Abstract:
The client application provides VCR functionality in a data-centered video multicast network ( 36 ) through active prefetching of the fragments from broadcast channels. The player ( 10 ) has a user interface ( 12 ) that supports a plurality of VCR functions ( 14–26 ). Plural buffers ( 32 ) and their associated loaders ( 34 ) download video data under control of a manager module ( 38 ). The manager module accesses the playback pointer ( 40 ) and employs feasible point calculation module ( 42 ) to cause the loaders to selectively download segments so that VCR functions can be supported while maintaining continuous playback after the VCR function is performed.

Description:
This is a continuation of patent application PCT International Ser. No. 09/25422, filed Oct. 28, 1999, which claims benefit of Provisional application Ser. No. 60/106,167, filed Oct. 29, 1998. 

   BACKGROUND AND SUMMARY OF THE INVENTION 
   The present invention relates generally to video on-demand multicasting. More particularly, the invention relates to a technique for providing VCR functionality (e.g., play, pause, fast forward, rewind, jump) in a data-centered video multicast or broadcast network. 
   Video on-demand service promises to create an entire new dimension in home entertainment possibilities. Whereas current pay-per-view broadcasts are confined to specific broadcast times, video on-demand relaxes adherence to strict schedules. Customers are able to request a specific video, selected from a database of choices, and have the video delivered essentially instantaneously or, in the worse case, after only a brief delay. 
   It is recognized that most requests for videos are from a small group of the currently most popular movies. Using multicast to send these movies is an efficient way to satisfy customer demand. Currently there are two basic approaches to providing multicast video on-demand. In the user-centered approach, the server allocates channels to one user or to a group of users and then sends the movie over the allocated channel or channels. In early unicast systems (a special case of multicast, with one user per group) a single channel was allocated for each user. More recent multicast systems allocate one channel to support several users. 
   In the data-centered approach, the server allocates channels to a movie or to a part of a movie. Through periodic broadcasts, a given channel broadcasts a movie repeatedly in cycles. Conventionally there are several different schemes for broadcasting in the data-centered paradigm. Pyramid broadcasting divides a movie into segments of exponentially increasing size and lets each channel broadcast each segment repeatedly. The segment is broadcast in the channel at a faster speed than playback speed. Skyscraper broadcasting modifies the pyramid approach, using a different distribution of segment sizes and broadcasting at the same speed as playback speed. Skyscraper broadcasting places an upper bound on the maximum weight of the segment size. This is done to reduce storage requirements at the client side (user). The number of multicast channels required does not depend on the number of requests from users. Thus the data-centered approach appears to be more scalable than the user-centered approach. 
   The multicast approach to supplying video on-demand from several users at one time, to some extent, sacrifices special requirements of each individual user. The multicast systems usually require the client to wait for a certain period before it can be served. This is referred to as the startup latency for the services. VCR functions such as pause, fast forward, fast rewind and jump to new location are difficult to provide in data-centered multicast systems. Although there have been attempts to provide VCR functionality, the solutions offered to date have not been able to provide scalability while at the same time guaranteeing seamless delivery. Typically, as the size of the system is scaled up, it becomes increasingly more difficult to provide seamless operation. Thus users experience jumpiness or breaks in delivery that detract from the enjoyment of the system. The present invention proposes a new scheme which can provide VCR functionality in data-centered multicast video on-demand systems. The scheme is scalable and is far more able to guarantee smooth, discontinuity-free delivery. The system employs separate buffers at each client, from which media content may be played back to support VCR functionality. A loader associated with each buffer downloads segments of the video stream from the video data server under control of a pre-fetch manager that intelligently determines what to pre-fetch into the buffer. The pre-fetch manager does this by monitoring the playback position pointer and controls the pre-fetching operation to maintain the playback position pointer within a predetermined range within the buffer. 
   The system also employs a calculation of feasible points based on a set of predefined rules. These feasible points are used in determining which operations are performed in a seamless fashion and also to identify when seamless operation may not be possible. In the latter case, appropriate user-friendly performance is initiated so that the user does not experience undue jerkiness or long delays or gaps in media delivery. 
