Scheduling MPEG-4 Video Streams Through the 802.11e Enhanced

7/27/2019 Scheduling MPEG-4 Video Streams Through the 802.11e Enhanced

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Scheduling MPEG-4 Video Streams through the 802.11e Enhanced

Distributed Channel Access

MichaelDitze 1, Kay Klobedanz 1, GuidoKamper 1, PeterAltenbernd 2C-LAB, Furstenallee 11, 33102 Paderborn, Germany 1

Fachhochschule Darmstadt, Haardtring 100, 64295 Darmstadt, Germany2

EMail: {michael.ditze, kay.klobedanz, guido.kaemper}@c-lab.de 1

[email protected]

AbstractThe upcoming IEEE 802.11e standard for Wireless LAN hasgained a lot of popularity, recently. It improves the MediumAccess Control (MAC) of the legacy 802.11 with regard toQuality of Service (QoS) by introducing the Enhanced Dis-tributed Channel Access (EDCA) and the HCF ControlledChannel Access (HCCA). In contrast to the legacy Dis-tributed Coordination Function (DCF), EDCAachieves QoSby providing independent transmit queues and MAC param-eters for each traffic class, and hence higher prioritized traffichas a higher probability for transmission. Crucial to the suc-cess of such a strategy is a scheduler that assigns the datatraffic to the respective transmit queues. This is all the moreessential in cases where priorities cannot be assigned stati-cally before run-time, but rather require dynamic and adapt-able priority assignment as this is the case for many multime-dia applications, e.g. MPEG-4 video streaming. This paper

develops and accommodates a new scheduler for EDCA intothe MPEG-4 Delivery Framework. The scheduler dynami-cally determines MPEG frame priorities and assigns datapackets to the EDCA transmit queues accordingly. We usethe NS-2 network simulator to show that we are able to in-crease the amount of timely transmitted frames significantlyand hence improve the QoS percepted by the user. To thebest of our knowledge this is one of very few scheduling ap-proaches that considers MPEG-4 related traffic priorizationin EDCA.

1. Introduction

Following its finalization in 1999, the wireless 802.11 stan-dard, also commonly referred to as WLAN, is increasinglybeing established as the wireless networking protocol that al-lows to interoperate devices and their services in many appli-cation domains, e.g. home domain. Crucial to the commer-cial success of 802.11 is the degree by which application re-quirements can be fulfilled by the networking protocol. Mul-timedia applications like MPEG video streaming representa significant share in many application domains and are in-creasinglybeingdeployedinmanyembeddedConsumerElec-tronics end-devices like cell phones, PDAs and set top boxes.MPEG-4 applications require QoS support on both, the end-device and the network carriers in order to guarantee the de-liveryof time-sensitive video data from the source to the sink.

Asthe legacy802.11 doesnot provide any QoSguarantees dueto the random-based medium access scheme in the Data Link

Layer, the 802.11e working comitee prepares major amend-ments to the 802.11 channel access regardless of the physicallayer underneath. 802.11e allows to assign prioritized trafficto traffic categories that exhibit different MAC parameters.The proper adjustment of these parameters for each trafficcategory results in different probabilities to gain the mediumaccess. In order to assigndata packets to the respective trafficcategorieswith regard to theirpriority derived fromthe appli-cation, a scheduler is required. The scheduler is easy to main-tain when priorities are assigned statically before run-time.Many multimedia applications e.g.video streaming, however,benefitfromdynamicschedulingpoliciesthatallowthesched-uler to adjust to the varying workloads that result from videocompression and user interactivity [1]. In cases where a dy-namic scheduling policy is deployed that may change prior-ities at runtime in order to adapt to the dynamic task con-ditions the scheduler becomes all the more the crucial entity

that ensures QoS maintenance. Dynamic scheduling policieshas been proven crucial for

This paper presents a new smart scheduler that dynamicallydetermines priorities for MPEG-4 frames and assigns cor-responding data packets to EDCA transmit queues accord-ingly. In contrast to other solutions we do not confuse the im-portance and the urgency of frame-tapes and hence schedulelower prioritized frames with a close deadline in the presenceof higher prioritized frame in case the latter can still makeits deadline. The dynamic priority assignment derives from amodification of the Least Laxity First approach which we al-ready used to suit processor scheduling for MPEG streamson end-devices [[2], citeditze2] and now adapt for network

scheduling. The laxity of a frame hereby denotes the amountof time a data packet can be delayed on its transmission andstillarrive within its deadline at the receivingend-device. Thescheduler relies on a statistical approach for Admission Con-trol for 802.11e [4] and may provide soft real-time guaran-tees.

