A Distributed Wireless MAC Scheme for Service Differentiation in WLANs

Transcription

1 A Disibued Wieless AC chee fo evice Diffeeniaion in WLAs A hesis subied in paial fulfillen of he equieens fo he degee of ase of Engineeing in Elecical and Eleconic Engineeing A he Univesiy of Canebuy, Chischuch, ew Zealand. A b d u l l a F i a g Depaen of Elecical and Copue Engineeing Univesiy of Canebuy Chischuch, ew Zealand ay 003

2 Absac obile counicaions is evolving due o he ecen echnological achieveens in wieless newoking. Today, wieless newoks eis in any fos, poviding diffeen ypes of sevices in a ange of local, wide aea and global coveage. The os widely used WLA sandad oday is IEEE 80.. Howeve, i sill has pobles wih poviding he Qo equied fo uliedia sevices using disibued ehods. In his hesis, a new disibued AC schee is poposed o suppo Qo in wieless LAs. In he schee, saions use CA fo channel access, wih collisions beween saions being esolved by sending a se of beacons in a pedefined anne, and viual collisions being esolved by schedules a he saions. The poposed AC schee is analyzed aheaically, fo wo-pioiy case, and he esuls obained ae validaed by siulaion. The aheaical odel esiaes he aveage delay epeienced by daa packes of pioiy one and wo unde diffeen condiions. A pefoance evaluaion sudy of he poposed AC schee as well as he IEEE 80. DCF, and IEEE 80.e EDCF AC schees is also done by eans of sochasic siulaion. I is found ha he esuls obained by siulaion ae in vey good ageeen wih he analyical esuls, heeby validaing he. oeove, he siulaion sudy evaluaed diffeen pefoance easues of hese AC schees. The esuls showed ha he IEEE 80. DCF schee does no suppo Qo, bu he poposed AC schee and he upcoing IEEE 80. EDCF boh do. In geneal, he esuls show ha he poposed AC schee pefos equally o bee han he cuen IEEE 80. DCF schee in evey case consideed. I is also found ha he poposed AC schee pefos equally well as he upcoing IEEE 80.e EDCF schee, in evey case consideed in his hesis. i

3 Acknowledgeens I gealy appeciae he help given by y supeviso, Associae Pofesso Hasha iisena. I would like o hank hi fo his consan guidance, suppo and encouageen houghou his hesis. I would also like o hank y co-supeviso, Associae Pofesso Kzyszof Pawlikowski fo his guidance, especially owads he copleion of his hesis. I a gaeful fo y colleagues a he ewok Reseach Goup a he Univesiy of Canebuy fo he updaed infoaion ha we shaed fo diffeen newoking aeas. In paicula, I would like o hank y wife fo he suppo houghou his hesis and also checking he conens of his hesis. I would no have his oppouniy wihou he scholaship povided by ZAID (ew Zealand Agency fo Inenaional Developen). Theefoe I would like o specially hank he. Finally, I would like o hank he os ipoan people in y life, y faily. You have always given e he sengh o ovecoe all he difficulies in life and I know you will be hee wheeve life akes e. ii

4 Table of Conens ABTRACT I ACKOWLEDGEET II TABLE OF COTET III LIT OF FIGURE VII LIT OF TABLE IX ABBREVIATIO X.0 ITRODUCTIO. BACKGROUD. GOAL OF THE THEI.3 OUTLIE OF THEI 3.4 REULT 4.0 WIRELE LOCAL AREA ETWORK 6. ITRODUCTIO 6. TYPE OF WLA 7.3 TECHOLOGIE UED I WLA 9.3. IFRARED 9.3. ICROWAVE LA PREAD-PECTRU LA 0.4 WLA ACCE TECHIQUE.4. FIXED AIGET.4.. FDA.4.. TDA iii

5 .4..3 CDA 3.4. RADO AIGET PURE ALOHA LOTTED ALOHA CA 6.5 IUE I WLA 8.5. QUALITY OF ERVICE 8.5. HIDDE AD EXPOED TERIAL PROBLE 0.6 UARY 3.0 WLA TADARD 3 3. ITRODUCTIO TO IEEE IEEE 80. PHYICAL LAYER D PHYICAL LAYER FH PHYICAL LAYER IR PHYICAL LAYER IEEE 80. EDIU ACCE COTROL UBLAYER GEERAL AC FRAE FORAT DITRIBUTED COORDIATIO FUCTIO (DCF) POIT COORDIATIO FUCTIO (PCF) IEEE 80.E PROTOCOL EHACED DITRIBUTED COORDIATIO FUCTIO (EDCF) HIPERLA TYPE HIPERLA TYPE UARY PROPOED AC CHEE FOR ERVICE DIFFERETIATIO OBJECTIVE OF THE PROPOED AC CHEE THE PROPOED AC PROCEDURE COLLIIO REOLUTIO EW TATIO AD ID UBER EHACEET UARY ATHEATICAL ODEL OF THE PROPOED AC CHEE 65 iv

6 5. TATIO HAVE OLY PRIORITY OE TRAFFIC BOTH TATIO AD CHAEL ARE FREE AVERAGE UBER OF COLLIDED TATIO CALCULATIG THE PROBABILITY OF THE ITUATIO CALCULATIG THE AVERAGE ERVICE TIE FOR THI ITUATIO THE TATIO I EPTY BUT THE CHAEL I BUY CALCULATIG THE PROBABILITY OF THE ITUATIO CALCULATIG THE TIE REAIIG TO ERVE PREVIOULY COLLIDED TATIO, R THE TATIO I BUY I ERVICE CALCULATIG THE PROBABILITY OF THE ITUATIO CALCULATIO OF THE AVERAGE ERVIC E TIE OF A TATIO AVERAGE DELAY OF THE PACKET, T 8 5. TATIO HAVE PRIORITY OE AD TWO TRAFFIC BUT THE CHAEL I OT FULLY OCCUPIED PRIORITY OE TRAFFIC PRIORITY OE QUEUE I OT ERVIG A PACKET AD THE CHAEL I FREE PRIORITY OE QUEUE I OT ERVIG A PACKET BUT THE CHAEL I BUY PRIORITY OE QUEUE I ERVIG A PACKET AVERAGE ERVICE TIE OF P PACKET PRIORITY TWO TRAFFIC PRIORITY TWO QUEUE I OT ERVIG A PACKET AD THE CHAEL I FREE PRIORITY TWO QUEUE I OT ERVIG A PACKET BUT THE CHAEL I BUY PRIORITY TWO QUEUE I ERVIG A PACKET AVERAGE ERVICE TIE OF P PACKET AVERAGE DELAY OF THE P AD P PACKET TATIO HAVE PRIORITY OE AD TWO TRAFFIC AD THE CHAEL I FULLY OCCUPIED PRIORITY OE QUEUE I OT ERVIG A PACKET BUT THE CHAEL I BUY FIDIG THE PROBABILITY OF THI ITUATIO TIE REAIIG TO COPLETE THE OGOIG COLLIIO REOLUTIO, R PRIORITY OE QUEUE I ERVIG A PACKET FIDIG THE PROBABILITY OF THI ITUATIO AVERAGE ERVICE TIE OF A TATIO AVERAGE DELAY OF P PACKET ATURATIO THROUGHPUT UARY 6.0 IULATIO ODEL AD REULT 3 6. ITRODUCTIO 3 6. DATA TRAFFIC ODEL THE WIRELE CHAEL ODEL 7 v

