Influence of Nework Lod on he Performnce of Opporunisic Scnning Mrc Emmelmnn, Sven Wiehöler, nd Hyung-Tek Lim Technicl Universiy Berlin Telecommunicion Neworks Group TKN Berlin, Germny Emil: emmelmnn@ieee.org, {wiehoel, lim}@kn.u-berlin.de Absrc This pper presens deiled performnce evluion of novel sochsic nework discovery pproch denoed s opporunisic scnning. Therein, sion only puses is communicion for n exremely shor ime inervl o scn for oher echnologies / sysems. The seleced scnning durion is shor enough no o be noiceble by higher lyer pplicions wih sric QoS consrins. We pply his concep o 82.-bsed neworks using power sve s one possible signling proocol o puse he communicion beween he scnning sion nd is inerlocuor. Wihin his pper, we presen deiled simulionbsed performnce nlysis of he opporunisic scnning pproch showing is effecs on rel-ime services such s VoIP srems. Our resuls show h opporunisic scnning behves surdy gins bckground rffic. This novel pproch enbles successful nework discovery wihin 85-milisecond ime spn while gurneeing QoS consrins of such higher lyer relime pplicion. I. INTRODUCTION Efficien nework discovery is one of he key funcionl elemens for wireless locl re neworks (WLANs). Hence, is mechnisms nd poenil improvemens hve inensively emerged over he ls decde. Hisoriclly, WLANs simply replced he wired Eherne connecion for sionry nodes. Communicion prners were mosly discovered during he iniil power-up phse of he sysem. Sring wih he suppor of mobile devices, such nework discovery occurred more frequenly nd ws even subjec o ime-consrins if relime pplicions, i.e. voice over IP elephone, were o be suppored. Sill, in he ps, such nework discovery ws rher seldom occurrence. Bu his hs chnged: new pplicion res s well s he ide of opering unlicensed WLAN devices wihin licensed frequency bnds impose new chllenges owrds nework discovery schemes. The issue on how o scn for new neworks very high frequency or even how o coninuously discover neighboring echnologies hs o be solved low cos. Exmples for new pplicion res requiring such frequen nework discovery re mnifold: 82. s he mos predominn WLAN echnology will move owrds he 6GHz frequency bnds [] nd discusses on how The uhors would like o hnk OPNET Technologies Inc. for providing he simulion environmen under he universiy progrm. This work ws prilly suppored by Europen Commission hrough projec EU-MESH (Enhnced, Ubiquious, nd Dependble Brodbnd Access using MESH Neworks), FP7 ICT-2532. o suppor highly mobile users [2] [4]. Due o he microcellulr rchiecure in he former nd he high hndover frequency in he ler cse, hndover is clerly no longer seldom occurrence. The sme pplies o nodes hving sofwre-defined rdio inerfce which enbles o consnly choose he nework for bes conneciviy nd o conduc hndover from one echnology o noher even during n ongoing communicion. Finlly, unlicensed devices opering in he whie spce specrum hve o relese he medi immediely if primry users pper. To ssure his, nework discovery / specrum sensing is mndory ccording o he FCC rules [5], [6]. One wy o ssure such primry user deecion is o recurrenly scn he specrum. In summry, ll hose pplicion scenrios require o scn for oher devices of he sme or differen echnology coninuously or high re while minining QoS consrins of he ongoing communicion. Opporunisic scnning is one cndide providing coninuous nework discovery. I periodiclly scns for lernive ccess echnologies while upholding QoS consrins in erms of ssuring mximum inerrrivl imes of user dgrms. The pproch behind opporunisic scnning is srighforwrd: he scnning sion leves is communicion chnnel only for very shor ime hence no noicebly ffecing he QoS consrins of ny higher lyer communicion. As he dwell ime on he scnning chnnel is very shor, opporunisic scnning cnno gurnee o deec n exising echnology wihin single scnning period. This mkes opporunisic scnning sochsic process wih high vribiliy. Also i requires signling proocol beween he scnning sion nd is inerlocuors in order o puse ongoing d exchnge MAC level. Though he generic concep of opporunisic scnning my be pplied o ny wireless echnology, his pper ssumes 82.