We implemented the scheduling policy on top of the 802.11eMAC into the NS-2 network simulator. Evaluation resultsprove that using this approach we are able to increase theamount of timely transmitted frames significantly comparedto traditional scheduling solutions. To the best of our knowl-edge this one of very few scheduling approaches for 802.11e.

Therestofthepaperisorganizedasfollows:Section2givesanintroduction on MPEG-4 and 802.11e. Section 3 presents re-lated work. Section 4 describes the new approach followed by


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a description on simulation in Section 5. Section 6 concludesthis paper with future work.

2. Introduction to 802.11e and MPEG

This section gives s short introductionon the general workingprinciples of the 802.11e EDCA and MPEG-4.

2.1. Introduction to 802.11e

The legacy 802.11 standard provides detailed medium ac-cess control and operation at the physical layer for WirelessLANs.Thefundamentalmediumaccessfunctionisreferredtoas Distributed Coordination function (DCF). DCF operatesin contention periods where each station may autonomouslyaccess the medium. Contention periods alternate over timewith contention free periods where a central Point Coordina-tor uses the Point Coordination Function (PCF) to poll sta-tions for medium access. As PCF is rarely implemented incurrent 802.11 chipsets, we will only consider DCF for the re-maining of this paper.

DCF defines a Carrier Sense Multiple Access - Colli-sion Avoidance (CSMA/CA) listen-before-talk schemewhere each station needs to sense the medium as be-ing idle for a DIFS-time (DCF Interframe Space) be-fore transmission. To keep multiple stations from accessingthe medium simultaneously in the same slot after a suc-cessful transmission, a random binary exponential backoffprocedure is performed where each station choses a back-off timer withina predefinedtemporal range referred to as theContention Window (CW) which is a multiple of the phys-ical slot time. The backoff timer is decremented while themedium is being sensed as idle. Once it reaches zero, the sta-tion re-initiates the transmission. Each transmitted packetwill be acknowledged by the receiving station in order to ac-

count for the unreliable wireless transmission medium. Incase a transmission remains unacknowledged for the dura-tion of a Short IFS (SIFS) which is shorter than DIFS, a colli-sion is presumed andthe packet is queuedfor re-transmission.According to the backoff procedure the CW is doubled af-ter each unsuccessful transmission as long as it does notexceed the maximum CW size and a new random back-off timer is set.

As medium access is controlled through a random-based ar-bitration scheme that does not support traffic differentiation, DCF does neither allow to prioritize traffic nor does it pro-vide timingguaranteesand hence QoSto the applications. Asa consequence, the 802.11e working group is in the process of

developing a new improved medium access scheme for con-tention periods referred to as Enhanced Distributed ChannelAccess (EDCA).

In contrast to DCF, EDCA allows to prioritize traffic by in-troducing four different AccessCategories (ACs)to each QoSstation (QSTA) [5]. Each AC maintains a separate transmitqueue and a dedicated channel access function that featuresAC-specific parameters. These parameters include differentvalues for minimumand maximumContention Windows, Ar-bitration Interframe Spaces(AIFS) and a Transmission Op-portunity(TXOP) duration [6].

AIFS, that is generally larger than DIFS, hereby denotes anindividual time for sensing the medium that can be adjusted

foreach AC. Hence, in order to allow fortraffic priorization in802.11e, higher priority ACs receive shorter AIFSs and lowerCWs to increase the probability of a successful channel access

(see Fig.1). The channel access itself remains similar to DCF.A TXOP is usually allocated by the QoS AccessPoint (QAP)and, in contrast to the legacy DCF, hereby grants a stationthe right to use the mediumat a defined pointin timefor a de-fined maximum duration. Higher prioritized ACs are grantedlarger TXOPs which results in a larger throughput per AC.

In case of an internal collision i.e. the backoff timer of at leasttwo ACs simultaneously reaches zero, an internal scheduler

grantsthe accessrightsto thehigher prioritizedAC andforcesthe other station to enter backoff procedure.

2.2. Introduction to MPEG

The MPEG standards developed by the Motion Pictures Ex-perts Group have grown to become a world-wide standardfor video compression reducing the workload on processorsand networks by exploitingthe intrinsic redundancy betweenconsecutive video pictures. MPEG-4 covers a wide area ofbit-rates ranging from below 64 Kbits/sec for applicationswith extremely low bandwidth up to 4 Mbit/sec for videostreamingapplications [7].As the allocated encoding bit-rate

in MPEG-4 is not fixed, it may be further increased.