7 6.3. IPLEETATIO OF THE ODEL I ATLAB IEEE 80.E EDCF IULATED CEARIO IULATIO ODEL IPLEETATIO OF IEEE 80. DCF IULATED CEARIO IPLEETATIO OF THE PROPOED AC CHEE IULATED CEARIO IULATIO ODEL IULATIO OUTPUT DATA AALYI ETHOD OF IDEPEDET REPLICATIO ERROR AALYI REULT AD COPARIO AALYTICAL AD IULATIO REULT OF THE PROPOED AC CHEE COPARIO OF THE PROPOED AC CHEE WITH OTHER AC CHEE AXIU ATURATIO THROUGHPUT THROUGHPUT ACHIEVED WITH CHAEL ERROR ERVICE DIFFERETIATIO ACHIEVED BY THE AC CHEE PERFORACE OF THE AC CHEE WITH VOICE TRAFFIC COCLUIO COCLUIO AD FUTURE WORK COCLUIO FUTURE WORK 6 REFERECE 64 APPEDIX A ATLAB CODE UED I THE IULATIO 68 vi

8 Lis of Figues Figue. Diffeen configuaions of WLAs 8 Figue. FDA whee diffeen channels ae assigned diffeen fequency bands Figue.3 TDA whee each channel occupies cyclically epeaing ie slos 3 Figue.4 CDA in which each channel is assigned a unique pseudo-ando code which is ohogonal o pseudoando codes used by ohe uses 4 Figue.5 Pue ALOHA concep showing eansission of collided pack es 5 Figue.6 loed ALOHA concep, showing eansission of collided packes 6 Figue.7 Eaples of hidden and eposed saion poble Figue 3. IEEE faily of poocols 4 Figue 3. IEEE 80. infasucue newok 5 Figue 3.3 IEEE 80. OI Refeence odel 6 Figue 3.4 D PLCP fae foa 7 Figue 3.5 FH PLCP fae foa 8 Figue 3.6 IR PLCP fae foa 9 Figue 3.7 Geneal AC fae foa 3 Figue 3.8 Geneal AC fae foa 3 Figue 3.9 uccessful daa ansission in he basic access ehod 34 Figue 3.0 uccessful daa ansission by using RT/CT ehod 35 Figue 3. An eaple of collision avoidance by using he backoff pocedue 36 Figue 3. PCF and DCF aleaion 38 Figue 3.3 Taffic caegoies (TCs) of a saion wih he schedule o esolve inenal collisions 40 Figue 3.4 Backoff pocedue of TCs of diffeen pioiy 4 Figue 3.5 Refeence odel of HIPERLA/ 4 Figue 3.6 The EY-PA poocol aciviies 43 Figue 3.7 HIPERLA/ AC fae foa 45 Figue 4. Diffeen pioiy affic of a saion wih a schedule o esolve inenal collisions 48 Figue 4. Channel accessing pocedue fo new and collided daa of Pi 49 Figue 4.3 Daa ansission using RT/CT 5 Figue 4.4 Daa ansission using RT/CT, in collision esoluion phase 5 Figue 4.5 Daa ansission wihou using RT/CT, in collision esoluion phase 5 Figue 4.6 aions infoaion able 53 Figue 4.7 Collision esoluion peiod fo collided daa of Pi 53 Figue 4.8 Pocedue of he collision esolving beacons phase of he collision esoluion peiod 54 Figue 4.9 Collision Resoluion fo a B ha has 3 saions 56 Figue 4.0 TP essage passing ehods in scheduled daa ansission 57 Figue 4. Pocedue o add a saion o a given B 60 Figue 4. Adding a new saion ino he B 6 Figue 4.3 Reoving non-esponding saions fo he B 6 Figue 4.4 A saion is checking whehe i is sill a ebe of he B 64 Figue 5. The AC odel fo he siuaion when all he saions in he B have only Pioiy one affic 66 Figue 5. When a packe aives o a saion he channel and he saion is epy 68 Figue 5.3 aus of he channel duing a ie ineval. 7 Figue 5.4 The scenaio when a packe aives o a saion, he channel is busy bu he saion is epy 76 Figue 5.5 Residual sevice ie 77 Figue 5.6 Residual sevice ie of he siuaion, when a packe aives o a saion, he saion is epy bu he channel is busy 78 Figue 5.7 The scenaio showing a saion is busy when a packe aives o he saion 80 Figue 5.8 Aveage delay epeienced by he pioiy one packes 83 Figue 5.9 AC odel fo saions, which has wo affic caegoies 84 Figue 5.0 When a packe aives o he pioiy one queue of a saion he channel and he queue is epy 85 Figue 5. aus of he channel duing a ie ineval. 88 vii

9 Figue 5. When a P packe aives o a saion he channel is busy bu he saion s P seve is epy 89 Figue 5.3 Residual sevice ie 9 Figue 5.4 Residual sevice ie fo his siuaion 9 Figue 5.5 When a P packe aives o a saion P seve is busy 93 Figue 5.6 When a packe aives o he pioiy wo queue of a saion he channel and he queue is epy 97 Figue 5.7 aus of he channel duing a ie ineval. 99 Figue 5.8 When a packe aives o he P queue of a saion, he P seve is epy bu he channel is busy 0 Figue 5.9 Residual sevice ie 0 Figue 5.0 Residual sevice ie fo his siuaion 0 Figue 5. When a packe aives o P queue of a saion he P seve is busy 06 Figue 5. Aveage delay of pioiy one and wo packes 0 Figue 5.3 When a P packe aives o a saion he channel is busy bu he saion s P seve is epy Figue 5.4 aus of he channel duing a ie ineval. Figue 5.5 Residual sevice ie 3 Figue 5.6 Residual sevice ie fo his siuaion 4 Figue 5.7 When a packe aives o a saion he saion is busy 6 Figue 5.8 Aveage delay of pioiy one and wo packes 9 Figue 5.9 The B is in sauaion condiion wih pioiy daa 0 Figue 5.30 auaion houghpu of he poposed AC schee Figue 6. CBR affic geneaion 6 Figue 6. -sae akov chain O-OFF affic odel 6 Figue 6.3 Backgound affic geneaion 7 Figue 6.4 Bus eo odel epesened by -sae coninuous-ie akov chain 7 Figue 6.5 Alenaing good and bad saes in he channel eo odel 9 Figue 6.6 Two pioiy level siulaion odel of he IEEE 80.e EDCF 30 Figue 6.7 iulaion odel of he IEEE 80.e EDCF 3 Figue 6.8 COTETIO UUCCEFUL pocedue in IEEE 80.e EDCF 33 Figue 6.9 COTETIO UCCEFUL pocedue in IEEE 80.e EDCF 34 Figue 6.0 iulaion odel of he IEEE 80. DCF 35 Figue 6. Two pioiy level siulaion odel of he poposed AC schee 37 Figue 6. iulaion odel of he poposed AC schee 38 Figue 6.3 Collision Resoluion pocedue 39 Figue 6.4a Thee pilo uns o find he nube of iniial obsevaions ha need o be discaded 4 Figue 6.4b The aiu siulaed ie esiaed was good enough fo 0 pecen of iniial daa o be discaded 43 Figue 6.5 Plo of esiaed ean values wih 95% confidence ineval 46 Figue 6.6 Copaison of siulaion and analyical esuls obained fo P affic in he poposed AC schee 47 Figue 6.7 Copaison of siulaion and analyical esuls obained fo P and P affic in he poposed AC schee 48 Figue 6.8 auaion houghpu of he poposed AC schee 49 Figue 6.9 auaion houghpu of he IEEE 80.e EDCF 50 Figue 6.0 auaion houghpu of he IEEE 80. DCF 50 Figue 6. auaion houghpus of he AC schees wih daa payload of 000 byes 5 Figue 6. Thoughpu of he AC schees wih pesence of channel eo, whee BER 0 eans ha BER G BER B 0, and BER > 0 eans ha BER G 0-0 and BER B Figue 6.3 evice Diffeeniaion achieved fo diffeen pioiy voice seas 53 in he poposed AC schee 53 Figue 6.4 evice Diffeeniaion achieved fo diffeen pioiy voice seas 53 IEEE 80.e EDCF 53 Figue 6.5 evice Diffeeniaion achieved fo diffeen pioiy voice seas 54 IEEE 80. DCF 54 Figue 6.6 Daa houghpu achieved fo each pioiy class of he poposed AC schee 54 Figue 6.7 Daa houghpu achieved fo each pioiy class of he IEEE 80.e EDCF 55 Figue 6.8 Daa houghpu achieved fo each pioiy class of he IEEE 80. DCF 56 Figue 6.9 Aveage Delay of he voice packes 57 Figue 6.30 Pefoance of O/OFF voice wih pesence of backgound daa affic 58 Figue A. Conens of he aached CD-RO 68 viii

