A New per-class Flow Fixed Proportioal Differetiated Service for Multi-service Wireless LAN* Meg Chag Che, Li-Pig Tug 2, Yeali S. Su 3, ad Wei-Kua Shih 2 Istitute of Iformatio Sciece, Academia Siica, Taipei, Taiwa mcc@iis.siica.edu.tw 2 Dept. of Computer Sciece, Natioal Tsig Hua Uiversity, Hsichu, Taiwa {lptug, wshih}@cs.thu.edu.tw 3 Dept. of Iformatio Maagemet, Natioal Taiwa Uiversity, Taipei, Taiwa suy@im.tu.edu.tw Abstract. I this paper, we propose a ew per-class Flow fixed proportioal differetiated service model (CLAF) ad a compaio medium access cotrol scheme for multi-service wireless LANs (WLANs). The scheme is based o the IEEE 82. framework. Differet from covetioal relative differetiated service, i CLAF, a fixed badwidth quality ratio is guarateed o per-class per-flow basis regardless of the traffic load of each service class. Specifically, each service class is assiged a umber of separate coordiatio periods, proportioal to the policy-based badwidth quality ratio for class isolatio. Each class is associated with its ow cotetio widow size which is dyamically adjusted i accordace with the umber of flows i the class i such a way to miimize collisio probability betwee flows of the same class. Simulatios results of the CLAF performace as well as a compariso with IEEE 82.e EDCF icludig the support of QoS-sesitive VoIP applicatios are preseted. The results show that the proposed scheme outperforms EDCF ad achieves better resource utilizatio efficiecy. It ca provide users a more predictive, affirmative service guaratees tha covetioal relative differetiated service like IEEE 82.e EDCF. Itroductio Wireless LANs (WLANs) are gaiig sigificatly i popularity ad beig deployed at a rapid rate. The beauty of WLANs is that they are scalable with low etry cost. As IEEE 82. based WLAN peetrates further, its Quality of Service (QoS) support for multimedia applicatios such as VoIP ad video streamig is importat ad critical to the success of wireless commuicatios, especially to produce profitable busiess. I the past, several QoS mechaisms based o the IEEE 82. wireless LAN framework have bee proposed. Most of them base their methods o tuig three differet parameters: a) duratio of the Iterframe Space (IFS)[][2][3][4]; b) legth of the cotetio widow ()[][2][5][6][7][8]); ad c) legth of the backoff timer [3][9] to provide service differetiatio amog differet service classes. The * This research was partly supported by NSC uder grats NSC-92-223-E-2-88 ad NSC 92-223-E--. R. Boutaba et al. (Eds.): NETWORKING 25, LNCS 3462, pp. 382 394, 25. IFIP Iteratioal Federatio for Iformatio Processig 25
A New per-class Flow Fixed Proportioal Differetiated Service 383 IEEE 82.e EDCF [] is a example that combies the first two approaches. The scheme provides a simple relative differetiatio of badwidth sharig betwee classes. The problems with such distributed priority-based approach are two folds. First, the chael badwidth sharig ratio betwee classes is a fuctio of total chael load ad the umber of active wireless statios i the etwork. Secod, there is o traffic regulatio ad performace guaratees to flows withi the same class. Third, the lower classes ca experiece starvatio effects if o restrictio is placed o the load of higher classes. Schemes [4][7][9] were also proposed to provide proportioal chael badwidth sharig betwee multiple service classes. Better tha 82.e EDCF, these schemes have the merit of prevetig lower priority traffic from access starvatio. But i all these schemes, the cotetio probability icreases as more flows requestig the same class of service. Other related works iclude some relative differetiated service models proposed for wired etworks, such as strict prioritizatio[], price differetiatio[] ad capacity differetiatio[2][3][4]. As illustrated i [5], relative differetiated service ca ot provide cosistet service differetiatio, because resource allocated to each service class does ot reflect actual class load variatio. I this paper, we propose a ew service mode called per-class Flow fixed proportioal differetiated service model (CLAF) ad its wireless medium access cotrol scheme. Differet from covetioal relative differetiated service model [][][2][3][4], CLAF provides a fixed proportio o the badwidth sharig betwee multi-class flows. Here, a flow is a uidirectioal sequece of packets uiquely idetified by the IP addresses ad