In contrast to its predecessors, MPEG-4 [7] allows for thedecomposition of video scenes into single audio-visual ob-

jects. Each object can be separately encoded and transmit-ted as a series of frames in one or several Elementary Streams(ES). ESs then pass the Sync Layer before they are trans-mitted through the Delivery Multimedia Integration Frame-work that encapsulates them into native transmission proto-cols, e.g. RTP/IP.

In order to exploit theredundancy in video streams, MPEG-4defines three particular types of Video Object Planes (VOP)that are temporal instances of an audio-visual object. TheseVoPs exhibit different compression ratios and are referred

to as I(ntrapicture)-VOPs, P(redicted picture)-VOPs andB(idirectionalpredicted picture)- VOPs.

I-VOPs serve as reference VOPs to P-and B-VOPs whereasP-VOPs are predicted VOPs that collect relevant informa-tion encoded in former I-VOPs. They also serve as referenceVOPs to B-VOPs. Consequently, I-VOPs and P-VOPs arealso referred to as reference VOPs. B-VOPs can be either for-ward or backward predicted and likewise exploit redundantinformation encoded in previous or subsequent VOPs.

Since B-VOPs can be either forward-, backward-predictedor a combination of both, the MPEG standard distinguishesthe order in which VOPs are encoded (Display Order) andthe order in which they are transmitted (Transmission Or-der). Fig.2 further illustrates the interdependencies amongthe different VOP-types in Transmission Order. The refer-ence VOP a particular VOP relies on for decoding is denotedby the shaded boxes on the top right and the arrows point-ing to that VOP.

A Group of VOPs(GOV) is a sequence of VOPs ranging fromone I-VOP to the next. It complies with Groupsof Pictures inMPEG-2. Even if MPEG does not standardize the GOV pat-tern, numerous streams often show the same fixed sequence.While fixed spacing and/or the use of GOVs is not requiredby the standard, it is so widely used, that a pair of parame-ters describes the spacing between I-VOPs and P-VOPs. As

each GOV may be self-contained, it is independent of oth-ers which allows for decoding without any knowledge aboutother groups.


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Figure 1. EDCA Parameters

MPEG-4 can be encoded in constant bit-rate (CBR) or vari-able bit-rate (VBR). Whereas CBR encodes every VOP atthe same fixed bit-rate, VBR allows the bit-rate to vary, andhence ensures the same steady picture quality even in scenesthat are hard to encode.As VBR reaches better compression-rates, we proceed on the assumption that MPEG-4 streamsare encoded likewise.

B1

I0

P1

I5

B1

P13

B2

P14

P2

P18

B1

P26

B2

P27

B2

I1

I

2

B1

P39

B1

P310

P3

P211

I

2

B1

I0

B2

I1

P1

I5

B1

P13

B2

P14

P2

P18

B1

P26

B2

P27

P3

P211

B1

P39

B1

P310

Display Order

Transmission Order

Forward Precedence Backward Precedence

Figure 2. Transmission and Display Order in a

GOV

3. Related Work

A lot of work has addressed the topic of scheduling for Mul-timedia Applications, recently, e.g. [[1],[3],[8],[9]]. This sec-tion describes previously published relevant work in the areaof Admission Control and scheduling for 802.11e.

Ansel et al. [10] present a new scheduling algorithm forthe contention-free Hybrid Coordinated Channel Ac-

cess (HCCA) in 802.11e that aims to be fair at CBR andVBR video flows. Their approach contains a node sched-uler anda QAPscheduler.The QAPscheduler exploits queuelength estimation for each QSTA to control the time allo-cation to stations. It estimates the varying queue lengthfor each QSTA before every service interval and com-pares this value to the ideal queue length. Based on a win-dow of previous estimation errors for each TC, the QAPadapts the computation of TXOPs allocation to a cer-tain QSTA. The QSTA may then redistribute the unusedtime among the different TCs.

In contrast to Ansel we introduce a scheduling mechanismthat operates under the contention-based EDCA which is

more practical as the past has shown that many manufac-tures do not implement contention-free MAC due to cost lim-itations. Further, [10] relies on an average sending rate for

VBR applications. Their approachbehaves well in stable con-ditions but is condemned to fail in situations with abruptworkloadchangesas it is often thecase in MPEG scene transi-tions.Besidesrelying on EDCA,we assumea maximum send-ing rate per frame-type over a predefined time window. Like-wise, we are able to estimate the sending rate per flow moreaccuratelyand henceachievea moreefficient resource utiliza-tion.