10 Lis of Tables Table 5.0 AC paaee descipions Table 5. uaized esuls showing he pobabiliy of a saion being sevice a a given posiion Table 5. uaized esuls of he above hee secions... 8 Table 5.3: The AC paaees Table 5.4 uaized esuls Table 5.5 uaized esuls...08 Table 5.6 uaized esuls of he above hee secions...7 Table 5.7 Paaees of he poposed AC schee... Table 6. Paaees of IEEE 80.e EDCF...3 Table 6. Paaees of IEEE 80. DCF...36 Table 6.3 Paaees of he poposed AC schee...37 Table 6.4 Eaple o show confidence ineval calculaion...45 i

11 Abbeviaions ACF ACK AIF AIFC A AIFC i AIF i AIIF AP BCCH BER BA B CAC CBR CDA CFP CP CR CRB CRC CRIF CA CA-CA CA-CD CT CW DBPK DCF DIF DiL DL DLC Auhenicaion Confiaion Fae Acknowledgeen Auhenicaion iniiaion fae Abiay Inefae pace fo auhenicaion collision esoluion Abiay Inefae pace fo Collided daa of pioiy i Abiay Inefae pace fo ew daa of pioiy i Auhenicaion iniiaion inefae space Access poin Boadcas conol channel Bi eo ae Basic sevice aea Basic sevice se Channel access conol Coninuous bi ae Code division uliple access Conenion fee peiod Conenion peiod Collision esoluion Collision esoluion beacon Cyclic edundancy code Collision Resoluion IF Caie sense uliple access CA collision avoidance CA collision deecion Clea o send Conenion window Diffeenial binay phase shif keying Disibued coodinaion funcion DCF inefae space Diec Link Downlink phase Daa link conol

12 DQPK D D-CDA D EDCF EIF E EV EY-PA FCCH FC FDD FD FDA FH-CDA FH GFK G HC HIPERLA IB IEEE IF IR I LA LLC AC A PDU AP AV PB PCF PDU PF PHY PIF Diffeenial quadaue phase shif keying Disibued syse Diec sequence CDA Diec sequence spead specu Enhanced disibued coodinaion funcion Eended inefae space Eended sevice se Eliinaion suvival veificaion Eliinaion yield non pe-epive pioiy uliple access Fae conol channel Fae check sequence Fequency division duple Fequency division ulipleing Fequency division uliple access Fequency hopping CDA Fequency hopping spead specu Gaussian shaped fequency shif keying Global yse fo obile Hybid coodinao High pefoance local aea newok Independen B Insiue of Elecical and Eleconics Enginees Inefae space Infaed Indusy, science and edicine Local Aea ewok Logical link conol ediu access conol eopolian aea newok AC poocol daa uni AC sevice access poin ewok allocaion veco o packe beacon Poin coodinaion funcion Poocol daa uni Pesisence faco Physical laye PCF inefae space i

13 PLCP PD PPB PP QB Qo RA RCH RLC RT DIF IF LRC RC TAID TC TDD TD TDA TP TR TXOP UL WEP WLA Physical laye convegence pocedue Physical ediu dependen Packe pesen beacon Pulse posiion odulaion Qualiy of evice suppoing B Qualiy of evice Radio access Rando access channels Radio link conol Reques o send cheduled Daa IF ho inefae space aion long ey coun aion sho ey coun aion idenificaion Taffic caegoy Tie division duple Tie division ulipleing Tie division uliple access Token Pass Token Receive Tansission oppouniy Uplink phase Wied equipen pivacy Wieless Local Aea ewok ii

14 C H A P T E R.0 Inoducion This Chape povides a geneal backgound and oveview of he hesis. The goals of he hesis ae pesened and a bief suay descibing he succeeding chapes is included. This chape concludes wih a bief discussion of he esuls of he hesis.. Backgound obile counicaion is fas evolving oucoe of ecen echnological achieveens in wieless newoking and poable devices, like noebook copues. Today, wieless newoks eis in any fos, poviding diffeen ypes of sevices in a ange of local, wide aea and global coveage. Ipoveens in boh wieless and wied newok echnologies like eegence of Voice-ove-IP (VoIP) have esuled in poviding uliedia sevices o obile uses. Wieless Local Aea ewoks (WLAs) ae deployed alos eveywhee oday. The WLA ake is gowing a annual ae of 40 o 60 %, accoding o he Wieless LA Alliance (WLAA) [pecalink 000]. The inenaional IEEE 80. sandad fo WLAs is he diving foce behind his gowh. The ai of he IEEE 80. coiee is o define a coon ai ineface (hough sandadizing he daa link, ediu Access Conol (AC) and physical laye funcions and specificaions) which is independen of he undelying wieless ansission echnologies [Toh 997]. uliedia applicaions ipose equieens on counicaion paaees, such as daa ae, dop ae, delay and jie. Cobining delay sensiive uliedia sevices, like voice, wih seas of delay non-sensiive daa affic ove he sae he backbone ceaes los of pobles. Today, newok anages ae looking fo a pacical soluion o povide he Qualiy of evice (Qo) equied fo such applicaions.. Goals of he Thesis Alhough, IEEE 80. sandad is he os widely used WLA sandad oday, i has pobles wih poviding Qo equied fo uliedia sevices. The IEEE 80. sandad poposes o use

