DBASE : A Distributed Bandwidth Allocation/Sharing/ExtensionProtocol for Multimedia over IEEE 802.11 Ad Hoc Wireless LAN

Shiann-Tsong Sheu” and Tim-Fang Sheu+

*Department of Electrical Engineering, Tamkang University, Tamsui, Taipei Hsien, Taiwan‘Institute of Communication Engineering, National Tsing-Hua University, Hsing Chu, Taiwan

stsheu@ee.tka.edu. tw, sunnie@ieee.org

Abstract—In ad hoc networks, carrier sense multiple access(CSMA) is one of the most pervasive medium access control(MAC) schemes for asynchronous data traffics. However,CSMA could not guarantee the quality of real-time traffics. Inthis paper, we will propose a distributed bandwidthallocation/sharing/extension (DBASE) protocol to supportmultimedia traffics with the characteristics of variable bit rate(VBR) and constant bit rate (CBR) over ad hoc WLAN. Overallquality of service (QoS) will be guaranteed in DBASE. Suchbandwidth allocation procedure is based on a contention processthat only occurs before the first successful access and areservation process after the successful contention. If any real-time station leaves, the reserved bandwidth will be released byDBASE immediately. The designed DBASE protocol will notonly allocate sufficient bandwidth for real-time stations but alsopermit them to extend bandwidth requirements on demand ifthere is any excess bandwidtb left. Moreover, the proposedDBASE is still compliant with the IEEE 802.11 standard. In thispaper, the system capacity of DBASE is analyzed and theperformance of DBASE is evaluated by simulations. Simulationsshow that the DBASE is able to provide high channel utilization,low access delay and small delay variation for real-time services.

1. INTRODUCTION

Since the beginning of the 1990s, wireless local areanetworks (WLANS) for the 900 MHz, 2.4, and 5 GHzindustrial, scientific, and medical (ISM) bands have beenavailable based on a range of proprietary products [1]. As thespeed and capacity of wireless networks increase, so does thedemand for improved quality of service (QoS) for real-timemultimedia applications. The IEEE 802.11 WLAN standard[2] includes a basic Distributed Coordination Function (DCF)and an optional Point Coordination Function (PCF). The DCFuses Carrier Sense Multiple Access with Collision Avoidance(CSMA/CA) as the basic channel access protocol to transmitasynchronous data in a contention period. An attractivefeature of CSMA/CA protocol is that it is simple toimplement however, the contention-based MAC protocolcannot provide delay guarantees to real-time traffic due toeach packet must contend for transmission. To provideperformance guarantees, some packet-radio networks haveopted for collision-free schemes based on polling, fixedassignments (using time or frequency division multiplexing),or reservation. The PCF in the IEEE 802.11 standard is apolling-based protocol. Real-time stations access channel in around-robin manner during a contention free period. However,the use of centralized scheme in PCF constrains the operation

of wireless LANs. Furthermore, several researchers [3-4]pointed out that such centralized protocol results in a poorperformance.

Due to these reasons, a distributed multiple accessprotocol for voice over WLAN was proposed [5-6]. In thisproposed scheme, voice stations sort their access rights byjamming the channel with pluses of energy, which is namedBB contention, before sending their voice packets. Sincevoice packets must be transmitted repeatedly in a constantinterval, sending bursts of energy for each packet will wasteconsiderable bandwidth. Moreover, the BB contention is nota regular scheme defined in IEEE 802.11 standard, and thus itis difficult to be overlaid on current CSMA implementations.Although the chaining scheme proposed in [6] is used toenhance the efficiency of i3B contention, the splitting ofchain, which occurs while a node ends a session or its packetis corrupted, will also reduce the efficiency. Furthermore, theasynchronous data might transmit its packet in the hole ofchain and make the access instants of real-time packetsstretched. These real-time packets might be dropped becausethe packet delay exceeds the delay-bound.

Another protocol supporting real-time multimedia trafficin a WLAN based on Group Allocation Multiple Access(GAMA) was proposed [7-9]. In GAMA, owing to at mostone new session could access the channel and reserve thebandwidth in a cycle, the medium access delay mightincrease sharply when the load becomes high. Besides, thereis no differentiation between the priorities of real-time andasynchronous data traffics. For the prior reserved members,they have the higher priorities to occupy more bandwidth,which might make the QoS of others decrease. Thus GAMAis not a fair protocol and doesn’t consider the overall QoS.Therefore, we propose a new distributed protocol, which willnot suffer the potential problems of antecedents, to supportmultimedia traffics over IEEE 802.11 ad hoc WLAN.

For simplicity, a station with non-real-time (real-time)traffic is denoted as rev-station (rt-station) throughout thispaper. To make sure the rt-traffic meets the delay restrictions,time-sensitive rt-packets always have higher priorities thanordinary nrt-packets. In the proposed protocol, the nrt-stations regulate their accesses to the channel according to thestandard CSMA/CA protocol. On the contrary, the rt-stationswill reserve the bandwidth and transmit their rt-packetswithout contention after the first access. At the first access

0-7803-7018-8/01/$10.00 (C) 2001 IEEE IEEE INFOCOM 2001

time, a modified CSMA/CA protocol with RTS/CTS controliiames is used for compatible with IEEE 802.11 standard. Tosupport both constant bit rate (CBR) service and variable bitrate (VBR) services over WLAN, the scare bandwidth will beefficiently allocated, shared, and released by our proposedDBASE protocol. Moreover, the DBASE also guarantees thequality of service (QoS) by limiting the maximum size of thereservation repetition interval.