Hertrichdeveloped a simple prioritizing scheme for the trans-mission of MPEG-4 video over the legacy 802.11 [12]. He pri-oritizes frame types and adapts the 802.11 data link layer pa-rameters such as the quality of the MPEG4 video at the re-ceiving end-device is kept as good as possible, even and es-pecially when non-optimal link quality affects radio trans-mission. Hence, he adjusts the amount of re-transmission at-tempts with regard to the frame priority, preferring I-VOPsover P- and B-VOPs.

Hertrich uses a static priority assignment scheme that doesonly consider the importance of particular frame-types butneglects their urgency. As a consequence lower prioritizedframeswill be skipped in the presence of a high priority frame

even if the low prioritized frame could have been transmittedwithout causing the latter to miss its deadline. Further, theapproach is restricted to the legacy 802.11 MAC. In our ap-proach we rely on a dynamic scheduling strategy that con-siders the urgency and the importance of a frame. Hence,frames are assigned pre-defined priorities that may dynami-cally change according to the frame laxity. Likewise, framesare transmitted if they are guaranteed not to interfere withhigher prioritized frames. Moreover, we exploit the differen-tiated traffic mechanisms as provided by the 802.11e.

As our scheduling approach relies on an accurate estimationof available bandwidth in order to prioritize data traffic ac-cordingly, the following approaches are also relevant for this

work:Bianchi provides an analytical model to compute the 802.11DCF saturation throughput in the assumption of ideal chan-nel conditions and a finite number of terminals [11]. The sat-uration throughput hereby denotes the maximum load a sys-tem can carry in stable conditions as the offered load in-creases. Bianchi first determines the stationary probabilityT that a station transmits a packet in a generic slot time andthen calculates the throughput by studying the events thatmay occur within the generic slot time.

Pong and Moors extend this approach to make it suitable for802.11e by estimating the per-flow throughput for EDCA [4].A flow is defined as the set of packets belonging to the same

AC. They determine the transmission probability of a flowand calculate its achievable throughput at saturation condi-tions as the proportion of time for transmitting data payload


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Figure 3. MPEG-4 EDCA Scheduler embedded

in the MPEG Delivery Framework

in respect to idle, collision and header transmission time dur-ing a cycle of frame exchange. We exploit this approach toestimate the medium access time for a packet in a particu-lar flow.

4. A MPEG Scheduler for EDCA

Fig.3 illustrates a simplified architecture that accommo-dates the MPEG-4 EDCA scheduler into the MPEG-4 De-livery Framework. The scheduler also complies with the lat-est 802.11e draft [5]. The compression layer encodes VOPsthat represent Audio Visual Objects and passes them via theSync Layer and the DMIF API directly to the new scheduler.The Sync Layer hereby performs time stamp assignment andVOP packet fragmentation.The task of thescheduler is to dy-namically determine the priority of each packet according toa scheduling policy that is suited for MPEG-4 and assign thefragmented VOP packets to the EDCA AC transmit queues.

4.1. Priorities and Access Categories As-signment

We use a priority-based scheduling policy that does not con-fuse theimportance of data packets andtheirurgency. UrgentMPEG-4 packets are those packets that have a close deadlinewhile important packets are required for further frame decod-ing. As a consequence,I-VOPs usually exhibit thehighest im-portance as the whole GOV cannot be decoded without hav-ing decoded this I-VOP first. B-VOPs on the contrary maytemporarily be more urgent as they appear more frequentlyand have closer deadlines (see Section 2.2). Correspondingto the EDCA ACs, the scheduler maintains 4 internal work-

ing queues WQ that feed the the transmit queues of eachAC.The scheduler deploys a Least-Laxity-First (LLF)-extendedscheduling policy in order to dynamically assign packets to

the queues. Initially, considering the importance, packets areassigned as follows:

Description Working QueueOrder

WQ[3] I- and P-VOPdata packets

optimized unique

priorities, prior-

ity orderWQ[2] B-VOP data

packets

optimized unique

priorities, prior-

ity order

WQ[1] all other datapackets

FIFO

Table 1. Access Categories for MPEG-4 data

packets

Similar to the 802.11e ACs, WQ[3]represents the highest pri-ority and WQ[0] the lowest priority queue. I- and P-VOPsdata packets are assigned to top priority WQ[3], whereas B-VOP packets that can be skipped occasionally in favour ofother data packets are inserted into the WQ[2] queue. Otherremaining packets that do not have a priority are insertedin the WQ[1] queue and hence do not need to undergo Ad-mission Control. The different priorization of VOP packetshereby adheres to the uni- and bidirectional predictive en-coding of MPEG.

