WITH rapid improvements of wireless technologies,

Transcription

1 JOURNAL OF SYSTEMS ARCHITECTURE, SPECIAL ISSUE: HIGHLY-RELIABLE CPS, VOL. 00, NO. 0, MONTH Adaptve GTS Allocaton n IEEE for Real-Tme Wreless Sensor Networks Feng Xa, Ruonan Hao, Je L, Naxue Xong, Laurence T. Yang, and Yan Zhang Abstract The IEEE standard s able to acheve lowpower transmssons n low-rate and short-dstance Wreless Personal Area Networks (WPANs). It supports a Guaranteed Tme Slots (GTSs) allocaton mechansm for tme-crtcal and delay-senstve data transmssons. However, the nflexble Frst- Come-Frst-Served (FCFS) GTS allocaton polcy and passve deallocaton mechansm sgnfcantly mpar network effcency. Ths paper proposes an Adaptve and Real-Tme GTS Allocaton Scheme (ART-GAS) to provde dfferentated servces for devces wth dfferent prortes, whch guarantees data transmssons for tme-senstve and hgh-traffc devces. The bandwdth utlzaton n IEEE based PAN s thus mproved. The proposed scheme s developed based on the IEEE Medum Access Control (MAC) protocol and s fully compatble wth the mplementaton of IEEE devces. It s applcable to varous real-tme systems bult upon wreless sensor networks. Smulaton results demonstrate that our ART-GAS algorthm sgnfcantly outperforms the exstng GTS mechansm specfed n IEEE n terms of success probablty, average delay, average watng tme, and CFP bandwdth utlzaton. Index Terms IEEE , Wreless Sensor Networks, Real Tme Systems, Medum Access Control, Guaranteed Tme Slot. I. INTRODUCTION WITH rapd mprovements of wreless technologes, Wreless Sensor Networks (WSNs) are attractng growng attenton from both academa and ndustry. The nterest s manly drven by the large amount of WSN applcatons, ncludng envronmental montorng [1], ndustral sensng and dagnostcs [], health care and data collecton for battlefeld awareness [3, 4], and most of them are developed by usng low-rate, short-dstance and low-cost wreless technologes. Among the well-known specfcatons, IEEE [5], whch was orgnally desgned for Low-Rate Wreless Personal Area Networks (LR-WPANs), has become one of the most promsng canddates for the next-generaton wreless network technology [6]. The IEEE standard provdes specfcatons for the Physcal Layer (PHY) and the Medum Access Control (MAC) sublayer [5]. Specfcally, t supports two types of channel access mechansms: beacon enabled mode and non-beacon enabled mode. The non-beacon enabled mode ams at provdng far access to all wreless nodes and does not support realtme applcatons. Whle n beacon enabled mode, the PAN Feng Xa, Ruonan Hao and Je L are wth School of Software, Dalan Unversty of Technology, Dalan 11660, Chna. E-mal: f.xa@eee.org. Naxue Xong s wth School of Computer Scence, Colorado Techncal Unversty, USA. Emal: nxong@coloradotech.edu. Laruence Tanruo Yang s wth Department of Computer Scence, St. Francs Xaver Unversty, Canada. E-mal: ltyang@stfx.ca. Yan Zhang s wth Smula Research Laboratory and Department of Informatcs, Unversty of Oslo, Norway. Emal: yanzhang@eee.org. coordnator generates perodc beacon frames wth a provson of a superframe structure, whch provdes applcatons wth a tme frame property. Further, The beacon enabled mode also makes t possble to obtan real-tme guaranteed servce by allocatng the Guaranteed Tme Slot (GTS) on a Frst- Come-Frst-Served (FCFS) bass, where a GTS s a perod of tme that can be reserved by a sensor node for ts data transmssons. Admttedly, the GTS mechansm s able to support tme-crtcal data transfers generated by repettve lowlatency applcatons and guarantee the relablty and performance of data delveres, and from ths perspectve, the desgn of beacon enabled mode seems perfect n these stuatons. Nevertheless, t stll has many weak ponts [7-10]. Frst, the fxed FCFS GTS schedulng mechansm may not satsfy the tme constrants of real-tme and hgh-traffc applcatons, whch are commonly seen n medcal or manufacturng sensor networks that montor and sgnal emergences. Second, the abuse of dedcated resources mght result n the excluson of other transmssons due to the schedulng nflexblty n low-latency data delvery, correspondng to network workload and applcaton needs. Thrd, starvaton mght be caused for devces wth low data transmsson frequences snce a fxed tmer s mantaned n IEEE for GTS deallocaton. Hence, to resolve the above ssues, t s crtcal to desgn a new GTS allocaton scheme that can adaptvely schedule GTSs to needy devces n an effcent and tmely manner. Ths paper manly focuses on the GTS allocaton mechansm n IEEE and proposes an Adaptve and Real- Tme GTS allocaton Scheme (called ART-GAS) to support tme-crtcal and delay-senstve wreless applcatons. The proposed scheme s developed based on the standard of the IEEE MAC protocol and completely follows the specfcatons defned n [5] wthout ntroducng any extra protocol overhead. A prelmnary verson of our ART-GTS was presented n [8]. Ths paper extends our prevous work wth substantally new contents ncludng (1) applyng the mechansm n a real scenaro of health care applcaton, () detaled descrpton of (mproved) mechansms and algorthms. Besdes, extensve numercal results and analyss are also presented here under the stuatons of dverse number of nodes. Major research contrbutons of ths paper are as follows. (1) It provdes dfferentated servces for devces n dfferent prorty levels, where both data-based prorty and rate-based prorty are consdered n GTS allocaton. Thus, data transmssons of tme-crtcal and hgh-traffc devces are better guaranteed. () An adaptve GTS schedulng mechansm s further proposed by utlzng a dynamc threshold prorty. Ths s based on both network load and applcaton needs (that s, prortes),

