Title of Invention

"STREAM MERGING FOR VIDEO ON DEMAND"

Abstract

A channel merging method for a video on demand system, said method comprises the steps of: in response to requests from a plurality of clients for a video program, establishing a root channel (SI) and at least one sub-channel (Sll), said root channel (SI) being established according to a request from a client that makes the earliest request, each of said sub-channels (Sll) being established in response to a request from a client that makes a later request; and monitoring variation of the number of the clients that are using each of said established channels, and maintaining the channel if the number of the clients using the monitored channel is not zero, and closing the channel if the number of the clients using the monitored channel becomes zero.

Full Text

CHANNEL MERGING FOR VIDEO ON DEMAND The present invention relates to a channel merging method, and more particularly to an optimized multicast delivery to a plurality of clients by using the merging of a plurality of channels delivering a video stream in a communication network. With the explosive growth of the Internet and the increasing power of computers, interest has grown in a class of application called video-on-demand (VOD), where clients can request media files (video, audio, data..., etc.) at any time for immediate or future watching. However, video-on-demand poses a new challenge, that is, a huge consumption of server bandwidth and network bandwidth. Traditionally, each request is served by a dedicated unicast stream, and the cost of the unicast based VOD system is enormous. The advent of channel merging technology creates a brand new model for VOD service, and its goal is to reduce the server bandwidth required to satisfy clients requesting a particular object video by having them simultaneously receive two or more streams. As clients receive and store the data for immediate watching purposes, the server can have one video object served to more than one user simultaneously by multicast and thus reduce both the network bandwidth and server bandwidth. Wanjium Liao et al: "The split and merge (SAM) protocol for interactive video-on-demand systems" Infocom X97. Sixteenth Annual Joint Conference of the IEEE Computer and Communications Societies. Driving the Information Revolution., Proceedings IEE Kobe, Japan 7-11 April 1997, Los Alamitos, CA, USA, IEE Comput.Soc., US, 7 April 1997 (1997-04-07), pages 1349-1356, XP010251958 ISBN: 0-8186-7780-5, teaches a video on demand system which supports multicasting operations and which uses a unicast Interaction stream to satisfy users' requests for interactions. Bommaiah, E.; Guo, K.; Hofmann, M.; Paul, S.: "Design and implementation of a caching system for streaming media over the Internet" Real-Time Technology and Applications Symposium, 2000. RTAS 2000. Proceedings., [Online] 31 May 2000 (2000-05-31), pages 111-121, XP002283106, teaches a video on demand system with a garbage collection function where a reference count is associated with a packet in a buffer and the reference count is equal to the number of consumers which are yet to consume the particular packet. Existent channel merging methods can be classified as three types: static broadcast, merge tree construction and event driven. The static broadcast, exampled by Skyscraper, broadcasts segments of a demanded object in several channels with a specified period and length. The advantage of the static broadcast is its simplicity and relatively high efficiency in very busy environment. However, the performance of the static broadcast is poor when the load of system is not high or the popularity of different objects is disperse due to its rigid resource allocation. The merge tree construction, exampled by Dyadic, dynamically constructs a merge tree when the new users arrive, with the nodes of tree representing channels. A channel is not allocated until it is really needed by a user. This method overcomes the drawbacks of the static broadcast by eliminating the waste of idle channel resources. However, as the merge tree is exclusively determined by the joining time of new users, it does not directly support VCR-like functions, i.e., random stop, pause, fast/back forward, etc. The Event-driven, exampled by SRMT (Simple Reachable Merge Target) and CT (Closest Target), dynamically determines a set of channels that the client should subscribe to when the client sends indications to the server of playing, stopping, jumping or merging events. VCR-like functions are supported by this method because the merge path for each client is dynamically adjusted according to user interactions. A method of merging of two channels will be described below in conjunction with Figs. 1, 2 and 3: At step 1, the VOD server I receives a request for playing a video program from a client A and, according to the request, sends the requested video program to the client A on the channel S6. At step 2, when receiving the same VOD request as that of client A after some time (T) from a client B, the VOD server 1 creates a channel Sll and informs the client B to get ready for receiving from the VOD server 1 the video program on the channel Sll and the channel S6. At step 3, the VOD server 1 sends the video program from its starting point (a) to the client B on the channel Sll, and the client B receives it, and meantime the client B receives on the channel S6 in synchronism with the client A and stores the subsequent part of the video program continuously sent from the VOD server 1. At step 4, the VOD