-bsed sysem using his novel scnning scheme for nework discovery. The rionl behind his choice is h 82. is he mos inexpensive nd mos widely deployed WLAN echnology which we believe will coninue o be he predominn echnology for WLANs opering in whie spce s well s in unlicensed, higher frequency bnds. Our relizion lso dheres o he consrins h ny sysem employing opporunisic scnning shll fully comply o he exising 82. sndrd which enbles n efforless
inegrion of his novel scheme in exising WLAN chipses / deploymens. This pper presens deiled performnce nlysis of Opporunisic Scnning using he 82. power sve o puse ny ongoing communicion while scnning for oher echnologies. We hereby exend our previous mhemicl nlysis of he opporunisic scnning scheme [7] which ssessed he smlles chievble service inerrupion cused by he scnning process nd invesiged requiremens for he choice of scnning inervls. Novelies of his conribuion cn be summrized s: ssessing he performnce limis while considering implicions of he 82.-rchiecure nd proocol such s delyed becons nd cler chnnel ssessmen ccouning for rndom bckoff due o busy medi; compring resuls for n idle communicion chnnel o nlyiclly derived performnce limis; evluing he influence of bckground rffic on he performnce of opporunisic scnning; nd qunifying he cos of opporunisic scnning including deiled discussion of he inroduced proocol overhed. The reminder of he pper is srucured s follows: Secion II inroduces our sysem model nd is followed by shor descripion of he opporunisic scnning pproch nd how i is pplied o 82. sysem using power sve s signling proocol. Emphsis is se on Secion IV which srs prphrsing he gols nd mehodology of he performnce evluion. Bsed on hese gols, we derive our simulion scenrio nd se of pplied merics nd conclude he performnce evluion secion wih deiled discussion of he resuls. In he end of he pper, Secion V plces opporunisic scnning wihin he conex of mos recenly published work on improved scnning schemes followed by summry nd oulook on our fuure work. II. SYSTEM MODEL We consider se of 82. ccess poins (APs) ech hving high cpciy connecion o he Inerne. These APs my be loced user premises (home neworks) or in highly populed urbn res (public ho spos). 82. devices hve sndrd complin implemenion of he MAC bu re no necessrily limied o opere only on he 2.4 nd 5 GHz frequency bnd defined in he sndrd. Hereby, our rchiecure implicily llows 82.-bsed devices o be run in he whie spce recenly opened for unlicensed operion. No consrins re imposed on he bckhul connecion excep h we ssume h he bckhul s medi ccess is sricly sepred from he ls hop. This ssumpion llows wide rnge of rchiecurl choices rnging from wired links (e.g., Eherne), over hving heerogeneous (wireless) echnologies for he ls hop nd bckhul (e.g., 82. nd WiMAX), up o using homogeneous echnologies on differen frequencies (e.g., 82.b/g vs. 82.). This ssumpion is fesible s bckhul conneciviy is usully se up by service provider which ries o void ny effecs of end-user-sysems on is bckhul echnology. Regrding he ls hop, ech 82. AP forms n infrsrucure bsic service se (BSS). All considered BSSs belong o he sme exended service se (ESS). BSSs my overlp nd hence frequenly opere on differen chnnels o reduce inerference. Also, he coverge re of BSS my overlp wih he one of secondry echnology (e.g. WiMAX or 3/4G nework). We ssume h ny presen echnology nnounces heir exisence regulr ime inervls, e.g. by brodcsing becon or frme heder (82. or WiMAX) or by recurring energy pern which cn be idenified bu no necessrily decoded by he scnning STA (fooprin-bsed deecion of primry users in whie spce). In our sysem model, users re 82.