   For a more complete understanding of the invention, its objects and advantages, refer to the following specification and to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is block diagram of the presently preferred embodiment for providing VCR functionality; 
       FIG. 2  illustrates a first broadcasting scenario, demonstrating how discontinuity occurs in the case of an unrestricted jump; 
       FIG. 3  illustrates a similar broadcasting scenario, showing how discontinuity can occur in the case of a fastforward action; 
       FIG. 4  illustrates Case 1 of the presently preferred set of rules for assessing feasible points; 
       FIG. 5  illustrates Case 2 of the presently preferred rules for determining feasible points; 
       FIG. 6  illustrates Case 3 of the presently preferred rules for assessing feasible points; 
       FIG. 7  is an example showing how the feasible point analysis is performed in accordance with the presently preferred embodiment. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A presently preferred embodiment of the system is illustrated in  FIG. 1 . The system includes a player module  10  that provides an associated user interface  12  by which the user can select among a set of VCR functions. Illustrated in  FIG. 1  are the following functions: jump backward  14 , fast rewind  16 , pause  18 , play  20 , slow motion play  22 , fast forward  24  and jump forward  26 . These VCR functions are exemplary of the type of functions that may be provided in a consumer product. Of course, other functions may also be provided, if desired. 
   For instance, a jog shuttle mechanism may be provided to allow the user to move forward or backward through successive frames of the video content. Also note that a stop function may also be included. However, in the presently preferred implementation, the pause function  18  provides nearly the same functionality as a stop function would. This is so because, unlike in a mechanical VCR, there is no requirement to unload the tape from the tape head or to halt the tape drive motors and capstan. Nevertheless, if desired, both pause and stop functions may be included, in which case the pause function might provide a freeze-frame function while the stop function would blank the screen, for example. 
   Player  10  accepts VCR commands from the user interface and plays back the video stream through a suitable monitor  30 . Player  10  obtains the video stream from one or more buffers  32 . In the presently preferred embodiment there is a loader module  34  associated with each buffer. The loader module is responsible for interfacing with the distribution network  36  to download the required video content, as will be more fully described below. 
   The loader modules are controlled by a manager module  38  that is responsible for implementing the selective downloading strategy of the system. As will be more fully described below, the manager is able to access the playback pointer  40  of player  10 . This pointer shows the current playback location within the buffer  32  that is currently supplying the video stream to the player for display on the monitor  30 . The manager module  38  includes a feasible point calculation module or calculation processor  42  that is responsible for assessing which user-requested functions may result in discontinuous service. The feasible point calculation module thus supplies manager  38  with the information needed to interpret the user&#39;s request in a modified fashion if the original request would result in undesirable playback (e.g., jumpy or discontinuous playback or long delay). 
   When the client wishes to download video segments from a broadcast channel on network  36 , the manager  38  of the preferred embodiment causes loaders  34  to load information into the buffers in two phases. The first phase is a pyramid phase (unequal segments) during which all loaders are used to download information to guarantee smooth playback in the normal forward direction. Once the buffers  32  have been loaded to support the initial forward play mode, manager  38  gradually phases the loaders  34  into the equal segment phase. In the equal segment phase, some of the segments may be fetched to support playback of backward segments (i.e., content that precedes the current playback pointer). 
   The manager selects the channels and segments to be loaded based on the current playback segment, as designated by the playback pointer  40 . The manager  38  assigns priority to each segment based on its proximity to the current segment and also based on whether loading is in the initial pyramid phase or in the later equal segment phase. 
   To illustrate, assume the current segment is k. If k is one of the pyramid phase segments, the first three segments in the forward direction (k, k+1, k+2) are assigned the highest priority. If k is one of the equal phase segments, the highest priority segments are the current segment k and those that immediately precede and follow it (k, k+1, k−1). The priorities of the remaining segments are determined by their distance from the current segment. The nearer to the current segment the higher its priority. In case of ties, the tie is broken in favor of the segment in the forward direction. If there are more than three loaders and buffers, allocation is based on the priority of the segment. 
   We will further consider here the special case where there are only three loaders and buffers. In this case manager  38  allocates or reallocates the loaders into situations: (a) startup and (b) after performing a VCR action. 
   In the startup situation the playback pointer lies at the boundary of the first segment to be played. Note that the boundaries of all segments are also boundaries of the first segment at startup. At this boundary both de-allocation and allocation can occur. If a loader finishes loading a segment, it is returned to the free loader pool (de-allocation). This de-allocation process is voluntary by the loader. Allocation can also occur after de-allocation. 