server 1 takes the channel S6 as the parent channel of the channel Sll (i.e., the channel to which the channel Sll will merge). When the video program received by the client B on the channel Sll reaches the beginning point (b) of the video program that it receives on the channel S6 and stores, i.e., when another time of T is passed, the channel Sll is merged into the channel S6. The VOD server 1 will close the channel Sll and stops sending the video program to the client B on the channel Sll. At this time, in the client B is stored the video program from the point (b) to a point (c). After the channel Sll (sub-video stream) is merged into its parent channel S6, if no other client is using the sub-channel Sll (i.e., the sub-channel of the channel Sll), then the sub-channel Sll will be terminated. At step 5, after the channel Sll is merged into the channel S6, while the client B continues to receive, on the channel S6, and stores the subsequent part of the video program sent from the VOD server 1 from the point (c) , it reads from the point (b) and plays back the video program stored in its local memory in a FIFO manner, enabling the playback of the video program on the client B to be continued. Although the event-driven method for channel merging is the most flexible method for controlling multicast channels, existing methods of this type have an evident drawback. If a channel is removed when it has merged into its parent channel or is cancelled due to stopping or jumping events, those clients subscribing to the sub-channels of this removed channel have to change the channels they have subscribed to. For example, the CT scheme simply chooses the latest video stream channel in the earlier video streams still in the system as the next target to be merged, and the merge target computed by CT is not always reachable, even if no further new sub-channel is created. The reason is that the target stream channel may itself merge with its target channel before it can be reached by later channels. In this case, later stream channel must select a new merge target again by using the CT algorithm. ' Furthermore, the operations of the target stream channel such as random stopping, pausing, fast-forwarding, etc. will also make it impossible for the later stream channels to merge and will force them to reselect their new parent channels. In order to inform affected clients of the change of merge tree, the video server must actively send a notification to the each of these clients. This could bring about the following disadvantageous effects: 1. Reverse notifications from a video server to clients significantly may increase the load of the video server, since the number of notifications is proportional to the number of affected clients and the frequency of unexpected channel stopping events. 2. Clients must be ready to accept incoming connections from unknown regions of the Internet, which increases the possibility for clients to be affected unexpectedly. 3. The reverse notifications may not be able to pass through the firewall with certain configurations. For example, if a client within a firewall tries to watch a video clip stored in a video server outside the firewall, the server will never be able to initiate the transmission of a notification to the client. The present invention provides a channel merging method for a VOD System as claimed in claim 1, and a corresponding apparatus and computer program product as claimed in further claims. Preferably, all the "channel merging events" proceed in the direction from the lowest-level sub-channel to the root channel, therefore no case will- occur that the channel which a client is using is removed. Even if a stopping event occurs directly, a channel will not be removed until all clients that use it (in the form of a sub-channel of the channel) explicitly release it. Therefore reverse notifications are avoided, and one client's behaviour will not affect other clients, and the load of both the VOD server and the network are reduced. Meantime, VCR-like functions, e.g., playing, stopping, seeking (fast/back forward), etc. are inherently supported by this method because the merge tree is dynamically constructed and adjusted when client-initiated events occur, and the merge path for each client is dynamically adjusted based on the merge tree. Another main advantage of the invention is that the method for controlling the VOD server is compatible with the request/response mode of HTTP, thus can be easily implemented upon HTTP. HTTP is an important protocol for exchanging application data on the Internet, and the advent of Web services further consolidates the trend that Internet-based applications should rely on HTTP as much as possible for data transmission. In addition, most firewalls are reluctant to let general traffic other than HTTP pass through. The above-mentioned advantages and other features of the present invention will become more apparent from the following detailed description in conjunction with the drawings, in which: •^ Fig. 1 shows a multicast network having a VOD server and a plurality of clients. Fig. 2 is a schematic diagram of a video program stream that performs channel merging. Fig. 3 is a schematic diagram of a channel merge tree. Fig. 4 is a timing diagram of request/response occurring between the VOD server and the client of Fig. 1. Fig. 5 is a diagram showing the structure of a channel merging apparatus configured in the VOD server according to the present invention. Fig. 6 is flow chart of the process of the VOD server in the case of occurrence