Design of an interactive video-on-demand system

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<ul><li><p>130 IEEE TRANSACTIONS ON MULTIMEDIA, VOL. 5, NO. 1, MARCH 2003</p><p>Design of an Interactive Video-on-Demand SystemYiu-Wing Leung, Senior Member, IEEE,and Tony K. C. Chan</p><p>AbstractWe design an interactive video-on-demand (VOD)system using both the clientserver paradigm and the broad-cast delivery paradigm. Between the VOD warehouse and thecustomers, we adopt a clientserver paradigm to provide an inter-active service. Within the VOD warehouse, we adopt a broadcastdelivery paradigm to support many concurrent customers. Inparticular, we exploit the enormous bandwidth of optical fibersfor broadcast delivery, so that the system can provide many videoprogram and maintain a small access delay. In addition, we designand adopt an interleaved broadcast delivery scheme, so that everyvideo stream only requires a small buffer size for temporarystorage. A simple proxy is allocated to each ongoing customer, andit retrieves video from the optical channels and delivers the videoto the customer through an information network. The proposedVOD system is suitable for large scale applications with manycustomers, and it has several desirable features: 1) it can be scaledup to serve more concurrent customers and provide more videoprograms, 2) it provides interactive operations, 3) it only requirespoint-to-point communication between the VOD warehouse andthe customer and it does not involve any network control, 4) it hasa small access delay, and 5) it requires a small buffer size for eachvideo stream.</p><p>Index TermsBroadcast delivery paradigm, client-server pro-gram, video-on-demand.</p><p>I. INTRODUCTION</p><p>A N INTERACTIVE video-on-demand (VOD) system pro-vides an electronic video rental service to geographicallydistributed customers [1]. It retrieves video programs from itsstorage and delivers them to the customers through an informa-tion network. The customers can select and watch video pro-grams at their convenient time and place, and they can interactwith the programs via interactive operations such as pause, fast-forward, and rewind.</p><p>A VOD system has to serve multiple customers concurrently,and therefore it must have a large enough capacity to providemultiple video streams. Many designs have been proposed forthis purpose and they can be classified into two categories:clientserver designandbroadcasting design.</p><p>A. ClientServer Design</p><p>The clientserver design adopts theclientserver paradigm.The system is composed of one or more servers [2], [3]. It main-tains a dedicated video stream for each ongoing customer. Whenthe customer performs an interactive operation, the system re-trieves and delivers the corresponding video for him. This de-</p><p>Manuscript received February 29, 2000; revised May 30, 2001. The associateeditor coordinating the review of this manuscript and approving it for publica-tion was Dr. Thomas R. Gardos.</p><p>The authors are with the Department of Computer Science, HongKong Baptist University, Kowloon Tong, Hong Kong (e-mail: ywleung@comp.hkbu.edu.hk; tkcchan@hkbu.edu.hk).</p><p>Digital Object Identifier 10.1109/TMM.2003.808818</p><p>sign can provide an ideal interactive service to the customers,but it needs dedicated resources (such as I/O bandwidth) tomaintain a video stream for each ongoing customer. For largescale applications with many customers, this design requireslarge amount of resources.</p><p>A clientserver design can use abatching policy[4][8] toserve more concurrent customers. The main idea is that thesystem waits for a time interval (calledbatch window) to collecta batch of requests for a video program. Then the system createsone video stream for this program and multicasts it to a batch ofcustomers. In this manner, one video stream can serve multiplecustomers simultaneously. However, the customers have to waitbefore starting a VOD session (the waiting time is calledaccessdelay) and they cannot perform (or can only perform some con-strained) interactive operations. Several batching policies havebeen proposed in the literature and they are as follows.</p><p> Danet al.[5] proposed that when the system can establisha new video stream, it selects the batch with the largestnumber of waiting customers and creates a video stream toserve all the customers in this batch. This batching policycan minimize themeanaccess delay, but some customersmay experience long access delay.</p><p> Danet al. [6] proposed to choose a shorter batch windowfor the more popular video programs. They developed ananalytical model and determined the window size for eachvideo program.</p><p> Almeroth et al. [7] proposed a batching policy that sup-ports some constrained interactive operations by bufferinga certain portion of the video program or joining the cus-tomer to another existing video stream.</p><p> Liao and Li [8] proposed a batching policy calledsplit-and-merge. When a customer performs an interac-tive operation, the system splits him from his originalvideo stream and attempts to create a new video streamfor him. If this is not possible, the customers waits. Oncethe interaction is done, the system attempts to merge thiscustomer back to an existing video stream via buffering.If this is not possible, the customer waits.