2 JOURNAL OF SYSTEMS ARCHITECTURE, SPECIAL ISSUE: HIGHLY-RELIABLE CPS, VOL. 00, NO. 0, MONTH 013 whch provdes a sgnfcant mprovement n terms of average delay and farness performance. Bandwdth utlzaton s thus mproved. (3) We also presents a careful evaluaton of ART- GAS performance n ths paper and further analyss s gven by comparng t wth the exstng IEEE MAC through smulatons. These evaluatons examne the protocols from an mplementaton perspectve and take nto account envronmental detals lke number of sensor nodes and traffc load. The remander of ths paper s organzed as follows. Secton II presents the related work, whch s followed by a bref overvew of the IEEE MAC protocol n Secton III. In Secton IV, we present basc prncples and mechansms of the ART-GAS algorthm, ncludng servce dfferentaton and GTS allocaton. The desgn and mplementaton ssues are descrbed n Secton V. The performance evaluaton, ncludng expermental results, s gven n Secton VI. Fnally, Secton VII concludes the paper. II. RELATED WORK The performance of the IEEE protocol has been subject of many research studes recently. These studes nclude the performance analyss of the Carrer Sense Multple Access wth Collson Avodance (CSMA/CA) protocol [9-11] n Contenton Access Perod (CAP), as well as the GTS mechansm [1-14] operatng n the Contenton Free Perod (CFP). Specfcally, some nterestng algorthms have been proposed to mprove the performance of IEEE MAC [15-18]. For example, energy s a valuable resource n sensor networks, and thus, tremendous research work s concernng MAC protocol optmzaton to conserve energy, throughput, and other smlar performance metrcs n the process [19]. In [15], Valero et al. propose an ncrementally deployable energy effcent MAC protocol based on IEEE to mnmze energy consumpton. L et al. [16] present a brand new approach by utlzng a Synchronous Low Power Lstenng technque to acheve lower power consumpton for low data rate applcatons. In [17], a channel feedback-based enhancement to the IEEE MAC s proposed that s sgnfcantly more scalable, showng a relatvely flat, slowchangng total system throughput and energy consumpton. Kojma and Harada [18] adopt a low-power mult-hop dataframe transmsson scheme to reduce power wastage due to perodc transmsson and a dynamc re-assocaton procedure for optmzng the allocated actve perod s used for low power consumpton over the entre system. These protocols are able to render sgnfcant mprovements n terms of energy effcency compared wth the standard IEEE MAC. However, they are typcally not sutable when applcatons demand better performance at the expense of some addtonal energy, especally n lfe-crtcal or safety-crtcal areas. In these cases, the ssue of Qualty of Servce (QoS) provsonng should be gven more attenton. Nowadays, wth the ever-ncreasng usage of IEEE networks n health care systems, communty medcal servces, alarm systems, etc., where QoS s to be emphaszed for emergency messages wth energy consumpton only a secondary ssue, some researchers begn to seek solutons to meet requrements of these applcatons. For nstance, Kobayash and Sugura [0] propose a Tmng Group Dvson method to acheve faster communcatons by optmzng the tradtonal CSMA/CA mechansm. Khan [1] adopts an Improved Bnary Exponental Backoff algorthm to avod collson and decrease latency, whch exhbts great QoS mprovement n IEEE In [], a backoff control mechansm s gven to mprove the delay performance as well as keep throughput enhancement n cluster-based WSNs. However, these solutons only take nto account the contenton access perod, whle gnorng the GTS schedulng n contenton free perod, whch may have much more sgnfcant mpact on QoS. In [3], Yoo et al. presents a real-tme message schedulng mechansm to schedule a large number of real-tme messages to meet ther tmng constrants. Chen et al. [4] propose an Explct Guaranteed tme slot Sharng and Allocaton scheme (EGSA) for beacon-enabled IEEE networks, whch s capable of provdng tghter delay bounds for real-tme communcatons. Addtonally, the authors of [5], [6], and [7] also provde effectve approaches for meetng delay constrants of tmesenstve applcatons. Whle these GTS schedulng solutons are able to guarantee real-tme data transmssons n IEEE networks, they have not devoted enough attenton to the ssue of bandwdth utlzaton, thus leadng to unnecessary wastage of GTS resources. Le et al. [8] propose an unbalanced GTS allocaton scheme (UGAS), whch dvdes the CFP nto dfferent duraton tme slots for dfferent bandwdth requrements to mnmze the under-utlzaton. In [9], a new GTS scheme s desgned to allow more devces to share bandwdth wthn the same perod. These two methods can mprove the bandwdth resource effcency but real-tme performance s not ensured n the meanwhle. Shrestha et al. [30] present an optmzatonbased GTS allocaton scheme that can mprove relablty and bandwdth utlzaton, as well as support tme-crtcal or delay-senstve data transmssons. However, t cannot adapt to dfferent traffc condtons n network, thus lackng flexblty n GTS allocaton. A delay-senstve GTS allocaton scheme s presented n [31] based on GTS usage feedback to satsfy the delay constrants of devces n dfferent traffc condtons. Nevertheless, prortes of devces are specfed as fxed values n ths approach so that t may not gve satsfactory performance n a dynamc envronment. In contrast to prevous solutons [3], the proposed ART- GAS algorthm n ths paper s based on a two stage approach. Servces are dfferentated n the frst stage through a dynamc prorty assgnment polcy. GTSs are scheduled n the followng stage to needy devces n an effcent manner. The mportant features of ART-GAS are as follows: (1) Both data-based prorty and rate-based prorty are consdered n servce dfferentaton, thus real-tme and hgh-traffc data transmssons are guaranteed; () GTS allocaton s hghly adaptve to network load and applcaton needs and can be easly appled to a dynamc envronment; and meanwhle, (3) GTS resources are more effcently utlzed. The detaled descrpton of ART-GAS wll be presented n Secton IV.