15 Chape : Inoducion wo diffeen ehods o access he channel. The disibued access ehod, Disibued Coodinaion Funcion (DCF), is used noally fo non eal-ie sevices. The cenalized access ode, Poin Coodinaion Funcion (PCF), is used fo eal-ie sevices, o povide highe pioiy han ha of non-eal ie sevices, as eplained in [Cocke e al. 00], [Chen and Yeh 00], and [Peick 997]. Howeve, he cenalized schee equies he eisence of access poins wih specialized funcions, and oeove, as i is concluded in [Visse and Zaki 995], he cenalized ode pefos pooly. Recenly, disibued ehods fo ipleening Qo in IEEE 80. sandad via sevice diffeeniaion have been poposed; see fo eaple [Kishnakua and obinho 996], [Aad and Caselluccia 00] and [Bay e al. 00]. On he basis of hese esuls, IEEE 80. Task Goup E cuenly defines enhanceens o IEEE 80. sandad, called IEEE 80.e, which inoduces Enhanced DCF (EDCF). Like he DCF, he EDCF is a disibued access ehod in which no saion has oe esponsibiliy han ohes o ainain he funcionaliy of he newok. The ain objecive of he EDCF is o suppo sevice diffeeniaion aong diffeen pioiy affic. As he os widely used WLA sandad, IEEE 80. sandad has pobles wih poviding he Qo equied fo uliedia sevices, he ain goal of his hesis is o develop an alenaive fully disibued ediu Access Conol (AC) schee ha suppos sevice diffeeniaion in WLAs. In addiion, his hesis also pesens: pefoance evaluaion of he sevice diffeeniaion achieved by he IEEE 80. poocol, pefoance evaluaion of he sevice diffeeniaion achieved by he upcoing IEEE 80.e poocol, an alenaive fully disibued ediu Access Conol (AC) schee ha suppos sevice diffeeniaion in WLAs, an analyical odel fo he poposed AC schee, and pefoance copaison of he poposed AC schee wih IEEE 80. and IEEE 80.e poocols. In ode o cove he above opics he hesis is divided ino seven chapes. The ouline of he chapes is given in he ne secion.

16 Chape : Inoducion.3 Ouline of Thesis This secion povides a bief oveview of he chapes in his hesis. The hesis is oganized as follows: Chape : This Chape povides a geneal backgound and oveview of he hesis. The goals of he hesis ae given and a bief suay descibing he oveview fo succeeding chapes ae pesened. This chape concludes wih a bief discussion of he esuls of his hesis. Chape : In he pas decade, a gea deal of eseach has been done in wieless counicaion. Thee have also been nueous new developens in his aea. This chape gives an oveview of he basic esuls abou wieless newoks. This chape biefly eplains he diffeen ypes of wieless newoks, wieless LA access echniques, and ajo issues egading wieless LAs such as Qualiy of evice and hidden and eposed einal pobles in wieless newoks. Chape 3: The os widely used wieless LA sandads oday ae IEEE 80. WLA and HIPERLA. This chape gives deailed descipions of he os widely used WLA sandads; he IEEE 80. WLA and HIPERLA. The chape also gives a bief inoducion o he upcoing WLA sandad IEEE 80.e. The IEEE 80.e is able o suppo Qualiy of evice in wieless envionens by ehod of sevice diffeeniaion. Chape 4: This chape is one of he ain chapes of he hesis. The chape poposes a fully disibued AC schee ha suppos sevice diffeeniaion in a wieless LA envionen. In his schee, saions use CA fo channel access, wih collisions beween saions being esolved by sending a se of beacons in a pedefined anne. Viual collisions, i.e. clashes beween daa packes of diffeen pioiy wihin a saion, ae esolved by a schedule inside he saion. This chape also pesens ohe elevan infoaion equied fo he poposed AC schee. Chape 5: This chape pesens a deailed aheaical odel of he poposed AC schee. In ode o siplify he odel, a aiu of wo pioiy classes ae consideed. The odel is deived o esiae he aveage delay epeienced by pioiy one and wo packes unde diffeen condiions. The assupions ade o siplify he odel ae given in he chape wih bief discussions. The chape also pesens an epession o esiae he sauaion houghpu 3

17 Chape : Inoducion of he pioiy one affic. This epession is used o esiae he sauaion houghpu of he poposed AC schee when he packe payload and he nube of saions in a given B ae given. Chape 6: This chape discusses a pefoance evaluaion sudy of he poposed AC schee as well as he IEEE 80. DCF, and IEEE 80.e EDCF AC schees by eans of sochasic siulaion. iulaion odels wee developed using ATLAB. The chape pesens he siulaion odels used and he esuls obained. The chape also copaes esuls obained analyically and by siulaion fo he poposed AC schee. Fuheoe, his chape also pesens a copaison sudy of he poposed AC schee wih IEEE 80. DCF and IEEE 80.e EDCF. Chape 7: This chape povides conclusions dawn fo he wok done in his hesis. This chape also suggess possible fuue wok ha can be done o epand he wok pesened in his hesis. Appendi A: The appendi conains ATLAB scips used fo he siulaion of he cuen IEEE 80. DCF, he upcoing IEEE 80.e EDCF, and he poposed AC schees. The scips conain ea infoaion abou he odels..4 Resuls Alhough, he IEEE 80. sandad is he os widely used WLA sandad oday, i sill has pobles wih poviding Qo equied fo uliedia sevices using disibued ehods. As a esul, disibued ehods fo ipleening Qo in IEEE 80. sandad via sevice diffeeniaion have been poposed by seveal eseaches. On he basis of hese eseaches, IEEE 80. Task Goup E cuenly defines enhanceens o IEEE 80. sandad, called IEEE 80.e, which inoduces new ehods o suppo sevice diffeeniaion aong diffeen pioiy affic by using a disibued ehod. As IEEE 80.e is sill no sandadized, he ain ai of his hesis is o develop a AC schee ha suppos Qo in wieless LAs. In he poposed schee, saions use CA fo channel access, wih collisions beween saions being esolved by sending a se of beacons in a pedefined anne, and viual collisions esolved by a schedule inside he saions. 4

18 Chape : Inoducion The poposed AC schee was analyzed aheaically and he esuls obained wee validaed by siulaion. I was seen ha he esuls obained by he aheaical odel wee in vey good ageeen wih he siulaion esuls. Boh he siulaion and analyical esuls of he poposed AC schee show ha he delay epeienced by he highe pioiy affic has less aveage delay copaed o he lowe pioiy affic. This eans ha he poposed AC schee is capable o suppo Qo in wieless LAs, by eans of sevice diffeeniaion. The esuls also showed ha he upcoing IEEE 80.e EDCF suppos sevice diffeeniaion bu he cuen IEEE 80. DCF does no. In geneal, he esuls show ha he poposed AC schee pefos equally o bee copaed o he cuen IEEE 80. DCF schee in evey single case consideed. I is also found ha he poposed AC schee pefos as well as he upcoing IEEE 80.e EDCF schee, in evey single case consideed in his hesis. 5