</p><p>B. Broadcasting Design</p><p>The broadcasting design adopts thebroadcast delivery par-adigm[9], [10] to serve many concurrent customers. There arethree broadcasting designs for VOD [11][13].</p><p>The first design is calledperiodic broadcasting[11]. Itbroadcasts multiple streams of the same video program atstaggered times periodically. To watch a video program, acustomer waits until a new video stream for this program isbroadcast and then he receives this stream. The system canserve many concurrent customers because many customerscan receive the same video stream from a broadcast channelsimultaneously. However, it has a long mean access delay or</p><p>1520-9210/03$17.00 2003 IEEE</p></li><li><p>LEUNG AND CHAN: DESIGN OF AN INTERACTIVE VOD SYSTEM 131</p><p>Fig. 1. VOD system architecture.</p><p>it requires many broadcast channels per video program. Forexample, if a 90-min video program is broadcast every 10 min,the mean access delay is 5 min and it requires nine broadcastchannels. To support interactive operations, a low-resolutionversion of each video program is prepared and stored, and it isdelivered to the customers upon requests using a clientserverparadigm.</p><p>The second design is calledstaggered VOD[12]. It is similarto periodic broadcasting, but it provides interactive operationsin a different manner. To perform an interactive operation, acustomer changes to receive another broadcast video stream ifit exists. To produce a good interactive effect, the staggeringinterval should be small but this would require a large numberof broadcast channels. For example, if the staggering intervalis 1 min, then a 90-min video program requires 90 broadcastchannels.</p><p>The third design is calledpyramid broadcasting[13]. It di-vides each video program into segments of increasing sizes,and broadcasts theth segment of all the video programs period-ically in the th broadcast channel. The bit rate of each broadcastchannel is significantly larger than the video playback rate, es-pecially when the number of video programs is large. For thisreason, every customer must have a fast receiver and a largestorage space.</p><p>C. A New Design</p><p>In this paper, we adopt both the clientserver paradigm andthe broadcast delivery paradigm to design an interactive VODsystem for large scale applications with many customers. Thissystem has several desirable features.</p><p> The system can be scaled up to serve more concurrentcustomers and provide more video programs.</p><p> The system provides interactive operations which are ap-proximations of the ideal ones. The customer can controlthe pause duration, the fast forward rate and the fast rewindrate.</p><p>(a)</p><p>(b)</p><p>Fig. 2. Multiple optical fibers can be used to provide a large number of videoprograms. (a) Every proxy is tapped to one of the optical fibers. (b) Every proxyis tapped to any one of the optical fibers via a directional coupler.</p><p> The system only involves point-to-point communicationbetween the VOD warehouse and a customer. This typeof communication can be supported by many existing net-work infrastructures. In contrast, some existing VOD de-signs assume that the network can support multicasting orbroadcasting.</p><p> The system does not involve any network control. Thisis important when the network is not owned and man-aged by the VOD service provider. In contrast, someexisting designs involve network control such as dynamicmulticasting.</p><p> The access delay is small (say, 30 s). In contrast, the accessdelay in the clientserver designs with batching policiesand the broadcasting designs is significantly longer (say,several minutes).</p></li><li><p>132 IEEE TRANSACTIONS ON MULTIMEDIA, VOL. 5, NO. 1, MARCH 2003</p><p> Each video stream only requires a small buffer size fortemporary storage.</p><p>II. SYSTEM DESIGN</p><p>A. Basic System Architecture</p><p>Fig. 1 shows the proposed VOD system. The customers areconnected to aVOD warehousethrough an information net-work. The information network can be a private network ownedby the VOD service provider, or it can be a public network. Acustomer makes requests via a low bit rate channel, and the VODwarehouse delivers the requested video to this customer via ahigh bit-rate channel. Both channels are point-to-point.</p><p>Within the VOD warehouse, thevideo archivesstore videoprograms. Each video program is organized intopages, whereevery page lasts for the same duration.</p><p>The video archives are connected to an optical fiber, whichprovides logical channels by wavelength division mul-tiplexing [14]. The pages of each video program are readfrom the storage and are broadcast cyclically over multipleoptical channels according to abroadcast delivery scheme.This scheme specifies the time and channel for broadcastingevery page. We will design two broadcast delivery schemes inSection III.