3 JOURNAL OF SYSTEMS ARCHITECTURE, SPECIAL ISSUE: HIGHLY-RELIABLE CPS, VOL. 00, NO. 0, MONTH Fg. 1. Beacon CAP CFP GTS GTS GTS Superframe Duraton (SD) Beacon Interval (BI) IEEE superframe structure Inactve Beacon III. OVERVIEW OF IEEE MAC PROTOCOL IEEE s a standard for low-rate, low-power and low-cost Personal Area Networks (PANs) [5]. It defnes two dfferent channel access methods: beacon enabled mode and non-beacon enabled mode. The non-beacon enabled mode s entrely contenton based. It uses unslotted CSMA/CA to manage access to the channel. There s no provson for servce requrement guarantees n ths mode of operaton. Whereas, the beacon enabled mode provdes a contenton-free GTS mechansm to support tme-senstve data transmssons. Hence, we only focus on the more commonly used beacon enabled mode n ths paper. In beacon enabled mode, beacon frames are perodcally sent by the PAN coordnator to dentfy ts PAN and synchronze nodes that are assocated wth t. The PAN coordnator defnes a superframe structure characterzed by a Beacon Interval (BI) and a Superframe Duraton (SD). Beacon Interval specfes the tme between two consecutve beacons, and ncludes an actve perod and, optonally an nactve perod. The actve perod, also called superframe, s correspondng to Superf rame Duraton and can be dvded nto 16 equally-szed tme slots, durng whch frame transmssons are allowed. Durng the nactve perod (f t exsts), all nodes may enter nto a low-power state to save energy. The superframe structure of beacon enabled mode s depcted n Fg. 1. BI and SD are determned by two parameters, the Beacon Order (BO) and the Superf rame Order (SO) respectvely, whch are broadcasted by the coordnator va a beacon to all nodes. BI and SD are defned as follows: BI = abasesuperframeduraton BO, for 0 BO 14 SD = abasesuperframeduraton SO, for 0 SO BO 14 In Eqs. (1) and (), abasesuperf rameduraton denotes the mnmum duraton of the superframe, correspondng to SO = 0. Ths value also corresponds to ms, assumng 50 kbps n the.4 GHz frequency band, whch wll be consdered throughout the rest of ths paper. (1) () The Superf rame Duraton can be further dvded nto CAP and CFP. The beacon s transmtted by the coordnator at the start of slot 0, and the CAP follows mmedately after the beacon. Durng the CAP, a slotted CSMA/CA algorthm s used for channel access. A node computes ts backoff delay based on a random number of backoff perods, and performs two CCAs (Clear Channel Assessments) before accessng the medum. In addton to non tme-crtcal data frames, MAC commands such as assocaton requests and GTS requests are transmtted n the CAP. In the CFP, whch s for the use of devces requrng dedcated bandwdth, the communcaton occurs n a TDMA (Tme Dvson Multple Access) style by usng a number of GTSs, pre-assgned to the ndvdual sensor nodes. Whenever a devce requres a certan guaranteed bandwdth for transmsson, the devce sends GTS request command usng CSMA/CA durng CAP. Upon recevng the request, the coordnator frst checks the avalablty of GTS slots n the current superframe, based on the remanng length of the CAP and the desred length of the requested GTS. The superframe shall have avalable capacty f the maxmum number of GTSs has not been reached and allocatng a GTS of the desred length would not reduce the length of the CAP to less than amncap Length. Provded there s suffcent capacty n the current superframe, the coordnator determnes, based on a FCFS fashon, a devce lst for GTS allocaton n the next superframe, and nforms the devces about the allocaton of slots n the GTS descrptor n the followng beacon frame. GTS deallocaton can be performed by the coordnator or by the devce tself. For devce ntalzed deallocaton, t sends GTS request wth characterstc type subfeld set to zero usng CSMA/CA durng CAP. From ths pont onward, the GTS to be deallocated shall not be used by the devce, and ts stored characterstcs shall be reset. In ths way, devces can return the GTS resources by explctly requestng that the PAN coordnator provde deallocaton. However, n most cases, the PAN coordnator has to detect the actvtes of the devces occupyng GTSs and determne when the devces stop usng ther GTSs. If the coordnator does not receve data from the devce n the GTS for at least n super frames, the coordnator wll deallocate the GTS wth startng slot subfeld set to zero n the GTS descrptor feld of the beacon frame for that devce, where n = 8 BO for 0 BO 8, and n = 1 for 9 BO 14. IV. ART-GAS: ADAPTIVE GTS ALLOCATION The objectve of ths secton s to descrbe the basc deas and mechansms of our ART-GAS scheme for the IEEE standard usng a star topology. As an example, consder an applcaton of health care wth wreless body sensor networks (see Fg. ). The PAN coordnator collects data from dfferent sensors deployed n the body of the people. These sensor devces can be electrocardogram (ECG), blood pressure, temperature, and accelerometer. Data from these sensors are collected perodcally. However, emergency data may be generated randomly and need to be transmtted mmedately. Furthermore, the GTS resources should be carefully

4 JOURNAL OF SYSTEMS ARCHITECTURE, SPECIAL ISSUE: HIGHLY-RELIABLE CPS, VOL. 00, NO. 0, MONTH LOW P P 40 d d HIGH P P 40 d d Devce PAN Coordnator P P +0 d d P P 0 d d 0 P P d d P P +0 d d IEEE Fg.. Star topology of a PAN MIDDLE allocated to needy devces wth hgher frequences of sendng data and the prevously allocated but unused GTSs should be reclamed n tme to mprove network effcency. To solve these problems, our ART-GAS adopts a prortybased GTS schedulng approach, whch s based on both the servce dfferentaton mechansm and the GTS allocaton mechansm. In the servce dfferentaton mechansm, devces are assgned two dfferent knds of prortes n a dynamc fashon: data-based prorty and rate-based prorty. Devces sendng data of greater mportance or wth real-tme requrements are gven hgher data-based prortes, whle devces wth hgher frequences of sendng data are gven hgher rate-based prortes. In other words, data-based prortes are assgned accordng to data, and rate-based prortes are assgned accordng to rate. In the GTS allocaton mechansm, a comprehensve polcy of utlzng the two prortes s proposed. GTSs are gven to devces n a decreasng order of ther prortes. Further, varous scenaros of dfferent databased prortes and rate-based prortes are dscussed, CAP and CFP traffc loads have also been taken nto consderaton. Addtonally, a starvaton avodance mechansm s presented to regan servce attenton for lower prorty devces. Fnally, we provde a GTS allocaton scheme whch can satsfy the needs of both tme-crtcal and hgh-frequency devces. Detals of the servce dfferentaton mechansm and the GTS allocaton mechansm are presented n the