of a "starting event" according to the present invention. Fig. 7 is flow chart of the process of the VOD server in the case of occurrence of a "jumping event" according to the present invention. Fig. 8 is flow chart of the process of the VOD server in the case of occurrence of a "merging event" according to the present invention. Fig. 9 is flow chart of the process of the VOD server in the case of occurrence of a "stopping event" according to the present invention. Unless noted specially, all such operations as starting, stopping, jumping, merging and etc. mentioned in the following is performed with respect to channels on which the same video program (object) are played. Fig. 1 shows a multicast network having a server I and a plurality of clients 4. The multicast network comprises a VOD server 1, the Internet 2, a firewall 3 and a plurality of clients 4, wherein the VOD server 1 and the clients 4 communicate with each other through the Internet 2 and the firewall 3. In the present invention, the client 4 sends requests to the VOD server 1 through the firewall 3 and the Internet 2, for performing the operations of VCR-like functions such as playing, stopping, pausing, fast/back forwarding on a video clip. The request/response operation occurring between the VOD server 1 and the clients 4 of Fig. 1 will be described below in conjunction with Fig. 4. Fig. 4 is a timing diagram of the request/response occurring between the VOD server 1 and the clients 4 of Fig. 1. In Fig. 4, each message contains the message type of the action to be performed, which is specified by a list of parameters. Here are currently five message types defined: OPEN, PLAY, PAUSE, MERGE and CLOSE. For each message sent by the client, the sever will send back a RESPONSE message. At step (1), by sending an OPEN message to the VOD server 1, the client 4 creates a session with the VOD server 1, the OPEN message containing a video ID for uniquely identifying the requested video file (video program) on the VOD server 1. If the session is successfully created, then the VOD server 1 will send back to the client 4 a RESPONSE message containing channel information, such as multicast address and port number, and the client 4 can receive the requested video program according to the channel information. At step (2), the client 4 sends a PLAY message to request to start playing the video program or resume from paused state. An offset can be specified as a parameter in the PLAY message to search a specified position of the video program. The RESPONSE message may contain additional information for the client 4, such as the information that indicates the client 4 to receive the requested program, and etc. At step (3), when the client 4 detects the merging of channels, it sends a MERGE message to the VOD server 1 (of course, the VOD server 1 can also derive the occurrence of a channel merging event by using its components such as the channel control unit 20 to compute the channel). The VOD server 1 will close channels that are not used and send back a RESPONSE message indicating additional channels to which the client 4 should join. At step (4), the client 4 can send a PAUSE message to the VOD server 1 to temporarily halt data transmission while playing the video program, and the VOD server 1 makes a corresponding response. At step (5), the client 4 can send a CLOSE message to the VOD server 1 to explicitly close the session with the VOD server I, and the VOD server 1 makes a corresponding response. If we model all the above-mentioned requests and notification messages into various events, then there are four types of event pertinent to the VOD server 1, i.e., "starting event", "jumping event", "stopping event" and "merging event". All these events can be operated through the above-mentioned five types of messages, i.e., OPEN, PLAY, PAUSE, MERGE and CLOSE. When the client 4 sends a request for an object (video program), a starting event happens, i.e., the client 4 requests to playa video clip at time "t" (using a PLAY message). A jumping event happens when the client 4 sends a fast-forward or back-forward request for an object. That is, the client 4 sends a request for playing the video program from time "t+s" or "t-s", where "s" is an offset time of an object (i.e., other parts of the requested video program) to be jumped with respect to time "t". At this time, the VOD server 1 creates a new channel to play the video program from time t-s (using the PLAY message), and meantime closes the channel that plays the video program from time "t" (using the CLOSE message, as will be described in detail below). A stopping event happens when the client 4 no longer needs an object, i.e., the channel on which the video program is played is closed (using a CLOSE message). A merging event happens when the client 4 has reached a merge point where the last channel pair the client 4 was watching have successfully merged (using the MERGE message and the CLOSE message), such as the case of the channel merging method described in the portion of the background art of the present invention, i.e., the beginning point (b) in Fig. 2 is the merged point. In order to implement the operations of VCR-like functions such as playing, stopping, pausing, fast forward/backward, the present invention provides a channel merging apparatus 40. The specific structure of the channel merging apparatus 40 according to the present invention will be described now in conjunction with Fig. 5. Fig. 5 is a view showing the structure of the channel merging apparatus 40 according to the