19 C H A P T E R.0 Wieless Local Aea ewoks The nube of copue-based syses have gown because of ecen advances in echnology and as a esul any diffeen ypes of copue newoks have been inoduced and ipleened. One of he is called he wieless Local Aea ewoks (WLAs). As is nae iplies, a local aea newok coves a liied geogaphical aea, unlike wide aea newok [Hoppe e al 986]. In he pas decade, a gea deal of eseach has been done in wieless counicaion. Thee have also been nueous new developens in his aea. This chape gives an oveview of he basic esuls abou wieless newoks. This chape biefly eplains diffeen ypes of wieless newoks, WLA access echniques, and ajo issues egading WLAs such as Qualiy of evice (Qo) and hidden and eposed einal pobles in wieless newoks.. Inoducion In ypical LAs he newok eleens such as seves, einals, pines and ohe peipheal ae conneced by a syse of coppe o opical fibe wies. These ae wied LAs. In he wied LAs he cabling cos can be as high as 40 pecen of he whole insallaion [anaaia and Lopez 993]. Each newok eension o econfiguaion would equie addiional wiing which is boh ie consuing and cosly. Tadiional wieless newoks, such as codless cellula elephones, paging syses, obile daa newoks, and obile saellie syses, have epeienced enoous gowh ove he las wo decades. Wih his gowh in wieless counicaion indusy, a new concep of pesonal wieless counicaion syse, WLAs has appeaed in he indusy due o he above enioned econoic facos. Unlike in wied LAs, he WLAs can offe poabiliy and lowe cos of insallaion. In geneal WLAs have hese advanages ove he wied LAs [Wesolowski 00].

20 Chape : Wieless Local Aea ewoks Fleibiliy WLA nodes can counicae wih each ohe wihin he newok coveage aea wihou ajo liiaions in he einal locaions. iplified Planning The newok planning is elaed o adio pa. Howeve configuaion of newok is no necessay. Possibiliy of a epoay newok configuaion Wieless counicaions open a possibiliy o consuc a local newok which is only needed epoaily. Like evey ohe achieveens, WLAs also have soe disadvanages as a esul of using a adio channel as he signal popagaion ediu. The ain disadvanages include: Lowe ansission qualiy Wied LAs which uses opical fibe as a ansission ediu has channel eo ae a os 0-0. Howeve in WLAs he eo ae in he adio channel can be as high as 0-3 and even wose. Cos of wieless equipen The coponens of he WLAs is sill uch highe han he equivalen coponens of wied LAs. Lowe level of daa secuiy The infoaion ansied on he adio channel can be ineceped uch easie han in wied LAs and also inappopiaely used WLA can be a souce of inefeence fo ohe sensiive devices such as edical equipen. Alhough hee ae disadvanages like hese in he WLAs, he WLA indusy is gowing apidly due o hei benefis. Today, hee ae wo widely acceped WLA sandads: IEEE 80. and HIPERLA. In each of hese WLA, he newok can opeae eihe as ad hoc newok o as an infasucue newok. These WLA configuaions and oe deails abou he WLAs ae given in he following secions.. Types of WLAs WLAs ay be divided ino goups accoding o hei newok configuaions. On his basis, he WLAs can eihe be infasucue-base wieless newoks o ad hoc wieless newoks. These wo ypes of WLAs ae shown in Figue.. An ad hoc newok is a collecion of wieless obile hoss foing a epoay newok wihou he aid of any esablished infasucue o cenalized adinisaion. This is he siples configuaion of he WLA. Two o oe obile hoss if a any ie ae wihin ange of each ohe, can se up an ad hoc independen newok. In ad hoc newoks, he obile saions 7

21 Chape : Wieless Local Aea ewoks counicae diecly wih each ohe. Evey obile saion ay no be able o counicae wih evey ohe saion due o ange liiaions. obile ad hoc newoks do no ely on any fied infasucue bu counicae in a self-oganized way. The ain advanages of he ad hoc newok ae ha i has no single poin of failue. A ajo disadvanage of he ad hoc newok is ha when opeaing acoss lage newoks, pefoance will degade unless he ansie powe is enhanced. Wieless infasucue-base newok Wieless ad hoc newok Figue. Diffeen configuaions of WLAs An inegaed wieless and wied LA is called an infasucue configuaion. In infasucue WLAs, he ipoan eleens ae newok Access Poins (APs) which ineface he wieless einals wih he wie-line newok infasucue. In os cases ansission is pefoed only beween access poins and wieless einals, so wo newok einals counicae via he appopiae access poins. The ain advanage of he newok is ha he newok can be designed o opeae elaively efficienly in he use of he signal ansission powe. o copaed o he ad hoc configuaion, use saions in he infasucue newok can each saions a wice he disance wih he sae signal powe. The ajo disadvanage of he newok is he pesence of a single failue poin. If he AP fails hen he whole newok will fail. 8

22 Chape : Wieless Local Aea ewoks.3 Technologies used in WLAs Thee disinc echnologies ae cuenly in use fo WLA syses: infaed ligh syses, cellula syses opeaing a 8-9 GHz [Buchholz e al 99], and unlicensed spead-specu syses opeaing in he I bands ( Hz,.4-.5 GHz, and GHz) [acus 99]..3. Infaed The ansission echnology used in infaed (IR) LAs is he sae as ha found in failia consue eleconic devices used fo eoe conol of elevision ses, VCRs, and ohe eneainen poducs, as well as in wieless keyboads and wieless pine connecions fo lapop copues. Tansies used in hese applicaions ae siple and inepensive LEDs, and eceives ae siilaly siple avalanche phoodiodes o p-ininsic-n phoodiodes. IR syses opeae a vey low powe levels [Pahlavan and Levesque 995]. As esul, sepaae IR LAs ae no likely o inefee wih each ohe, unless placed vey close ogehe. Unlike adio ansissions, IR signals will no peneae walls, and his is advanageous in applicaions whee signal confineen wihin a walled office is desiable. Disadvanages fo IR LAs ae ha soe applicaions have liied ange elaive o adio LAs, inefeence fo he sun and ohe ligh souces, and sensiiviy o shadowing. An Eaple of IR LAs is he InfaLA Token-Ring yse, anufacued by InfaLA Technologies, Inc. The syse uses he oken-ing uliple access poocol and is in fac fully coplian wih he IEEE 80.5 oken ing sandad..3. icowave LAs WLAs opeaing a ens of gigahez offe high daa aes han which ae cuenly achievable wih os IR LAs. The signal powe levels fo hese syses ae facions of a Wa [Pahlavan and Levesque 995], and he signals ae confined wihin a geneal aea in which he newok is Indusy, cience and edicine (I) 9