</p><p>There are proxiestapped to the optical fiber, where a proxyis a simple logical unit for reception and transmission. Whena customer initiates a new VOD session, the system allocatesa free proxy to him. The proxy receives the requested videofrom the optical channels, and transmits it to the customer atthe video playback rate through an information network. Whenthe customer terminates the VOD session, the associated proxywill be released.</p><p>B. Scalability</p><p>As the VOD service becomes more popular, the system has toserve more concurrent customers and provide more video pro-grams. The proposed VOD system is scalable to cope with thesefuture expansions.</p><p>To serve more concurrent customers, we add more proxies.It is not necessary to modify the existing hardware, and it isonly necessary to modify the software setting to manage a largernumber of proxies (e.g., to record which proxies are free).</p><p>To provide more video programs, we add storage and opticalfibers if the existing ones are not sufficient. When there are mul-tiple fibers, each proxy can be tapped to one of them [Fig. 2(a)],or it can be tapped to any one of them via adirectional coupler[15] [Fig. 2(b)].</p><p>If the VOD service covers a wide area (e.g., an entire country),we can replicate the VOD warehouse in distributed sites andeach warehouse serves its nearest customers. This can avoidusing long-distance channels, and reduce the propagation delayto give a better response time for interactive operations.</p><p>III. B ROADCAST DELIVERY SCHEMES</p><p>In this section, we design two broadcast delivery schemesfor broadcasting each video program over the optical channels.</p><p>Fig. 3. Basic broadcast delivery scheme.</p><p>These schemes are calledbasic broadcast deliveryand inter-leaved broadcast delivery. We let be the bit rate of each op-tical channel and be the video playback rate.</p><p>A. Basic Broadcast Delivery</p><p>For clarity, we adopt the following case for explanation. Avideo program consists of pages and these pages arebroadcast over channels. Generalization to anyandis straightforward.</p><p>1) Delivery Schedule:Fig. 3 shows the basic broadcast de-livery scheme. Time is divided into cycles where all the pagesare broadcast once in a cycle. Each cycle is further divided into</p><p>slots where one page is broadcast in one slot in a channel.We let the duration of a cycle and a slot beand respectivelywhere . The first three pages are broadcast in channel 1one after the other. Similarly, the next three pages are broadcastin channel 2; and the last three pages are broadcast in channel 3(see Fig. 3).</p><p>Fig. 4(a) shows how the proxy retrieves the pages from the op-tical channels, and Fig. 4(b) shows how to deliver these pages tothe customer. First of all, the proxy tunes its receiver to channel1, and waits until the beginning of the coming cycle. Then it re-trieves page 1 from channel 1, and at the same time delivers thispage to the customer at the video playback rate(a slowerrate) through an information network. After retrieving page 1,the proxy does not retrieve in the next slots but it con-tinues to deliver the remaining portion of page 1 to the customer[see Fig. 4(b)]. In the second and third cycles, the proxy doessimilar steps to retrieve and deliver page 2 and page 3 respec-tively, and the details are shown in Fig. 4(a) and (b). After re-trieving page 3 from channel 1, the proxy does not retrieve inthe next slots (i.e., cycle 4) but it continues to deliver theremaining portion of page 3 to the customer. At the same time,it tunes its receiver to channel 2 so that it will be able to retrievepage 4, page 5, and page 6 in the fifth, sixth, and seventh cy-cles respectively. Then, the proxy tunes its receiver to channel3 to retrieve page 7, page 8, and page 9 in the ninth, tenth, andeleventh cycles respectively. In this manner, the proxy can de-liver video to the customer continuously [Fig. 4(b)].</p><p>2) Buffer Size, Tuning Time, and Slot Duration:The proxyretrieves video at the channel bit rate (say, 50 Mbps) for oneslot and then waits for one cycle. At the same time, it continu-ously delivers video at the video playback rate (say, 1.5 Mbps).</p></li><li><p>LEUNG AND CHAN: DESIGN OF AN INTERACTIVE VOD SYSTEM 133</p><p>(a)</p><p>(b)</p><p>Fig. 4. Basic broadcast delivery for VOD. (a) Proxy retrieves the shaded pages. (b) Proxy delivers the retrieved pages to the customer.</p><p>Therefore, it must have buffer for temporary storage. To deter-mine the buffer size required, we analyze the buffer occupancyin the proxy. Fig. 4(c) shows the buffer occupancy versus time,as follows.</p><p> In the first slot of the first cycle, the proxy retrievespage 1 and delivers it to the customer simultane-ously. Since the retrieval rate is faster than the de-</p><p>livery rate, the buffer occupancy is increasing withtime until it reaches a maximum at the end of thisslot. The maximum buffer occupancy is equal to</p><p>retrieval rate delivery rate duration of a slot.</p><p> In the subsequent slots, the proxy does not re-trieve any page but it still delivers the remaining portion</p></li><li><p>134 IEEE TRANSACTIONS ON MULTIMEDIA, VOL. 5, NO. 1, MARCH 2003</p><p>(c)</p><p>Fig. 4. (Continued.) Basic broadcast delivery for VOD. (c) Buffer occupancy in the proxy versus time. The buffer size required b...</p></li></ul>

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