The rest of the paper is organized as following. Section 2describes the DBASE MAC protocol consisting of DCF fornrt-traftlcs and the distributed bandwidth allocation/sharing/extension protocol for rt-traffics over WLAN. The systemcapacity of DBASE in WLAN is analyzed in Section 3.Simulation models and results of the proposed DBASE arediscussed in Section 4. Finally, Section 5 summarizes theconclusions.

2. DBASE PROTOCOL

In this section, we will describe the access procedures fortransmitting nrt-packets and rt-packets separately. In DBASE,we divide the frames into three priorities as the standard does.The frames of different priorities have to wait different inter-fiame spaces (IFSS) before they are transmitted. The shortIFS (SIFS) is used by immediate control flames, whichalways have the highest priority, such as clear to send (CTS)and acknowledge (ACK). The priority IFS (PIFS) is used bythe rt-frames, such as reservation frame (RF) and the requestto send (RTS) of voice and video packets. The DCF IFS(DIFS) is the longest IFS and used by the nrt-frames, whichalways have the lowest priority, such as the data packets. Theaccess procedure for transmitting asynchronous data packetsis based on the CSMA/CA protocol and is briefly describedin section 2.1. We control the rt-stations to access themedium by a modified CSMA/CA protocol, which still canbe overlaid on the IEEE 802.11 standard implementation. Theaccess procedure for rt-sources is described in section 2.2.

2.I The Access Procedure for Asynchronous Data Stations

The basic access method for nrt-stations is based onconventional DCF [2]. A nrt-station with a data packetwaiting for transmitting needs to monitor the channel untilthe medium is determined to be idle for the duration of aDIFS period and the backoff procedure is finished. After then,the nrt-station generates a random backoff time for the datacontention window. The data backoff time (DBT) is derivedby

DBT = rand(a,b) x Slot_time,

where rand(a, b) returns a pseudo random integer withininterval [a, b], which b grows exponentially for eachretransmission attempt and the range of b is ilom predefineb~,. to b~u. By the IEEE 802.11 standard, a is set to O; b~,nand b.m are set as 32 and 1024 for simplicity. That is

b= b~,. X 2’ ~ brew.

where r is the number of retransmission times. The Slot_time,which is defined as the time needed for a station to detect apacket, to accumulate the time needs for the propagationdelay, the time needed to switch from the receiving state tothe transmitting state (~_TX_Turnaround_Time), and thetime to signal to the MAC layer the state of the channel (busydetect time) [10]. The backoff time counter is decreased aslong as the channel is idle, and froze while the mediumbecomes busy. The nrt-station transmits its packet (or RTS ifany) only when its backoff time counter becomes zero. Whenthe destination receives the packet correctly, it will transmitan ACK to the source within a SIFS.

2.2 The Access Procedure for Real-time Stations

In DBASE, the rt-stations contend for the medium by therequest packets (RTSS) to join the Reservation Table (RSVT)and reserve the needed bandwidth. The RSVT is a virtualtable built and maintained by each rt-station respectively, andit records the information of all rt-stations that have finishedthe reservation procedure successfully. The informationincludes the access sequence, the MAC address, the packetlength, the service type and the required bandwidth of eachrt-station. After a rt-station (STA~~) joins into the RSVTsuccessfidly, it will not need to contend the medium anymore during this whole session. In order to maintain thecorrect access sequence, each rt-station needs to be equippedwith a sequence ID (SID) register and an active counter (AC).The SID is used to record the access order among all activert-stations and the AC is used to record the total number ofactive rt-stations at this moment.

Now, we will describe how the rt-stations reserve,allocate, share and extend the bandwidth in the following foursubsections.

2.2.1 Reservation Procedure

If STA~~ intends to start a session at time t,itwill monitorthe channel for detecting the Reservation Frame (RF) in theinterval (t,t+DmJ, where D.u means the smallest maximaltolerance delay among all active rt-connections. D~a stronglydepends on the characteristic of rt-service. Parameter D~m isusually a constant value and predefine in system. The RF isa broadcast ti-ame and used to announce the beginning ofcontention free period (CFP). The RF is sent by the first rt-station (SID= 1) and this rt-station has the right to send its rt-packet first in the CFP. This rt-station which has theresponsibility to initiate the CFP (i.e. by issuing a RF flame)periodically is named as contention fkee period generator(CFPG). We consider that the repetition interval is equal toD.a because the real-time information delayed for more thanD.a will cause unacceptable quality and must be discarded.The RF flame mainly carries the information of the numberof active rt-stations (AN) in the basic service set (BSS) andthe information of all rt-stations recorded in the RSVT ofCFPG.