port umbers of the source ad destiatio statios, as well as the IP protocol type. For example, suppose there are two service classes. Class has oe flow ad Class 2 has two flows. I the case that the per-class flow badwidth quality ratio policy is 2:, accordig to the CLAF service model, the Class flow will receive /2 of the chael capacity ad each Class 2 flow will receive /4 of the chael capacity. The badwidth share ratio of Class flow ad Class 2 flow is 2:. The proposed medium access cotrol scheme for CLAF complies with the IEEE 82. framework. To achieve per-class flow fixed badwidth share proportio i a multi-service wireless LANs, separate umber of coordiatio periods are allocated to differet service classes to achieve class isolatio i chael access. Secod, the umber of coordiatio period allotted to a service class is proportioal to the class s badwidth share defied i the policy-specified badwidth quality ratio. Durig the coordiatio period, both upstream ad dowstream flows of the same class coted for packet trasmissio. Third, each service class is associated with a differet class cotetio widow size whose value is adaptive to the umber of flows i the class. Distributed flows of the same class will follow the baselie CSMA/CA protocol to resolve cotetio. Uder the scheme, flows of differet service classes access the wireless chael i a distributed, coordiated way. The advatages of CLAF iclude class isolatio, prevetio of lower priority traffic from access starvatio, ad the adaptatio of chael resource allocatio to differet service classes based o their actual traffic load.
384 M.C. Che et al. A commo problem with the IEEE 82. CSMA/CA-based medium access cotrol scheme is that the scheme does ot provide access poit (AP) the capability i chael access that reflects the traffic load at the AP. This is especially crucial for may APs servig as Iteret gateways as well as for etworks with asymmetric uplik ad dowlik traffic patter. I these eviromets, AP usually carries may more dowlik flows tha ay other wireless statio. It ofte results iefficiet resource utilizatio ad low overall chael throughput. I our proposed per-class flow fixed proportioal differetiated service, this problem is resolved by allocatig chael resources o per-class, per-flow basis. Moreover, such allocatio is i accordace with the actual umber of flows i the etwork. The paper is orgaized as follows. I Sectio 2, the proposed per-class Flow fixed proportioal differetiated service model (CLAF) is preseted. I Sectio 3, the medium access cotrol scheme is described i detail. I Sectio 4, the simulatio results of the CLAF performace are preseted. The, we compare the results with IEEE 82.e EDCF, icludig the support of QoS-sesitive VoIP applicatios. Sectio 5 gives the coclusio. 2 CLAF - The per-class Flow Fixed Proportioal Differetiated Service Model I this sectio, we preset the per-class Flow fixed proportioal differetiated service model for wireless local area etworks. Differet from covetioal relative differetiated service model, CLAF provides fixed badwidth share proportio to idividual flow of differet service classes. How badwidth share betwee flows is defied i a system parameter called badwidth quality ratio which is set by the etwork admiistrator as a resource maagemet policy. The CLAF service model is formulated as follows. Cosider a wireless local area etwork supportig K service classes. Let ϕ k be the target badwidth quality ratio parameter; ad ω k be the target throughput measuremet of a class k flow. The model imposes costrais of the followig form for all classes: ω : : ω2 :...: ωk = ϕ : ϕ2 :... ϕ K () where ϕ > ϕ2 >... > ϕk. The higher classes have the larger badwidth shares. Let the total umber of flows i service class k is N k, k=,, K. Give the ivariat badwidth quality ratio, the aggregate throughput of each service class chages as the umber of flows admitted to the class varies. We have the aggregate badwidth quality ratio of the K service classes, deoted as B }, as follows. B { k : B2 :...: BK = N ϕ : N 2 ϕ2 : Κ : N K ϕ (2) K For example, cosider a wireless etwork supportig two service classes with ϕ : ϕ2 = 4 :. The policy says that every Class flow would be guarateed four times as much the badwidth share as of a Class 2 flow. If there are two Class flows ad three Class 2 flows i the etwork, the aggregate badwidth share ratio would be 8:3.
