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M u l tic a s t V i d e o - o n -D e m a n d S e r v i c e s *Huadong MaCollege of Com puter Science & T echnology

Beijing U niversity of Posts and Telecomm.Beijing 100876. Chinarnhd @ b upt.edu.cn

Kang G. ShinReal-Time Computing LaboratoryDepartment of EEC STh e University of MichiganAnn Arbor, MI 48109-2122 , US Akgshin @ eecs. u mich, edu

A B S T R A C TT h e s e r v e r ' s s t o ra g e I / O a n d n e t w o r k I / O b a n d w i d t h s a r e t h e m a i nbo t t l eneck o f VoD se rv ice . M ul t ica s t o f fe r s an e f f ic ien t means o fd i s t r ib u t i n g a v i d e o p r o g r a m t o m u l t i p l e c l ie n t s , t h u s g r e a t l y i m -p r o v i n g th e V o D p e r f o r m a n c e . H o w e v e r , t h e r e a re m a n y p r o b l e m st o o v e r c o m e b e f o r e d e v e l o p m e n t o f m u l t i c a s t V o D s y s t e m s . T h i spape r c r i t i ca l ly eva lua te s and d i scusses the recen t p rog ress in deve l -op ing mu l t ica s t VoD sys tems . W e f i r s t p re sen t the concep t and a r -ch i tec tu re o f mu l t ice s t VoD, and then in t roduce the techn iques u sedi n m u l t i c a s t V o D s y s t e m s . W e a l s o a n a l y z e a n d e v a l u a t e p r o b l e m sre la ted to mu l t ica s t VoD se rv ice . F ina l ly , we p re sen t open i s sue s onmul t ica s t VoD as poss ib le fu tu re re sea rch d i rec t ions .

K e y w o r d s : Q u a l i t y - o f - S e r v i c e ( Q o S ) , s c h e d u li n g , V C R - l i k e i n -te rac t iv i ty , mu l t ica s t , Video -o n -D ema nd (VoD)

1 . I N T R O D U C T I O NA t y p i c a l V i d e o - o n - D e m a n d ( V o D ) s e r v i c e a ll o w s r e m o t e u s e r s t op lay back any on e o f a l a rge co l lec t ion o f v ideos a t any t ime . l~yp -ica l ly , these v ideo f i l e s a re s to red in a se t o f cen t ra l v id eo se rve rs ,a n d d i s t r i b u t e d th r o u g h h i g h - s p e e d c o m m u n i c a t i o n n e t w o r k s t ogeograph ica l ly -d i spe rsed c l ien t s . U pon rece iv in g a c l i en t ' s se rv icereques t , a se rve r de l ive rs the v ideo to the c l ien t a s an i soch rono usv i d e o s l re a m . E a c h v i d e o s t r e a m c a n b e v i e w e d a s a c o n c a t e n a t i o no f a s to rage - l /O "p ipe" and a ne twork p ipe . Thus , su f f ic ien t s rn rage -F O b a n d w i d t h m u s t b e a v a i l a b l e f o r c o n t i n u o us t r a n s f e r o f d a t a f r o mthe s to rage sys tem to the ne twork in te r face ca rd (NIC ) , wh ich mus t ,in turn , have enough ba ndw id th to fo rward da ta to c l i en ts - Thus , av i d e o s e r v e r h a s t o r e s e r v e s u f f ic i e n t I / O a n d n e t w o r k b a n d w i d t h sbe fo re accep t ing a c l i en t ' s r eques t . W e de f ine a s e r v e r c h a n n e l a st h e s e r v e r r e s o u r c e r e q u i r e d t o d e l i v e r a v i d e o s t r e a m w h i l e g u a r a n -

tee ing a c l i en t ' s con t inuous p laybac k .T h i s t y p e o f V o D s e r v i c e h a s a w i d e s p e c t r u m o f a p p l i c at i o n s ,s u c h a s h o m e e a t e r ta i n m e n t , d i g i t a l v i d e o l i b r a r y , m o v i e - o n - d e m a n d ,* T h i s w o r k w a s p a r t l y d o n e d u r i n g H u a d o n g M a ' s v i s i t t o R T C La t t h e U n i v e r si t y o f M i c h i g a n . T h e w o r k w a s s u p p o r t e d i n p a r t b yt h e U S A N S F u n d e r G r a n t E I A - 9 8 0 6 2 8 0 a n d t h e N a t u r a l S c i e n c eFound a t ion o f Ch ina unde r G ran t 69873006 .

d i s tance lea rn ing , t e le - shopp ing , news-on-demand,a n d m e d i c a l i n -fonna t ion se rv ice . In genc ra l , the VoD se rv ice can bc cha rac te r izedas fo l lows .L o n g - l i v e d s e s s i o n : a V o D s y s t e m s h o u l d s u p p o r t l o n g - l iv e d s e s -s i o n s; f o r e x a m p l e , a t y p i c a l m o v i e - o n - d e m a n d s e r v i c e u s u -a l l y l a s t s 9 0 - 1 2 0 m i n u t e s .H i g h b a n d w i d t h r e q u i r e m e n t s : f o r e x a m p l e , s e r v er s t o ra g e I / O a nd

n e t w o r k b a n d w i d t h r e q u i r em e n t s a r e 1 .5 M b p s ( 3 - 1 0 M b p s )f o r a M P E G - I ( M P E G - 2 ) s t re a m .S u p p o r t f o r V C R - l l k e / n t e r a e t / v i t y : a c li e n t r e q ui r es t he V o D s y s-tem to o f fe r VC R- l ik e in te rac t iv i ty , such a s the ab i l i ty to p lay ,f o r w a r d , r e v e r se a n d p a u s e . O t h e r a d v a n c e d i n t e r a c t i v e f e a -tu re s inc lude the ab i l i t y to sk ip o r se lec t adve r t i semen ts , in -v e s t i g a t e a d d i ti o n a l d e t a il s b e h i n d a n e w s e v e n t ( b y h y p e r m e -d i a l i n k ), s a v e t h e p r o g r a m f o r a l a t e r re f e r e n c e , a n d b r o w s e ,s e l e c t a n d p u r c h a s e g o o d s .Q o S - s e n s i l i v e s e r v i c e : t h e Q o S t h a t V o D c o n s u m e r s a n d s e r v i c ep r o v i d e r s m i g h t c a r e i n c l u d e s s e r v i c e l a t e nc y , d e f e c t i o n ra t e ,in te rac t iv i ty , p layb ack e f fec t s o f v ideo s , e tc .

A c o n v e n t io n a l T V o D ( T r u e V i d e o - o n - D e m a n d ) s y s t e m u s e s o n eded ica te d channe l fo r each se rv ice reques t , o f fe r ing the c l ien t theb e s t T V o D s e r v i c e . H o w e v e r , s u c h a s y s t e m i n c u r s v e r y h i g h c o s ts ,e s p e c i a l l y i n t e r m s o f s t o r a g e - I / O a n d n e t w o r k b e n d w i d t h s . M i n e -o v e r, s u c h a V o D s e r v i c e h a s p o o r s c a l a b i l i t y a n d l o w p e r f o r m a n c e / c o s te f f ic iency . A l thoug h the conven t iona l app roach s im p l i f ie s the im-p lemen ta t ion , no t sha r ing channe ls fo r c l i en t r eques t s wi l l qu ick lye x h a u s t t h e n e t w o r k a n d t h e s e r v e r F O b a n d w i d t h . I n f a ct , t h en e t w o r k - I /O b o t t l e n e c k h a s b e e n o b s e r v e d i n m a n y e a r l i e r sy s t e m s ,s u c h a s T i m e W a r n e r C a b l e ' s F u l l S e r v i c e N e t w o r k P r o j e c t i n O r -l a n d o [ 6 9 ], a n d M i c r o s o f t ' s T i g e r V i d e o F i l e s e r v e r [ 12 ]. I n o r d e r t os u p p o r t a l a r g e p o p u l a t i o n o f c l i e n ts , w e t h e r e f o r e n e e d n e w s o h i -f inns tha t e f f ic ien t ly u t i l i ze the se rve r and ne tw ork re sou rces .C l e a r l y , th e p o p u l a r i t y o r a c c e s s p a t t e r n o f v i d e o o b j e c t s p l a y s a ni m p o r t a n t r o l e i n d e t e r m i n i n g t h e e f f e c ti v e n e s s o f a v i d e o d e l i v e r ytechn ique . Becau se d i f fe ren t v ideos a re reques ted a t d i f fe ren t ra te sand a t d i f fe ren t t imes , v ideos a re u sua l ly d iv ided in to ho t (popu la r )a n d c o l d ( l e s s p o p u l a r) , a n d r e q u e s t s f o r t h e t o p 1 0 - 2 0 v i d e o s a r eknow n to cons t i tu te 60 - -80% o f the to ta l demand . So , i t i s c ruc ia l toi m p r o v e t h e s e r v i c e e f f i c i e n c y o f h o t v i d e o s .T h u s , r e q u e s t s b y m u l t i p l e c l i e n t s f o r t h e s a m e v i d e o a r r i v i n gw i t h i n a s h o r t t i m e i n t e rv a l c a n b e b a t c h e d t o g e t h e r a n d s e r v i c e dus ing a s ing le s t ream. Th is i s r e fe r red to a s h a t c h i n g . Th e m u l t i c a s tfac i l i ty o f modern com mu nica t ion ne tw orks [25 , 26 , 60 ] o f fe r s ane f f ic i e n t m e a n s o f o n e - t o - m a n y d a t a t r a n sm i s s i o n . T h e b a s i c i d e at M u l t i c a s t a l s o c o v e r s m u l t i p o i n t - t o - m u l t i p o i n t c o m m u n i c a t i o n , b u t

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Classification I Featuresis to avoid u'ansmitting he same packet more than once on each linkof the network by hav ing branch routexs duplicate and then send thepacket over multiple downstre am branches. Multicast can signifi-cantly improve the VoD performance, because it reduces the required network bandwidth greatly, thereby de-creasing the overall network load; alleviates the workload of the VoD server and improves thesystem throughput by batch ing requests; offers excellent scalability which, in turn, enables servicing alarge n umber of clients; and provides excellent cost/performance benefits.