0-7803-7018-8/01/$10.00 (C) 2001 IEEE IEEE INFOCOM 2001

A new active rt-station STA~~ will detect the RF fi-ame inthe interval (t, t+D~J if there is any active rt-station alreadyreserved the bandwidth. If a RF flame is not received duringthe interval (t, t+D~J, it means that there is no active rt-station. Thus, when the channel still perceives idle in the

interval (t+D.w+&, t+D.a+c +PIFS] and none RF frame isdetected, STA~~ will execute the backo~jscheme. The symbolc denotes the remaining transmitting time of the current PDU(protocol data unit) at the time instance t+Dmm. Thecontention process is still based on the CSMA/CA protocol.The real-time backoff time (RBT) of a rt-station is defined asfollowing:

RBT = rand(c, d) x Slot_time,

where the function rand(c, o returns a pseudo randomnumber in the range from c to d. In our scheme, c and d areset to O and 3, respectively. The RBT time counter isclecreased as long as the channel is idle, and suspended whilethe medium is sensed busy. If RBT reaches zero, STA~~willtransmit its RTS to the destination station to setup aconnection. If no collision occurs, the rt-station STA~~,whichsends RTS tlame, will receive the CTS ffame within SIFS,and then STA~~ will play the role of the CFPG in network.Meanwhile, it sets both its SID and AC to one and transmitsthe RF frame and its rt-packet right away. In the proposedDBASE, the RTS/CTS control frames for rt-traffics will begenerated only before the first successful access. The lengthclf the first rt-packet of each session is limited as the averagelength decided by the traffic type of this session.

To prevend nrt-station’s transmissions from disturbing rt-station’s transmissions, the relationships among three spacesand two contention windows are shown in Figure 1 and mustsatis& the following two constrains:

SIFS + Slot_time s PIFS,

PIFS + Slot_time + max{RBT} < DIFS + min{DBT}.

In this paper, the propagation delay can be ignored becausethe diameter of a Basic Service Area (BSA) is only on thecinderof 100 feet [10].

Contrarily, if collision occurs when transmitting RTS, theI’-persistent scheme is used to decide whether the collidedstations insist on accessing channel in the next Slot_time.Such collision is detected by a rt-station not receiving its CTSwithin the following duration of SIFS after transmitting theRTS frame. The collided rt-station will retransmit the RTS inthe following Slot_time with a probability p. With aprobability q= l-p, it will defer at least one Slot_time andcontend at the next real-time contention window. That is, itwill recalculate the RBT by the following equation

RBTP = rand(c + 1, d) x Slot_time.

This scheme will efficiently reduce the contentionresolving period. As soon as the CFPG is being generated,

non-real-tlmc

. DIFs___ contention windowH

real-t]me contention (DBT) ‘+::34 g~ b

,/”’ ,,,‘ ,,, ,/L-—E!!??__ Bwkoff ,/ ‘ Busyv b*”

SlOi_tlme

Figure 1. The relations between three interframe spaces (SIFS,PIFS and DIFS) and two contention windows (DBT and RBT).

other rt-stations will detect the RF and the rt-packet of theCFPG and then content for the second access position in theRSVT. If a new active rt-station (STAK~)detects the RF framein the interval (t,t+D~J, itknows that at least one active rt-station already reserved the bandwidth. To avoid disturbingthe rt-stations access channel in the CFP, a new rt-station thatwants to join into the RSVT by contention must wait until theCFP finishes. The length of CFP can be calculated by theinformation in RF. During the waiting period, STA~~monitors the activity of channel. If the channel idles aSlot time during CFP, it implies a rt-station disconnectssess~n and the CFP should be decreased. Afler the CFPfinishes, STA~~ follows the backoff scheme to contend for itsreservation by sending a RTS. While in the backoff periodRBT, STA~~ should keep monitoring the channel to checkwhether any rt-station joins into the RSVT successfidly. Untilthe RBT becomes zero, STA~~ will send its RTS. If nocollision occurs, the content of AC will be increased by oneand the SID of STA~~ is set as the content of AC. At thismoment, it transmits its first rt-packet right away. Based onthis access procedure, each rt-station will increase their ACby one as soon as it listens a CTS during the real-timecontention period. On the contrary, if collision occurs, thecontention resolution also follows the P-persistent scheme asmentioned above. To make sure the rt-packets can betransmitted periodically and the repetition cycle will not belonger than D.a, we define a parameter RP~,, to limit thecontention period.

Figure 2 demonstrates how to add a real-time session intoa RSVT by contending the medium for its reservation. Figure2(a) is the case that no collision occurs and Figure 2(b) showsa case that a collision occurs. In Figure 2(a), we assume thereare 5 stations where STA 1 and STA3 want to transmit rt-packets to STA2 and STA4, respectively. Moreover, STA5has a nrt-packet that is waiting for transmission. In this case,if no RF is received by STA1 and STA3 after listening to thechannel for D.m, they believes that no rt-station exists. Then,if STA1 and STA3 detect an idle period of PIFS after adetecting period D.a, each of them generates a backoff time(RBT) and starts to count down. We assume that RBTs~Al issmaller than RBTs~A3.When RBTs~Al counts down to zero,STA1 sends out a RTS and waits for its CTS. If STA1receives a CTS within SIFS, there is no collision occurredand STA1 adds into the RSVT successfully. Because STA1 isthe first active station in the RSVT, ithas the responsibility to

0-7803-7018-8/01/$10.00 (C) 2001 IEEE IEEE INFOCOM 2001

11Ieal.tim.cml!c”tmr, .On.real.thlle

Wi”dnw :;’,’,’Cmtmtio”widow

?)7s SIRS DBS ~w~ I%d%.1.,-payload

I

1A 1 1 ~ ““./.,””’”TS RF .I-pa,kcSTA1 RF Wack t

CY$ ‘laSTA2 @

STA3 r,.pack , “.pmkelT

STA4 TC w @ c

STA5 T nrt.data,,,

(a) without collisionPIPS SIFS PIFS

4 #“’”$\ 4w ,,.pckct RF .I-pmket tb

c-iSTA2 c cl

,,.pack.t ..packtb

STA4 T c

. bm-dehFDm=

(b) with a collision

1%

Figure 2. An example of a new rt-station joining into the RSVT.

transmit a RF before ita rt-packet. After STA1 finishestransmitting its first rt-packet, STA3 continues to count down.When RBTs~N counter becomes zero, STA3 sends out itsFLTS and waits for receiving its CTS. When its CTS isdletected, it means STA3 has already reserved its bandwidthand joined into the RSVT. When a nrt-station (STA5) detectschannel being idle for DIFS, it is clear that no rt-station wantstransmit and STA5 can try to send out its RTS a soon asDBTsTA5 counter becomes zero.