followng sectons. A. Servce Dfferentaton In the servce dfferentaton mechansm, each devce s adaptvely assgned a data-based prorty and a rate-based prorty by the PAN coordnator, accordng to the mportance of data and past transmsson feedback respectvely. Assume that there are N devces n an IEEE based PAN, and that there are N d (0, 1,..., N d 1 ) data-based prorty numbers and N r (0, 1,..., N r 1 ) rate-based prorty numbers dynamcally assgned to the N devces. The data-based prorty number assgned to the devce n s defned as P dn, and the rate-based prorty number assgned to the devce n s defned as P rn, then we have 0 P dn N d 1 and 0 P rn N r 1. In our ART-GAS, a larger prorty number represents a hgher Fg. 3. Set real-tme restrctons or data excepton occurs Data excepton occurs on devces wth real-tme restrctons State transton dagram Devce Real-tme restrcton s canceled or data go back to normal status Real-tme restrcton s canceled and data go back to normal status prorty for GTS allocaton. Devces wth hgher data-based prortes are assumed to send tme-crtcal data, such as alarms and emergency messages, and devces wth hgher ratebased prortes are consdered to have more recent traffc and thus havng hgher probabltes to transmt ther data n the followng superframe. The prorty numbers of a devce are nternally mantaned by the PAN coordnator. 1) Data-based Prorty: In the MAC modfcatons, we set the total number of data-based prortes, N d to 60 and classfy all devces nto three data-based prorty levels accordng to whether there are exceptons or real-tme requrements related to the data. The process s depcted by the state transton dagram n Fg. 3. Each devce has three states: LOW, MIDDLE, and HIGH, correspondng to ts three levels respectvely, and LOW has the data-based prorty numbers from 0 to 19, MIDDLE has the numbers from 0 to 39, and HIGH has the numbers from 40 to 59. States LOW, MIDDLE, and HIGH are defned as follows: LOW: All devces are placed n the LOW state ntally. There are nether transmsson tme restrctons nor exceptons n any of these data n ths state. Ther dfferent data-based prorty numbers are only determned only by ther degree of mportance wthout any other consderatons. For example, data of heart rate wll have a hgher data-based prorty than data of temperature n terms of mportance. MIDDLE: Devces n the MIDDLE state are sendng data that have real-tme requrements or ndcate that certan exceptonal phenomenon related to the data value occurs durng the current superframe. For example, f the data

5 JOURNAL OF SYSTEMS ARCHITECTURE, SPECIAL ISSUE: HIGHLY-RELIABLE CPS, VOL. 00, NO. 0, MONTH Fg. 4. STEP-1 All devces are ntalzed to the LOW state, whch ndcates there s no exceptons or real-tme restrctons. STEP- When a devce detects that an excepton occurs or real-tme restrcton s set, t wll swtch the current state to MIDDLE and ncrease ts data-based prorty by 0. STEP-3 For devces n the LOW state, whenever an excepton occurs, and meanwhle, the real-tme restrcton s set, the MIDDLE state s skpped and the HIGH state s set. Correspondngly, Pd wll be ncreased by 40 nstead of 0. For devces n the MIDDLE state, whenever an excepton occurs upon data wth real-tme restrctons, the HIGH state s set wth Pd ncreased by 0. STEP-4 For devces n the MIDDLE state and the HIGH state, whenever requrements of the partcular state cannot be satsfed, the devce wll be set to the matchng state and ts data-based prorty wll be decreased by 0 or 40. Basc steps of the state transton scheme value of temperature (we assume there s no real-tme restrctons n ths case) exceeds the normal upper bound value 37 C, then we consder that an excepton occurs, and the correspondng data-based prorty of temperature wll be ncreased and the MIDDLE state s set. Whenever the real-tme restrctons are canceled or the data value goes back wthn the normal nterval, the devce wll leave the MIDDLE state for ts orgnal LOW state. HIGH: The state HIGH means that there are certan exceptons n data wth real-tme restrctons. Ths represents the hghest level of data-based prorty, whch always ndcates an emergency message or an alarm. In ths case, we wll set HIGH state to the devce and ncrease ts data-based prorty n order to prvlege the tme-crtcal data over the less crtcal data, and meanwhle, related adaptve performance can be supported. Fg. 4 shows the basc steps of the state transton scheme. The three states are swtched dynamcally as shown n Fg. 4. ) Rate-based Prorty: In addton to the data-based prortes, rate-based prortes are also dynamcally assgned to each devce by the PAN coordnator accordng to ts recent transmsson feedback. Ths method provdes a good estmate of the future GTS usage behavors of devces. Hence, GTS resources can be allocated to the needy devces wth hgh frequences of sendng data, accordng to ther dfferent ratebased prortes. A waste of GTS resources s avoded n ths way. Before presentng detals of the rate-based prorty assgnment polcy, we defne CSMA/CA ht and CSMA/CA mss, GTS ht and GTS mss as follows: CSMA/CA ht and CSMA/CA mss: If one devce has attempted to access the channel n the CAP of the current superframe, the devce s defned to have a CSMA/CA ht, no matter whether the attempt was successful or not. Otherwse, the devce s consdered to have a CSMA/CA mss. GTS ht and GTS mss: If one devce has ssued a successful GTS request n the CAP or transmtted data wthn ts allocated GTS to the PAN coordnator durng the perod of the current superframe, the devce s defned to have a GTS ht. Otherwse, the devce s consdered to have a GTS mss [33]. It s obvous that a CSMA/CA ht or GTS ht ndcates more recent traffc for a certan devce, whereas a CSMA/CA mss or GTS mss represents a comparatvely lght traffc. Consequently, we can know about the recent data transmsson behavors of devces through the occurrence of CSMA/CA ht/mss and GTS ht/mss. Then the rate-based prortes can be set dynamcally accordng to the transmsson feedback. Specfcally, whenever a CSMA/CA ht or GTS ht occurs on a devce, the rate-based prorty number wll be ncreased by the PAN coordnator, and the prorty of GTS allocaton for the devce upgrades. On the other hand, upon occurrence of a CSMA/CA mss or a GTS mss, the PAN coordnator decreases the rate-based prorty number of the devce to reduce ts opportunty for obtanng the GTS resources. In ths way, devces wth more frequent data transmssons wll have larger probabltes to obtan GTS allocaton n the subsequent superframe, whle devces wth a lght recent traffc wll have smaller probabltes of ganng GTS resources. Furthermore, when a CSMA/CA ht/mss or a GTS ht/mss occurs, devces should not be treated equally f they have dfferent rate-based prorty numbers. For example, devces wth