In spite of these advantages, multicast VoD (MVoD) introducesnew and difficult challenges, as listed below, that may make the sys-tem more complex, and may even degrade a particular customer'sQ o S .

I t i s d i ff i cu l t o s u p p o r t V C R - l i k e i n te r a ct i v it y i t h m u l t i c a s tVoD service while im proving service efficiency. Batching makes the clients arriving at different times share amulticast stream, which may incur a long service latency (orwaiting time) causing some clients to renege. A single VoD stream from one server cannot support clients'

heterogeneity due ma inly to diverse customer premise equip-ments (CPEs). A multica st session makes it difficult to manage the syste mprotocol an d diverse clients. Multicast VoD introduces the complex legal issue of copy-right protection.

A multicast VoD system must therefore overcome the above draw-backs without losing its advantages. This paper critically reviewsthe recent progress in multicast VoD (inclu ding general VoD tech-piques) and discusses open issues in multicast VoD.The rema inder of this paper is organized as follows. Section 2introduces the concepts and architectures of a multica st VoD system,and analyzes the problems in developing it. Section 3 reviews theimplemen tations of multicast VoD. Section 4 discusses the issuesrelated to multic ast VoD service. Finally, Section 5 summarizes thepaper and discusses open issues in im pleme nting multicast VoD.2 . O V E R V I E W O F M V O D S E R V I C E2 . 1 T h e t a x o n o m y o f V o D s y s t e m s

A true VoD system supports a user to view any video, at any timeand in any interactive mode. Based on the amou nt of interactiv-ity and the ability of controlling videos, VoD systems are classi-fied as Broadcast (No-VoD), Pay-Per-View (PPV), Quasi Video-On -Dema nd (QVoD), Near Vide o-On- Deman d (NVoD), True Video-On-Demand (TVoD) [56] which are listed and compared in Table1.O b v i o u s l y , TVoD is the most ideal service. For TVoD service,

the simplest scheme of scheduling server channels is to dedicate achannel to each client, but it will require too many channels to beaffordable. Since a client may be willing to pay more for TVoD ser-vice t h a n for non-TVoD service, sharing a chan nel among clients is areasonab le way to improve the VoD performa nce and lower clients 'cost. In fact, rnulticast can sup port all types of VoD services whileconsum ing much less resources.for our purpose in this paper, it suffices to consider only one-to-many communication.

No-VoD

PPV

Q V o D

NVoD

TVoD

similar to broadcast TV, in which theuser is a passive particip ant and hasno control over the session.in which the user signs up and paysfor specific programming, si milar toexisting CATV PPV services.in which users are grouped based on athreshold of interest. Users can performrudimentary temporal control activitiesby switching to a different group.function s like forward a nd reverse aresimulated by transitions in discretetime intervals. This ca pability can beprovided by multiple channels with thesame programming skewed i n time.the user has complete con~ol over thesession presentation. The user has full-f u n c t i o n VC R c a p a b i l i t i e s , ncludingforward a nd reverse play, freeze, a ndrandom positioning.

Tab le 1 : C lass i f i ca tion o f V o D sy s t em

2 . 2 V C R i n t e r a c t i v i t y o f V o DInteractivity is an essential feature of V o D service. After theiadmission, customers can have the following types of interactionsPlay/Resume, S top/Pause/Abort, Fast Forward/Re wind, Fast SearchSearch, Slow Motio n as identified in [55].A TVoD service may also provide the support for other interactions such as R e v e r s e and S l o w R e v e r s e , which correspond to a presentation in the reverse direction, a t normal or slow speed. Usuallywe d on't consider them as part of the usual interactive behavior of acustomer.We classify interactive operations into two types: (1) for war dinteractions, such as Fast Forward and Fast Search; (2) backwardinteractions, such as Rewind, Reverse Search, Slow Motion, andStop/Pause. This classification depends on whether the playb ack

rate after interactive operations is faster than the norma l playback onot. In order to understa nd the limited support provided by defaulin multicast VoD systems, one can identify two types of interactivity: cont inuous or d iscont inuous interac tion [7]. Continuous interactive functio ns allow a customer to fully control the duration of alactions to support TVoD service, whereas discont inuous interactivfunctions allow actions to be specified only for durations that areinteger multiples of predetermined time increme nt to support NVoDservice. Note that the size of discont inuity is a measure of the QoSexperienced by the customers from NVoD service.From the implementation's perspective, we also categorize interactions as in teract ions wi th p ic ture or in teract ion wi thou t p ic tureFast/Reverse Search and Slow Motion are typical interactions withpi~ure, whereas Fast Forward and Rewind are typical interactionwithout picture. In general, it is easier to impleme nt interactionwithout picture because it requires less system resource.2 . 3 T h e a r c h i t e c t u r e o f m u l t i c a s t V o D s y s t e m s2 . 3. 1 T h e r e f e r e n c e m o d e l o f V o D s y s t e m s

The Digital Audio-v isual Council (DAVIC) founded in 1994 isa non-profit organization which has charged itself with the task ofpromoting broa dband digital services by the timely availability ointernatio nally-agr eed specifications of open interfaces an d proto

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c l ~

F'---] ' -(c P S - S P 3 i I I: s P s . s p s - s c , ~Del i ' v l . ~ I D , d i v = 3

- ~ I k l 4 ' r m m l = s r -

~ ~ , II "

s c s

) - " ' [ ~ 1) - " { ~ 7

Figure 1: DAVIC reference model . . . . . . . . . . . . . . . . . . . . . . i

iii Ii k ~ov,a= ) a=~o . ;l |; ;; a ' Y ~i ;l ;i i|i1 |ii L stag= l i! 1i . . . . . . . . . . . . . . . . . . . . . . . . [

F i g u r e 2: A mult icast VoD system

cols that maximize interoperability across counlries and applicationsor services. According to the DAVIC reference model shown in Fig-ure 1 [2g], a VoD system generally consists of the following entities:C o n t e n t Pro vider System (CPS) owns and sells video programsto the service provider;Service Pro dd er System (SPS) is a collection of system functionsthat accept, process and present info rmat ion for delivery to aservice consumer system;Service Con sume r System (SCS) is responsible for the primary func-tions that allow a consumer to interact with the SPS and areimplemented in a customer premise equipmen t (CPE);CPS-SPS and SPS-SCS network provider.

A consumer generates a request for service to the provider, whowill obtain the necessary material from the program (content) providerand deliver it to the cons umer using the network provider's facilities.The SPS acts as an agent for consumers and can access the varioustypes of CPS. The network, CPS, and SPS can be the same organi -zation , but they are generally different. DAVIC-based VoD systemshave been developed, such as the one in [72], ARMIDATM [53],the NIST VoD system [46], KYDONIA [19], and the BroadbandInteractive VoD system at Beijing Telecommunications.The reference model is also suitable for specifying the architec-ture of MVoD (Multicast Video-on-Demand) systems. Consider atypical MVoD delivery system shown in Figure 2 [8, 36]. Con-sumers make program requests to the manager server (Service Provider).A request is received and queued by the manager server until thescheduler is ready to allocate a logical channel to deliver videostreams from a video object storage to a group of consumers (m~ -ticast group) across a high-speed network. The manager server or-ganizes the media server and network resources to deliver a video

E

lW

F i g u r e 3 : Hierarch ica l amh itec ture o f a VoD systemslream into a channel. A channel can be either a unicast or multicastchannel. The m edia server receives consumer requests for video ob-jects via the manager server, processes them, and determines whenand which channels to deliver requested video objects to the con-s u m e r s .Each consumer accesses the system by a CPE which includes aset-top box (STB), a disk and a display monitor. A consumer is con-nected tn the network via a STB, which selects one or more networkchannels to receive requested video objects according to the server'sinslracfions. The received video objects are either sent to the displaymonitor for immediate playback, or temporarily stored on the diskwhich will later be rewieved and played back.2 .3 , 2 H i e r a r c h i c a l V o D s y s t e m s

Large-scale VoD systems require the servers to be arranged as adistributed system in order to support a large number of concurrentsl~eams. If the system is hierarchical, an end-node server handlesthe requests from a particular area, the next server in the hierarchytakes the requests over for end-node servers if they cannot handlethem. This architecture provides the cost efficiency, reliabi lity a n dscalability of servers. Generally, servers are either tree-shaped [61]or graph-structured [77, 78] in Figure 3. The graph-structured sys-tem often offers good QoS for handling the requests, but the man-agement of requests, videos and streams is complicated in the sys-tem. The tree-shaped system can easily manage requests, videos andsl~eams, but it offers poorer QoS than the former. In order to eval-uate the effectiveness of disl]-ibution strategies in such a hierarchy,the authors o f [40] investigated how to reduce storage and networkcosts while taking the customers' behaviors into account.Although some of hierarchical architectures are original ly de-signed for unJcast VoD services, they can also be used for multicastVoD to further improve the efficiency of service.2 . 4 P r o b l e m s w i t h m u l t i c a s t V o D

Given below are the desired properties of a multlcast VoD system.F_~ciency: The system should impose a minima l additional burdenon the server and the network, and should sufficiently utilizecritical resources on the server and the network.Real-TUne: The system should respond to the consumer requestsand ~ansmit the requested videos in real time.Scalabil~y: The system should scale well with the number of clients.i n t e r a c t l v ~ : T h e system should provide the clients full control ofthe requested video by using VCR-like interactive functions.Reliabili ty: The system should be robust to failures in the serverand the network, and easy to recover from failures. The trans-mission of messages and video streams should also be reli-able.

