZegBee MAC protocol.

1. Introduction

Continuous real time health monitoring based on body sensor networks (BSNs) has a great potentialfor the care of patients. They consist of severaldistributed network devices containing sensor unitapplied to collect and process data and communicatewith other device using a radio equency channel[Ilyas and Mahgoub, 2005]. I 802.5.4/ZigBeeis a standard for low rate, and low power wirelesspersonal area networks in which the contention based

and schedule based MAC schemes are applied as itsMA stadar I, 20] t is base o carriersese multiple access with collision avoidance(CSMA/CA). A ZigBee node competes with all othernodes in its network range for access to the channelfor transmission. Thus the network performancedepends on their data packet rate nd number ofnetwork nodes. The chanel utilization issignicantly aected by back-o time and packetcollision. Successful channel access probability is animportant factor for reliable data trnsmission andecient packet latency. If a node cannot access thechannel aer several back-off attempts, it wastes

transmission time and loses the data packet.

98-1-444-88-0/10/$00 010 EEE

This work analyzes the eects of back-o parametersand dierent network components (number ofnetwork devices, and size of data payload) on the performance of un-slotted CSMA/CA operation of aZigBee MAC protocol. The paper adopts the notationin Table 1. We use I 802.15.4 and ZigBeealteatively in this paper. Section 2 provides anoverview of a ZigBee MAC protocol. Section 3 performs the nalysis of un-slotted CSMA/CAoperations. Analytic results for total back-o time

and probability of accessing the channel aredescribed. Section 4 concludes the paper.

Table 1. Notation and symbols

The number of nodes in WPANb e e o ac-o

e eosBr macM BE 3-8B macinBE 0- 8NB Te ube o back-os wit

ital vaue of zeo

Contetion Winow lengh

B Back-oexponets macMCSMA Back{ 0-5R Average number of bac-oCl aacaacy 250kbps

Most eecve aacaacy macFrameRetres 0-7

TU aUntBackflerod 20,TACK The asmission tie lengt o

CK ame us a te-frae 12sacg eioTC The pero of clear channel 8I

assessments

TAA Te mea asisson eo odata rame.TR T uaoud (RX to T

T R Tuaou tie-(TX toRX 12T,T Sbol time 6 Js

T L1FS tie 40,T SIFS tie ,T,/ acket tasission elay

WJ/n' Daasamli ieTA Aveae back-o esTA Su o TA o eac eosT Total backoffperiod ieP Probabiiy of access the channelPc Pobabiiy o cael beig le Pobabt o acke asittig


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2. General IEEE 802.15.4 MAC protocol

In the CSMACA algorithm in Fig. 2.1, a random back-o period easily causes an unnecessary wasteof bandwidth and increases power consumption.

Therefore, ZigBee suggests that the initial BE value(=macMinBE) is set to 3. However, if the networkload is heavy, this approach leads to high collision rate, high power consumption and low network toughput. Non-beacon enabled Zigbee networksystems use an un-slotted CSMA-CA chael accessmechanism. Our proposed system uses it for sound transmission of human heart beat monitoring. InCSMA-CA, each time a device needs to transmit data,it waits for a rndom number of unit back-o periodsin the range {, 2

} before performing a CCA(Clear Chanel Assessment) step, whereBE can havea value between

and =macMBE. By default,

they are 3 and 5, respectively.

Initially, the back-o exponent (BE) is set to 03.

One symbol period is equal to 16 s at 24 GHzI 802.154ZigBee standards. The CCA time

period (TCCA) is dened as 8 symbol periods andaUnitBackoeriod (T) is dened as 20 symbol

periods. Note that a back-off period is the time required to transmit 20symbols, where a symbol iseuvalent to 4 bts, n a 25 Kbs channel. sng the default value and assuming that the channel isfound to be idle by the rst channel access attempt,an idle channel access time can be calculated as:

+ 7 + 8 7 3 J + 8J 368 (1)

Note that Intialbackoeriods is dened by the product of a random number om [0, (2 1)] andTR In (1), the random number is selected as 7 toderive the maximum time delay.

Aer the CSMA wait is over, the node determines if the channel is idle. This CCA is performed over the time duration of 8symbols. If the channel is busy( fails), the node incrementsBE value up to a

pre-ened and repeats the MA proceure andCCA to transmit data packets. If the availablechannel cannot be found ( fails) even aerpredened macMSBacko reattempts, aCAF (channel access failure) is declared and rtherattempt is not processed to transmit data packets.