-bsed sions (STA) loced wihin BSS nd re conneced o he Inerne vi heir ssocied AP. Under he bes-conneced nework prdigm, STAs my coninuously choose mong lernive links, i.e. noher 82. AP or secondry echnology. Thus, hey coninuously scn in order o deec lernive echnologies or evlue he link properies on oher frequencies. Such coninuous scnning is lso used o deec primry users for whie-spce operion. III. OPPORTUNISTIC SCANNING APPROACH The generl concep of opporunisic scnning is driven by he prdigm h semless conneciviy (s seen by he end user) does no men gurneeing zero-dely, inerrupion-free communicion bu limiing he durion nd frequency of possible inerrupions o n upper bound no ffecion he QoS consrins of he user pplicion, hence no being noiceble o he user. As resul, our novel scnning pproch is driven by wo min consrins: The pproch should enble rel-ime communicion wih smll pcke inerrrivl imes nd hrd QoS consrins requiring low pcke loss nd relive smll exr dely MAC. Such pplicions include, e.g., voice over IP (VoIP) s well s elemery pplicions. Second, he scnning pproch shll only pssively scn he scnned chnnel. This ssures h opporunisic scnning does scle wih he number of sions employing his pproch nd does no (unproducively) ffec ny communicion on he scnned chnnel. The following subsecions highligh he generl ide of he opporunisic scnning pproch nd show how opporunisic scnning cn be pplied o n IEEE 82. bsed WLAN. A. Generl ide of opporunisic scnning In generl, opporunisic scnning ims periodiclly leving he curren communicion chnnel only for very shor ime o conduc scnning procedure s indiced by he scn-inervls (SI) in Fig.. The pproch hereby ssumes h he he sysem / echnology o be discovered nnounces is exisence regulr ime inervls on he scnned chnnel If BSS does no belong o he sme ESS, cliens my no use his BSS for roming. Hence, deecing such BSSs is concepully idenicl o he deecion of ny oher echnology (e.g. he presence of primry user for whie spce operion) which is no used for communicion purposes.
Fig.. Generic Opporunisic Scnning pproch ( becon ). 2 Thus, wo unique phses comprise he scn inervl: d exchnge nd scn phse. In order o void pcke loss, he former lso involves signling o ny inerlocuor o puse ongoing rnsmission (sleep-procedure) before i ends, s well s o coninue sending d (wke-up procedure) in is beginning. Depending on he implemenion, opporunisic scnning llows o prioriize eiher he scnning or he d exchnge process. The former gurnees minimum scn durion nd hence sops he d exchnge even if user d pckes re pending for rnsmission. Such prioriy could be given in whie spce environmen where he upmos gol is deecing he primry user. The ler would lwys exchnge ny pending user d pckes even he cos of reducing he scn durion nerly o zero. Thus, his second mode is suible if QoS-consrined d exchnge is vlued higher hn nework discovery nd ws hence our choice for his pper. B. Applicion of opporunisic scnning o 82. WLAN The power sve feure of IEEE 82. [8] llows sion o signl is inerlocuors o hold (nd buffer) ny pending rffic. Though, i does no deem he signling sion o cully go ino power sve mode. Hence we herein use his ime for pssively scnning noher chnnel. I should be noed h in mos common implemenions, he sleeping sion only reurns from power sve fer he recepion of 82. becon which would resul in unccepbly long sleeping imes. Neverheless, rrely used sequence of power sve signling messges llows he sion o resume communicion ny ime. This enbles us o use he exising, sndrd complin power sve feure o pply opporunisic scnning o 82. nework. Hereby, STA signls o he AP h i will go ino sleep mode for mos n becon inervls. Neverheless, he STA my reurn from is sleep mode ny ime before his period expires. 