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system should provide efficient support for copyrightprotection in U'ansmittingvideo streams to multiple clients.t o d e a l w i t h h e t e r o g e n e i t y : T h e system should deal with het-erogeneous networks and CPEs.

system should provide "fair" scheduling of videoswith diffe rent popularitie s so as to treat all customers "fairly."In order to meet the above requirements, we must solve the fol-The first problem is how to deal with the coupling between systeme hatching inte rval Increasing the batching inter-server and networ k resources significantly at the ex-

short-time will d iminish the benefits of multicast VoD.this tradeoff, we mu st shorten all requests' waiting

The second proble m is how to support scalability and interactiv-y. Support for full interactivity requires an "individualized" serviceD. We need aon-dema nd service in multicas t VoD systems with-

mic viability.The third proble m is how to guarantee customers' QoS with lim-bandwidths. In multicast VoD, customers' QoS can be ex-customers' d e f e c t i o n r a t e due to long waits,action blocking probability and playback effect. How-er, since system resources are limited, we must strive to maximizeheir utilization.Moreover, the multicast VoD service generally favors popular

icast VoD framework is also of importance to the fairness of service.

. I M P L E M E N T A T I O N O F M V O D3.1 Storage organizationThere are two types of servers: m anager and media servers. Themanager server (Service Provider) is responsible for billing and con-nection management, while the med ia server, the focus of this sec-tion, handles real-time retrieval and delivery of video streams.The ma in challenge in the design of video server is how to utilizestorage efficiently. When designing a cost-effective video storage,one must conside r issues, such as place ment of data on disks, diskbandwidth and disk-access QoS.We consider the following main storage requirements.

The VoD server requires a large storage capacity. A 100-minute MPEG- 2 video with a wausfer rate of 4 Mbps requiresapproximately 3 GBytes of storage space. Video objects are difficult to handle due to their large vol-ume/size and stringent requirement of real-time continuousplayback.

Most existing studies consider the use of multiple disks orga-nized in the form of disk-farm or disk-array. A video server typi-cally uses disk-array for large video data. When de signing such adisk-array base d VoD server, we must deal with several conslra intson resource allocation to provide scalability, versatility, and load-balancing. Scalability is defined as the ability to absorb significant

workload fluctuations and overloads without affecting admission la-tency, while versatility is defined as the ability to reconfigure theVoD server with a min imal disturbanc e to service availability. High-level versatility is also desir able for expandability, to ensure that newdevices can be added easily. Each video can be stored on a singledisk or stripped over multiple disks.There are two basic types of storage organization. T he first typecompletely partitions the storage amon g different movie tides. Sucha storage system is said to have a c o m p l e t e l y - p a r t i t i o n e d (CP) orga-nization, and may be found in small-s cale VoD servers which storeOne .Movie title Per Disk (OMPD). T he se cond type completelyshares the storage amo ng different movie rifles, whi ch is said to havea c o m p l e t e l y - s h a r e d (CS) organization. VoD servers store movierifles using fine-grained striping (FGS) or coarse-grained swiping(CGS) [66] of videos across disks in order to effectively utilize diskbandwidth. In FGS (si milar to RAID-3), the stripe unit is relativelysmall and every retrieval involves all n disks that behave like a singlelogical disk with bandwidt h nB (B is the bandwidth of one disk). InCGS, each rel3-ievalblock consists of a large stripe unit which is readfrom onl y a single disk, but different disks can simultaneously serveindepen dent requests. CGS with parity informa tion maintai ned onone or several dedicated disks corresponds to RAID- 5 [11, 67].CP organizations typically trade availability m disks can fail orbe brought off-line for update witho ut affecting the entire service mfor increased latency a nd cosily, inefficient use o f storage capacity.CS organizations ensure a very low latency and high storage uti-lization, but reconfigurations risk the availability of the entire VoDserver. Studies in [3, 18, 33, 64] have shown that video stripi ng im-proves disk utilization and load-balancing, and hence inciea ses thenumb er of concurrent streams. [3, 64] considered both CGS andFGS, and concluded that the former can support more concurrentvideo streams than the latter. Thi s is because a disk has a relativelyhigh latency for data access (10--20 ms), and a sufficient amount ofvideo data must be transferred in each disk access in order to im-prove the utiliza tion of the effective disk lransfer bandwidth.3 .2 User-centered scheduling strategies

A conven tional VoD system assumes the u s e r - c e n t e r e d schedul-ing scheme [4, 84] in which a user eventually acquires some ded-icated bandwidth. It can be achieved by providing (1) a sufficientbandwidth equal to an object c onsumpt ion rate multiplied by thenumber of users, or (2) less bandwidth, for which the users com-pete by negotiating with a scheduler. The con sumption rate of avideo object is equal to the amou nt of bandwidth necessary to viewit continuously. Whe n a client makes a request to the server, theserver sends the requested object to the client via a dedicated chan-nel. This scheme incurs high system costs, espec ially in terms ofserver storage-I/O and network bandwidths. To maximally utilizethese channels, researchers have proposed efficient scheduling tech-niques [21, 33, 44, 52, 62, 63, 65, 85]. Thes e techniques are saidto be 'laser-cantered,'" because channels are allocated to users, notdata or objects. Thes e simplify the implementatio n, but dedicatinga stream to each viewer will quic kly exhaust the network-I/O band-width.3.3 Data-centered scheduling strategies

To address the network-I/O bottlenec k faced by the user-centeredscheduling, one can use the data-centered scheduling which dedi-cates c h a n n e l s t o v i d e o o b j e c t s , i n s t e a d o f u s e r s . I t a l l o w s u s e r s toshare a server stream by batc hing their requests. That is, requestsby multiple clients for the same video arriv ing within a short timeinterval can be hatched togethe r and served by u sing a sing le stream.The data-centered scheme has the potential for dramatically re-

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B atching policyMaximum Queue Length(MQLF) [22]First-Come-First-Served-Firs t (FCTS) [22]Maximum Factored QueueLength First (MFQLF)[5]Look-Ahead-Maximize

-Batch (LAMB) [34]

Features Comparisonrequests for the video with the largest maximizing the server throughputnumber of pending requests to serve first, but unfairness to unpopular videos.the oldest request (with the longest fairness but a lower systemwaiting time) to serve next. throughput.the pending batch with the largest size a throughput close to that of MQLFweighted by the factor, (the associated without compromising fairness.access frequency)-I /2,to serve next.a channel is allocated to a queue if andonly if a head-of-the-line user is aboutto leave the system without being served

maximizi ng the numbe r of admittedusers in a certa in time windowbut unfairness to some requests.

Group-Guaranteed Server server capacity is pre-assigned to meeting a given performance objectiveCapaci ty (GGSC) [82] groups of objects for the specific group.'lhble 2: Mu lticas t batching policies

ducing the network and server bandwidth requirements. The data-centered multicast VoD service can be either client-initiated or server-initiated [36]. In the clien t-init iated service, channels are allocatedamong the users and the service is initia ted by clients, so it is alsoknow n as a schedu led or client-pull service. In the server-initiatedservice, the server channels are dedicated to individual video ob-jects, so it is also called aperio dic broadcast or server-push service.Popular videos are broadcast periodically in this scheme, and a newrequest dynamically oin s, with a small delay, the stream that is be-ing broadcast. In practice, it is efficient to use hybrid batching thatcombines the above two schemes.3 . 3. 1 C l i e n t - i n i ti a t e d m u l t i c a s t s c h e m e s