23 Chape : Wieless Local Aea ewoks o be used. The naue of popagaion a hese fequencies is such ha signals ae significanly consained by dense sucues like concee floos o any building walls. As a esul, signal ansission can be vey saisfacoy houghou an open office aea bu confined enough by floos and walls o pei euse of he sae fequencies in ohe poions of he sae building, wihou inefeence beween syses. The ain disadvanage of he 8-9 GHz echnology is ha he adio ansceives fo his band ae ahe specialized and epensive. An eaple of icowave LA poduc is Alai and Alai Plus syses, which ae deployed in icocells, uch like cellula elephone syses [Feebug 99]. These poducs opeae a 8 GHz band and povide copaibiliy wih he IEEE 80.3 Ehene poocol..3.3 pead-pecu LAs pead-pecu WLAs ae designed o use in he 900 Hz, GHz, and 5 GHz I bands and can be opeaed wihou he need fo FCC licensing. These syses ae liied o powe levels less han W and is ypically inended o povide signal coveage up o abou This coveage aea is geneally lage han ha povided by eihe IR o icowave LAs. The use of spead-specu and ansission, cobined wih effecive uli-use access poocols such as CA 3, ake i feasible o deploy uliple syses in he sae geneal aea, even hough signal coveage is ovelapping. The dawback of unlicensed opeaion is ha one us be able o opeae in he pesence of inefeence fo ohe uses of he sae bands. The IEEE 80. coiee has developed a WLA sandad, IEEE 80. WLA, fo opeaion a ansission aes of bis/sec and bis/sec in he GHz band. The sandad specifies he wo daa aes fo boh fequency-hopped and diec-sequence speadspecu (FH and D) adio ansission. The bi ae have been since inceased o bps, bps, and 54 bps in he new sandads; IEEE 80.a, IEEE 80.b, and IEEE 80.g. The Fedeal Counicaions Coission (FCC) is an independen Unied aes govenen agency, diecly esponsible o Congess. The FCC was esablished by he Counicaions Ac of 934 and is chaged wih egulaing inesae and inenaional counicaions by adio, elevision, wie, saellie and cable. 3 Caie ense uliple Access (CA) 0

24 Chape : Wieless Local Aea ewoks.4 WLA Access Techniques Regulaing he access o he shaed link is an ipoan ask in he ansission of packes on he WLAs o achieve efficien opeaion and good pefoance. Uses in he wieless newok seldo have need o access a channel fo a long peiod of ie. The schees ae needed fo poviding uli-use access o he fequency and ie esouces of he newok in an odely anne and in a way ha iniizes ansission ovehead while aiizing oveall newok capaciy. These schees ae called ediu Access Conol (AC) poocols. The AC poocol egulaes he access o he channel by giving each node a chance o ansi is packe. This conol is necessay since hee is a coon ediu which is shaed by any nodes. Wihou his conol seveal nodes could be ansiing siulaneously poducing couped essages. The wo ajo caegoies of edia access conol poocols fo WLAs ae fied assignen and ando assignen. These wo ajo caegoies ae biefly eplained in he following subsecions..4. Fied Assignen In fied assignen echniques, access o he coon channel is independen of use deands and soe poion of he esouces of he channel is assigned o each use in a saic, pedeeined anne. In os fied assignen ehods hee is no ovehead, in he fo of conol essages. Howeve, due o fied assignen, pas of he channel igh be idle even hough soe uses have daa o ansi [Ro and idi 990]. The os well known fied assignen echniques ae he Fequency Division uliple Access (FDA), Tie Division uliple Access (TDA), and Code Division uliple Access (CDA). These echniques ae biefly descibed below. A good discussion of hese echniques can be found, fo eaple, in [Rappapo 996]..4.. FDA FDA is he os widespead syse, and he os copehensible. This echnique is buil upon he well known fequency-division ulipleing (FD) schee fo cobining non ovelapping use channels fo ansission as a wide-bandwidh signal. FDA assigns

25 Chape : Wieless Local Aea ewoks diffeen individual channels o individual uses. As seen fo Figue., each use is assigned a unique fequency band o channel. These channels ae assigned on deand o uses who equess he sevice. Duing daa ansission, ohe uses canno shae he sae fequency band. FDA is used in he cellula obile elephone syses and in VHF and UHF land obile adio syses. FDA is also he os coon fo of uliple access in saellie newoks [Bhagava e al 98]. Code Channel Channel Channel 3 Channel Fequency Tie Figue. FDA whee diffeen channels ae assigned diffeen fequency bands.4.. TDA TDA syse divides he adio specu ino ie slos as shown in Figue.3, and in each ie slo only one use is allowed o eihe ansi o eceive. TDA schee is build upon ie-division ulipleing (TD). In fied assignen TDA opeaion, a ansi conolle seves o assign uses o ie slos, and an assigned ie slo is held by a use unil he use eleases i. A he eceiving end, a use saion synchonizes o he TDA signal fae, and eacs he ie slo designaed fo ha use. Two diffeen fos of TDA ae being used in wieless syses; ie-division duple (TDD), and fequency-division duple (FDD). In a TDA/TDD syse, alenaing ie slos on he sae caie fequency ae assigned o he fowad and evese diecions of he counicaion. Howeve, in TDA/FDD syses, siila fae sucue would be used solely fo eihe

26 Chape : Wieless Local Aea ewoks fowad o evese ansission, bu caie fequency would be diffeen fo he fowad and evese link. TDA is he access ehod used in cuen Euopean cellula syses, G 4, and he Unied aes cellula syse, I-54. Code Channel Channel Channel 3 Channel Tie Fequency Figue.3 TDA whee each channel occupies cyclically epeaing ie slos.4..3 CDA The hid access ehod is called code division uliple access (CDA). This can be viewed as a hybid cobinaion of FDA and TDA. In CDA, uliple uses opeae siulaneously ove he enie bandwidh of he ie-fequency signaling as shown in Figue.4. In CDA syses, he naowband essage signal is uliplied by a vey lage bandwidh signal called he speading signal. The speading signal is a pseudo-ando noise code sequence ha has a chip ae which is odes of agniudes geae han he daa ae of he essage. Each use has is own pseudoando codewod which is appoiaely ohogonal o all ohe codewods. The speading signal is used o povide sufficien degees of feedo o be able o sepaae he use signals in he ie- fequency doain. The wo coon CDA echniques used ae diecsequence CDA (D-CDA) and fequency hopping CDA (FH-CDA). 4 Global yse fo obile 3

27 Chape : Wieless Local Aea ewoks Tie Channel Channel Channel 3 Channel Code Fequency Figue.4 CDA in which each channel is assigned a unique pseudo-ando code which is ohogonal o pseudoando codes used by ohe uses Eaples of a WLA using spead-specu echnique is he CR s WaveLA [Tuc 99], and he IEEE 80.. The IEEE 80. uses D and FH echnologies opeaing in.4 GHz I bands..4. Rando Assignen The fied assignen ehod is suiable when each use has a seady flow of daa o be ansied. Howeve, if he infoaion o be ansied is ineied o busy in naue, fied assignen access ehods can esul in counicaion esouces being wased os of he ie. On his conas, ando assignen access ehods povide a oe fleible and efficien way of anaging channel access fo counicaion of sho essages. In ando assignen echniques, he enie bandwidh is povided o he uses as a single channel o be accessed andoly. In his ehod, he success of a ansission is no guaaneed in advance. The eason is ha wheneve wo o oe uses ae ansiing on he shaed channel siulaneously, a collision occus and he daa canno be eceived coecly. This being he case in his ehod, packes ay have o be ansied and eansied unil evenually hey ae coecly eceived. In designing his ype of access ehod, he ain concen is ansission scheduling. The ando access is pobably he iches faily of ediu access poocols. any local aea newoks oday use ando assignen ehods due o is sipliciy so ha hei ipleenaion is saighfowad. 4