In Figure 2(b), we only assume STA1 and STA3 want totransmit rt-packets to STA2 and STA4 at the same timerespectively. In this case, if RBT~~Alis equal to RBTs~N, acollision will occur because STA1 and STA3 send out theiramm RTS at the same time. Then P-persistent scheme will beapplied. When STA1 and STA3 don’t receive their CTSwithin SIFS, they have the probability p to persist ontransmitting their RTS in the next Slot_time. Suppose thatSTA1 gets the right to persist on transmitting but STA3 doesnot, STA1 will retransmit its RTS right away and STA3 willgenerate a new backoff time RBTPsT~. If STA1 successfullyreceive its CTS, it will send out a RF and its rt-packet. Assoon as STA3 counts down RBTPsAT~to zero, it begins tosend out its RTS flame just as the previous case. If anycollision occurs, the rt-stations will repeat the P-persistentscheme under the constrain that the following contentionprocess will not cross the RPmboundary.

After art-station STA~~ has joined into the RSVT and thepassing time from the last access is over the RPm., it willstart to monitor the radio channel for its contention-freeaccess. If the STA~~ is the CFPG (SID= 1), it will issue theFWframe as soon as the channel is detected idle for a PIFS.

3 4 4

Owation control(101) ~

1 ‘ 1 Aibi’)NOEF

..~........ . ---------...............

MACHeader ‘m yg~ A&bess 1 Adress 2 Adress 3 s:”#; Adress40m301 1

Figure 3. The duration field formatofanIEEE802.11 MPDU.

Also it will be the first one to transmit rt-packet. On the otherhand, as soon as the other STA~~ receives the RF, STARTwillupdate its RSVT by the broadcast information in the RF andthe access instant of each session will be decided. When thechannel is idle for a Slot_time, we assume that a rt-stationwith AID (i.e. SID = AID), which should deliver its packet atthis moment, has stopped transmitting. Each following stationwith larger SID will shift forward its access sequence in theRSVT. Due to the characteristic of the multimedia traffic, it isreasonable to release the reserved bandwidth when a stationtears down a rt-session. If the channel is still idle for the nextSlot_time, the release step will be repeated. After each stationfinishes sending its packet in the current cycle, it still keepsmonitoring the channel to check whether any session behindit tears down or any new session succeeds to add into theRSVT.

2.2.2 Allocation Procedure

To filly utilize the resource in a distributed system, theinformation of RSVT can be got and updated by the RF frameand by checking the duration field of MAC header in eachMAC PDU (MPDU). The duration format in a MAC headeris shown in Figure 3. This duration field consists of five sub-fields: control field, type field, next degree (ND) field,extension flag (EF) and raise degree (RD) field. The firstthree bits in the duration field of a packet is control field. Weassume that the control field with ‘101‘ indicates that thispacket is a rt-packet. The value ‘101’ has been reserved bystandard The 4-bit type field indicates what type of the rt-packet is. (e.g. voice, video, MPEG bit stream, etc.) Thesetraffic types are predefine in system. Each session utilizesthe ND field to inform other stations the demandedbandwidth at the next cycle. The request is decided by theamount of buffered packets in each station. Since ND fieldonly occupies 4 bits, the required bandwidth is represented as16 degrees for each service type. To reduce the overhead ofdescribing the length of required bandwidth, let u(i) denotethe unit bandwidth of the multimedia type i of a session. Thenthe required bandwidth can be described by identifying howmany bandwidth unit u(i) a station needs. The u(i) can beobtained by considering two characteristic parameters: PBR(i)(peak bit rate) and MBR(i) (minimal bit rate) of multimediatype i. That is, we have

[

PBR(i) – MBR(i) D~=u(i) = x

16 1UST x CDR “

0-7803-7018-8/01/$10.00 (C) 2001 IEEE IEEE INFOCOM 2001

where UST is the unit slot time and CDR is the channel datarate. In this equation, a minimal amount of bandwidth isreserved for each session in each cycle.

Let MS(i) and ND(i) respectively denote the minimumnumber of UST needed for an active session in every cycleamd the required bandwidth degree (excluding the minimumguaranteed bandwidth MY(i)) of a session at thewith service type i. Thus, we have

next cycle

‘s(i)=MBR(i)x[us:::DR1il

QLAD(i) = —

u(i) ‘

where QL denotes the queue length of each rt-session whichis measured in UST. The derived ND will be no mare than 16.As long as art-station has the right to access the channel, theND is calculated immediately according to the queue lengthin its buffer. Based on this concept, in proposed DBASE,after rt-stations update their RSVTS, each station will beaware of the reserved bandwidth for each session in everyCFP. Now we will describe how to dynamicallyaJlocate/share/extend bandwidth for each session withclifferent bandwidth requirements in WLAN.