2 Plese noe h he erm becon includes ny kind of fooprin idenifying echnology rnging from decodble signling pcke (s known from homogeneous echnologies, i.e. 82.) up o unique energy pern (whose conens cnno be inerpreed bu only recognized s known from he primry user deecion concep in he whie spce). B e c o n AP STA D exchnge Fig. 2. DIFS D UL M n = rnd(,cw min ) SIFS signl_sleep-psm A C K STA: doze se... sleeping STA AP: TIM[AID_s]= LisenInervl (n * BeconInervl) sleep-psm sleep-process-psm Sleep procedure of PSM-STA Figures 2 nd 3 illusre he resuling proocol deils for he wke-up nd sleep process. Acully, ll he signling informion cn be piggy-bcked in he rnsmission of pending up-link d pckes. The only overhed for his pproch comes if no uplink d is pending which null-d pcke hs o be rnsmied. Also he sndrd requires power sve sions o explicily reques ll buffered pckes in he downlink using PS-Poll frme [8]. For he ske of briefness, he reder is referred o [7] for deiled discussion of he signling procedure. Therein, he heoreicl performnce limi of he pproch is derived nd prime numbers s opiml choices for he scn inervl re recommended. A. Gols nd mehodology IV. PERFORMANCE EVALUATION The overll im of his performnce evluion is o qunify he behvior of opporunisic scnning pplied o n IEEE 82. WLAN sysem used for rel-ime services, i.e. VoIP communicion. Hereby, focus is se on hree min specs: Firsly, we consider n idle chnnel (no oher STAs compeing B e c o n
AP AP: TIM[AID_s]= becon SIFS UL A C K M o r e = AP: TIM[AID_s]= becon LisenInervl (n * BeconInervl) D M n = D exchnge STA sleeping STA wke STA wi signl- STA: doze se STA: wke se wkeup- PSM sleep-psm wkeup-process-psm Fig. 3. STA iniied wke-up procedure Fig. 4. Simulion Scenrio, sic cse wih bckground lod for chnnel ccess) in order o isole he effec of he scnning scheme iself on he rel-ime rffic; secondly, we revel how bckground rffic furher influences he performnce nd surdiness of our novel scnning scheme; nd hirdly, we qunify he overhed ssocied wih opporunisic scnning. The evluion of he performnce for idle chnnel condiions cs s bseline nlysis showing how opporunisic scnning iself influences he iner-rrivl ime of user d pckes. Also, we herein show how long our novel pproch needs o discover one exising AP given probbiliy. The ler resuls revel wors cse bound on he performnce of our pproch s we consider excly one exising AP. Obviously, i kes less ime o discover les one AP if severl APs independenly rnsmi becons on he scnned chnnel. In second sep, we im clssifying he influence of bckground rffic showing how i ffecs opporunisic scnning: poenilly busy chnnel will no only dely he rnsmission of user d, bu lso hinder he (piggy-bcked) signling informion, nd even procrsine he becon being rnsmied fixed ime-inervls. The finl secion of his performnce evluion qunifies he proocol overhed in erms of ddiionl signling informion imposed by opporunisic scnning. All he former performnce specs re ssessed by mens of simulion considering ll relevn proocol specs of 82. [8] such s cler chnnel ssessmen (CCA) funcionliy nd rndom bck-off, delyed becons due o busy medi, nd rnsmi-receive-urnround ime of he RF fron-end. B. Simulion scenrio Our simulion scenrio considers wo djcen 82. ccess poins (APs) opering on non-inerfering chnnels nd hving n overlpping coverge re (c.f. Fig. 4). Boh APs un-synchronously rnsmi becons o nnounce heir bsic service se (BSS). Becon rnsmissions re scheduled regulr ime inervls (rge becon rnsmission ime, TBTT) s defined by he 82. sndrd [8] bu my be delyed if he AP s CCA funcionliy indices he medi o be busy. Boh APs re conneced vi Eherne o VoIP server; hereby, he wired connecion does no impose ny hroughpu consrins. The STA employing opporunisic scnning for nework discovery is plced in he overlp of he djcen cells. I is ssocied wih one of he wo APs which is lso used