28 Chape : Wieless Local Aea ewoks Biefly discussed below ae he os coonly used ando access ehods: () he basic ALOHA schee; () enhanced vesion of he basic ALOHA, sloed-aloha; (3) Caie ense uliple Access wih Collision Deecion (CA-CD); (4) Caie ense uliple Access wih Collision Avoidance (CA-CA). A good discussion of CA, and ohe uliple access echniques can be found, fo eaple, in [Haond and O Reilly 988]..4.. Pue ALOHA This poocol deives is nae fo he ALOHA syses, a counicaion newok developed by Abason and his colleagues a he Univesiy of Hawaii [Abason 970]. The basic idea of pue ALOHA is siple. Uses ansi iediaely wheneve hey have daa o send. If he ansission is successful he eceive sends an acknowledgen o he sende wihin a ie peiod equal o one popagaion ie. Popagaion ie is he ie i akes a packe o avel fo he sende o he eceive and back again. If no acknowledgen is eceived, ha eans he packe is los due o a collision in he channel and he packe will be sen again wih a andoly seleced delay o avoid epeaed collisions. The opeaion of he pue ALOHA is shown in Figue.5. Use Use 3 3 Use Toal Collision Reansission Reansission Figue.5 Pue ALOHA concep showing eansission of collided packes.4.. loed ALOHA I can be shown ha he aiu houghpu achieved by he pue ALOHA is 0.84; see fo eaple [Waland 99]. Hence, he efficiency achieved by he pue ALOHA is vey low. The sloed ALOHA schee was poposed o incease he efficiency of he ALOHA ehod [Robes 975]. In he schee, he ie ais is divided ino ie slos wih duaions equal o he ie 5

29 Chape : Wieless Local Aea ewoks aken o ansi a packe on he channel. All he uses ae hen synchonized o hese ie slos, so ha when a use einal geneaes a daa packe, he packe is held and ansied in he ne ie slo. In he sloed ALOHA, he ineval of vulneabiliy o collision fo any packe is educed o one packe ie fo wo packe ies in he pue ALOHA, as shown in Figue.6. Use Use 3 3 Use Toal Collision Reansission Reansission Figue.6 loed ALOHA concep, showing eansission of collided packes As he ineval of vulneabiliy is educed by a faco of wo, he aiu houghpu of he sloed ALOHA eaches o 0.368; see fo eaple [Pahlavan and Levesque 995]. o he efficiency of he sloed ALOHA is inceased copae o he pue ALOHA. loed ALOHA is used in wieless daa counicaion applicaions whee long ansission delays ae encouneed. The poocol is used in aellie counicaion newoks and ohe daa counicaion syses as well CA The efficiency of he ALOHA schees ae low due o he fac ha uses ake no accoun of wha ohe uses ae doing when hey aep o ansi daa packes, and his leads o a high ae of packe collisions. A bee esul can be obained if a use saion lisens o he channel befoe aeping o ansi a packe. This echnique is used as he basis of seveal poocols eed caie sense uliple access (CA). Thee ae seveal coon poocols of his ype. These poocols ae biefly descibed below. The siples fo of CA is one in which each use einal wih daa o ansi fis lisens o he channel o deeine if ohe uses ae ansiing. If he channel is busy, he use einal lisens coninuously, waiing unil he channel becoe idle, and hen sends a daa packe iediaely. This poocol is called -pesisen CA, due o is ansission saegy, which 6

30 Chape : Wieless Local Aea ewoks is o ansi wih pobabiliy as soon as he channel is available. The objecive of he schee is o avoid collisions wih packes of ohe uses. onpesisen CA is anohe ype of CA. In his ype, he use saion does no sense he channel coninuously while he channel is busy. Insead, afe sensing he busy condiion, i wais a andoly seleced ineval of ie befoe sensing again. As wih -pesisen CA, a use wih daa o send begins ansiing iediaely when he channel is sensed o be idle. The andoized waiing ies beween channel sensing eliinae os of he collisions ha would esul fo uliple uses ansiing siulaneously upon sensing he ansiion fo busy o idle condiion. This leads o uch highe houghpu values copaed o -pesisen CA. Anohe vaiaion of he CA is p-pesisen CA. The schee is only usable in sloed channels. In he schee, when a use saion has daa o send, i senses he channel. If he channel is sensed idle, i ansis wih pobabiliy p. Wih pobabiliy - p he saion defes acion o he ne ie slo, whee i senses he channel again. If ha slo is idle, he saion ansis wih pobabiliy p o defes again wih pobabiliy - p. This pocedue is epeaed unil he packe is ansied o he channel is sensed o be busy. When he channel is sensed o be busy, he saion hen senses he channel coninuously, and when he channel becoes idle, he saion sas he above pocedue again. Beside CA, soe newoks have addiional feaues, naely, Collision Deecion (CD) and Collision Avoidance (CA). CA-CD is used only in he wied newoks. The collision deecion ehod is no suiable fo wieless newoks because a saion is unable o lisen o he channel fo collisions while ansiing. The ain eason is he lage diffeence beween ansied and eceived signal powes a he saion. As a esul soe WLAs use CA-CA insead of CA-CD. CA-CD - In CA-CD, he use can deec inefeence aong seveal ansissions while ansission is in pogess and abo ansission of hei collided packes. If his can be done sufficienly fas hen he duaion of an unsuccessful ansission would be shoe han ha of a successful one. The opeaion of all CA-CD poocols ae idenical o he opeaion of he coesponding CA poocols, ecep ha if a collision is deeced duing ansission, he ansission is aboed and he packe is scheduled fo ansission a soe lae ie. 7

31 Chape : Wieless Local Aea ewoks CA-CA - As enioned befoe, CA-CA is used in wieless newoks insead of CA- CD. One eaple of a wieless newok which uses his pocedue is IEEE 80. WLAs. In CA/CA, he collision avoidance pa is done by using he ando backoff pocedue. In his pocedue, if a saion wans o send a daa fae, he saion iniially senses he saus of he channel. If he saion senses ha he channel is busy hen he saion wais unil he channel becoes idle. When he channel is sensed o be idle, he saion calculaes a ando backoff ie. Afe waiing his aoun of backoff ie, he saion ansis he packe. If he packe ansission is unsuccessful he saion epeas he above pocedue choosing he ando backoff ie fo a lage ineval, unil he packe is successfully ansied..5 Issues in WLAs As he WLA ake gows, new feaues ae equied o ee use deands. Wih inceasing echnologies, people deand o use daa-inensive, ie-sensiive daa affic like audio and video aound wieless newoks. Howeve, oday, hese affic ae cobined and seviced as a single daa sea wih elaively siple ypes of daa like e. o he equied feaues of hese affic is no e. As a esul of his, Qualiy of evice (Qo) has becoe a ajo issue in he wieless newoks. Fuheoe, new pobles like hidden and eposed saion pobles ae found wih he use of WLAs. These wo issues of he WLAs ae discussed in he following secions..5. Qualiy of evice In he las few yeas, he IEEE 80. and HIPERLA wieless LAs have been inceasing in populaiy. Today hese WLAs, specifically IEEE 80. WLAs have been ipleened in any diffeen envionens such as offices, hoes, and ohe places. Howeve hese WLAs sill need o be ipoved in he sense of Qo suppoed by he. Qo is a newoking e ha is a bi oe cople han i igh sound. Qo efes o he concep of being able o conol and easue daa ansission aes, o houghpu, and eo aes. Fundaenally, Qo enables you o povide bee sevice o ceain flows. This is done by eihe aising he pioiy of a flow o liiing he pioiy of anohe flow. Deliveing e and ohe elaively siple ypes of daa aound a newok does no necessaily equie cople Qo 8