To fairly utilize the resources, if the demand of each rt-station for bandwidth in the next cycle does not excess itsawerage bandwidth requirement (AVD), the demandedbandwidth will be allocated. Let AVD(i) denote the averageclegree of a multimedia type i. The A VD(i) can be obtainedfrom the ABR(i) (average bit rate) of multimedia type i. Thatis,

~ ~(j)= (ABR(i) - A4BR(i)) x ~m

u(i) x UST x CDR

If the requested bandwidth is larger than A VD(i), only theaverage bandwidth quota of its multimedia type i will be firstallocated. Therefore, the maximal bandwidth reserved for allactive rt-sessions in every CFP is the sum of A VDS of allactive rt-sessions, and simply described as the CFPmX.If theCFP~U is larger than the actually required bandwidth, there isresidual bandwidth that can be shared by some overloaded rt-stations. In the next subsection, we will describe how residualbandwidth is shared.

Because the interference exists in the wirelessenvironment and the delay bound for the rt-traffics is limited,an efficient retransmission scheme should be designed toimprove the quality of rt-traffics and the stability of aclistributed system. In DBASE protocol, after passing CFP,the CFPG will broadcast the retransmission mapping (RTM)frame. The RTM is a bit mapping to inform all rt-stationswhich stations can retransmit their packets. For example,RTM is “001”, it means that the rt-station whose SID = 3 canretransmit its r-t-packet after RTM frame when AC = 3. The

length of the retransmitted packet is limited as the negotiatedaverage packet length (AVD). If the retransmission succeeds,every rt-stations monitoring the channel can detect the NDfield of the retransmitted packet and this information will berecorded into each RSVT, Otherwise, the ND field for thisretransmitted station in the RSVT will be set as AVD(j) if themultimedia type of the retransmitted station is j. During theretransmission scheme, the rt-stations still need to checkwhether the time instant is over the boundary of RPmXor not.If the time exceeds the boundary, the retransmission schemewill be terminated immediately.

2.2.3 Sharing Scheme

Before reallocating residual bandwidth for the overloadedrt-stations, each active rt-station will first accumulate thespare bandwidth from those sessions whose requests are lessthan A VD. Let CD be the required bandwidth degree of thecurrent cycle, which is copied horn the ND that has beeninformed in the previous cycle. For simplicity to demonstratethe following equations, we omit the service type parameterin notations A VD, CD, MS and u. Let SS denote the numberof USTS that can be shared by those rt-stations whosedemanded degree excesses its A VD. (The SS can be treated asthe residual bandwidth in a contention free cycle.) Therefore,we have

SS = f(A VDk – min(A VDk, CDk )) x ui ,k=l

To fairly share the residual bandwidth among over-loadedsessions, the proportional approach is used. That is, the exactnumber of reserved unit slot (RS) for sessionj is

RSJ =

1(CD] - A VDj ) x U] 1 XSS+AVDIXU1~(CDk -A VDk) xukCDk>AVDk~MS + 2x SIFS+ TACK

UST, where CD] > A VD1

CD, X1.t, +MS+ 2X S1:;TACK

1 , where CD] < A VD1where TACKis the time needed for transmitting the ACK frame.According to the above equation and the RSVT in each activert-station, the packet length of each session can be calculatedby each rt-station individually. Consequently, we note that theaccess instant of each reserved real-time session and thelength of reserved period can be got easily.

2.2.4 Extension Procedure

In the case that the burst traffic arrives just after a rt-station issued the last bandwidth reservation information, thedelay bound for the excess data may be violated. To solve

0-7803-7018-8/01/$10.00 (C) 2001 IEEE IEEE INFOCOM 2001

[~ undetectaipacket SIFS

gzEEEg smF& ~F,pp \ .’ 1,:’ ;,/-’

? c RT B TTD

ND-s ND+ I-ID+ ND=5

RPmCCYC16Dm‘4$ JJAsp_E~:- -~’+-’

Figure 5. Superfiame structure.(a) Interferenceoccurs in CFP.

Becauseofcolhsiono- m rcbanwnissm7 proposed DBASE.

SIFSy +>,

Duicd,themkseta$ -~IFS PIFS AVE

‘Due to the characteristic of time-sensitive for rt-services,!1

1 R ;:;,%A ~~. C R ?\#,;, T T D w~hno~ ~ ~ ~ ~ ~. . ..

ND-5 ND-5 ND=5 I ‘AVDCD=5 CD=5RP_

bit rate (Mbps) bit rate (Mb}s) .MaximumBit Rate

‘/c 3 m

44~,time(ms)

(exponentialdistributed) stateholdongtime(exponentialdistributed)

Figure 6. The source model for VBR.

Table 1. Numerical values for the CBR model.

Table 2. Numerical value for the VBR model.

Parameter ~Maximum Bit Rate 420 KbpsMinimum Bit Rate 120 KbpsMean Bit Rate 240 KbpsMean State Holding Time 160 msecMean Video Transmission Time 180 secMaximum Packet Delay 75 ms

Table 3. The characteristics of two trat%c types for simulations.

mDEEllma3EKlmmmmlmCBR I 64 Kbps ] 64 Kbps I 64 Kbps I 15 0 0VBR 1420 Kbps1240 Kbpsl 120 Kbpsl 28 6 I 5

Parameter L.-X&-JConversation Length 180 sec

Table 4. System parameters of simulations.