o rely n ongoing VoIP cll (G7 codec employing 2 ms pckeizion wihou silen suppression). Employing opporunisic scnning for nework discovery, he STA senses on he neighbor AP s chnnel for becon rnsmissions. Bckground rffic is incresed by coninuously dding ddiionl STAs (BG-STAs) ech feuring one ongoing VoIP cll. In ech sep, one bckground STA is dded per AP nd is plced wihin he ler s coverge such h STAs re in communicion rnge of ech oher. Such plcemen voids hidden erminl effecs s RTS-CTS is disbled for he considered VoIP service. This pproch clerly isoles he opporunisic scnning scheme o be he cuse of ny observed effecs. All simulions re conduced using OPNET Modeler Wireless Relese 4PL2. The 82. model of he simulion librry is enhnced by power sve feures complin o [8] s well s by nework discovery scheme ccording o he opporunisic scnning pproch. C. Merics The following merics re used hroughou he performnce evluion: Becon recepion probbiliy qunifies he number of scnning emps / ime required o successfully receive becon given probbiliy. Iner-rrivl ime (IAT) of user d is he ime beween consecuively received user dgrms. Pcke loss ccouns for he percenge of los (user d) pckes. Averge scn durion idenifies he verge ime spen on single scnning emp on he neighbor AP s chnnel. Averge d exchnge durion qunifies he ime required o rnsmi nd receive ll buffered (user d) pckes fer pusing he communicion for n opporunisic scnning emp. Null D frme re qunifies he proocol overhed s null d frmes hve o be used o crry signling informion incse user d is no pending o piggy-bck signl informion for he power sve procedure.
STATIC Scen Cse 2:CDF s funcion of IAT <SI=3ms> CDF s funcion of scnning ime <Bi=ms>.9.9.8.8.7.7 y: P [X x].6.5.4 P [ X becon found ].6.5.4.3.3.2.2. 5 5 2 25 3 35 x: IAT [ms]. Anlysis Simulion.5..5.2.25 Time o find becon frme becon found [s] Fig. 5. Influence of Opporunisic Scnning on IAT of VoIP flow Fig. 6. Required ime o find becon: nlysis vs. simulion PS Poll frme re qunifies he proocol overhed s, ccording o he sndrd, he STA hs o poll for buffered downlink d if employing power sve mode operions. D. Resuls Previous numericl nlysis of he heoreicl performnce limis for idle chnnel condiions showed h prime numbers shll be chosen for he scn inervl in order o gurnee successful recepion of becon fer finie ime spn [7]. Accordingly, we show resuls for prime scn inervls in beween 3ms nd 5ms. The smlles vlue gurnees service re less hn he VoIP pckeizion re nd he lrges is in he order close o he 5-milisecond service inerrupion ccepble for voice communicion. ) Performnce for idle chnnel condiion: Opporunisic scnning corsely divides he ime for ech STA (independenly) ino scnning period nd d exchnge period. As his devision is periodiclly repeed fixed re, we expec VoIP pcke o rrive he STA muliples of he scn inervl. Figure 5 shows he cumulive disribuion funcion (CDF) of he IATs for scn inervl of 3ms. For he 2ms-pckeizion re of he VoIP pplicion, he wo modes of he IAT s disribuion lie expecingly he wo muliples closes o he scn inervl (SI), nmely 3ms nd 26ms. Also, we observe h slighly more pckes re rnsmied wih every second SI. The resuling sligh righ skewness of he disribuion is for he reson of he chosen SI: The 26-millisecond muliple of he SI is simply closer o he expeced IAT of 2ms s he 3-millisecond muliple. Choosing differen SI of 7ms would resul in lef skewness s rnsmission would occur 7ms nd 34ms. Also noe, h he medin being he correc single index of cenrl endency for such skewed disribuions [9] is 2ms nd perfecly corresponds o he ypicl iner-rrivl ime of he considered VoIP flow. Figures 6 illusres for n idle he probbiliy o successfully deec he neighbor AP s becon fer given ime spn. For he chosen SI of 3ms, he 99%-percenile is found less hn 2ms ol scn ime which corresponds o receiving he becon no ler hn