hgh rate-based prortes, whch stay n a hgh traffc level, can tolerate more easly temporarly-unstable transmsson behavors. Such devces are slghtly demoted to lower rate-based prortes upon occurrence of a CSMA/CA mss or a GTS mss. Whereas, devces wth comparatvely low rate-based prortes are demoted more greatly for the same CSMA/CA mss or GTS mss. Addtonally, when there s a CSMA/CA ht or a GTS ht, devces wth lower rate-based prortes wll be more greatly promoted to hgher prortes to receve GTS servce as soon as possble. As a result, starvaton of such a low-prorty devce can be avoded. Ths rate-based prorty assgnment polcy focuses on whether devces have contnuous data to be transmtted over the GTSs. The devces wth consecutve transmssons are favored by our scheme, and for a devce that s dle for a perod of tme, ts rate-based prorty wll be greatly degraded by the PAN coordnator and the unused GTSs wll be reclamed mmedately for hgh traffc devces. In ths way, GTS resources are more effcently used by devces. Let us assume that devce mantans the followng parameters at the begnnng of the t th superframe. Pr t 1 and Pr t are the rate-based prortes n the prevous superframe and the current superframe respectvely. N t 1 CSMA/CA,ht and N t 1 GT S,ht are the numbers of CSMA/CA ht and GTS ht that occur on devce n the prevous superframe, respectvely. Assume that each devce receves the beacon at the begnnng of the current superframe, then the rate-based prorty of devce,.e. Pr t, wll be updated as follows: P t r = P t 1 r +H CSMA/CA (P t 1 r M CSMA/CA (Pr t 1 ) + H GT S (P t 1 ) M GT S (P t 1 r ) r ) (3)

6 JOURNAL OF SYSTEMS ARCHITECTURE, SPECIAL ISSUE: HIGHLY-RELIABLE CPS, VOL. 00, NO. 0, MONTH M CSMA/CA (P t 1 r and H GT S (P t 1 r ), M GT S (Pr t 1 ) can be determned by: ), H CSMA/CA (Pr t 1 ), Prevous superframe t 1 P r H CSMA/CA (P t 1 r M CSMA/CA (P t 1 r M GT S (P t 1 r H GT S (P t 1 r ) = λ CSMA/CA,mss P t 1 r (4) ) = λ GT S,mss P t 1 r (5) ) = λ CSMA/CA,ht P t 1 r ) = λ GT S,ht P t 1 r N t 1 CSMA/CA,ht (6) N t 1 GT S,ht (7) where λ CSMA/CA,mss, λ GT S,mss, λ CSMA/CA,ht, and λ GT S,ht are all constants, and λ CSMA/CA,mss, λ GT S,mss, λ GT S,ht, and λ CSMA/CA,ht are greater than zero. As an example, consder Pr t 1 1 = 5 for devce 1, and = 10 for devce. Suppose there s a GTS mss for both P t 1 r devces durng the (t 1) th superframe (CSMA/CA ht and CSMA/CA mss are not consdered), then at the begnnng of the t th superframe, ther rate-based prortes can be obtaned from: P t r 1 = P t 1 r 1 P t r = P t 1 r = P t 1 r 1 = P t 1 r M GT S (Pr t 1 1 ) λ GT S,mss = 5 λ Pr t 1 GT S,mss /5 1 M GT S (Pr t 1 ) λ GT S,mss = 10 λ Pr t 1 GT S,mss /10 In the above case, devce has a hgher prevous ratebased prorty than devce 1. When there s a GTS mss n the current superframe, the two devces prortes are both decreased. However, they are decreased by dfferent values, λ GT S,mss /5 for devce 1 and λ GT S,mss /10 for devce. Ths s because devces wth hgher rate-based prortes, whch ndcate consecutve transmssons, can tolerate temporarly unstable transmsson behavors n a relatvely easy way. Such devces wll be slghtly demoted to lower prortes on the occurrence of a GTS mss, compared wth the low prorty devces. Fg. 5. presents the flowchart of the rate-based prorty assgnment process n a superframe. B. Guaranteed Tme Slots Allocaton Accordng to the IEEE Specfcaton [1], on recept of a GTS request, the PAN coordnator shall frst check f there s avalable capacty n the current superframe, based on the remanng length of the CAP and the desred length of the requested GTS. GTSs shall be allocated f the maxmum number of GTSs (seven) has not been reached and allocatng a GTS of the desred length would not reduce the length of the CAP to less than amncap Length. Here we descrbe the GTS allocaton mechansm for our ART-GAS algorthm. The proposed mechansm wll modfy the FCFS GTS allocaton polcy n IEEE standard, and optmze the tradtonal GTS deallocaton scheme whch may lead to starvaton of lght-traffc devces. In our GTS allocaton mechansm, the GTS schedulng crtera are based on the prorty numbers (data-based prortes Fg. 5. Y GTS ht? N Current superframe CSMA/CA ht? t t 1 t 1 t t 1 t 1 1 Pr P r H GTS Pr P r P r M GTS Pr t t 1 t 1 / Pr P / r H CSMA CA Pr t t 1 t 1 3 P P M P 4 r r CSMA CA r Rate-based prorty assgnment process and rate-based prortes), the superframe length (dependng on the BO value), and the GTS capacty of the superframe, compared wth the orgnal GTS mechansm. In ths method, tme-crtcal devces wth hgher frequences of sendng data are prvleged over non tme-crtcal and lght-traffc devces. Detals of the algorthm are presented as follows: At frst, we classfy all devces nto three states, LOW, MIDDLE, and HIGH, accordng to ther data-based prorty levels descrbed n Secton III(A). For each state, we defne a GTS schedulng crteron, whch determnes whether the GTS resources wll be allocated to certan devce. Crtera for dfferent states are dfferent, hence servce for devces wth LOW, MIDDLE and HIGH data-based prorty levels wll be dfferentated. Let P denote the prorty of devce to obtan GTS allocaton, and GTSs shall be gven to devces n a decreasng order of ther prortes. P L, P M, and P H denote the mnmum prorty of a devce to be allocated the GTS resources n state LOW, MIDDLE and HIGH respectvely. In other words, GTSs can be allocated to devce only when one of the followng requrements s met: For devce n LOW state: P P L ; For devce n MIDDLE state: P P M ; For devce n HIGH state: P P H. For devces n the HIGH state, whch means the hghest level of data-based prortes, the real-tme guarantee for emergency messages s the prmary concern, whereas the frequences of data transmsson are consdered to be less mportant. In ths scenaro, we defne P as P = P d and P H as P H = Mn(P ) = 40. Hence there wll be no extra lmtatons for devces n the HIGH state whenever they have data to transmt, provded that there s avalable capacty n the current superfame. For devces n the MIDDLE state, both the data-based prorty and rate-based prorty are consdered to determne a GTS allocaton. The prorty P s defned as P = P d P r, whch s based on the devce s data-based prorty P d and N t P r Y