I Parameter l~mChannel data rate CDR ] 11 MbpsMaximum delay D .QI 25 msSlot time Slot time 20 usUnit slot time UST [0 us

In the first subsection, the traffic models used in thesimulations are introduced. In the second subsection, theperformance measurements are defined. In the last subsection,the simulation results are presented and some discussions aremade. in order to investigate the multimedia over WLAN, weconsider the WLAN with 1lMbps, which WLAN adapter hasbeen announced recently. Each simulation run sustains 3x106Slot_times.

4.1 Traffic Models

In order to evaluate the performance of the proposedprotocol, three different traffic models are considered:

Voice Trafjc Model (CBR): The voice traffic is usuallyconsidered as a service with constant bit rate (CBR) traffic. Inthe simulations, the voice traffic is modeled as a two-stateMarkov process with talkspurt and silent-gap states. Eachvoice source is assumed to equip a slow speech activitydetector (SAD) [11-12]. In the talkspurt state, we considerthat the voice source generates a continuous bit-stream. Onthe other hand, in the silent state, there is no packet to begenerated. The duration of talkspurt and silent-gap bothfollow exponential distribution with the mean duration equalto 1 and 1.35 seconds respectively. A voice packet is assumedto be dropped if it suffers a delay longer than D.a (=25ms).We assume that the data rate of the voice traffic is 64Kbps.The parameters of voice model are summarized in Table 1.

Variable-bit-rate J4deo Traf@ Model (VBR): This modelis a multiple-state model (shown in Figure 6) where a stategenerates a continuous bit stream for a certain holdingduration [13-14]. The bit rate values for different states areobtained from a truncated exponential distribution. Thisdistribution is defined with a minimum and a maximum bit

Short interfkame space SIFS 10 usPriority interframe space PIFS 30 usDCF interframe space DIFS 110 usMAC header HMAC 272 bitsPHY header H 128 bitsThe packet length of RTS T,,~::DR 288 bitsThe packet length of CTS TC,,XCDR 240 bitsThe packet length of ACK T.C.XCDR 240 bitsBasic backoff window size of nrt-sessions ~Lnb 32Maximum backoff window size of nrt- b

1024sessionsPersistent probability P. 0.8

rate values. The holding times of the states are assumed to bestatistically independent and exponential distributed. Weassume that each state has the same mean holding time. Inour simulations, we considered 16 levels to approach the bitrates. Table 2 summarizes the numerical values used for theVBR model. Table 3 shows the characteristics of two traffictypes for simulations.

Data Traffic Model: We assume that data packets arrive ateach station following the Poisson process with the meanvalue Z=(W’/iV’~, where L means the arrival rate per nrt-station. The total data load p can be estimated as~ = anrl x EIT~~~lOa~], where A “r~ is the mean arrival rate of

total asynchronous packets . Through the simulations, the

EITJ~-lOaJ] is set as 744 ~s (=8184 bits/CDR), A is 0.1,

buffer size is fixed at 100 packets and the length ofasynchronous packet is fixed at 8184 bits.

According to the traffic models defined above, the trafficparameters of DBASE can be summarized in Table 4.

0-7803-7018-8/01/$10.00 (C) 2001 IEEE IEEE INFOCOM 2001

1 I I

0.8 —. .— —

‘-’’’-’n

+ VBR=IOla VBR=20

g 0,4 a VBR=3 o

x VBR=400.2

o~0,1 0.3 0.5 0.7 0.9 1.1 1.3 1.5

Dataload

(a) VBR real-timeservices

0.s r— ,

0.6

Pfl ‘

, .y~- ‘ ‘ ~*-

m

+ CBR=50

El E CBR=l 00

$ 0.4 ,*’ a CBR=150

c? + CBR=200- 0.2 y II - CBR=250 ]o~ - CBR=300

0.1 0.3 0.5 0.7 0,9 1.1 1.3 1.5Data load

(b) CBR real-timeservices

Figure 7. A comparison of the goodput, as the number of real-time sessions varies. Show two traffic types respectively.

4.2 Performance Measurements

The performance measurements used in our analysis aredefined as follows:l Goodput: the goodput is defined as the percentage of the

time used by both rt- and nrt-stations to successfullytransmit their pure payload data. The control andmanagement signals, the idle time, the collided packets,the error packet caused by the interference and theheader bits are excluded from the goodput.

l Packet delay dropped probability (PDDP): the packetdelay dropped probability is defined as the flaction ofdiscarded packets due to the delay-sensitivity of the rt-traffics.

4.3 Simulation Results

To obvious the performance of DBASE, we change thenumber of rt-stations and the traffic types. Figures 7 and 8plot the goodput and PDDP of CBR and VBR trafficsrespectively. The number of nrt-stations N“” is fixed at 10 andthe asynchronous data load (p) increases fi-om 0.1 to 1.5. Thenumber of VBR sessions ranges from 10 to 40, and thenumber of CBR sessions ranges from 50 to 300.