present invention. The channel merging apparatus 40 according to the invention is disposed inside the VOD server 1, comprises: a channel selecting unit 10 for receiving requests for a certain video program from a plurality of clients, and for creating a root channel (SI) and at least one sub-channel-(Sll) in response to the requests, said root channel (SI) being created according to a request of a client that makes the earliest request, each of said plurality of sub-channels (Sll) being created in response to the request of a client that makes a request later, the channel selecting unit 10 further seeking a parent channel satisfying with conditional expressions (1) and (2) to be described later for a sub-channel in the channel merging process; and a channel control unit 20 for performing such operations as the creating, merging and closing of channels according to the request of the client 4 and the selection result of the channel selecting unit 10. In addition, the above-mentioned channel merging apparatus 40 can be connected to the VOD server 1 operatively, instead of being disposed inside the VOD server 1. Meantime, the channel selecting unit 10 and the channel control unit 20 can be a single unit, such as a CPU in a computer, for executing an executable program stored in ROM or RAM or other storage medium (not shown) in the computer to effect the functions corresponding to the channel selecting unit 10 and the channel control unit 20. The channel control unit 20 further comprises a count unit 22, which marks the number of the clients 4 using each channel with a count parameter (ref_num) to effect the control function of the channel control unit 20. When a merging, jumping or stopping event happens to said each channel and its sub-channels, the count unit 22 decreases the value of the count parameter, and if the value of the count parameter is zero, the channel whose value of the count parameter is zero is closed at the side of the VOD server I. If the value of the count parameter is not zero, then the channel is maintained at the VOD server 1, and the client having performed the merging, jumping or stopping event no longer receives the program played on the channel. In the present invention, the channel selecting unit 10 in the VOD server 1 creates a root channel (SI) and at least one sub-channel (Sll) in response to the requests for a certain video program from a plurality of clients as shown in Fig. 3, said root channel (SI) being created according to the request of the client that makes the earliest request (such as client A), each of the a plurality of sub-channels (Sll) (and also S5 and S6, for example) being created in response to the request of clients that make requests later, and the root channel SI and the, plurality of sub-channels Sll (and S5 and S6, etc.) form a tree structure. Certainly, all of the above-mentioned requests are those satisfying the VOD condition. All the above-mentioned channels transfer multicast streams, from the VOD server 1 based on the request of the client 4, and the multicast streams of each channel can be received by all the clients 4. Each client 4 receives at most two channels at the same time, of which one is initiated for the client 4 itself and another one was initiated for a previous client for example the first client requesting the object only receives a video program from a channel si (as shown in Fig. 3), while the second client receives the video program from channels s2 and si simultaneously, in which si is selected as the parent channel of s2 (as shown in Fig. 3). All clients must be capable of storing the received video program on a local storage (not shown). For each created channel, the VOD server 1 monitors the variation of the number of the clients 4 using the channel through the channel control unit 20 of the present invention. If the number of the clients 4 using the monitored channel is not zero, the channel is maintained; if the number of the clients 4 using the monitored channel is zero, i.e., no client 4 is using the channel, then the VOD server 1 closes the channel through the channel control unit 20 of present invention. Here, the channel control unit 20 in the VOD server 1 detects the merging of two channels (video streams) either by its own calculation or by receiving messages from the client 4 where channel merging occurs. And the merging process continues until all sub-channels have merged into their root channel (the first channel created for the same video program). It is assumed that the length of the video program is infinite here. The channel merging method according to the present invention will be described below with reference to the drawings. Referring to Fig. 3 again, Fig. 3 is a schematic diagram of a channel merge tree. Each node represents a channel, where the first channel created for a certain video program is defined as a root channel. A higher level channel is defined as the parent channel of a lower level, and in contrast the lower level is defined as the sub-channel of the higher level channel. As shown in Fig. 3, SI is the root channel, S6 is the parent channel of Sll, while Sll is the sub-channel of S6, and channels S6, S10 and Sll are a collection of the posterity channels of S5. If the channel control unit 20 in the VOD server 1 creates a new channel, which we assume as channel Sll, under the request of the client 4 (starting or jumping of a channel), then the channel selecting unit 10 immediately seeks the parent channel S6 for the channel Sll. If the parent channel S6 is found, then the parent channel 