A New per-class Flow Fixed Proportioal Differetiated Service 385 3 The Medium Access Cotrol Scheme The medium access cotrol scheme for the CLAF service model cosists of three parts: a) the baselie chael access procedure for the cotedig flows of the same service class; b) the coordiatio of the chael access betwee differet classes; ad c) the protocol for the joi ad leave of a flow. 3. Baselie Itra-class Chael Access Procedure I CLAF, the medium access cotrol procedure for flows of the same class follows the basic backoff procedure as defied i the Carrier Sese Multiple Access with Collisio Avoidace (CSMA/CA) protocol of the IEEE 82. Distributed Coordiatio Fuctio (DCF) mechaism. Whe a statio has a packet to trasmit, it geerates the radom backoff timer from the rage of to the curret Cotetio Widow-. The backoff timer is decremeted while the wireless medium is sesed idle ad is froze whe it is busy. Whe the backoff timer couts dow to zero, the statio trasmits the packet. If two or more statios trasmit at the same time, collisio occurs. I our scheme, a collided statio does ot double cotetio widow size for retrasmissio. The flow waits for the ext coordiatio period of the class it belogs to retrasmit the packet. 3.2 Iter-class Chael Access Scheme 3.2. The Size of the Class Cotetio Widow A ew distributed coordiatio algorithm is used to regulate the chael access betwee classes. First, each service class is associated with a class cotetio widow, deoted as k (t) whose size is determied based o the umber of flows i the class at time t as follows: ε ( t) = ( N ( t)) (3) k ε is the base cotetio widow fuctio, where ε is the target maximum collisio probability i a coordiatio period ad N k (t) is the umber of flows i class k at time t. Flows belogig to the same service class use the same class ε cotetio widow size i their backoff timer computatio. A example of specificatio is give i Table. k Table. A example of the base cotetio widow fuctio ε Number of flows 2 3 4 5.25 4 8 5 Number of flows 6 7 8 9 8 22 25 29 32.25
386 M.C. Che et al. 3.2.2 The Class Frames ad Coordiatio Periods EQ(3) oly specifies the rage of the backoff time for flows of the same class. To achieve per-class flow fixed badwidth quality ratio, two structures - Class Frames ad Coordiatio Periods - are used. First, the chael access time axis is divided ito a sequece of superframe. Each superframe cosists of K class frames, oe for each service class. A superframe always begis with the class frame of the highest priority class. For the k th service class, its class frame will cosist of ϕ k coordiatio periods. I the case that a service class has zero flow, its class cotetio widow size will be set to zero. It results i zero duratio of the correspodig class frame. I other words, the chael access will immediately cotiue to the ext lower service class without waste of chael capacity. Withi a class coordiatio period, flows of the correspodig class follow the basic itra-class chael access backoff procedure for packet trasmissio. Fig. depicts the iter-class chael access structure. Fig.. The iter-class chael access structure assumig K service classes 3.3 Chael Access ad Packet Schedulig at a Statio At the begiig of a superframe, the QoS AP broadcasts a Beaco message, icludig a list of the curret class cotetio widow sizes to use by wireless statios. The packet schedulig algorithm used at a wireless statio is give i Fig. 2. I the algorithm, a backlogged flow makes oe sigle chael access attempt i each coordiatio period of its associated class s Class Frame. Fig. 3 shows a possible implemetatio of the packet schedulig algorithm i the embedded kerel. The scheduler picks out the ext packet from the flow with the miimum remaiig backoff time to trasmit i the curretly-served service class