Further, we prioritize data packets in the working queues ofeach AC in priority order, serving packets with the highestpriority first. Priorities are derived as follows:

WQ[3] gives I-VOP data packets priority over P-VOPdata packets. If packets belong to different streams buthavethe same priorityand share thesame deadline,theybuild ordered sets of data packets. Within each such setthe lower-sized VOP packets are scheduled first. This isto ensure that large-sized VOPs do not interfere low-sized VOPs with the same priority. Other schedulingpolicies may chose to prefer VOPs of streams that ex-hibit a higher priority within the ordered set and henceallow for a higher throughput rate of these streams.

WQ[2] also builds ordered sets of B-VOP packets incase they belong to different streams and share the samedeadline and priority. Similar to WQ[3], lower sized B-VOPs are preferred.

Building ordered sets has a an impact of the throughput per-formance ofMPEG-4 data packets as it allows to usethe avail-able bandwidth more efficiently or permits to distinguish be-tween streams with different priorities.

4.2. Scheduling Policy

Data packets in WQ[3] and WQ[2] are concurrently sched-uled by an extension to the LLF algorithm. This policy thatis often applied in real-time operating systems schedules themost urgentdata packet, i.e. the data packet with the small-est laxity Y. The laxity hereby denotes the maximum time

a data packet can be delayed on its transfer to be transmit-ted and decoded at the receiver within its deadline. In addi-tion to that our policy


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skips B-VOP packets in WQ[2] if their deadline has al-ready passed or exhibit a negative urgency Y.

may move single data packets from WQ[2] to WQ[3] incase they can be transferred and decoded without caus-ing the latter to miss their deadline and are kept in or-der of appearance. Due to the forward precedences inMPEG, WQ[2] VOP packets may only be inserted be-fore those VOP packets that WQ[2] depends on for de-

coding. prefers lower-sized VOPs in case two VOPs in the same

AC are equally prioritized and share the same deadline.

4.3. Admission Control and ThroughputAnalysis

Thescheduling approach relieson an Admission Control thatfounds on a throughput analysis for 802.11e. The through-put analysis estimates the available throughput at satura-tion conditions [4](see Section 3). It exploits the statisticalcollision probability for a flow in an AC in order to computethe transmission probability in a slot and may likewise de-

rive the achievable throughput j for (ACj) on the propor-tion of time for transmitting data payload in respect to idle,collision and header transmission times. We use the availablethroughput analysis per AC in order to determine if WQ[2]packets can be moved to WQ[3] (and hence AC[3]) withoutcausing WQ[3] packets to miss their deadline

TheACfirstdetermines the transmissionprobabilityof a flowi (fi) that can be estimated as [4]

P(txinaslot|fi) =2(1 2pi)

(1 2pi)(W+ 1) + piW(1 (2pi)b)(1)

thereby assumingpi as thelong term collisionprobability andW as the CWmin size for fi. b denotes the maximum backoffstage.

We then define the expected processing Time Tp[i]j for eachdata packet i in a Working Queue j as the expected time re-quired to gain medium access TMAC[i]j in ACj, its expectedtransmission time TTi and the worst-case decoding time ofthe frame the packet belongs toCi at the receiving device

Tp[i]j = TMAC[i]j + TTi + Ci (2)

As we assume a static scenario where devices do not moveand the transmission channel does not suffer overlay inter-ference from other transmission protocols operating in thesame GHz range, we can assume TT to be constant for eachpacket. Further,Ci can be derived from a Worst-Case Execu-tion Time Analysis forMPEGdecoding [13]. Themedium ac-cesstime for eachpacket ina WQ can thusbe computedas thefraction of all packet sizes that precede a packet in the trans-mit queue and the working queue and the estimated achiev-able throughput

TMAC[i]j =

i1k=0

sk|kWQj kACj

j(3)

wheresi denotes the size ofa packet i.UsingEq.1andEq.2wecan now simply determine the urgencyY[i]j ofa data packet iin Working Queue j as

Y[i]j = Di Tp[i]j (4)

where Di denotes the deadline of a packet i and derives fromthe frame-per-second rate of the video stream. As a conse-quence, the scheduler may move data packets from WQ[2] toWQ[3], if the estimated processing time Tp2 of data packetsbelonging to a VOP is less than the urgency Y[i]3 of a corre-sponding packet i in WQ[3].