6 is returned; otherwise, a message of "can not find the parent channel" is returned. The method executing the above-mentioned operations is as follows: Step 1: by using the following expression (1), find a nearest root channel such as SI from the collection of active root channels to which the channel Sll will possibly merge, that is: Min(Sll.start_time-Sl.start_time) wherein in which, min(Sll.start_time-Sl.start_time) indicates a minimal value among the difference values between the start time (Sll.start_time) of the channel Sll and the start time (SI.start_time) of each root channel in the collection of the root channels, and object-length/2 indicates a half of total length (total time) of the played video program, and wherein the start time (start_time) indicates the start time of this channel. Therefore, the above expression means that the minimal value among the difference values between the start time of the channel Sll and the start time of a certain root channel SI in the collection of the root channels is less than the 1/2 total length (total time) of the played video program. In this case, the root channel SI is considered to be reachable by its sub-channel Sll and can act as the root channel SI of the channel Sll; otherwise, or if the difference value between the start times is larger than the 1/2 total length (total time) of the played video program, this root channel is considered to be unreachable, and therefore a message of "can not find the root channel" is returned, with the channel Sll being taken as a new root channel. If an effective root channel SI is found by using the above-mentioned condition, the method proceeds to the following step 2. Step 2: if the collection of the posterity channels of the root channel SI is defined as S, the posterity channels described here include the sub-channel S5 whose direct parent channel is root channel SI, and sub-channel S6 of the channel S5. Here the channel Sll is defined as the sub-channel of the channel S6, and meantime it can be known that the channel Sll is also a posterity channel of the channel S5, the rest being deduced by analogy. By using the following expression (2), the channel S6 is found in the collection S (wherein the channel S6 is the parent channel of the channel Sll, and both channels S6 and Sll are within the posterity channel collection S and are posterity channels of the root channel SI: min (Sll .start_time-S6. start_time) The above expression means that a parent channel S6 to which the channel Sll will merge is found for the channel Sll, and this parent channel S6 should satisfy the following condition: the minimal value among the difference values between the start time of the channel Sll and the start time of the candidate parent channel S6 is less than the difference value between the start time of the parent channel S6 and the start time of the parent channel S5 of the parent channel S6. If there exists a channel S6 satisfying the above-mentioned condition, then the channel S6 is returned as the parent channel of the channel Sll in the next merging. Otherwise, or if no channel satisfying the above condition can be found, the root channel SI is returned as the parent channel of the channel Sll. Obviously, if only the above condition is satisfied, it is ensured that the parent channel S6 will not merge into the parent channel S5 of the parent channel S6 before the channel Sll merges into its parent channel S6. That is to say, at the time a channel S6 merges into its parent channel S5, each sub-channel of the channel S6, such as S10 and Sll, has merged into its parent channel S6. Therefore, from the time then knowing that the channel S6 merged into its parent channel S5 by calculation or by receiving the message from the client 4, the VOD server 1 closes the channel S6 so that the usage of other clients is not influenced. This is because if the selection of the channel S6 satisfies the above-mentioned condition, no other client is using the channel S6 at this time, i.e., channels S10 and Sll satisfying the above-mentioned condition have both merged into the channel S6. Thus, it is ensured that all the channel merging starts in a direction from the lowest sub-channel to their root level. That is, the channel will not be removed until all the clients 4 using a certain channel explicitly release the channel (in the form of the sub-channel of the channel). The operations of the VOD server 1 and its units therein in response to such 4 types of events as starting, jumping, merging and stopping will be described below in conjunction with Figs. 6,7 ,8 and 9. First, the operations of the VOD server 1 and its units therein in response to a starting event will be described. As shown in Fig. 6, when the client 4 initiates a starting event at time "t" (i.e., the client selects to play a video clip): At step 100, the channel selecting unit 10 creates a new channel Sll to play back the video program, and sets in the channel Sll start_time=t, object_offset=0 (an object offset time indicating the offset time when the channel starts, i.e., the offset of the start time of the channel with respect to the channel starting at time "t"), and ref_num=l (reference value indicating the number of the clients), where start time=t indicates the channel starts at time "t", object_offset=0 indicates the channel has no offset, and ref_num=l indicates only one client is using the channel. At step 102, the parent channel of the channel Sll is sought by the channel selecting unit 10. At step 103, it is determined whether or not there exists