wihin he 5h scn emp. The corresponding vlue for rdiionl pssive scnning would be 99ms wih he implicion of cusing service inerrupion of ms s compred o opporunisic scnning coninuously gurneeing he QoS consrins of he rel-ime pplicion. Also, being for only 2ms wihin he overlpping re of APs in order o gurnee becon recepion is n exremely smll required dwell ime even if he scnning STA were mobile. Finlly, compring he resuls wih he heoreicl findings presened in [7], we noe h he effecs of delyed becons, rndom bck-off due o busy medium, nd rx-x-urnround imes of he RF fron-end impose only neglecfully smll effec on he resuls hence enbling performnce predicion for ny chosen rge becon rnsmission ime nd scn inervl by using he heoreicl findings for his specil cse. 2) Influence of bckground rffic: Opporunisic scnning by nure is vulnerble o bckground rffic s n incresed nework lod wihin BSS my cuse delyed becons. Also, signling required o swich from wke o doze se in order o conduc n opporunisic scn emp my be delyed due o busy medium. As our novel scnning scheme primrily ims providing d exchnge opporuniy every scn inervl o gurnee coninuous communicion pplicion level, incresing he ime spen on signling ny se chnge uomiclly reduces he effecive ime vilble for scnning noher chnnel for ech scnning emp. In firs sep, we deermine he mximum number of STAs simulneous serviceble by he AP. Figure 7 illusres he pcke loss probbiliy if in ddiion o he opporunisic scn STA erminls re dded o he BSS. We noice no pcke loss up o ol of STAs in he sysem (9 BG-STAs nd one OP-Scn-STA). Hving (= + ) STAs in he sysem sures he WLAN nd we noice pcke loss due o buffer overflows MAC level. Wih n ddiionl simulion, we verified h pcke loss does no occur for ol of STAs even if ll STAs employ opporunisic scnning
.9.8.7 SI=3ms SI=3ms SI=5ms oppscnsta: AVG[Pcke Loss] <R=Mbi/s,seeds>.9.8 Opp.Scn STA : CDF of ReqScnTime <SI=3ms, R=Mbi/s, seeds>.6.7 y: #Pcke Loss.5.4.3 Y: P [ X x ].6.5.4.2.3.. 2 4 6 8 2 BG STA.2 3BG STA 5BG STA. 8BG STA 9BG STA BG STA.2.4.6.8.2.4.6.8 x: scntime [s] Fig. 7. Pcke Loss Probbiliy Fig. 9. Influence of bckground rffic on ol scn ime Y: P[X x].9.8.7.6.5.4.3 Opp.Scn STA : CDF of IAT <SI=3ms, R=Mbi/s, seeds> BG STA.2 3BG STA 5BG STA. 8BG STA 9BG STA BG STA 5 5 2 25 3 35 4 45 5 x: IAT [ms] y: D Exchnge Durion [ms] 4 35 3 25 2 5 5 oppscnsta: AVG[DExchnge durion] <R=Mbi/s,seeds> SI=3ms SI=3ms SI=5ms 2 4 6 8 2 Fig. 8. Influence of bckground rffic on IATs of VoIP d flow Fig.. Sic Scenrio: Averge d exchnge durion of opscnsta simulneously. Considering published work by oher uhors [], [], our novel scnning scheme reduces he mximum number of supporble STAs only by one STA he gin of gurneeing QoS pplicion level nd conducing coninuous nework discovery. For n incresed bckground lod, he chnnel ccess dely increses nd is more nd more domined by he 82. bck-off procedure. Hence, he d exchnge beween he AP nd he opporunisic scnning STA is less deerminisiclly bound o muliples of he scnning inervl s shown in Fig. 8. The disribuion of he IAT of VoIP pckes chnges from wo-modl o uni-modl for incresed nework lod. The righ skewness reduces he sme ime o lmos zero for n lmos sured chnnel. For n unsured medium, he medin remins 2ms regrdless of he nework lod. We only noice n increse of he 99%-percenile of he IAT from 28ms o 4ms. The incresed bckground lod resuls in longer periods spend on exchnging user d per scn inervl. Thus, less ime is cully spen per scnning emp reducing he becon recepion probbiliy. Figure 9 illusres h he 99%- percenile incresed from 3ms for single VoIP bckground flow o up o 85ms for highes chievble sysem lod. In spie of his noiceble increse, his ime represens he longes possible ime required o gurnee deecion of he oher cell while upholding n ongoing, inerrupion-free communicion on pplicion level. For compleeness, Figures nd illusre he effec of n incresed bckground lod on he on effecive scn durion nd d exchnge durion for differen scn inervls. 