Figure 7 shows the relations of the goodput and the dataload under different number of rt-stations. In Figure 7, wecan find that the goodput can be up to about 80% for VBRtraftlcs and 67’%for CBR traffics. We know that the overheadcaused by headers of packets will be quite obvious when thepayloads of packets are small. Therefore, the saturatedgoodput of VBR is higher than that of CBR since the packetlength of VBR (average length= 58 UST) is longer than that

0.01tOa%assssaas S@ @as

0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5Data load

(a) VBR real-time services

‘:G

0.1 0.3 0.5 0.7 0.9 1,1 1.3 1.5DataIoad

(b) CBR real-time services

.+ CBR=50

w CBR=I 00

A

a cBR=150

.Y CBR=200

+ CBR=250

l CBR-300

Figure 8. A comparison of the packet delay dropped probability(PDDP) of real-time stations, as the number of real-time sessionsvaries.

of CBR (average length = 15 UST). Because the number ofdata stations is fixed at 10, the increasing data load will onlyincrease the queue length of the data buffer but not thecollision probability when the traffic load is saturated.Accordingly, the goodput will not decrease after the goodputis saturated. In Figure 7(b), because the payload length of nrt-session is much larger than that of CBR sessions, the curveswith different number of CBR sessions cross each other.While the number of CBR stations decreases, the number ofnrt-packets transmitted successfully will become more.Therefore the saturated goodput with 50 CBR sessions ishigher than others when the asynchronous data load is heavy.

Figure 8 plots the PDDP of CBR and VBR trafficsrespectively. According to the Section 3, we can estimate thesaturated capacity of the system by those simulationparameters. If the traffic type is CBR, the saturated capacityis about 112 CBR rt-sessions. Similarly, the saturatedcapacity for the VBR traffic model is about 39 VBR rt-sessions. In Figure 8(a), since 40 VBR sessions are just alittle over the saturated number of stations, the PDDP is notzero but it is still very small. The derived PDDP is about zerofor other cases. Because the reservation process is applied,packets will be dropped only when the rt-sessions are still inthe contention procedure. Therefore, the PDDP will notincrease rapidly even though data load is still increasing andthe rt-sessions are over the saturated capacity (39). In Figure8(b), we find that the PDDP is not over 3% even when thenumber of CBR sessions is up to 200, which is over thesaturated active sessions of CBR traffics. This is because theON-OFF model is used in the CBR traffic model.

0-7803-7018-8/01/$10.00 (C) 2001 IEEE IEEE INFOCOM 2001

1 I I

()~20 60 100 140 180 220 260 300

Numberof red-time sessions

Figure 9. A comparison of the goodput of DBASE and DCF fora variety of traffic types in the clear radio environment.

In papers [15-16], authors concluded that the packetizedvoice communications can tolerate only a small amount(1%-3%) of dropped packets before suffering a large qualitydegradation. Based on these criteria, in Figure 8(b), themaximum number of CBR stations can be up to 200. We findthat the asynchronous data load does not affect the PDDPbecause the rt-traffic has a higher priority than the nrt-traffic.The low PDDP of DBASE implies that DBASE can performa high quality of service even when the traffic load is heavy.

To compare our proposed protocol DBASE with the IEEE802.11 MAC protocol based on pure DCF, the followingsimulations are made. For simplicity, the conventionalprotocol based on IEEE 802.11 DCF is named as DCF. InFigure 9, we assume that the number of nrt-sessions is stillfixed at 10 and the A is 0.1. The curves DCF(CBR) andDBASE(CBR) indicate the performance of DCF and DBASEfollowing the CBR rt-traflic model respectively and so as theDCF(VBR) and DBASE(VBR) following VBR rt-trafficmodel. For the CBR model, when the active number of rt-sessions is low, the DBASE performs similar to DCF.However, as the number of CBR sessions is larger than 100,the DBASE performs much better than DCF. This is becauseDBASE will reserve the bandwidth and the increasingcontentions caused by the increasing number of rt-sessionswill not dominate the channel resource. For the VBR model,the DBASE also performs much better than DCF because ofthe same reasons mentioned above. We emphasize that thederived goodput of DBASE(VBR) can get up to 90%. Thisimplies that almost network resources are fblly utilized byDBASE protocol.

To show the effect of the interference in the wirelessenvironment on the DBASE and DCF, we set the packet errorrate (PER) as 0.1. Meanwhile, we assume that the number ofnrt-session is zero. In Figure 10(a), the curves show that thegoodput of DBASE is higher than that of DCF under thesame traffic types. We know that a higher PER will result in alower goodput. However, the goodput of DBASE(VBR) canalso hold on 80’% even though the PER is up to 0.1 but thegoodput of DCF(VBR) decreases to about 30%. In Figure10(b), we find that the PDDP of DCF is much higher than

o~ I

20 60 100 140 180 220 260 300

Number of real-timesessions

(a) The goodput of DBASE and DCF

b DBASE(CBR)

M DBASE(VBR)

a DCF(CBR)

x DCF(VBR)

20 60 100 140 180 220 260 300

Numberof real-timesessions

(b) The PDDP of DBASE and DCF

Figure 10. A comparison of goodput and PDDP of DBASE andDCF in the interfering environment (PER= 0.1).

that of DBASE under the same traffic types. Figure 10(b)show that the PDDP of DBASE(CBR) will begin to increasewhen the number of CBR sessions is larger than 220, whichis the saturated capacity under the consideration of ON-OFFmodel and PER=O. 1. However the PDDP of DCF(CBR) willincrease rapidly as long as the number of CBR sessions islarger than 100. Similarly, the PDDP of DBASE(VBR)increases at about 40 VBR sessions and PDDP of DCF(VBR)is up to about 20’%while the number of VBR sessions is only20. This is because the rt-trafics are delay-sensitive but thePER and contentions make the packet delay probability ofDCF increase extremely. Since the bandwidth is reserved forthe rt-packets to retransmit in DBASE, the packet error ratedoes not influence the PDDP of DBASE seriously when thenumber of rt-sessions is small. Although DBASE has theretransmission scheme to reduce PDDP of the reserved rt-sessions, the new rt-sessions using CSMAICA to reservebandwidth still makes the PDDP increase as the rt-traffic loadincreases.