32 Chape : Wieless Local Aea ewoks echaniss. Hence, in he pas, sic easues fo Qo did no ae because he daa was no uliedia and he end-use would no noice o be aeially affeced by laencies. Today, he os widely used WLA sandad is IEEE 80.. IEEE 80. uses wo diffeen ehods o access he channel as will be eplained in chape 3. Is disibued access ehod, Disibued Coodinaion Funcion (DCF), is used noally fo non eal-ie sevices and he cenalized access ode, Poin Coodinaion Funcion (PCF), is fo eal-ie sevices o povide highe pioiy han fo non-eal ie sevices. Howeve, he cenalized schee equies he eisence of access poins wih specialized funcions. The cenalized schee has he dawback in ha sense ha failue of he access poin leads o failue of he whole newok. o in cenally conolled newoks, he pefoance of he newok vey uch depends on he coec opeaion of he access poin. Due o his eason, uses pefe a disibued conol schee. The basic access ehod of IEEE 80. AC is DCF. The faes in DCF do no have pioiies, and hee is no ohe echanis o guaanee an access delay bound o he saions. In ohe wods, eal-ie applicaions like voice o live video affic have o copee wih non ealie daa affic o access he channel. As a esul, eal-ie affic suffes in he sense of laency. To ovecoe his poble, he IEEE 80. sandad needs ipoveen so ha diffeen pioiy affic can be suppoed in DCF faes. Today, eseaches have poposed seveal diffeen appoaches o suppo Qo wihin he faewok of he IEEE 80. WLA sandad. obinho and Kishnakua poposed a schee called Blackbus, wih he ain goal of iniizing delay fo eal ie affic [Kishnakua and obinho 996]. Blackbus equies ha all high pioiy saions y o access he ediu wih consan inevals and also equies he abiliy o ja he wieless ediu fo a peiod of ie. Low pioiy saions use he odinay DCF access echanis of IEEE 80.. One of he dawbacks of his schee is ha i suppos only wo pioiy affic classes. Ohe ehods o suppo Qo in IEEE 80. sandad by eans of sevice diffeeniaion have been poposed. These ehods include: Backoff inceasing funcion: Each pioiy class affic has diffeen backoff inceen funcion. DCF Ine Fae pace (DIF): Each pioiy class affic is assigned a diffeen DIF. aiu fae lengh: Each pioiy class affic has a aiu fae lengh allowed o be ansied a once. 9

33 Chape : Wieless Local Aea ewoks By using hese diffeeniaion echaniss, Task Goup E of he IEEE 80. woking goup ae cuenly woking on an eension o he IEEE 80. sandad called IEEE 80.e. The goal of his eension is o enhance he access echaniss of IEEE 80. and povide a disibued access echanis ha can povide sevice diffeeniaion. All he deails have no ye been finalized, bu a new access echanis called Enhanced DCF (EDCF), which is an eension of he basic DCF echanis, is used o achieve Qo. The deail abou he IEEE 80.e sandad and ohe widely used wieless LA sandads, IEEE 80. and HIPERLA ae given in chape Hidden and Eposed Teinal Pobles Collision deecion has caused any pobles in newoking and his is paiculaly he case wih wieless newoks. Collisions occu when wo o oe nodes shaing a counicaion ediu ansi daa ogehe, he wo signals coup each ohe, and he esul is gabage. This has always been a poble fo copue newoks and he siples poocols ofen do no ovecoe his poble. oe cople poocols check he channel befoe ansiing daa. This is vey siple wih Ehene as i eely involves checking he volage on he wie befoe ansiing. Coupled wih his, he hidden and eposed einal poble has been idenified in wieless syses and has caused pobles in collision deecion. The Figue.7 illusaes hese wo siuaions. In Figue.7a, he ansission anges of obile einal A and C do no allow he o hea each ohe, bu can boh be head by einal B in beween. The einals A and C ae hidden einals wih egads o each ohe. As a esul, einal A igh sense he channel o be idle alhough einal C is ansiing. o einal A igh sa o ansi even if einal C is ansiing. If his happens, he assage eceived a he einal B will be couped because ansission fo boh einal A and C will inefee a ha poin. Hence he essage is unusable by einal B. This poble is called he hidden einal poble in wieless newoks. In ode o eove he hidden einal poble, soe wieless LA sandads have inoduced he fou-way handshake pocedue based on RT and CT. In his pocedue, a souce einal, a einal which wans o send a packe, fis sends a RT essage o he desinaion einal. The desinaion einal esponds o his by sending a CT essage o he souce einal. The 0

34 Chape : Wieless Local Aea ewoks RT and CT essages conain infoaion abou how long he channel will be busy fo he daa ansission. o all he einals which ae in ange of he souce einal will know how long he channel will be busy fo RT essage and all he einals which ae in ange of he desinaion einal will know how long he channel will be busy fo CT essage. Hence, if he CT essage is eceived coecly by he souce einal, all he einals in ange of boh souce and desinaion einals will efain fo ansiing. Then he souce einal can send he daa packe wihou a hidden einal poble. On he ohe hand, soe ohe WLAs paially avoid he hidden einal poble by ensuing he channel sensing ange o be uch highe han he eceiving ange. A B C D A B C D a. Hidden saion poble b. Eposed saion poble Figue.7 Eaples of hidden and eposed saion poble In Figue.7b he eposed einal poble is descibed. A node is eposed when i is wihin he sensing ange of he sende bu ou of he inefeing ange of he eceive. In he case of he figue, einal C is ansiing a packe o he einal D. A he sae ie, einal B wans o send a packe o einal A. Bu einal B is waiing unil he ansission of einal C is coplee because einal B is in ange of einal C. Howeve, he einal B eally does no have o wai unil he ansission of einal C is coplee because a einal A, he ansission sengh of saion C will be vey weak copaed o ansission sengh of saion B and so he einal A can eceive coecly he ansission of he einal B alhough he einal C is ansiing a he sae ie. This siuaion is called he eposed saion poble because soe saions efain fo ansiing alhough hey eally do no have o. The use of RT and CT access ehod iniizes he eposed einal poble bu can no eove copleely. Hence i can deeply affec he pefoances of uli hop wieless ad hoc newoks.