3) Qunificion of proocol overhed: In he previous subsecion, we lredy qunified he overhed ssocied wih opporunisic scnning: he mximum number of supporble VoIP STAs wihin BSS is reduced by one. In he following, we highligh he cuse for his reducion. If d is pending for uplink rnsmission, opporunisic scnning does no impose ny overhed s ll he signling informion is conined in sus bis lredy presen in he 82. frme heder. Only if he STA does no hve o rnsmi d o he AP, i hs o send null d frme signling is
45 4 oppscnsta: AVG[Scn durion] <R=Mbi/s,seeds> SI=3ms SI=3ms SI=5ms 62 6 Overhed of sending PsPoll Frmes 35 y: Scn Durion [ms] 3 25 2 y: Overhed [#sen frmes/s] 58 56 54 5 5 2 4 6 8 2 52 SI=3ms SI=3ms SI=5ms 5 2 3 4 5 6 7 8 9 Fig.. Sic Scenrio: Averge scn durion of opscnsta Fig. 3. Number of sen Ps-Poll frmes per seconds y: Overhed [#sen frmes/s] 9 8 7 6 5 4 3 2 SI=3ms SI=3ms SI=5ms Overhed of sending Null D Frmes 2 3 4 5 6 7 8 9 Fig. 2. Number of sen Null D frmes per seconds se chnge for new scnning emp. Figure 2 illusred he influence of he bckground lod on he null d frme re for vrious scn inervls. The smlles considered scn inervl of 3ms resul in unproporionlly higher overhed s he 2ms-pckeizion re of he VoIP flow is lrger hence resuling frequenly in n empy d queue if signling informion is pending. The influence of he bckground lod is neglecfully smll; even for high nework lods, we observed h pending user d re rnsfered bulk sill cusing empy queues if signl informion hs o be rnsmied. Even hough he 82. power sve feure provides n enirely sndrd complin men of signling communicion puses used for opporunisic scnning emps, i hs one disdvnge: ech pcke buffered he AP for downlink rnsmission hs o be requesed by he STA using PS-Poll frme [8]. Accordingly, we expec ps-poll frme re of 5fps cused by he 5 VoIP pckes per second imposed by he pplicion. The medin of recorded frme res is consnly ner his esime regrdless of he bckground lod. Bu we observe n incresed righ-skewness of he disribuion of he recorded frme res: occsionl ouliers increse he men from 5fps o pproximely 6fps s illusred in Fig. 3. The incresed bckground lod cuses more pckes o be queued he AP which re hen ll polled for rnsmission in very shor ime. V. RELATED WORK Regrding nework discovery (probing) being he mos inensively sudied hndover phse, we only summrize mos novel reserch resuling in he bes known, published reducion of is durion. Rmni nd Svge [2] developed he SyncScn lgorihm which coninuously rcks nerby ccess poins using very shor lisening periods. In conrs o our novel opporunisic scnning scheme, SynScn follows deerminisic pproch s i synchronizes he shor lisening periods wih he periodic rnsmission of he becons of neighbor APs. This requires knowledge when such becon rnsmission occurs which is relized by cenrlized pproch ccumuling such knowledge. In conrs, opporunisic scnning does no require such synchronizion nor cenrlizion. Similr o our pproch, Rmni nd Svge buffer pckes he AP during scn emps. Bsed on SyncScn, Singh, Awl nd Sohi [3] s well s Chui nd Yue [4] presen n enhnced pproch feuring disribued ccess poin coordinion scheme. The ler enbles he synchronizion of becon rnsmissions wihin given ime period for ll APs in disribuion sysem opering on he sme chnnel. Even hough he ssocied dely is only in he order of 5 ms, he presened scheme is only pplicble o single-frequency neworks which we consider s limiion. The originl SyncScn lgorihm nd is enhncemen only know when becon of ny AP should be rnsmied. Hence, STAs scn for priculr AP s becon rnsmission even if hey re no wihin he ler s coverge. DeuceScn [5], [6] overcomes his drwbck including neighborhood grphs in he decision when o scn for becon rnsmissions.