ad forwards the packet to the MAC layer. I the embedded kerel, software ca be desiged to effectively maage per-flow FIFO queueig ad the associated backoff timers. Fig. 4 is a example that illustrates the proposed scheme i itra- ad iter-class chael access. There are two service classes with ϕ : ϕ2 = 3:. There are two A A statios - A ad B. Statio A has two Class flows ( f, ad f, 2 ), ad oe Class 2 A B B flow ( f 2, ); ad Statio B has a flow of each service class ( f, ad f 2, ). Usig the base cotetio widow specificatio i Table, we have the Class ad 2 cotetio widow size as =8 ad 2 =4, respectively. The backoff times of these flows at both statios are give i Table 2. Note that i this case the Class cotetio widow size is eight. Therefore, each Class coordiatio period spas eight idle time slot duratio to coordiate the start ad ed of a coordiatio period of a service class. This is ecessary because i wireless LAN, each statio does ot kow the actual umber of backlogged flows of each service class i the other statios but idividual class cotetio widow sizes. Eve though they kow, if
A New per-class Flow Fixed Proportioal Differetiated Service 387 there is a collisio, it is difficult to ifer how may statios were ivolved i the collisio. Therefore, the cotetio widow size i terms of umber of idle time slots is used for distributed coordiatio period sychroizatio. All statios moitor the chael evets ad cout the umber of idle slots to execute the proposed CLAF chael access scheme i a distributed ad coordiated fashio. Statio_ Packet _Schedulig_ad_Chael_Access_ Algorithm /* used by a wireless statio to determie packet schedulig sequece of flows of multiple classes i each superframe */ O receivig the Beaco Message { } for (i = to K) { /* For each Class Frame i */ for (j = to ϕ i ) { /* for each Coordiatio Period of Class i */ b = ; /* previous backoff timer */ prev } /* ed of j } /* ed of i Γ = φ ; /* iitialize u-served backlogged flow set */ if ( B φ ) /* B i : the set of locally backlogged flows of Class i. */ i Geerate a differet radom backoff time for each flow i B i, i.e. Γ = { b, } m B /* b i,m : backoff timer of flow m of class i */ i m i statiobackofftime = ; /* iitialize statio s backoff timer */ while ( Γ φ ) { Select a flow x such that b i, x = mi{ bi, m bi, m Γ} ; statiobackofftime = ( bi, x bprev ); Follow the baselie chael access procedure; Whe statiobackofftime couts dow to zero { Trasmit the head-of-lie packet from flow x to the etwork; /* remove flow from u-served backlogged flow set */ Γ = Γ \{ b i,x} ; = ; b prev b i, x }/* ed of whe */ } /* ed of while */ /* All backlogged class i flow have bee served. */ statiobackofftime = i b prev ;/* remaiig coordiatio period */ Follow the baselie chael access procedure; Cout dow statiobackofftime to zero; Fig. 2. The packet schedulig algorithm for chael access at a wireless statio I a coordiatio period, if a collisio occurs (as occurred i CP,2 i Fig. 4), all collided flows will retrasmit their packets i the ext coordiatio period. If the
388 M.C. Che et al. collisio takes place i the last coordiatio period of a Class Frame, retrasmissio will be deferred to the ext superframe. Hece, i CLAF, there is o expoetial backoff time computatio for packet retrasmissio. Fig. 3. The implemetatio architecture of the packet schedulig ad queueig at a wireless statio i the CLAP service model Fig. 4. The packet schedulig sequece ad chael access evets Table 2. Backoff times of the example i Fig. 4 Statio A B Class Class Class 2 Class Class 2 Flow Idex A f, A f, 2 Backoff Time 2 6 4 4 3 7 - - 3-5 - 3.4 Procedure for Flow Joi ad Leave I CLAF, the class cotetio widow size is dyamically adjusted based o the umber of flows of the service classes. To keep track of the umber of flows, a cotrol frame is used. As show i Fig. 5, each superframe is followed by a cotrol frame i which statios sed Re-associatio Requests to the QoS AP specifyig the umber of ew flows to joi ad the umber of flows to leave for each service class. A f 2, B f, B f 2,