Using a client-initiatedmulficast, when a server channe l becomesavailable, the server selects a batch to multicast according to thescheduling policies in Table 2,The equally-spaced batching mechanism has a fixed maximumservice latency and supports NVoD interactivity, but its usually-large service latency may cause some clients to renege. In orderto reduce the service latency, dynamic m ulticast has been proposed,where the multicast tree is expanded dynamically to accommodatenew requests.For example, Adaptive Piggybacking [39] allows clients arrivingat different times to share a data stream by altering the playbackrams of iwpmgress requests (for the same object), for the purposeof merging their respective video streams into a single stream thatcan serve the entire group of merged requests. This approach canlower the service latency as compared to simple hatching. But it isrestrictive in that the variation of the playback rate must be within,say 5%, of the normal playback rate, or it will result in a perceivabledeterioration of QoS. This limits the numbe r of streams that can bemerged.Chaining [78] is also a generalized dynamic multicast techniqueto reduce the demand on the network-I/O bandwidth by cachingdata in the clien t's local storage to facilitate future multicasts. Thus,data are actually pipel ined through the client stations residing at thenodes of the respective chaining tree, and the server sexves a "chain"

of client stations u sing only a single data stream. The advantage ofchaining is that not every request has to receive its data directly fromthe server. A large amount of video also becomes available fromclients located throughout the network. This scheme scales wellbecause each client station using the service also contributes its re-sources to the community. Hence, the larger the chaining trees, themore effective the application can utilize the aggregate bandwidth.The authors of [16] present stream tapping that allows a client togreedily "tap" data from any stream on the VoD server containing

video data s/he can use. This is accomplished through the use ofa small buffer on the CPE and requires less than 20% of the diskbandwidth used by conventional systems for popular videos.To elimina te the service latency, patching was introduced in [42].The objective of patching is to substantially improve the number ofrequests each channel can serve per lime unit, thereby sufficientlyreducing the per-customer system cost. In the patching scheme,channels are often used to patch the missing portion of a serviceor deliver a patching stream, rather than multicasting the video i nits entirety. Given that there is an existing multicast video, whento schedule another multicast for the same video is crucial. Thetime period after a multicast, during which patching must be used,is called the patching window [14]. Two simple approaches to set-ring the patching window are discussed in [42]. The first one usesthe length of the video as the patching window. That is, no multicastis initiated as long as there is an in-progress multicast session for thevideo. This approach is called the greedy patching because it Iriesto exploit an in-progress mulficast as much as possible. However,an over-greed can actually reduce data sharing [42]. The second ap-proach, called the grace patching, uses a pa tching stream for the newclient only if it has enough buffer space to absorb the skew. Hence,under grace patching, the patching window is determined by theclient buffer size. Considering such factors as video length, clientbuffer size, and request rate, the authors of [15] generalized patchingby determining the optimal patching window for each video. An im-proved form of patching, called as the transition patching [15], useseither a patching slream or a transition stream and improves per-formance without requiring any extra download bandwidth at theclient site. Other optimal patching schemes were described in [31,75]. In patching, a clien t might have to download data on both reg-ular multicast and patching channels simultaneously. To implemen tpatching, a clien t station needs three threads: two data loaders todownload data from the two channels, and a video player to playback the video.The contro lled CIW P (Client-Initiated-With-Prefetching) 36] isanother multicast technique similar to patching and tapping for nearinstantaneous VoD service. The novelty of the controlled CIWP isthat it uses a threshold to control the frequency of multicasting acomplete video stream. It uses simple FCFS channel scheduling sothat a client can be informed immediately of when its request willbegin service.3 , 3 .2 S e r v e r - i n i t i a t e d b a t c h i n g

In sexver-inifiated batching, the bandwidth is dedicated to videoobjects rather than to users. Videos are decomposed into segmentswhich are then broadcast periodically via dedicated channels, and

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Scheduling strategy Features Typical methodsClient-initia ted the channels are allocated among the users. Adaptive Piggyba cking [39], Patching [42](scheduled, the multicast tree can be expanded dynamic ally Chaining [78], Tapping [I 6] andclient-pull) to accommodate new requests so that the Controlled CIWP [36], etc.service latency is minimize d (ideally zero).Server-initiated(periodic broadcast,server-push)

Hybrid scheduling

the channels are dedicated to video objects,the videos are divided into segments whichare then broadcast periodically via dedicatedchannels, the worst-case service latencyexperienced by any client is less than theinterval of broadcasting the leading segmentthe overall performance is improved by comb iningclient-initiated and server-initiated strategies.

Only two download channels:EB [22], PB [83], PPB [4].SB [43], GDB [35], DSB [29],etc.More than two download channels:Harmonic broadcasting [48],Staircasescheme [49] andFB [47, 81], etc.Controlled multicast [36], Catchingand Selective catching [37], etc.T a bl e 3 : T h e s u mm a ry o f e x i s t i n g d a ta -ce n l e re d a p p ro a ch e s

hence, it is also called periodic broadcast. Although the worst-caseservice latency experienced by any subscriber is guaranteed to beless than the interval of broadcasting the leading segment and isindependent of the curr ent numbe r of pending requests, this strategyis more efficient fo~ popular videos than for unpopular ones due tothe fixed cost of channels.One of earlier periodic broadcast schemes was the Equally-spacedinterval Broadcasting (EB) [22]. Since it broadcasts a given video atequally-spaced intervals, the service latency can only be improv edlinearly with the increase of the server bandwidth. The author of[10] also proposed the staggered VoD which broadcasts multiplecopies of the sam e video at staggered times. To significantly reducethe service latency, Pyramid Broadcasting (PB) was introduced in[83]. In PB, each video file is partitioned into the segments ofgeometrically-increasing sizes, and the server capacity is evenly di-vided into K logical channels. T he i-th chann el is used to broadcastthe i-th segments of all videos sequentially. Since the first segmentsare very small, they can be broadcast more frequently through thefirst channel. This ensures a smaller waiting time for every video.A drawback of this scheme is that a large buffer - - which usuallycorresponds to more than 70% of the video - - must be used at the re-ceiving end, requiring disks for buffering. Furthermore, since a veryhigh transmission rate is used for each video segment, an extremelyhigh bandwidth is required to write data to the disk as quickly as itreceives the video. To address these issues, the authors of [4] pro-posed a technique called Permutation-based Pyramid Broadcasting(PPB). PPB is similar to PB except that each channel multiplexesits own segments (instead of tr ansmitting them sequentially), and anew stream is started once every short period. This strategy allowsPPB to reduce both disk space and I/O bandwidth requirements atthe receivers. However, the required disk size is still large due tothe exponential nature of the data fragmentation scheme. The sizesof successive segments increase exponentially, thus causing the sizeof the last segment to be very large (typically more than 50% ofthe video). Since the buffer sizes are determined by the largest seg-ment, using the same data fragmentation scheme proposed for PBlimits the savings achievable by PPB. I n PPB, a client needs to tunein different logical subeharmels to collect its data for a given datafragment if the maxi mum savings in disk space is desirable.To reduce the disk costs in the clie nt side, the authors of [43] in-troduced Skyscraper Broadcasting (SB) which uses a new data frag-mentatio n technique and proposes a different broadcasting strategy.In SB, K channels are assigned to each of the N most popular ob-jects. Each of these K channels transports a specific segment of thevideo at the playback rate. The progression of relative segmentssize on the channel, {1,2,2,5,5,12,12,25,25,52,52,105,105 ... }. is

bounded by the width parameter W, in order to limit the storagecapacity required at the client end. SB allows for simple and effi-cient implementation, and can achieve a low service latency whileusing only 20~ of the buffer space required by PPB. The authorsof [35] provided a framework for broadcasting schemes, and de-signed a family of schemes for broadcasting popular videos, calledthe Greedy Disk-conserving Broadcasting (GDB). They sysmmati-cally analyze the resource requirements, i.e., the number of serverbroadcast channels, the clie nt storage space, a nd the client I/O band -width required by GDB. GDB exhibits a tradaoff between any twoof the three resources, and outperforms SB in the sense of reduc-ing resource requirements. The Dynamic Skyscraper Broadcasting(DSB) in [29] dyna mical ly schedules the objects that are broadcaston the skyscraper channels to provide all clients with a precise timeat which their requested objects will be broadcast, or an upper bou ndon that time if the delay is small and reaps the cost/performance ben-efits of the skyscraper broadcasting .The above broadcasting schemes ge nerally assume that the clientI/O bandwidths are limited to download data from only two chan-nels. If the client can download data from mo re than two channels,there are methods available that can efficiently reduce the servicelatency with less broadcasting channels. For example, a broadcast-ing scheme based on the concept of harmonic series is proposed in[48, 50]; the scheme doesn't require the bandwidth assigned to avideo equal to a multiple of a channel's bandwidth. For a movie oflength of D minutes, if we wan t to reduce the viewer's waiting timeto DIN minutes, we only need to allocate H(N) video channels tobroadcast the movie periodically, where H(N) is the harmonic num-ber o f N . i. e . , H C N ) = 1 + + . . . + T h e s ta i, a s, s c h e m e i u[49] ca n reduce the storage and disk transfer-ram require ment at theclient end. However, both the staircase and har monic schemes can-not serve bufferless users. I n [47, 51], a scheme calle d Fast Broad-casting (FB) is proposed, which ca n further reduce the waiting timeand the buffer requirement. Using FB, if a STB does not have anybuffer, its user can still view a movie insofar as a longer waitingtime is acceptable. The authors of [81] proposed two enhance mentsto FB, showing how to dyna mical ly change the num ber of channelsassigned to the video and seamlessly perform this transition, andpresenting a greedy scheme to assign a set of channels to a set ofvideos such that the average viewers" waiting time is minimal .3.3.3 Hybri d multicast scheduling

All practical scheduling policies are guided by three primary ob-jectives: minimi ze the re neging probability, minimize average wait-ing time, and be fair. It was shown in [22, 23, 36, 37] that a hy-brid of the above two techniques offered the best performance. For