the parent channel. At step 104, if no parent channel is found, then the channel control unit 20 takes the channel Sll as a new root channel (i.e., sets root-flag=l (a root mark indicating whether the channel is a root channel)), and this channel Sll is the only channel that the client 4 should watch. Otherwise, at step 105, if the parent channel S6 is f.ound, then the client 4 must watch the channel Sll and its parent channel S6 simultaneously. At step 106, the above operation information is sent to the client 4 in response to the starting request initiated by the client 4. Next, the operations of the VOD server 1 and its units therein in response to a jumping event will be described. As shown in Fig. 7, when the client 4 initiates a jumping event at the object offset time "s" with respect to time "t" (i.e., the client 4 performs VCR-like functions such as fast forward/backward at time "t-s"): At step 200, the channel selecting unit 10 creates a new channel Sll to play back the video program starting from time "t-s", and sets in the channel Sll start_time=t-s, object_offset=s, and ref_num=l, where start_time=t-s indicates the channel starts from time "t-s", object_offset=s indicates the offset time of the start time of the channel with respect to the channel starting from time "t" is "s", and ref_num=l indicates only one client is using the channel. At step 202, if the channel starting at time "t" is S4, then the value of the count parameter of the channel S4 is decreased, i.e., because the channel S4 previously used by the client 4 has been caused to perform a stopping operation, the number of the clients using the channel S4 is currently decreased by 1. At this time, if the count parameter of the channel S4 is zero, indicating that no client is using the channel S4 currently, then the channel control unit 20 closes the channel S4; and in contrast, if at this time the count parameter of the channel S4 is not zero, indicating that there are still some clients 4 using the channel S4, then the channel S4 can not be closed, so as to be used continuously by other clients using the channel S4 (such as S8 and S9). But the client 4 having invoked the jumping event no longer uses the channel S4. Instead, it turns to the channel Sll to watch the video program. At step 203, the parent channel of the channel Sll is sought by the channel selecting unit 10. At step 204, it is determined whether there exists the parent channel. At step 205, if no parent channel is found, then the channel control unit' 20 takes the channel Sll as a new root channel (i.e., sets root_flag=l), and this channel Sll is the only channel that the client 4 should watch. Otherwise, at step 206, if the parent channel S6 is found, then the client 4 must watch the channel Sll and its parent channel S6 simultaneously. At step 208, the above operation information is sent to the client 4 in response to the starting request initiated by the client 4. Next, the operations of the VOD server 1 and its units therein in response to a merging event will be described. As shown in Fig. 8, when the client 4 initiates a merging event (i.e., the merging of the sub-channel Sll into the parent channel S6 occurs): At step 300,' the channel control unit 20 decreases the value of the count parameter of the sub-channel Sll. That is, since the channel Sll used by the client 4 has merged into its parent channel S6, the number of the clients using the channel Sll is currently decreased by 1. At this time, if the count parameter of the channel Sll is zero, indicating that no client is using the channel Sll currently, then the VOD server 1 closes the channel Sll; in contrast, if at this time the count parameter of the channel Sll is not zero, indicating that some clients are still using the channel Sll, then the channel Sll can not be closed, so that other clients can use the channel Sll continuously. However, the client 4 initiating the merging event no longer uses the channel Sll, and jumps to the channel S6 to watch the video program. The value of the count parameter of the channel Sll is the number of clients using the channel Sll. For example., if the count parameter is 1 (i.e., ref_num=l), it is indicated that one client 4 is using the channel Sll. If the count parameter is 5 (i.e., ref_num=5), it is indicated that five clients 4 are using the channel, all of these channels being the posterity channels of the channel Sll. At step 302, the parent channel of the channel S6 is sought by the channel selecting unit 10. At step 303, it is determined whether or not there exists the parent channel. At step 304, if no parent channel of the channel S6 is found, then the channel S6 is taken as a new root channel (i.e., sets root_flag=l), and this channel S6 is the only channel that the client 4 should watch. Otherwise, at step 305, if the parent channel S5 of the channel S6 is found, the client 4 must watch the channel S6 and its newly determined parent channel S5 simultaneously. At step 306, the above operation information is sent to the client 4 to respond to the merging request initiated by the client 4. Next, the operations of the VOD server 1 and its units therein in response to a stopping event will be described. As shown in Fig. 9, when the client 4 initiates a stopping event (i.e., the client 4 performs a stopping operation of VCR-like functions), or an object (video program) reaches an ending point (i.e., termination): Assuming that a client is using the channel Sll to watch a video program, then at step 400, it is determined whether or not the video program