7 JOURNAL OF SYSTEMS ARCHITECTURE, SPECIAL ISSUE: HIGHLY-RELIABLE CPS, VOL. 00, NO. 0, MONTH rate-based prorty P r. Furthermore, we defne the threshold prorty P M as follows: µ M ϕ M ( N P ) =1 P M = N BO (8) where µ M and are both constants, and µ M > 0, 0 < 1. N s the number of devces n the current IEEE based PAN. P M s used here to flter unnecessary GTS allocatons. It N =1 s dynamcally adjusted and manly depends on N and BO. BO s an ndcaton of CAP and CFP traffc load. As BO ncreases, there s a hgher probablty that a great number of devces have requested the GTS servce n a superframe. Based on our servce dfferentaton mechansm, the devces requestng GTS allocaton are assgned bg prorty numbers, even though they only have one request n the whole superframe. To prevent the scarce GTS resources from beng dstrbuted to those devces wth extremely low frequency GTS requests n such a long superframe, a strcter threshold s needed. In ths case, P M wll be set to a larger value to flter low prorty devces. On the other hand, wth a small BO, the value of P M can be decreased and the lmtaton N P ϕ( P ) =1 for the devce selecton can be relaxed. N represents an average level of devces prortes n a superframe. ϕ s N a functon of P. Its output s a partcular percentage =1 of the nput value, whch can be 80%, 90%, 100%, 110% or 10% of the gven N P. The value depends on the =1 overall level of devces prortes. When N P has a =1 larger value, the correspondng percentage mght be 80% or 90%. On the contrary, the percentage mght be 110% or N 10% when a relatvely small P s obtaned. The ϕ =1 functon s desgned to provde mult-level restrctons for devces n dfferent traffc condtons. Specfcally, when most of the devces have low prortes, the nput value N P =1 s smaller, and there s no need to allocate too many GTS resources for the devces. Too much dedcated bandwdth for GTS usage leads to resource wastage and to the degradaton of the overall system performance. Instead, the GTS bandwdth should be transferred for contenton-based accesses n CAP. In ths case, a larger percentage s used to ncrement ϕ and the threshold value wll also be ncreased, renderng a strct lmtaton for devces n the current superframe. Otherwse, a smaller output of functon ϕ s obtaned, whch means a more relaxed restrcton for devces n the hgh-prorty envronment. For devces n the LOW state, rate-based prortes are consdered to be more mportant snce they all stay n low databased prorty levels. In ths case, P s defned as P = P r. As n the MIDDLE state, the threshold value P L s defned as: µ L ϕ L ( N P ) =1 P L = N BO (9) However, ths s a more restrcted lmtaton for GTS allocaton compared wth the MIDDLE state snce we have dfferent µ M and µ L values. The P values and the threshold prortes of states HIGH, MIDDLE and LOW respectvely are gven below: HIGH : P = P d, P H = Mn(P ) N MIDDLE : P = µ M ϕ M ( P ) =1 P d P r, P M = N N BO µ L ϕ L ( P ) =1 LOW : P = P r, P L = N BO V. ALGORITHM DESIGN AND IMPLEMENTATION In ths secton, we descrbe the desgn and mplementaton of the algorthm of our ART-GAS scheme. From an applcaton perspectve, ART-GAS wll be nvoked before the PAN coordnator transmts the beacon frame to the devces wthn ts transmsson range. A. Algorthm Descrpton Here we present a Status Management Algorthm and a Prorty Flter Algorthm for the ART-GTS scheme proposed n the prevous secton. Algorthm 1 Status Management Algorthm 1: Assume that there are N devces n the WPAN : type Devce = (d, S, RT, InRange) 3: type DevceSetT ype = (D, Devces n the current WPAN ) 4: DevceSetT ype DevceSet 5: Devce D 6: for devce D DevceSet do 7: RT F ALSE 8: InRange T RUE 9: S LOW 10: end for 11: 1 1: whle N do 13: f RT = T RUE and InRange = F ALSE then 14: S HIGH 15: else f RT = T RUE or InRange = F ALSE then 16: S MIDDLE 17: else 18: S LOW 19: end f 0: + 1 1: end whle Algorthm 1 s the Status Management Functon of devces n the IEEE based PAN. It mantans a set of actve devces, denoted as DevceSet, and s nvoked at the begnnng of each Beacon Interval. Whenever a change

8 JOURNAL OF SYSTEMS ARCHITECTURE, SPECIAL ISSUE: HIGHLY-RELIABLE CPS, VOL. 00, NO. 0, MONTH occurs on certan devce concernng real-tme requrements and data exceptons, ts status wll be changed correspondngly. Parameter RT wll be T RU E f real-tme restrcton s set, otherwse RT wll be F ALSE. Smlarly, parameter InRange s set to T RUE or F ALSE dependng on whether the data value beyond the normal boundary. In ths way, all the devces are swtched flexbly to dfferent states accordng to dfferent condtons of data transmssons (.e., dfferent databased prortes). In the GTS Allocaton Mechansm (descrbed n Secton III(B)), the Prorty Flter functon s called, takng the followng parameters as nputs: the number of allocated tme slots T, the number of devces n the current WPAN, the set of such devces (DevceSet), and each devce s state (S ) and prorty (P ). If a devce s prorty s not lower than ts threshold value, the PAN coordnator wll allocate GTS resources to the certan devce, provded that the GTS capacty s not overloaded (that s, the maxmum length of seven tme slots s not reached). Otherwse, any new GTS requests shall be rejected snce no more GTS can be allocated. Algorthm Prorty Flter Algorthm 1: Assume that there are N devces n the current WPAN : The number of allocated GTS T 3: DevceSet = {D 1, D,, D N } 4: P = {P 1, P,, P N } 5: P H = Mn(P ) 6: P M = µ M ϕ M ( N P ) =1 N N BO µ L ϕ L ( P ) =1 7: P L = N BO 8: whle T < 7 do 9: Fnd a devce D whose P s the maxmum number n P 10: f S = HIGH and P P H then 11: Allocate GTS to devce D and T T + 1 return T RUE 1: else f S = MIDDLE and P P M then 13: goto lne 11 14: else f S = LOW and P P L then 15: goto lne 11 16: else 17: Devce D wll not be scheduled n the current superframe return F ALSE 18: end f 19: end whle Algorthm presents the Prorty Flter functon. It returns a Boolean value statng whether or not to accept a devce s GTS request accordng to ts transmsson states and prortes. The return value s set to T RUE f requrements of the GTS allocaton are satsfed, otherwse, t wll be set to F ALSE. As we have mentoned before, devces n states HIGH, MIDDLE and LOW have dfferent threshold prorty values. Only when the devce s prorty exceeds or equals to ts correspondng threshold, wll the devce be scheduled n GTSs of the current superframe. B. Implementaton Issues From a practcal pont of vew, ART-GAS can be easly granted nto the IEEE protocol wth a mnor change. In ths secton, we dscuss some mplementaton ssues of our ART-GAS algorthm n comparson wth the orgnal IEEE GTS allocaton mechansm. To mplement the ART-GAS n the IEEE based PAN for adaptve and tmely GTS allocaton, some extra records for devces are requred, compared wth the orgnal IEEE GTS mechansm. For example, the prorty numbers ncludng data-based prorty, rate-based prorty, and overall prorty are needed for each devce and they are mantaned n the PAN coordnator. Also, states nformaton s also recorded for makng decsons on GTS allocaton. One thng to menton s that once the N s ncremented, the memory space requred for recordng addtonal nformaton should also be ncreased. However, all IEEE devces can mplement our ART-GAS algorthm wthout any modfcaton. In other