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example, the Catching proposed in [37] i s a combination of peri-odic broadcast and client-initiatedprefix retrieval of popular videos.There are many hybrid schemes to improve the overall performanceof multicast VoD. The selective catching in [37] further improves theoverall performance by combining catching and controlled multicastto account for diverse user access patterns. Because most demandsare on a few very popular movies, more channels are assigned topopular videos. However, it is necessary (and important) to supportunpopular videos. We assume that scheduled multicasts are used tohandle less popular videos, while the server-initiated scheme is usedfor popular videos. In this approach, a fraction of server channelsam reserved and pre-allocated for periodically-broadcasting popu-lar videos. The remai ning channels are used to serve the rest ofthe videos using some scheduled mu.lticasta. This hybrid of server-initiated and client-initiatedschemes achieves better overall perfor-l na nc ~ .The existing data-centered approaches are summarized in Table3.3, 4 M u l t i c as t m u t i n g a n d p r o t o c o ls

There has been extensive research into multicast routing algo-rithms and protocol [17, 73, 86]. Multicast can be implementedon both LANs and WANs. Nodes connected to a LAN often com-municate via a broadcast network, while nodes connected to a WANcommunicate via switched networks. In a broadcast LAN, Irans-mission from any one node is received by all the other nodes on thenetwork, so it is easy to implement multicast on a broadcast LAN.On the other hand, it is challenging - - due mainly to the problemof scalability - - to implement mnlticast on a switched network. To-day's WANs are designed to ma inly support unicast communication,but in future, as multicast applications become more popular andwidespread, there will be a pressing need to provide efficient multi -cast support on WANs. In fact, the multicast backbone (MBone) ofthe Internet is an at tempt toward this goal.For multicast video transmissions, one of die key issues is QoSmuting which selects routes with sufficient resources to provide therequested QoS. For instance, the multicast VoD service requires itsdata throughput to be guaranteed at or above a certain rate. Thegoal of QoS muting is twofold: (1) meet the QoS requirements forevery admitted connection, and (2) achieve global efficiency in re-source utilization. In most cases, the problems of QoS routing areproven to be NP-complete [87]. Routing strategies can be classifiedas source routing, distributed or hierarchical muting. Some heuris-tic QoS routing algorithms have been proposed (see [17, 86] for anexcellent survey of existing mnlticast QoS rout ing schemes).In an effort to provide QoS for video transmissions, a number ofservices have been defined in the Internet. A Resource ReservationProtocol (RSVP) has been developed to provide receiver-in itiatedfixed/shared resource reservation for unicast/multicast data flows[13] after finding a feasible path/tree to satisfy the QoS require-ments. Furthermore, a protocol framework for supporting continu-ous media has been developed: RTP (Real-Time Protocol) [74] pro-vides support for timing information, packet sequence numbers andoption specification, without imposing any additional error controlor sequencing mechanisms. Its companion control protocol, RTCP(Reai-Time Control Protocol), can be used for gathering feedbackfrom the receivers, again according to the application's need.3 .5 T h e c l ie n t - en d s y s t e m

Customer premise equipments (CPEs) include set-top boxes (STBs),disks, and display monitors, where a disk or a RAM is used as abuffer. As an example, the disk space of 100 MB can cache about1 0 minutes of MPEG-1 video. Such a disk space costs less than

$10 today. The high cost of a VoD system is due mostly to the net-work costs. For instance, the cost o f networking contributes morethan 90% of the hardware cost of the Time Warner's Full ServiceNetwork project.The clien t's STB, from software perspectives, generally containsa main control thread, video stream receiver threads and a videoplayer thread. A clien t is connected to the network via a STB. Themain control thread processes the client's service request by sendinga message indicating his desired video to the server. It then forks thevideo stream receiver threads to select one or more network chan-nels to receive and decompress video data according to the server'sinstructions. The received video data are either stored on the diskor sent to the display monitor for immediate playback. The displaymonitor can either retrieve stored data from the disk or receive datadirectly from a channel.The CPE buffer plays important roles as follows.

Supporting the VCR interactions of a customer [2, 7, 71]. Theinteractionprotocols for multicast VoD are designed by usingthe CPE buffer. Providing instant access to the stored video program so as tominimize the service latency [37, 70]. Preloading and caching,based on the video stored in the CPE buffer, can reduce theservice latency. Reducing the bandwidth required to transmit the stored video[37, 70]. Because some video data reside i n the CPE buffer,the overall bandwidth requirement of transmitting videos is

r e d u c e d . E l i m i n a ti n g t h e a d d it i o n a l b a n d w i d t h r e q u i r e d t o gu a r a n t eejitter-free delivery of the c o m p r e s s e d video s t r e a m .

These func tions are discussed in detail i n the following subsections.3 .6 S u p p o r t f o r i n t e ra c t iv e f u n c t i o n s

One of the important requirements is to offer VCR interacfivity.In order to support customers' interactive behavior in multicest VoDservice, there have been efficient techniques proposed by a combi-nation of tuning and merging as well as us ing the CPE buffer andI-channels (see Table 4). The authors of [113] introduced tun/ng instaggered VoD which broadcasts multip le copies of the same videoat staggered times. Intelligen tly toning to different broadcast chan-nels is used to perform a user interaction. However, not all inter-actions can be achieved by jumpi ng to different streams. More-over, even if the system can emulate some interactions, it cannotguarantee the exact effect the user wants. Other solutions to VCRinteractivity are proposed in [7] and [24], especially for handling apause/resume request. Support for conti nuous service of pause oper-ations was simulated for a NVoD server, bu t merge operations fromI-channels to batching- (or B-) channels were either ignored [24] ordid not guarantee continuity in video playou t [7]. [7] proposed theuse of the CPE buffer to provide limited interactive functions. In or-der to i mplemen t the interactivity of multicast VoD services, moreefficient schemes have been proposed. For example, the SAM proto-col [54] offers an efficient way for TVoD interactions , and all thoseintroduced in Section 2 are provided by allocating the I-channels assoon as a VCR action request is issued. W hen playout resumes, theVoD server attempts to merge the users back into a B-channel byusing a dedicated synch buffer located at access nodes and partiallyshared by all the users. Should this attempt fail, a request for a newB - c h a n n e l is then initiated.The drawback of the SAM protocol requires an excessive num-ber of I-channels, thus causing a high blocking rate of VCR inter-actions. The authors of [2] improved the SAM protocol by using

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Level of interactionNVoD

Featuresthe interactive functions are simula tedby transitions in discrete time interval.

Typical n~thod stiming [to ]Limited TVoD the continuous interaction times are that supported by CPE buffer [7]limited by the ave, lable resource.TVoD full control the durations of SAM [54], Improved SAM [2]all continuous interactions. BEP [57], SRMDRU [71]

'Table 4: 'The summary of interaction schemas for multicast VoDthe CPE buffer and active buffer management, and hence, more in-teractions can be supported without 1-channel allocation. The BE P(Best-Effort Patching) scheme proposed in [57] presents an efficientapproach to the imple mentat ion of conti nuous TVoD interactions.Compared to the other methods, BEP aims to offer zero-delay (orcontinuous) service for both request admission and VCR interaction,whereas the SAM protocol just supports continuous VCR interac-tions without considering service admission. Moreover, BEP uses adynam ic technique to merge interaction streams with a regular mul-ticast stream. This technique significantly improves the efficiencyof multicas t TVoD for popula r videos.The authors of [71] proposed another scheme called the Single-Rate Mul t icas t Double-Rate Unicaxt (SRMDRU) to minimize thesystem resources required for supporting full VCR f unctionali ty ina multicast VoD system. This scheme also supports TVoD service,so customers can be served as soon as their requests are received bythe system. It forces customers in unic ast streams (on the I-channel)to be served by multic ast streams a gain after they resume from VCRoperations. The idea is to double the transmission rate of the uni-cast stream so that the customer of normal playback can receivethe frame synchronized with the transmitting frame of a multicastgroup.4 . I S S U E S R E L A T E D T O M V O D S E R V I C E4 .1 Q o S o f m u l t ic a s t V o D

The effectiveness of a video delivery technique must be evaluatedin terms of both the server and network resources required for de-livering a video object and the expected service latency experiencedby the clients. Reducin g the service latency is an important goalin designing effective scheduling strategies. The existing dynamicmnlticast and periodic broadcast schemes reviewed in Section 3.3are shown to achieve good performance.Beside.s the dynamic scheduling schemes, other schemes, such ascaching and preloa ding, have been proposed to reduce the servicelatency. In [30, 76], proxy servers are used to cache the initial seg-ments of popular videos to improve the service latency. Becausenearly all broadcast protocols assume that the CPE buffer is largeenough to store up to 40 or 50 % of each video (abo ut 50 minutes ofa typical movie), the part ia l pre loading proposed in [70] uses thisstorage to preload anticipated customers' requests, say, the first 3minutes of top 16 to 20 videos. It will provide instantaneous accessto these videos and also reduce the bandwidth required to broadcastthem as well as the extra bandwidth required to guarantee jitter-free delivery of the compressed video signal. It differs from proxycaching in that the preloaded portions o f each video will reside in-side the C.PE buffe r rathe r than at a prox y server.The customers' defection rate is closely related to the servicelatency, and is inversely proportional to the server throughput, oran average num ber of service requests granted per program. Theshorter the service latency, the lower the defection rate becomes a ndthe higher the server throughput is. Another important QoS parame-

ter is the VCR acti on blocking probability. All the existing muiticas tTVoD protocols covered in Section 3.6 aim to reduce the blockingprobability or discontinuity of VCR inte ractions.4 . 2 Client heterogeneity