5. CONCLUSIONS

In this paper, we proposed a distributed bandwidthallocation/sharing/extension (DBASE) protocol to supportmultimedia traffic over ad hoc WLAN. In DBASE, rt-stations can reserve and free channel resources dynamically.The system capacity of proposed DBASE was analyzed.Simulation results show that the proposed protocol cansupport both constant bit rate and multimedia services of

0-7803-7018-8/01/$10.00 (C) 2001 IEEE IEEE INFOCOM 2001

variable bit rate in ad hoc WLAN. Simulation results also [12]demonstrate that the DBASE performs very well and muchbetter than the conventional IEEE 802.11 standard. Thechannel efficiency of DBASE is up to about 90°/0 for [13]

supporting variable bit rate (VBR) traffics. Besides, the delaydropped probability of rt-packets, caused by the delaylimitation and noise interference, is very low even though the

total traffic load is heavy. [14]

REFERENCES

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]

Richard van Nee, Masabiro Morikura, Hitoshi Takanashi,Mark Webster, Karen W. Halford, “New High-Rate

[15]

Wireless LAN Standards”, IEEE CommunicationsMagazine, Volume: 3712, pp. 82-88, Dec. 1999.IEEE 802.11, “Part 11: Wireless LAN Medium AccessControl (MAC) and Physical Layer (PHY) Specifications”,IEEE 802.11 Std, Aug. 1999.

[16]

W.K. Kuo, C.Y. Chan and K.C. Chen, “Time BoundedServices and Mobility Management in IEEE 802.11Wireless LANs”, Proceedings of the 1997 IEEEInternational Conference on Personal WirelessCommunications, pp. 157-161, 1997.Jaime Sanchez, Ralph Martinez, and Michael W. Marcellin,“A Survey of MAC Protocols Proposed for Wireless ATM”,IEEE Network, vol.11 6, pp. 52-62, Nov.-Dee. 1997.J. L. Sobrinho and A. S. Krishnakumar, “DistributedMultiple Access Procedures to Provide VoiceCommunications over IEEE 802.11 Wireless Networks”,Proceedings of the IEEE GLOBECOM’96, vol. 3, pp. 1689-1694, 1996.Joao L. Sobrinho and A. S. Krishnakumar, “Quality-of-Service in Ad Hoc Carrier Sense Multiple Access WirelessNetworks”, IEEE Journal on Selected Areas inCommunications, vol. 178, pp. 1353-1368, Aug. 1999.Andrew Muir and J.J Garcia-Luna-Aceves, “SupportingReal-Time Multimedia Traffic in a Wireless LAN”,Proceedings of the SPIE Multimedia Computing andNetworking, 1997.Andrew Muir and J.J Garcia-Luna-Aceves, “GroupAllocation Multiple Access with Collision Detection”,Proceedings of the INFOCOM’97, Kobe, Japan, pp. 1182-1190, April 1997.Andrew Muir and J.J Garcia-Luna-Aceves, “GroupAllocation Multipke Access in Single-Channel WirelessLANs”, Proceedings of the Communication Networks andDistributed Systems Modeling and Simulation Conference,Phoenix, Arizona, 1997.F. Cali, M. Conti, E. Gregon, “IEEE 802.11 Wireless LAN:Capacity Analysis and Protocol Enhancement”,Proceedings of the IEEE INFOCOM’98, San Francisco,USA, pp. 142-149, APril 1998.Antonio Iera, Antonino Modafferi and Antonella Molinaro,“Access Control and Handoff Management in Multi-tierMultimedia Wireless Systems”, IEEE WirelessCommunications and Networking Conference WCNC’99,vol. 3, pp. 1518-1522, 1999.

Chunhung Richard Lin and Mario Gerla, “AsynchronousMultimedia Multihop Wireless Networks”, Proceedings ofthe IEEE IiVFOCOM ’97, vol. 1, pp. 118-125, 19997.Ian F. Akyildiz, David A. Levine, and Inwhee Joe, “ASlotted CDMA Protocol with BER Scheduling for WirelessMultimedia Networks”, IEEE/A CM Transactions onNetworking, vol. 7, no. 2, pp. 146-158, April 1999.M. Decina and T. Toniatti, “Bandwidth Allocation andSelective Discarding for a Variable Bit Rate Video andBurst Data Cells in ATM Networks”, ht. J. Digital AnalogCommunication System, vol. 5, no. 2, pp. 85-96, Apr.-June1992.H. Heffes and D. M. Lucantoni, “A Markov ModulatedCharacterization of Packetized Voice and Data Traffic andRelated Statistical Multiplexer Performance,” IEEEJournal on Selected Areas in Communications, vol. SAC-4,no. 6, pp. 856-868, Sep. 1986.K. Snrarn and W. Whitt, “Characterizing SuperpositionArrival Processes in Packet Multiplexer for Voice anddata,” IEEE Journal on Selected Areas in Communications,vol. SAC-4, no. 6, pp. 833-846, Sep. 1986.

0-7803-7018-8/01/$10.00 (C) 2001 IEEE IEEE INFOCOM 2001