words, ART-GAS s fully compatble wth IEEE applcatons. Further, our proposed scheme s developed based on the standard of IEEE MAC protocol, whch totally follows both the message type and flow defned n IEEE specfcatons. The users of our ART-GAS for IEEE based devces would only need to change the legacy GTS allocaton/deallocaton of the PAN coordnator to ART- GAS. Another advantage of our ART-GAS n mplementaton s the lower cost of GTS request loss. When traffc load n CAP s heavy and GTS resources are almost fully occuped, loss of GTS requests would lead to long watng tme for the lost requests to be granted to GTS. Such negatve performance are manly attrbutable to the FCFS GTS allocaton and passve GTS deallocaton mechansm n the orgnal IEEE Nevertheless, our ART-GAS sgnfcantly allevates ths mpact. The P r wll be quckly ncreased as long as successful CSMA/CA hts or GTS hts are ssued n followng superframes. In ths case, a sngle loss of GTS request s more tolerable. Moreover, as the CAP traffc load ncreases, collsons would frequently occur and GTS requests wll be more rarely delvered to the PAN coordnator as a result. However, ths concern can be slghtly relaxed by ART-GAS, whch comes from the dynamcally adjusted threshold prortes. Increase of the threshold value wll result n shorter CFP and longer CAP for devces competton. A. Smulaton Setup VI. PERFORMANCE EVALUATION In ths secton, we present a smulaton study based on an accurate model of IEEE usng OMNeT++ smulator, to assess the performance of the proposed ART-GAS algorthm. On the OMNeT++ platform, we can smulate nearly all the connectons of the network object model and make the complex traffc network and topology hghly practcal to the stuaton n realty. The model lbrary of OMNeT++ can also satsfy the large-scale sensor network smulatons on demand,

9 JOURNAL OF SYSTEMS ARCHITECTURE, SPECIAL ISSUE: HIGHLY-RELIABLE CPS, VOL. 00, NO. 0, MONTH TABLE I SIMULATION PARAMETERS Parameters Value Network topology Star topology Synchronzaton mode Beacon enabled Carrer frequency.4 GHz Transmtter power 1 mw Carrer sense senstvty -85 dbm Transmsson range 17 m Number of devces 10 and 0 Frame sze 17 Bytes (default) Transmsson rate 50 Kbps macmaxbe 5 (default) macmnbe 3 (default) MaxNB 4 (default) MaxF rameretres 3 (default) Beacon Order 6 (default) Superframe Order 6 (default) χ 0.15/s χ h 0.35/s TABLE II SIMULATION SCENARIOS Scenaro Data-based prorty Rate-based prorty Scenaro Scenaro Scenaro Scenaro Scenaro whch s an mportant reason for choosng OMNeT++ as our smulator. In the smulaton, a star topology wth sngle PAN coordnator and N devces deployed n the area of 1000 mm 1000 mm s consdered. N can be 10 or 0. Each devce s allocated at most one GTS slot, and accordng to the IEEE Specfcatons, the maxmum GTS number n a superframe s seven. The packet arrvals for each devce form a Posson stream, and each new arrvng packet shall trgger the ssuance of a GTS request n the superframe. If there are no suffcent GTS resources for the request, the devce wll ressue the request for the packet n the subsequent superframe. Addtonally, two traffc types generated by devces are consdered: heavy traffc and lght traffc. χ h and χ represent respectvely the nterarrval rates for the heavy-traffc and lghttraffc devces. In the smulatons, we have χ h = 0.35/s and χ = 0.15/s. Such rate settngs are reasonable n IEEE based WPANs, snce IEEE targets lowrate wreless communcatons. In addton, let N h denote the number of heavy-traffc devces. Thus the GTS traffc load wll be Γ = N h χ h +(N N h )χ l. Table I lsts the nput parameters for our smulaton model. We develop a smulaton model-path Loss Model to nvestgate the performance of our ART-GAS algorthm. Based on [34], the path loss model s sutable for both narrow band (NB) and Ultra-wde Bandwdth band (UWB). We assume that the human body s 171cm hgh and hs weght s 63kg on average. The path loss wth 400MHz can be calculated as the followng formula: P L(d) [db] = a lg d + b + c + N (10) Wth the assumpton P L(d, f) [35] n db for body surface to body surface propagaton at dstance d (150 mm < d < 1000 mm) and frequency f =.4GHz (400 MHz < f < 500 GHz), the followng formula can be obtaned from Eq. (10): P L(d, f) [db] = 7.6 lg d 46.5 lg f Q (11) where Q s the shadowng component whch follows lognormal dstrbuton wth standard devaton of 4.1 db. The performance metrcs analyzed n ths paper are the followng. Success Probablty: Ths metrc s computed as traffc correctly receved by the network analyzer dvded by overall traffc. It reflects the degree of relablty acheved by the devce for successful transmssons. We denote by S(Γ) the success probablty as a functon of traffc load Γ. Average Delay: It s the average delay experenced by a data frame from the start of ts generaton by the applcaton layer to the end to the end of ts recepton by the analyzer. We denote by D(Γ) the average delay as a functon of traffc load Γ. Average Watng Tme: A devce average watng tme W can be defned as the length of tme t has to wat before a GTS allocaton. W s also a functon of traffc load Γ, denoted by W (Γ). CFP Bandwdth Utlzaton: It s the percentage of tme perod n the CFP that s used for data transmssons. Idle perod mght occur due to excessve GTS allocaton, nflexble GTS deallocaton, FCFS GTS schedulng polcy, and the lke. We denote by B(Γ) the CFP bandwdth utlzaton of traffc load Γ. B. Numercal Results and Analyss In ths secton, we wll present a careful evaluaton and analyss of our ART-GAS algorthm. In comparson, smlar assessments of the tradtonal IEEE MAC protocol and ART-GAS wth no threshold are also provded. We ran the smulaton 10 tmes. Snce the fluctuaton of results s very weak, we prefer to provde the average values. Detaled performance assessments are gven n four parts accordng to the prevously mentoned metrcs. 1) Success Probablty (S): Performance of data transmssons wth dfferent data-based and rate-based prortes are gven n ths secton, n terms of success probablty. We consder fve scenaros presented n Table II wth 10 nodes and 0 nodes envronment respectvely. Fve randomly selected nodes are adapted to the fve scenaros and each scenaro s smulated wth the ART-GAS algorthm. The scenaros can be appled to applcatons wth prortes lke health care, alarm system. For example, scenaro 5 s for heart rate. When someone s heart rate s abnormal, t s an emergency and t s assgned a hgher data-based prorty. It s obvous n Fg. 6 that devces wth dfferent prortes are clearly dfferentated n performance of data transmssons. Ths s because the devces wth hgher data-based prortes