As the multicast network expands, there will be various types ofend devices ranging fTom simple palm-top personal digital assis-tants (PDAs) to powerful desktop PCs or HDT V receivers of multi-cast VoD. Sinc e there will be mul tiple VoD transmissio n rates orpaths, the sen der alone cann ot meet the possibly conflicting de-mands of different receivers. Distributing a uni form representationof the video to all receivers could c ause low-capa city regions of thenetwork to suffer from congestion, and some receivers' QoS canno tbe me t even when there are sufficient network resources to providebetter QoS for these receivers.In the context of VoD, scalability also applies to the server's abil-ity to support the data requirements of multip]e terminal types. Oneway to solve this problem is proxy-based Wanscoding, where datastreams are individua lly transformed according to the specificationof each requesting receiver [32]. However, it typically imposes anadministrative burde n because it is not transpa rent to end users.Proxies are also difficult m deploy because a user be hind a con-strained network link might not have access to the optimal locationof a proxy. There was a proposal to use active networks to solve thisproble m by offering a commo n platform for such services as part ofthe basic network service model [80], but there remain many issuesto be addressed before such an infrastructure can be deployed. Fur-thexmore, transcoding proxies must be highly reliable and sea lablewhich cart be very costly [32].Another efficient solution to heterogeneity is the use of layeredmedia formats. This scheme encodes source data as a series of lay-ers, the lowest layer being called the base layer and higher layersbeing called the enhancement layers . Layered encoding can be ef-fectively combined with multicast transmission by sending differentlayers for different multicast groups. Conse quently, a receiver usingonly the basic multicast service (i.e., joining and leaving multicastgroups) can indivi dually tailor its service to match its capabilities,independe ntly of other receivers. Tiffs basic framework was laterrefined in a protocol architecture called Receiver-dr iven LayeredMuI t i c ast ( R L M ) [59]. In RLM, a receiver searches for the optimalnumber of layers by experimentally joining and leaving multicastgroups much in the same way as a TCP source searches for the bot-tleneck transmiss ion rate with the slow-start congestion avoidancealgorithm [45]. The receiver adds layers until congestion occursand backs off to an operating poi nt below this bottleneck.The two solutions to heterogeneity are summa rized in Table 5.4 . 3 F a i r n e s s o f m u l t i e a s t V o D service

Fairness is one of the performance me~ cs in VoD service, mean-ing that every client request shou ld be fairly treated regardless whetheit is for a hot video or not. In [42], the unfairness of a multicastVoD system is expressed as a f unctio n of the defection rate, that is,

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S o l u t i o n [ F e a t u r e s C o m p a r i s o nA c t i v e I r a n s c o d i n g d a t a s t r e am s a r e i n d i v i d u a l l y t r a n s f o r m e d a c c o r d i n g a n a d m i n i s t r a t i v e b u r d e n ,t o t h e s p e c i f i c at i o n o f e a c h r e q u e s t i n g r e c e i v e r d i f f i cu l t t o d e p l o y p r o x i e sL a y e r e d m u l t i c a s t e n c o d e s s o u r c e d a t a a s a s e r i e s o f l a y e r s , s e n d s c o m p l e x a d a p t a t i o n s c h e m ed i f f e re n t l a y e r s f o r d i f f e re n t m u l t i c a s t g r o u p s , i n c l i e n t - e n d

Table $ : The solutions of handling client heterogeneity

V / - = ,h e r e d r d e n o t e s t h e d e f e c t i o n r a t e f o r v i d e o i , d i s t h em e a n d e f e c t i o n r a t e , a n d N i s t h e n u m b e r o f v i d e o s. A l t e r n a t iv e l y ,t h is p r o p e r t y c a n a l s o b e m e a s u r e d b y v i d e o s e r v i c e l a t e n c ie s .T h e f a i r ne s s i s m a i n l y r e l a t e d t o s c h e d u l i n g a n d r e s o u r c e a l l o c a -t ion . W h en se lec t ing a schedu l ing s t ra tegy , we ma ke i t a s fa i r a sp o s s i b l e. T h e f a i r n e ss o f c e r t a i n b a t e h i n g s c h e m e s a r e s u r v e y e d i nSec t ion 3 .3 . How ever , schedu l ing s t ra teg ie s l ike va r ious pe r iod icb roadcas t s , a re on ly fo r popu la r v ideos , and the fa i rness depends ont h e s c h e d u li n g s c h e m e u s e d f o r c o l d v i d e o s a n d t h e b a n d w i d t h a l -l o c a t i o n a m o n g h o t a n d c o l d v i d e o s . U n f o r t u na t e ly , t h e r e a r e o n l ya ve ry few a t temp ts to ana lyze the fa i rness o f ex i s t ing schedu l ingschemes [42 ]. Ho w to a s su re the fa i rness o f p rac t ica l schedu l ings c h e m e s , p a r t i c u l a r l y f o r h y b r i d s c h e m e s , a n d m a k e t h e o p t i m a lb a n d w i d t h r e s o u r c e a l l o c a t io n a r e o p e n i s s u e s .4 . 4 C u s t o m e r b e h a v i o r

U n d e r s t a n d i n g c u s t o m e r b e h a v i o r s i s n e c e s s a r y t o e f f i c i e n t l y d e -s ign a mu l t ica s t VoD sys te m and take d i f fe ren t s t ra teg ie s to d i f fe ren tv i d e o s a t d i f f e r en t t i m e s . M o d e l i n g c u s t o m e x s ' b e h a v i o rs i n c l u d e sv ideo s e lec t ion d i s t r ibu t ion , va r ia t ions o f v ideo p opu la r i ty , and theu s e r i n t e r a c ti o n m o d e l .4 .4 .1 V 'uteo se le c t ion d i s t r ib u t ion

F o r s h o r t - te r m c o n s i d e r a t i o n s, m o s t r e s e a r ch e r s a s s u m e t h a t t h ep o p u l a r i t y o f v i d e o s f o l l o w s t h e Z i p f d i s t r i b u t i o n [8 8 ] , t h a t i s , t h ep roba b i l i ty o f choos ing the i - th v ideo i s 11_,~_- ~ = , w h e r e N i s t h et o t a l n u m b e r o f v i d e o s i n t h e s y s t e m , a n d z i s c a l l e d t h e akewfactor.Typ ica l ly , r e sea rche rs a s sum e tha t the skew fac to r be se t to 0 .271[5 , 22 ] . T h is num ber i s ob ta ined f rom the ana lys i s o f a u se r accesspa t te rn f i-om a v ideo ren ta l s to re [5 ] . Tha t i s , mo s t o f the dem and( 8 0 % ) i s f o r a f e w ( 1 0 t o 2 0 ) v e r y p o p u l a r v i d e o s .4 . 4 . 2 T i m e - v a r i a t i o n o f v i d e o p o p u l a r i t y

In a rea l VoD sys tem, requ es t a r r iva Is a re u sua l ly nons ta t iona ry .V a r i a t io n s i n t h e r e q u e s t r a t e c a n b e o b s e r v e d o n a d a i l y b a s i s , b e -t w e e n " p r i m e t i m e " ( e . g ., 7 p . m . - 1 0 p . m . ) a n d " o f f -h o u r s " ( e .g . ,e a r l y m o r n i n g ). O n a l a r g e r t i m e s c a l e (e . g ., o n e w e e k o r m o n t h ) ,m o v i e p o p u l a r i ti e s m a y c h a n g e d u e t o n e w r e l e a s e s o r l o s s o f c u s -tom ers ' in te re s t in cu r ren t t i t l e s . A t the same t ime , the d i f fe ren ttypes o f cus tomers (e .g . , ch i ld ren and adu l t ) have d i f fe ren t p r im et imes . In [1 ] , a t ime d i s t r ibu t ion mo de l i s exp ressed a s s inuso ida l :X( t ) = Xo + A s i , ( -~ )

X , ( t ) = p , ,X o + p m A s = ( ~ ) = X m +A~an( '~)w h e r e ~ i s t h e d a l l y a v e r ag e a r r iv a l r a t e , A ( > 0 ) i s t h e a m p l i t u d e,y = -~ (T be ing a 7A-hour pe r iod ) , and Pm the p o p u l a r i t y o f m o v i et i t l e m . M o r e g e n e r a l m o d e l s o f n o n s t a t i o n a r it y h a v e b e e n p r o p o s e din [ f l, 40 ] fo r the lon g - te rm pop u la r i ty o f mov ie . W e ca l l these t i rne -d e p e n d e n t c h a n g e s o f m o v i e p o p u l a r i t y t h e l if e - c y c l e o f t h e m o v i e .T h e a u t h o rs o f [ 4 0] o b s e r v e d t h a t t h e l o n g - t e r m b e h a v i o r o f a m o v i efo l lows an exponen t ia l cu rve p lu s a random e f fec t . [8 ] a l so a ssumedt h a t v a r i a ti o n s i n w o r k l o a d a r e e x p o n e n t i a l f u n c t io n s w i t h d i f f e re n tave rage in te r -a r r iva l t imes .