ZigBee provides a variable "macMSBacko that regulates the number of transmission

trials. It sets macMBacko to 4, i.e., a transmitter is allowed to access the chael 4 times

consecutively before it declares access faile anddrops the packet.

If CCA succeeds, the node changes the mode omtransmitto receive (TX-to-R turnaroun to obtain

the ACK packet om a coordinator. Also, aer thelatter receives the data packet om a node, it changes the mode om receive to transmit (R-to-TX

turnarounto send the ACK packet to the former.

Typically, ZigBee uses the haduplex system. Inother words, it cannot perform both transmit (Tand receive ( operations at the same time. TheR-to-TX and TX-to-R turnaround time is denedas 12 symbols.

Fig. 2.1 IEEE 802.15.4 CSMA-CA protoco ow

In the CSMA-CA algorithm, each node shallmaintain tee parameters for each transmissionattempt i.e., NB, W, and BE. NB is the Number ofBack-os with initial value of zero. The algoritm is

required to back-o before attempting the currenttransmission. NB value should be initialized to 0 before each new transmission attempt. W is theContention Window length, dening the number ofback-o slots that need to be clear of channel activitybefore the transmission can commence. This value is

initialized to 2 before each transmission attempt andeset t when the channe s assesse t be busyBis the Back-o xponent, which is the variable thatdetermines the number of back-off slots a deviceshall wait before attempting to assess a channelsstatus. It is chosen randomly in the range of 0 to (2

R 1). For a non-beacon mode, un-slotted CSMA-CAis used. Thus MAC sub-layer initializes NB and BEwithout w

The next step is to decide rndom waiting delay forcollision avoidance. It is the product of the back-operiod and a random number om [0, 2

1]. IfBE

set to 0, the collision avoidance procedure is disabledat the rst iteration, and the node performs the CCA

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directly without waiting. BE is incremented eachattempt for the channel is sensed busy. If more nodesjoin the same network area, the trac becomes heavier. Then BE must increase to reduce datacollision in CSMA-CA operations. It caot exceed a

predetermined value, which can be reached by thecompeting nodes at most aer 5 = 5, defaultvalue) transmissions of other nodes. If is set as 5by default, the back-o period is a random number in[0, 31] multiplied by aUnitBackoeriod in all

remaining nodes waiting to access the chanel.

In this process, is more critical than for the backo delay distribution. Its impact can changewith dierent network enviroments as chaacterized by trac, interference, size, and data payloads. If achannel is available in un-slotted CSMA-CA, MACsub-layer begins transmitting a packet ame. If

is

set as a smaller value, e.g., 4, the average back-odelay can decrease and MAC processing time is so.Consequently, power consumption can be reduced

The maximum effective data capacity, denoted as, is dened as the maximum achievable data ratefor a user application in the absence of any kind ofcross trac or interference by other systems withdierent communication standards and those usingsame equency range (2.4Hz). We can calculateand test the effective data capacity, denoted as ,under several conditions. Short addresses are used to reduce the size of a packet. Optional acknowledge

ames ( are enabled and the back-o exponentBE is set to "0. At 2.4Hz PHY layer, thetransmission duration of 1 byte = 2symbols =32

can be calculated for a single hop connectionbetween two devices, under the ideal conditions. For the MAC layer to process the data received om thePHY layer, each data packet is followed by an interame spacing (). Depending on the size of the

U (MAC protocol data unit), L (a long )and (a short ) can be used for ame spacing.If it is larger than 20 bytes, L is used. Otherwise, is selected. An IFS takes 640 s (40symbols)and an SIFS 192 s (12 symbols). From Fig. 2.2, thea bew aa ame ad i oreondiacknowledgement ( is same as the Rx-to-Txtuaround time calculated before, i.e., 192 s.

From ZigBee, ACK is an optional ame for transmission. Thus we can have dierent results. If NACK (No ACK) is applied, the receiver does notneed to send ACK packet to the transceiver forconrmation. Therefore, the receiver skips the tuaround (Rx to TX) te (12 symbols), andtransceiver does not wait for ACK (22 symbols).

Packet maximum size is 133 bytes long, which is the peak size allowed and includes 6 byte overhead. Its

packet transmission takes the channel time of 340symbols (266 symbols packet transmission + 12symbols for taround time (Rx-Tx) + 22 symbolsfor ACK transmission + 40 symbols for TUFS . Henceit can take at most 183.8 (62500/340) packets per

second with a 2.4 Hz channel of 250 Kbps (62500symbols/sec) capacity.