10 JOURNAL OF SYSTEMS ARCHITECTURE, SPECIAL ISSUE: HIGHLY-RELIABLE CPS, VOL. 00, NO. 0, MONTH Success Probablty (S) Traffc Load ( ) Scenaro 1 Scenaro Scenaro 3 Scenaro 4 Scenaro 5 Average Delay (D) Scenaro 1 Scenaro Scenaro 3 Scenaro 4 Scenaro Traffc Load ( ) Fg. 6. Success probablty of devces wth dfferent prortes (10 nodes) Fg. 8. Average delay of devces wth dfferent prortes (10 nodes) Success Probablty (S) Scenaro 1 Scenaro Scenaro 3 Scenaro 4 Scenaro Traffc Load ( ) Average Delay (D) Scenaro 1 Scenaro Scenaro 3 Scenaro 4 Scenaro Traffc Load ( ) Fg. 7. Success probablty of devces wth dfferent prortes (0 nodes) Fg. 9. Average delay of devces wth dfferent prortes (0 nodes) or rate-based prortes are prvleged to obtan suffcent GTS resources and ther threshold prortes are also more relaxed than other devces. Therefore, tme-senstve and hgh-traffc data transmssons can be ensured wth hgh relablty. Furthermore, through comparson of Fgs. 6 and 7, we can easly observe that when the traffc load Γ doubled wth 10 more nodes added to the WPAN, success probablty wll decrease, snce hgher traffc load also means more severe competton, thus leadng to more collsons. In ths case, CAP tend to be more ntensely occuped and t s more dffcult for a devce to obtan GTS resources n CFP. Hence, t becomes much easer for each node to have a transmsson falure. However, hgh-prorty devces experence a much smaller change than those low-prorty ones. The reason s that a sngle GTS mss or CSMA/CA mss can be more tolerable for devces wth hgher prortes due to the adaptve desgn of prorty assgnment descrbed n Secton IV, hence ther prortes are more slghtly decreased when transmsson falure occurs. ) Average Delay (D): Smlarly, devces wth lower prortes have greater average delays (Fgs. 8 and 9). Ths s because low prorty devces have a smaller probablty of obtanng GTS resources than hgh prorty devces. Thus a longer tme perod wll be needed for data transmssons. Also, from Fg. 8 to Fg. 9, the average delay D ncreases as the traffc load Γ doubled snce a hgher traffc load leads to worse competton envronment for data transmssons, whch s for the same reason as the decreased success probablty n change of traffc load. Furthermore, hgh-prorty devces tend to have a slghter ncrease n D, whle low-prorty devces wll experence a much more sgnfcant change when traffc load doubled. Ths phenomenon s also attrbutable to the servce dfferentaton mechansm and prorty flter mechansm n ART-GAS, whch enable hgh-prorty devces have a better chance to obtan GTSs. Ths s also a clear ndcaton that devces wth hgher prortes are better able to adapt themselves to an envronmental change and hence, tme-crtcal and hgh-traffc data transmssons are guaranteed from ths perspectve. 3) Average Watng Tme (W): Addtonally, we compare the proposed ART-GAS wth another two algorthms n terms of average watng tme W. The frst algorthm s the GTS allocaton mechansm specfed n orgnal IEEE standard, whch dstrbutes GTS resources n a Beacon Interval

11 JOURNAL OF SYSTEMS ARCHITECTURE, SPECIAL ISSUE: HIGHLY-RELIABLE CPS, VOL. 00, NO. 0, MONTH to all devces n a FCFS polcy, regardless of ther mportance and transmsson rates. The second algorthm, named nonthreshold ART-GAS, s largely the same as the GTS allocaton algorthm provded n ART-GAS, but wth no threshold values. That s, GTSs are allocated to devces n a decreasng order of ther prortes (P ) as long as the GTS capacty s not overloaded. Our evaluaton consders two scenaros, 10 nodes envronment n Fg. 10, and 0 nodes envronment n Fg. 11. We can fnd n these two fgures that ART-GAS has the best performance among the three algorthms. It obtans the lowest value of average watng tme at any gven traffc load. Ths s due to the adaptve GTS schedulng mechansm descrbed n Secton IV, where the threshold value s able to be dynamcally adjusted to network load and bandwdth resources are more effcently utlzed by needy devces wth less wastage. The non-threshold ART-GAS has an extremely smlar performance wth ART-GAS n Fg. 10. However, when the devce number N s ncreased to 0, the ncreasng rate of W n ART- GAS s consderably smaller than that of non-threshold ART- GAS and IEEE Ths ndcates that our proposed ART-GAS provdes more resstance to the ncreased traffc load as a result of ts adaptve GTS schedulng. Adjustment of the threshold prorty enables a better data-transmsson envronment, especally for hgh-prorty devces. In ths case, the non-threshold ART-GAS curve s no loner smlar to that of ART-GAS, snce t lacks the threshold restrcton, whch serves to allevate CAP competton when traffc load s hgh. There s another nterestng phenomenon n Fg. 11. As traffc load ncreases, the W of IEEE slghtly decrease after a sgnfcant ncrease. It s easy to understand that the ncrease n W s because of the nflexble FCFS GTS allocaton and passve deallocaton. In ths case, most of the GTS resources are occuped by heavy-traffc devces for a long tme, whch probably leads to the starvaton (or almost) of lght traffc devces. As traffc load becomes close to the maxmum value (all devces are n heavy traffc status), the number of starvng devces wth the lght traffc load decreases, and thus the average watng tme wll slghtly decrease. However, for ART-GAS and non-threshold ART-GAS, starvaton s hardly to occur due to the flexblty n GTS schedulng, hence no such phenomenon can be observed n these cases. 4) CFP Bandwdth Utlzaton (B): Our evaluaton of CFP bandwdth utlzaton B also consders the three algorthms mentoned prevously, ART-GAS, non-threshold ART-GAS, and IEEE In Fg. 1, we can see that n lowtraffc envronment, as the traffc load Γ ncreases, bandwdth utlzaton of all the three algorthms wll also ncrease. Because more unused GTSs are allocated to devces, whch are requestng GTS resources, the CFP bandwdth utlzaton s ncreased. However, n ths case, ART-GAS and non-threshold ART-GAS have much better performance than the tradtonal IEEE , snce these two algorthms provde a servce dfferentaton mechansm for heavy-traffc and lght-traffc devces. The servce dfferentaton mechansm allocates GTSs accordng to devces states and prortes and GTSs are more fully used by needy devces wth hgher data transmsson frequences. Whle n standard IEEE MAC, the n- Average Watng Tme (W) Fg. 10. Average Watng Tme (W) Fg. 11. CFP Bandwdth Utlzaton (B) Fg ART-GAS non-threshold ART-GAS IEEE Traffc Load ( ) Effect of traffc load on average watng tme (10 nodes) ART-GAS non-threshold ART-GAS IEEE Traffc Load ( ) Effect of traffc load on average watng tme (0 nodes) IEEE non-threshold ART-GAS ART-GAS Traffc Load ( ) Effect of traffc load on CFP bandwdth utlzaton (10 nodes) flexble FCFS-based GTS allocaton treats all devces equally, leadng to sgnfcant bandwdth wastage. Furthermore, when the number of devces N s ncreased to 0 (Fg. 13), we can observe that both the IEEE