F i g u r e 4 : V C R i n t e r a c t i v e m o d e l4 . 4 .3 I n t e r a c t i o n m o d e l

S o m e i n t e r a c t i o n m o d e l s h a v e b e e n p r o p o s e d i n [ 2, 2 7 , 5 5 ] . I n[ 27 ], t h e b e h a v i o r o f e a c h u s e r i s m o d e l e d b y a t w o - s t a t e M a r k o vp r o c e s s w i t h s t a te s P B ( p l a y b a c k ) a n d F F / R e w . T h e t i m e s s p e n t b yt h e u s e r i n P B a n d F F / R e w m o d e s a r e e x p o n e n t i a l l y di s t ri b u t ed . T h et w o - s t a t e m o d e l i n [ 5 5 ] a s s u m e s t h a t t h e u s e r a c t i v i t y is i n e i t h e r th en o r m a l o r t h e i n t e r a c t i o n s t a te . B u t t h e s e t w o m o d e l s a r e t o o s i m -p l e t o r e p r e s e n t t h e d e t a i l s o f u s e r i n t e r a c t i o n s. T o b e r e a l i s t i c , am o d e l s h o u l d c a p t u r e t h r e e s p e c i f i c p a r a m e t e r s f o r t h e i r p o t e n t i a l l ys i g n i f ic a n t i m p a c t s o n t h e p e r f o r m a n c e o f t h e V o D s e r v e r : ( 1 ) t h ef r e q u e n c y o f re q u e s t s f o r V C R a c t i on s ; ( 2 ) t h e d u r a t i o n o f V C R a c -t .ions ; (3 ) the b ia s o f in te rac t ion behav io r . Co ns ide r in g these pa ram -e t e rs , t h e a u t h o r s o f [ 2 ] p r o p o s e d a V C R i n t e r a c t io n m o d e l . I n t h i sm o d e l , a s e t o f s t a t e s c o r r e s p o n d i n g t o t h e d i f f e r e n t V C R a c t i o ns a r ed e s i g n e d d u r a t i o n s a n d p r o b a b i l i t i e s o f t r a n s i t io n i n g t o n e i g h b o r i n gs t a te s . T h e i n i t i a l s t a te i s P l a y , a n d t h e i n t e r a c t i o n s y s t e m r a n d o m l yt rans i t s to o the r in te rac t ive s ta te s o r s tays in the P lay s ta te acco rd -i n g t o t h e b e h a v i o r d i s l r ib u f i o n . T h e u s e r r e s i d e s a t e a c h i n t e r a c t io ns t a t e f o r a n e x p o n e n t i a l l y . d i s t r i b u t e d p e r i o d o f t i m e .A s s h o w n i n F i g u r e 4 , t r a n s i t i o n p r o b a b i l i t i e s Pi( i = 0 , . . . , 9 ) a r ea s s i g n e d t o a s e t o f s t a te s c o r r e s p o n d i n g t o d i f f e r e n t V C R a c t io n s . I ti s i m p o r t a n t t o n o t i c e t h a t t h e a b o v e - m e n t i o n e d p a r a m e t e r s a r e c a p -t u r ed b y t h is r e p r e s e n t a t i o n o f v i e w e r s ' a c t iv i t y . F i n d i n g r e p r e s e n t a -t ive va lues fo r Pi(i ----0 , . . . , 9 ) i s s t i l l an open i s sue . Fo r t r ac tab i l i ty ,c u s t o m e r s a r e c l a s s i f i e d t o b e Very nteractive (V I) or Not Very Inter-active (NVI). Their in te rac t ions can b e s i rnu laW xl by tak ing d i f fe ren tpa ram e te r va lues [2 ].4 . 5 E v a l u a t i o n o f mult icast VoD systems

B y b a t c h i n g m u l t i p l e r e q u e s t s a n d s e r v i n g t h e m w i t h a s i n g l em u l t i c a s t , t h e s y s t e m c a p a b i l i t y f o r h a n d l i n g a l a r g e n u m b e r o f r e -q u e s t s c a n b e g r e a t l y i m p r o v e d a t t h e e x p e n s e o f i n c r e a s e d a d m i s -s i o n d e la y . F o r z e r o - d e l a y a d m i s s i o n a n d V C R i n t e r ac t i o n s , m o r ec h a n n e l s a r e r e q u i r e d . I t is i m p o r t ~ t t o e v a lu a t e m u l t i c a s t V o D i nte rms o f th roughpu t , r e sou rce requ i remen t , and e f f ic iency . Thesee v a l u a t io n r e s u lt s w i l l i n f l u e n c e p r i c in g , s y s t e m m a n a g e m e n t a s w e l l

A C M S I G C O M M 3 9 C o m p u t e r C o m m u n i c a t i o n R e v ie w

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a s r e s o u r c e s h a r i n g .4 . 5. 1 T h e s e r v ic e t h r o u g h p u t

F o r N V o D s e r v i c e , w e n e e d t o e v a l u a t e t h e t h r o u g h p u t o f m u l -t i c a s t V o D . C o m m o n a s s u m p t i o n s i n c l u d e t h e P o i s s o n a r r i v a l o fc l i e n t s ' re q u e s t s , a n d c u s t o m e r s ' w i l l i n g n e s s t o w a i t fo r a f i x e d a m o u n to f t i m e , s a y , 5 m i n u t e s , b e f o r e w i t h d r a w i n g t h e i r r e q u e s ts . T h e a u -t h o r s o f [ 1 ] m o d e l e d t h e c u s t o m e r s ' p a t i e n c e w i t h a n e x p o n e n t i a ld i s t r i b u t i o n , i . e . , a c u s t o m e r a g r e e s t o w a i t f o r x u n i t s o f t i m e o rm o r e w i t h p r o b a b il i t y p ( % ~ ) = e - | , w h e r e is t he a v e r a g e t i m ec u s t o m e r s a g r e e d t o w a i t . I n g e n e r a l , t h e p a t i e n c e r a te a = ~ c a nb e a s s u m e d i n d e p e n d e n t l y o f t h e v i d e o s r e q u e st e d . B a s e d o n t h isa s s u m p t i o n , t h e a u t h o r s o f [ 1 ] c o n v e r t e d t h e p r o b l e m o f c a l c u l a t in gt h e n u m b e r o f c u s t o m e r s w a i t i n g b e t w e e n t w o c o n s e c u t i v e s e r v ic e si n t o t h a t o f m a k i n g a t r a n s i e n t a n a l y s i s o f t h e M / M I ~ " s e l f - s e r v i c e "q u e u e i n g s y s t e m w i t h a r r i v al r a t e Z , a n d s e l f - s e r v i c e w i t h a n e g -a t i v e e x p o n e n t ia l d i s t r i b u t i o n w i t h r a t e f t .. T h e y t h e n d e r i v e d t h es e r v e r t h r o u g h p u t a n d t h e a v e r a g e l o s s r a t e f o r e a c h m o v i e . I n [ 7 9 ] ,t h e u s e r w a l t - t o l e r a n t b e h a v i o r i n s o m e b a t e h i n g s c h e m e s , s u c h a sF C F S ( f i r s t - c o m e - f i rs t - s e r v e ), M Q L ( t h e m a x i m u m q u e u e l e n g t h ) ,M a x - B a t c h a n d M i n - I d l e , h a v e a l s o b e e n i n v e s t i ga t e d , th e p r o b l e mo f m a x i m i z i n g t h e s y s t e m t h r o u g h p u t i s f o r m a l l y d i s c u s s e d a n d t h ef u n c t i o n a l e q u a t i o n d e f i n i n g t h e o p t i m a l s c h e m e i s d e r i v e d .F o r T V o D s e r v i c e i n m n l t i c a s t V o D , t h e r e is , t o o u r b e s t k n o w l -e d g e , n o l i t e r a t u r e o n e v a l u a t i n g t h e s e r v i c e t h r o u g h p u t . I t i s s t i ll a no p e n i s s u e .4 . 5 .2 B a n d w i d t h r e q u i r e m e n t s o f T V o D s e r v ic e

S o m e m u l t i c a s t V o D p r o t o c o l s c a n s u p p o r t T V o D services i n z e r o -d e l a y a d m i s s i o n i f e n o u g h c h a n n e l s a r e u s e d . H o w t o e v a l u a t e t h ec h a n n e l r e q u i r e m e n t s d e p e n d s o n t h e u n d e r l y i n g m u l t i c a s t s c h e d u l -i n g s c h e m e . F o r e x a m p l e , [ 1 4 ] p r e s e n t s t h e o p t i m a l p a t c h i n g p e r f o r -m a n c e , a n d [ 1 5 ] a d d r e s s e s th e b a n d w i d t h r e q u i r e m e n t o f t r a n s i ti o np a t c h i n g . [ 5 8 ] p r o p o s e s a m e t h o d f o r a n a l y z i n g t h e u s e r in t e r a c -t i v i t y a n d e v a l u a t i n g t h e n u m b e r o f c h a n n e l s r e q u i r e d f o r m u l t l c a s tT V o D s e r v i c e. T h e r e s u l t s d e t e r m i n e t h e r e la t i o n s h i p s a m o n g t h ec l i e n t s ' b e h a v i o r s , t h e s y s t e m r e s o u r c e s , a n d t h e T V o D s e r v i c e p r o -t o c o l. H o w e v e r , r i g o r o u s l y a n a l y z i n g t h e r e q u ir e m e n t s o f c h a n n e l sf o r T V o D p r o t o c o l s s u p p o r t i n g z e r o - d e l a y a d m i s s i o n a n d f u l l V C Ri n t e r a c t i o n s i s s t i l l a n o p e n i s s u e .4 . 5 . 3 B a n d w i d t h c o s t v s . Q o S