A New per-class Flow Fixed Proportioal Differetiated Service 389 The QoS AP will reply a Re-associatio Respose to the statio with a status code idicatig whether the joi request is accepted or ot. Based o the results, the QoS AP computes ew class cotetio widow sizes, if ecessary ad updates all wireless statios i the ext Beaco message. No statios are allowed to sed Associatio ad Re-associatio messages i a superframe. Durig the cotrol frame, statios use a separate cotetio widow size deoted as CotrolFrameCoteetioWidow mi to radomize chael access backoff times whe submittig the joi/leave requests. Fig. 5. A chael access roud cosists of a superframe for multi-class packet trasfer plus a cotrol frame for otifyig flow joi ad leave 3.5 Base Cotetio Widow Fuctio Give the umber of flows of a service class, we wat to fid the cotetio widow size ( ε ( ) ) such that the probability of collidig flows is upper bouded by a small value ε i a coordiatio period. The computatio is based o the Iclusio- Exclusio Priciple ad is summarized as follows: Step : Compute the mea collisio umber by usig ε Step 2: Compute the expected umber of collided flows uder differet pair (, ), where. Here, ad p ( i) = Ck C F k F(, ), k!( F( k, k)), ( ) = i =, i 2 i, E k = i, [ Coll] = 3 p i=, i ( i) * i i i, ( ) Ci Ci i!( i) (5) i= Step 3: To get ε ( ), we ca fid smallest w that satisfies E [ Coll < ε from a sequeces of { E [Coll]}. The ε ( ) = w. 4 Performace Evaluatios ] I this sectio, we provide a performace evaluatio of the proposed per-class Flow fixed proportioal differetiated service model (CLAF) ad its medium access cotrol (4)
39 M.C. Che et al. scheme via simulatios. The simulator is implemeted i C[6]. The parameter values of the baselie Itra-class Chael Access procedure used i the simulatios are the same as those i 82. (see Table 3) i s-2[7] cofiguratio. Cosider N wireless statios ad a QoS AP. All commuicatio betwee statios must be forwarded by the QoS AP. Packets are fixed of legth K bytes. Table 3. IEEE 82. PHY/MAC parameters used i sumulatio SIFS µs Slot_time 2 µs DIFS 5 µs ACK Size 4 bytes MAC Header 28 bytes PreambleLegth 44 bits PLCP Header Legth 48 bits Rate Mbps 4. Fixed Proportioal Badwidth Differetiatio Fig. 7 shows the fixed badwidth sharig proportio is eforced betwee per-class flows regardless of the traffic load of idividual classes. Fig. 6 is the simulatio cofiguratio. There are three service classes with ϕ : ϕ2 : ϕ3 = 3: 2 :. Iitially, the badwidth quality ratio betwee Class flow ad Class 3 flow is approximately 3:. The throughputs of flows of the same class are closely equal. At time 5, two Class 2 flows joi the etwork. The resultig badwidth sharig ratio chages to 3:2:, while the actual throughput received by per-class flow is reduced due to the ew flows. Whe the ew flows leave, the throughput share restores to the iitial state. The aggregate throughput of each service class is show i Fig. 8, as the product of the class s badwidth share proportio ad the umber of admitted flows i the class. Fig. 6. The etwork cofiguratio of Fig.7 4.2 Performace Compariso with IEEE 82.e EDCF To illustrate the performace differece betwee CLAF ad EDCF, previous experimetal sceario is repeated for EDCF usig s-2. The miimum cotetio widow sizes for the Class, 2 ad 3 are 6, 32 ad 48, respectively. Fig. 9 shows the throughputs of flows of differet classes. The throughput proportio is odetermiistic ad has o direct relatioship with the miimum cotetio widow size. Moreover, throughput performace of idividual flow fluctuates heavily.