reaches an ending point, i.e. whether the video program has terminated. If the object (video program) reaches an ending point, then the channel control unit 20, at step 402, closes the channel Sll being used by the client 4, and directly releases all the resources of the channel. If the video program does not reach the ending point, then at step 404, the channel control unit 20, as described above, decreases the value of the reference value of the channel Sll. If the reference value of the channel Sll is zero, then the channel Sll is closed and all the resources of the channel are released; and in contrast, if the reference value of the channel Sll is not zero, then the channel control unit 20 does not close the channel Sll, so that other clients can use the channel Sll continuously, and the client having invoked the "stopping event" no longer uses the channel Sll. CLAIMS 1. A channel merging method for a video on demand system in which a server sends a video program to a plurality of clients, wherein said method comprising the steps of: (1) in response to requests from a plurality of clients for a video program, establishing a root channel (SI) and a sub-channel (Sll), said root channel (SI) being established according to a request from a client that makes an earliest request, said sub-channel (Sll) being established corresponding to a request of a client that makes a later request, wherein on each of the root channel and the sub-channel the server transmits a multicast stream of the video program, wherein the client that makes the later request receives two-multicast streams of the video program, one stream on the root channel and the other stream on the sub-channel, and wherein the client that makes the earlier request receives the multicast stream of the video program on the root channel; and (2) monitoring variation of the number of the clients that are using each of said established channels, and maintaining the channel if the number of the clients using the monitored channel is not zero, and closing the channel if the number of the clients using the monitored channel becomes zero. 2. The channel merging method according to claim 1, wherein said root channel (SI) and said sub-channel (Sll) are established in response to a play starting request or program jumping request from a client. 3. The channel merging method according to claim 1, wherein said root channel and said sub-channel form a tree structure. 4. The channel merging method according to claim I, wherein said step (2) includes the sub-steps of: (2-1) indicating the number of clients that are using each channel as a count parameter; (2-2) decreasing the value of said count parameter in response to occurrence of an event of merging, jumping or stopping of said 'each channel and sub-channels thereof; (2-3) closing said channel on the server side if the value of said count parameter becomes zero. 5. The channel merging method according to claim 4, wherein said channel is maintained on the server side if the value of said count parameter is not zero, and said clients having sent the event of merging, jumping or stopping no longer receive the programs being played on said channel. 6. The channel merging method according to claim 2, wherein if said client's request at time t is a starting request, a start time parameter is set to be t and an object offset parameter is set to be 0 in the sub-channel (Sll). 7. The channel merging method according to claim 2, wherein if said client's request at time t is a jumping request, and the object offset time of said jumping is s, a start time parameter is set to be t and an object offset parameter is set to be s in the sub-channel (Sll), and a stopping operation is performed on the channel on which said video program is played at time t and has been received by the client. 8. A channel merging apparatus which carries out the method of any preceding claim. 9. A computer program product stored on a computer readable storage medium, for, when run on a computer system, instructing the computer system to carry out the method of any of claims 1-7. 10. The channel merging method according to claim 2, wherein if said client's request at time t is a starting request, a start time parameter is set to be t and an object offset parameter is set to be 0 in the sub-channel (Sll). 11. The channel merging method according to claim 2, wherein if said client's request at time t is a jumping request, and the object offset time of said jumping is s, a start time parameter is set to be t and an object offset parameter is set to be s in the sub-channel (Sll), and a stopping operation is performed on the channel on which said video program is played at time t and has been received by the client. 12. The channel merging method according to claim 5, wherein if said stopping operation is due to the ending of said video program, said sub-channel is directly closed and all the resources of said sub-channel are released. 13. The channel merging method according to claim 6, wherein when secondary sub-channels of said sub-channel (Sll) are established, steps (1-1) and 15 (1-2) are repeated to take said sub-channel (Sll) as the parent channel of the secondary sub-channels, and the value of the count parameter of said sub-channel (Sll) is increased by I. 14 . A channel merging apparatus which carries out the method of any preceding claim. 15. A computer program product stored on a computer readable storage medium, for, ^hen run on a computer system, instructing the computer system carry out the method of any of claims 1-13.