A 1 h o u g h m o s t s e r v e r - i n i t i a t e d m u l t i c a s t V o D s c h e m e s a r e n o t f o rp r o v i d i n g T V o D s e r v i c e , t h e y s t r iv e t o m a k e a b e t t e r t r a d e o f f b e -t w e e n t h e c h a n n e l c o s t a n d t h e s e r v i c e l a t e n cy . F o r e x a m p l e , m o s tp e r i o d i c b r o a d c a s t i n g s c h e m e s t r y to r e d u c e t h e s e r v i c e la t e n c y a n di m p r o v e th e t h r o u g h p u t a t l o w c h a n n e l c o s ts . T h e p e r f o r m a n c e o fr e l a t e d p e r i o d i c b r o a d c a s t i n g s c h e m e s h a v e b e e n d i s c u s s e d i n [ 4 , 3 5,4 3 , 8 4 ] .4 . 6 O t h e r issues

B e s i d e s t h e m e n t i o n e d i s s u e s , t h e r e a l s o a r e o t h e r i m p o r t a n t i s -s u e s i n m u l t i c a s t V o D s e r v i c e , s u c h a s :Copyright protection: A t y p i c a l s o l u t i o n t o v i d e o c o p y r i g h t p r o -

t e c t i o n is a n e n c r y p t e d p o i n t - t o - p o i n t d e l i v e r y to e n s u r e t h a to r d y p a y i n g c u s t o m e r s r e c e i v e t h e s e r v i c e , b u t i t i s i n e f f ic i e n tf o r m u l t i c a s t V o D . I n [ 4 1 ] a s i m p l e s c h e m e i s p r o p o s e d t op r o v i d e s i m i l a r p r o t e c t i o n fo r t h e c o n t e n t , w h i c h c a n b e u s e de f f i c i e n tl y w i t h m u l t i c a s t a n d c a c h i n g . I n t h a t s c h e m e , t h em a j o r p a r t o f t h e v i d e o i s i n t e n t i o n a l l y c o r r u p t e d a n d c a n b ed i s t r i b u t e d v i a m u l t i c a s t c o n n e c t i o n s , w h i l e t h e p a r t n e c e s s a r yf o r r e c o n s t r u c t i n g t h e o r i g i n a l i s d e l i v e r e d t o e a c h r e c e i v e r i n -d i v i d u a l ly . T h e a u t h o r s o f [ 2 0 ] p r o p o s e d a n e f f i ci e n t s e c u r e

m n l t i c a s t p r o t o c o l w i t h c o p y r i g h t p r o t e c t io n . T h e r e a r e a l soo t h e r p a p e r s t h a t a d d r e s s e d t h e p r o b l e m i n r e c e n t m n l t i m ~ d i as e c u r i t y c o n f e r e n c e s . H o w e v e r , t h e c o p y r i g h t p r o t e c t i o n f o rm u l t i e a s t V o D s e r v i c e n e e d s t o b e s t u d i e d f u rt h e r.Hdeo replacement: The m a i n t e n a n c e o f s t o r a g e i s a m a i n s e r v e rm a n a g e m e n t r u s k i n g e n e r a l V o D s y s t e m s . D u e t o th e l i m -

i t e d s t o r a g e c a p a c i t y , it i s n e e e s s a r y t o r e p l a c e o l d , u n p o p u l a rv i d e o s b y n e w p o p u l a r v i d e o s . H o w t o s e l e c t v i d e o s t o re p l a c ed e p e n d s m a i n l y o n t h e h i t r a t i o o f v i d e o s . P o p u l a r i t y - b a s e da s s ig n m n n t a n d B S R ( B a n d w i d t h - t o - S p a c e R a t i o ) - b a s e d a s s i gn -m e n t a r e c o n s i d e r e d a s t h e im p o r t a n t p o l i c i e s . M o r e o v e r , t h er e p l a c e m e n t o p e r a t i o n f o r o n e v i d e o s h o u l d n ' t a f f e c t t h e s er -v i c e o f o t h e r v i d e o s r e s i d i n g i n t h e s a m e s e r v e r . T h i s p r o b l e mi s r e l a t e d t o s t o r a g e o r g a n i z a t i o n , a n d [ 3 ] d i s c u s s e d t h e e f f ec to f v i d e o p a r t i t i o n i n g s t r am g i e s .

5. CONCLUSI ON AND FUTUR E DIRECTIONA s V o D s e r v i c e b e c o m e s p o p u l a r a n d w i d e l y - d e p l o y e d ., c o n s u m e r sw i l l li k e l y su f f e r f r o m n e t w o r k a n d s e r v e r o v e r l o a d s . N e t w o r k a n ds e x v e r- I /O b a n d w i d t h s h a v e b e e n r e c o g n i z e d a s a s e r i o u s b o t t l e -n e c k i n t o d a y ' s V o D s y s t e m s . M u l t i c a s t is a g o o d r e m e d y f o r t h isp r o b l e m ; i t a l l e v i a t e s s e r v e r b o tf l e u e ~ k s a n d r e d u c e s n e t w o r k t r a f-t i c , t h e r e b y i m p r o v i n g s y s t e m t h r o u g h p u t a n d s e r v e r r e s p o n s e t i m e .W e d i s c u s s e d t h e s t a t e - o f - a r t d e s i g n s a n d s o l u t i o n s t o t h e p r o b l e m sa s s o c i a t e d w i t h m u l t i c a s t V o D . W e a l s o i l l u s t r a t e d t h e b e n e f i t s o fm u l t i c a a t V o D , a n d f e a s i b l e a p p r o a c h e s t o i m p l e m e n t i n g it . A l -t h o u g h m u l t i c a s t V o D c a n m a k e s i g n i f i c a n t p e r f ~ T n a n c e i r n p ro v v -m e n t s , t h e r e a r e s t i l l s e v e r a l o p e n i s s u e s t h a t w a r r a n t f u r t h er r e -

s e a r c h , i n c l u d i n g :Effective s c h e d u l in g a n d r o u t i n g s c h e m e s , a n d V C R - t y p e i n -t e r a c t i o n s f o r m u l t i c a s t T V o D . A d h o c s c h e m e s m a y a c h i e v ea b e t t e r tr a d e - o f f b e t w e e n Q o S a n d s y s t e m c o s t s w h i l e p r e -s e r v i n g s c a l a b i l i t y a n d i n t e r a c ti v i t y .E f f i c i e n t a c t i v e C P E b u f f e r m a n a g e m e n t . R i s h i g h l y d e p e n -,dento n c u s t o m e r s ' b e h a v i o r , a n d u t i l i z i n g th e C P E b u f f e r w i l ls i g n i f i c a n t ly i m p r o v e Q o S .F a i r n e s s o f V o D s e r v i c e . T h e r e i s a t r a d c o f f b e t w e e n f a i r n e s sa n d t h r o u g h p n t f o r p r a c t i c a l s c h e d u l i n g s c h e m e s , p a r t i c u l a r l yf o r h y b r i d s c h e m e s . W e n e e d t o a c h i e v e t he o p t i m a l b a n d -w i d t h r e s o u r c e a l l o c a t i o n w i t h o u t l o s s o f f a i r n e s s.K n o w l e d g e o f r e a l i s t i c c u s t o m e r s ' b e h a v i o r i s e s s e n t i a l t o t h ed e s i g n o f m u l t i c a s t V o D p r o t o c o l s a n d r e s o u r c e a l l o c a t i o n .O t h e r i s s u e s r e l a t e d t o c u s t o m e r s ' b e h a v i o r a r e t h r o ug h p u t ,s c a l in g , a n d s c h e d u l i n g .A n e f f i c ie n t t h e o r e ti c a l f r a m e w o r k f o r e v a l u a t i n g t h e p e r f o r -m a n c e o f m u l f i c a s t V o D s e r v i c e , e s p e c i a l l y fo r m o d e l i n g a n da n a l y z i n g m u l t i c a s t T V o D s e r v i c e t o s u p p o r t b o t h z e r o - l a t e n c ya d m i s s i o n a n d f u l l V C R i n t e r a c ti v i t y .D e v e l o p i n g s t a n d a r d p r o t o c o l s f o r m u l t i c a s t V o D f o r p r a c t i -c a l a p p l i c a t i o n s . T h e e x i s t i n g p r o t o c o l s f o r m u l t i c a s t r o u t in g ,s c h e d u l i n g a n d V C R i n t e r a c t i o n s c a n b e v i e w e d a s a b a s i s t oa c h i e v e t h i s g o a l . T h e D A V I C p r o t o c o l a l s o p r o v i d e s a g e n -e r a l r e f e r e n c e f r a m e w o r k , b u t p r o t o c o l s f o r m n l t ti c a st T V o Ds t i ll n e e d t o b e d e v e l o p e d f u r t h e r .

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