A New per-class Flow Fixed Proportioal Differetiated Service 39 Fig. 7. The per-class flow throughput performace usig CLAF ( ϕ ϕ : ϕ 3: 2 : ) : 2 3 = Fig. 8. The per-class aggregate throughput performace of Fig.7 Fig. 9. The per-class flow throughput performace usig EDCF ( ϕ ϕ : ϕ 3: 2 : ) : 2 3 =
392 M.C. Che et al. Fig.. The per-class aggregate throughput performace of Fig.9. Comparig the result with Fig. 7, CLAF ca guaratee a fixed badwidth proportio to idividual flow of differet service classes ad achieve more stable throughput performace at both the flow ad class levels. Furthermore, the aggregate throughput of the EDCF is lower tha that of the CLAF as show i Fig.. It shows that CLAF is more efficiet i the resource utilizatio. 4.3 VoIP over Wireless LAN I this experimet, we wat to show the merit of CLAF i the support of real-time multimedia applicatios. There are two kids of VoIP calls - G.7 (Class ) ad G.729 (Class 2). Each call is betwee a wireless statio ad a wired host coected to a LAN o the other iterface of the QoS AP. Hece, each VoIP call geerates a Fig.. The average throughput performace for VoIP usig CLAF ad EDCF
A New per-class Flow Fixed Proportioal Differetiated Service 393 uplik ad a dowlik flow i the wireless LAN, oe for each directio. The offered loads of the G.7 ad G.729 VoIP flow are.8kbps ad 44.8Kbps, respectively (icludig RTP/UDP/IP/MAC packet header overhead). Hece, the badwidth quality ratio is set to 9:4. I EDCF, both G.7 ad G.729 flows belog to the same access category 3 (AC3), ad the miimum ad maximum cotetio widow is 7 ad 5 respectively. Fig. compares the average throughput of a VoIP flow betwee CLAF ad EDCF uder differet umbers of VoIP calls. Oe ca see that the throughput performace of both G.7 ad G.729 flows are well protected uder CLAF regardless the umber of VoIP calls. For EDCF, the VoIP throughput performace is acceptable whe there is o cogestio withi the class (i.e. less tha te calls). For CLAF, the maximum umber of calls i this case is fourtee calls (i.e. 28 flows), more tha te calls for EDCF. CLAF ca ot oly provide idividual flow QoS but also achieves higher overall throughput performace. 5 Coclusios I this paper, we have preseted a ew per-class Flow fixed proportioal differetiated service model (CLAF) ad its medium access cotrol scheme for multiservice wireless local area etworks. Differet from covetioal differetiated services, CLAF provides a) policy-based fixed proportioal differetiated service; b) such fixed proportioal service differetiatio is o per-class flow basis; ad c) each class cotetio widow size is adjusted to reflect the actual traffic load of the class. The simulatio results show that the proposed scheme successfully provides per-flow fixed proportioal differetiated service betwee multiple service classes. Secod, we compare throughput performace betwee CLAF ad IEEE 82.e EDCF. The proposed scheme outperforms EDCF i terms of providig fixed proportioal badwidth share to idividual flows of differet service classes. It also achieves higher chael throughput. I the VoIP simulatio rus, the scheme ca as well better support real-time applicatios with QoS costraits. I summary, CLAF provides users a more predictive, affirmative service guaratees tha covetioal relative differetiated service like IEEE 82.e EDCF. Referece. IEEE 82.e draft/d4., Part : Wireless LAN Medium Access Cotrol (MAC) ad Physical Layer (PHY) Specificatios: Medium Access Cotrol (MAC) Ehacemets for Quality of Service (QoS), 22 2. I. Ada ad C. Castelluccia, Differetiatio Mechaisms for IEEE 82., i proceedigs of INFOCOM 2 3. J. Deg ad R. S. Chag, A Priority Scheme for IEEE 82. DCF Access Method, i IEICE Tras. Commuicatios, vol. E82B, No., 999 4. W. Pattara-atikom, S. Baerjee ad P. Krishamurthy, Starvatio Prevetio ad Quality of Service i Wireless LANs, i Proceedigs of WPMC 22
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