4.4. EDCA Scheduler Activity Diagram

Fig.5 describes the transmission process MPEG-4 VOPs inUML notation. MPEG-4 VOPs are generated by the MPEG-4 Compression Layer or by a respective MPEG-4 traffic gen-erator in our simulation environment. The scheduler decidesin whichqueue each VOP will be inserted accordingthe staticpriority assignment scheme as introduced in Section 4.2. Thescheduling policy checks the deadline of eachgenerated VOP,inparticularB-VOPsthat exhibit thelessimpact onthe videoquality, and may decide to reject VOPs that are expected tomiss their deadlines. In case of B-VOPs the scheduler fur-ther determines the average processing time of the B-VOPwhich is then compared with the urgency values of queuedAC[3]-packets as determined in Eq.2-4. In case the transmis-sion time of the B-VOPs is less than the urgency of subse-quent packets in AC[3], the B-VOP is inserted in AC[3]. Oth-erwise it will be sent by the AC[2]-queue with a lower prior-ity.

sendersender

MPEG4Traffic Generator

MPEG4Traffic Generator

framegenerate next

frame

sender

active

ready tosend

analyse frametype

SchedulerScheduler

XOR

I-VOP P-VOP B-VOP

Processing Time of

B-VOP less thanAC[3] urgency?

B has to beput into AC[2]

B can be

put into AC[3]

AND

XOR

AC[3] QueueAC[3] Queue AC[2] QueueAC[2] Queue

AND

B-VOP expectedto make

deadline?

XOR

Reject B-VOP

Figure 4. NS-2 MPEG-4 Scheduler Implementa-

tion


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Figure 5. NS-2 MPEG-4 Scheduler Implementa-

tion

5. Testbed and Evaluation

We evaluated the new scheduling policy with the NS-2 net-work simulator. As a MPEG-4 traffic generator, we chose amodel that exploits the Transform Expand Sample Method-ology [15] to fill the working queues of the scheduler that isimplemented as a class in NS-2. Fig.5 illustrates the archi-tectural design of the scheduler implementation in NS-2. Thetraffic generator uses additional separate NS-2 output agentsfor each VOP-type. This allows to generate graph diagramsin order to analyze the throughput behaviour per VOP-type.The traffic generator then passes the frame packets to thescheduler. The scheduler is built on top of the 802.11e im-plementation as described in [14]. It routes the VOPs to theEDCA AC queues according to the scheduling policy intro-duced in the last section. Eachof the AC agents then uses the

mandatory sendmsgmethod to transmit the queued packets.Furthermore, at the receiving client, we extended the NS-2Loss Monitor byan outp()-functionand bya separate counterfor MPEG-4 VOPs and traffic. This allows us to compare thegenerated video traffic throughput and the actual data re-ceived at the client, and hence the packet loss can be accu-rately determined.

Atthe receiver nodea secondLossMonitorrecords theincom-ing data and frames. Due to those records and the informa-tions of the first LossMonitor we can evaluate the behaviourand results of the scheduling.

The first experiments showed promising results and will beavailable when further progress in the simulation model will

have been made.

6. Summary and Future Work

This paper presented work on a new scheduling policy for802.11 EDCA. It is especially designed to dynamically pri-oritize MPEG traffic and assign it to the EDCA ACs in or-der to improve the resource utilization and the timely trans-fer of MPEG data frames. It relies on a statistical analysis ofthe available throughput per flow at saturation conditions.

Infuture wewillfurther evaluatethe policy with theNS-2sim-

ulator. We will further develop more accurate methods to es-timate the MPEG traffic per AC in order to determine theurgency more precisely.

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[9] Bavier, A., Peterson, L., Mosberger, D.: BERT : AScheduler for Best Effort and Realtime Tasks. Tech-nical Report TR-587-98, Princeton University, August1998.

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[11] Bianchi, G.: Performance Analysis of the 802.11Distributed Coordination Function In IEEE Journalon Selected Areas in Communications, Vol 18, No.3,March 2000.

[12] Hertrich, D.: MPEG4 video transmission in wire-less LANs: Basic QoS support on the data link layer of802.11b Minor Thesis, Technical University of Berlin,October, 2002.

[13] Altenbernd, P., Burchard, L., Stappert, F.: Worst-Case Execution Times Analysis of MPEG-2-Decoding12th Euromicro Conference on Real Time Systems,Stockholm, Sweden.

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Scheduling MPEG-4 Video Streams Through the 802.11e Enhanced