To calculate for a single-hop connection network, the size of an U is set to 127 bytes (itsmaximum size). We can set an U's size as 113bytes (U = 100 bytes) for heartbeat sound. TheK ame size is 11 bytes. For this scenario, there isno back-o delay andBE=O.

bytes (Max) bytesLong packet frame (PPOU Long aket frae PPDU

rrllll]sI

UF s !-Duralion = 5A4ms- Fig. 2.2 Duration for one data ame by LIFS with ACK

As shown in Fig 2.2, the total time between two longpacket ames otalis given by

otal =Tlongrame+Ttadtime +Tackjrame + = ms(2)

where

g = 33 32!s = .26msTack!re=IIx32!s=0.352ms,

n 0 =O.6ms

bytes (fOl' HB. Sound aa)Lg packe rame PPDU Log pacet rame PPDU.---1

T4 Draio OIB. Soud a 44

Fig.2.3 Duration for one data ame without ACK.

As shown in Fig.2.3, the total time between two longacket ames with NACK Ttotal for heart beat sounddata with NK is given by

(3)By applying NK to access the heart beat sounddata, we can reduce the duration time of one data

packet transmission.

According to [Sun, et a!., 2006]

C appliatiodata C 67 65 (4) X HY s

otal

where Tappcatiodata = 114 32s =3648 s, the time it takes to send the application data via the layer,


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3

and PY = 250b

The theoretical maximum hear beat sound datathroughput for single-hop transmission is given by

T kCHB =- x CPHY =1 82.48 's (5)Iota'where THB= 100 32s = 3200 s.

Therefore, the maximum eective data capacityavailable for a user is only 67% of the PHY data rate(250 kbps) via theoretical analysis. For heart beatsound data, it is only 72% of the PHY data rate.

In the real test using CC2430 sensor modules forcontinuous sound data (100 byte payload), the packetperiod time is 12 ms (with ACK) and 9 ms (withoutACK) for reliable transmission of sound data. This is

because that the modules have a low-performance processing unit, i.e., 8051 MC. Therefore, theachievable maximum toughput for continuoussound data is only 79 Kbps or 84 packet/sec withACK, and 105 Kbps or 112 packet/sec without ACK.

Analysis of un-slotted CSMA/CA

m m For the uplink process between a node andcoordinator, we consider its packet transmissiondelay Tpd It includes the back-o period, packet

transmission time, coordinator's taround timeswitching om transmitting to receiving, ACK

transmission time, and IFS time withSIFS= 12 andIFS= 40symbols.

The average back-o time of each transmnattempt consists of several back-o periods. Thenumber of back-off attempts is limited up to thepredened macMBacko, and depends on the network trafc. Clear Chanel Assessment timeTCCA = 8 symbols transmission time. TA is transceiver's transmitting to receiving tuaroundtime (12symbols).

.-Tc - - . Tp: T1, TACK TI"r:C..d pack.Frm. ' ( ' .(1- PcjaI)

1 (9)The probability P means the one that a node cansuccesslly access the channel. In (9), b is thenumber of back -o attempt periods.

, . _(-) (1

The probability of chanel being idle P in a clearCCA period can be calculated as

= l_yThe transmitting probability (q isTpacketq =-_!._-Tpackt ,amplingwhere mplng is data sampling time, i.e.,

Lpy l o>$

(11)

..... (12)

samlin"g ..... (13)In our BSN, we apply 4 KHz as the sampling rateand a sample has one byte data.

TAj is the sum of the average back-o times. It mayconsist of several back-o periods and depends onboth parameters of a node and trac load.

Because each back-o attempt delay period iscalculated om a random number between and 2B

1) multiplied by unitbackoperiods, we use theaverage back-off time in each range as

0


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TAB = [2BE .i1xunitbackofperiods

(14)Also, thejth back-o time for the number of chaelaccess attempts can be calculated as

[iRE+J_l TA} 2-B+}-1 i unitbackoeriods

..... (15)Based on the above analysis, we can obtain theaverage back-o attempts depending on the dierent

payload size and the number of network devices asshown in Fig. 3.2.

...==: 99J.