Including he ms chnnel swich dely s observed by [2], DeuceScn cn reduce he ssocied dely o pproximely 3.5ms. As well s our Opporunisic Scnning, ll of hese bes known prcices uphold compibiliy wih legcy 82. implemenions. Even hough heir performnce qulifies hem s well for rel-ime pplicions, ll of hem required more knowledge of he infrsrucure in erms of precisely guessing becon rnsmissions, requiring sricly synchronized becon rnsmissions mong neighboring ccess poins, or ssume perfec knowledge regrding neighborhood informion for ll APs. Insed, Wu e. l. do no impose such consrins on he infrsrucure. Their procive scn pproch [7] cively scns ( possibly prioriized) lis of chnnels periodiclly before conneciviy is los nd immediely swich o noher chnnel when conducing hndover. They uilize smll scn inervl of ms which is used o cively scn for lernive APs using probe-requess nd -responses. Such ggressive scnning is known o reduce he nework ime spend in nework discovery o few milliseconds. In single-clien experimen, he uhors show h heir scheme is suible for VoIP communicion. Neverheless, he uhors did no evlue he impc of heir scheme on he scnned nework nd hence do no defe he concern h cive scnning schemes do no scle for lrge number of cliens. VI. SUMMARY & FUTURE WORK This pper pplies he novel opporunisic scnning scheme o 82. bsed communicion neworks. Power sve is hereby used s signling proocol o puse communicion beween STA nd AP for very shor ime while serching for oher sysems / echnologies in he STAs viciniy. Wihin his conex, we presened comprehensive performnce evluion of he proocol using VoIP s n exmple for relime communicion. Resuls show h opporunisic scnning resuls in bi-modl disribuion of user pcke iner-rrivl imes (IATs) wihou ffecing he IAT s medin corresponding excly o he 2ms pckeizion of he considered G7 voice codec. The 99%-percenile of recorded IATs lies less hn 4ms. Feuring gurneed nework discovery wihin 85ms wihou noiceble service inerrupion pplicion level, opporunisic scnning is fully cpble of supporing rel-ime pplicions such s VoIP. Cos-wise, he mximum number of simulneously supporble users in he WLAN cell is only reduced by one. We conclude h opporunisic scnning is well ble of coninuously monioring STAs environmen while upholding QoS consrins for rel-ime services. Hence, i is suible s (nework) discovery scheme for vrious pplicion scenrios such dynmic link selecion for lod blncing beween differen APs of he sme or differen echnologies, for fs hndover pplicions, nd even for coninuously scnning for primry users if 82.-bsed nework were o opere in he whie spce. This universl pplicbiliy of opporunisic scnning o he vrious pplicion scenrios is only chievble s his novel, sochsic scnning pproch does no impose ny requiremens upon he nework o be discovered. 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