Documents:

2431-DELNP-2005-Abstract-(25-03-2008).pdf

2431-delnp-2005-abstract-03-04-2008.pdf

2431-delnp-2005-abstract.pdf

2431-DELNP-2005-Claims-(25-03-2008).pdf

2431-delnp-2005-claims-03-04-2008.pdf

2431-delnp-2005-claims.pdf

2431-DELNP-2005-Correspondence-Others-(05-03-2008).pdf

2431-DELNP-2005-Correspondence-Others-(25-03-2008).pdf

2431-delnp-2005-correspondence-others-03-04-2008.pdf

2431-delnp-2005-correspondence-others.pdf

2431-delnp-2005-correspondence-others03-04-2008.pdf

2431-delnp-2005-description (complete).pdf

2431-DELNP-2005-Drawings-(25-03-2008).pdf

2431-delnp-2005-drawings.pdf

2431-delnp-2005-form-1-03-04-2008.pdf

2431-delnp-2005-form-1.pdf

2431-delnp-2005-form-13-03-04-2008.pdf

2431-delnp-2005-form-18.pdf

2431-DELNP-2005-Form-2-(25-03-2008).pdf

2431-delnp-2005-form-2-03-04-2008.pdf

2431-delnp-2005-form-2.pdf

2431-DELNP-2005-Form-3-(25-03-2008).pdf

2431-delnp-2005-form-3.pdf

2431-delnp-2005-form-5.pdf

2431-DELNP-2005-GPA-(25-03-2008).pdf

2431-delnp-2005-gpa-03-04-2008.pdf

2431-delnp-2005-pct-304-03-04-2008.pdf

2431-delnp-2005-pct-304.pdf

2431-delnp-2005-pct-401.pdf

2431-delnp-2005-pct-408.pdf

2431-delnp-2005-pct-409-(25-03-2008).pdf

2431-delnp-2005-pct-request form.pdf

2431-delnp-2005-pct-search report.pdf

2431-DELNP-2005-Petition-137-(25-03-2008).pdf