'

t"'................__..n5 :\ n . . . . . . . . . . . 1

n=10

n=5

n=1

n nbe of neok ece

yo ye

Fig.3.2 Impact ofLpayloa and on the average nmber ofback-off attempts

Fig. 3.2 shows that the average number of back-offattempts is smaller for a longer payload ame than ashorter one. The number of average back-oattempts increases with n. The advantage of smallaverage back-o attempts is to reduce the

transmission delay by transmitting a long data ameinstead of separated small data ames.

If just one node is communicating with thecoordinator, it does not need to compete the channelaccess and is not affected by its payload size. Thusaverage number of back-off attempts is 1. As n

increases, they have to compete for the channelacces with eac oter For ts reason the averagenumber of back-o attempts approachesmacMSBacko. Because a long data ameoccupies the periods on the channel longer than asmall one during its transmission, other deviceswaiting for chanel access have more back-offattempts oen than the case with the transmission ofa short data ame. By increasing back-o attempts,

back-o exponent increases for each such attempt.This leads to longer back-o delays. From Fig. 3.2,we can reason that the payload size once over 40 bytes aects the average back-o attempts only

slightly. A large n leads to the maximum back-oattempts regardless of payload.

E E m

The back-o exponent BE is a critical parameter inthe back-o algorithm of CSMA-CA. It is used as an

estimate of the random back-o delay before trying to access the chael. As described before, in MACoperation, the CSMA channel access wait timedepends on BE. For every transmission attempt, BEis initialized to be and each CCA failure increasesBE by 1 until it reaches Therefore, the channelaccess wait duration depends on how many CCAfailures have already been processed prior to thecurrent attempt. It is related to the number ofback-o period times and TCCA'

The back-o delay time is determined by a random number om 0 (2

1). Thus, we use the mean value of backo delay (14, 15) for in the following discussions.

Total back-o period time is

= [I ] V HI) =)= (2(BMI) x x .32ms12B'H -

J

1 x [((I)I] x x .32ms

(16)where average 's integer part = and action part =v (If =3.5, then =3 and v = 0.5).

Total back-o time for chanel access and d a t a transmission is sum of all back-o attempt timesThus, is aected by and BE To simpli ourcalculation, we set macMBacko as 5. Table2 shows BE for each back-off attempt as dierent (08) and (38). If average back-off time in each

range is applied and is 5, the shortest isachieved as (0,1,2,3,3) for each back-o attempt BEwhen

= 0 and

=3. increases with

and

Aer BE reaches each back-o attempt keepshe s . Th raon i tha CSMA-CA compE and for each back-o attempt. If they are equa, aer several CCA failures, E has to keep thesame value as for any remaining back-o attempt.Fig. 3.3 shows the total back-off time periods fordifferent parameters. Fig. 3.3(a) shows the impact ofincreasing om 0 to 8 on as payload increases. Note that n=5, macMBacko=5, and E8.Decreasing BE can reduce . Increasing payloadsize can reduce it as well. However, aer certain size(515 bytes), reducing T with increasing payloadsize becomes less signicant.


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Tabe 2 erentE value or eac baco attet eens on aE (weemaMaxCSMABakof 5)lIacMinBE aMaxBE-3 aMaxBE-4 aMaxBE-5 aMaxBE-6 aMaxBE-7 aMaxBE-8 .2"d,3,/'.5 rl21(,3,'51 r2m,3,I51 r 3Y,/'51 S/ 2( 3-5" ' ,)I( 3'Yl1

0 0,1,2,3,3 0,1,2 3,4

,2,3,3,3 ,2,3,4,4

2 2,3,3,3,3 2,3,4,4,43 3,3,3,3,3 3,,4,4,4

4 4,3,3,3,3 4,,4,4,4

5 5,3,3,3,3 5,,4,4,4

6 6,3,3,3,3 6,,4,4,4

7 7,3,3,3,3 7,,4,4,4

8 8,3,3,3,3 8,,4,4,4

- - MinBO cuIc - - :uuI __ MinBE

!:--"----: 7 9 yd (y)

(a)

40 \ macfuxCSAI Jlucko!=5J

' ula '-aMaxS=E=-:!e aMaxBE


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importat. For example, emergency data have to be transmitted reliably without delay in any kinds ofsituations including heavy network trac. This posesa signicant challenge to researchers and engineers.It will require more research. The ture work also

includes setting up a testbed to veri our analysisresults.

12n 10 08 0E 0 02 00

09o 08 07 0 0 0

--=5, =5A _ , 5-- '=," =5-- 5," 5-- '= ," =5

bkoff ttempta)

03 02 01

00-036

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Zigbee Body Sensor