   During the pyramid phase, the three loaders are allocated to the next unassigned segments in line. During the equal segment phase, if the play point is in the earlier half of the current segment (the current segment being designated segment k), the three loaders are assigned to segments k−1, k, k+1. If the play point is in the latter half of the current segment, the three loaders are assigned to segments k, k+1, k+2. If the contents of the segment to be played is already in the buffer no loading occurs for that segment. Later downloading segments may overwrite earlier segments already in the buffer. However, if the playpoint is in to-be-overwritten segment in the buffer, the manager  38  prevents it from being overwritten beyond the playpoint. We call this playpoint protection. 
   In the second situation (after every VCR action is performed) a somewhat different sequence proceeds. During the pyramid phase, the manager checks whether the loaders are assigned to k, k+1, k+2. If this is the case, no actions are taken. Otherwise, the manager will enforce preemptively the assignment. During the equal segment phase, if the playpoint is inside the buffer, no actions are taken. The relative position of the playpoint is adjusted when the downloading point (channel broadcast point) comes to the boundaries of the equal segments. If the playpoint goes outside the buffer (the current segment assumed to be k), the manager reallocates the loaders to segments k, k+1 and k−1. This may cause an involuntary de-allocation of loaders. 
   The manager  38  of the preferred embodiment controls the loading of buffers  32 , based on the playback pointer  40 , striving to maintain the playback pointer within a pre-determined range within the buffer. In the presently preferred embodiment, the manager causes information to be fetched and stored in the buffers so that the playback pointer remains at or near the center of the buffer after the initial pyramid phase loading is completed. Doing this allows the system to provide continuous VCR functionality with a very high probability that the user&#39;s request can be smoothly carried out. In contrast with other schemes, the present system empowers the client to actively pre-fetch nearby frames around the playback point and to locally manufacture the contents required to implement most VCR actions by appropriately accessing the local buffers  32 . 
   To account for situations in which a user&#39;s VCR command may not be feasible, manager  38  includes a feasible point calculation module  42 . The preferred implementation employs a set of rules to decide whether a given destination point is feasible or not, and how to respond accordingly. To provide high quality VCR functionality and seamless playback, the system strives to guarantee that the user can jump to a desired destination point p and then resume normal playback to the end of the movie without discontinuity. Given the particular channel broadcasting situation and the buffer conditions at the client, not all points are feasible points. For example, as illustrated in  FIG. 2 , an unrestricted jump can lead to discontinuities of later playback even if there is no subsequent VCR action. As  FIG. 2  illustrates, assume that a user issues a VCR action to jump to position c of segment k outside of the client&#39;s buffer and that the channel k happens to broadcast at point c. If the jump is allowed, after an elapse of time t, the buffer will only have c k  e k  and c k+1  d, however the client needs to playback from e k  (or b k+1 ) to c k+1 , which is not available and results in discontinuous playback. 
   Even if the video frame contents are in the buffer, some VCR actions can lead to discontinuity after normal play is resumed.  FIG. 3  illustrates this. In  FIG. 3  a fastforward action is initiated. Assume that the channel broadcasting point is c i  for channel i. The contents of channel k are in the buffer and contents of channel k+1 are not in the buffer. The current playback point is bk. If a fastforward command is issued with length of e k  e k , all the contents are in the buffer. Assume that the fastforward of this length takes time t. If the system performs this action to the full length, it will take time t to the playback position of e k . At this point, only b k  e k  and c k+1  d in the buffer, but b k+1  c k+1  is needed to continue normal play and is not available. 
   To address the issues illustrated in  FIGS. 2 and 3 , the feasible point calculation module determines whether a given point is feasible or not and then determines how to find the nearest point that is feasible. The presently preferred embodiment distinguishes three following cases according to the relative size of the segments involved. 
   Assume that the size of segment i is s i . The beginning position and end position of segment i and b i  and e i , respectively. We assume that b i =0 for all 1≦i≦K. At any time each channel is broadcasting a specific frame in the segment. This point is denoted as c i  and we have b i ≦c i ≦e i . 
   If the destination point p is located at or before the channel point c i , we call the segment containing the desired destination current segment and we assume it is segment k and broadcast in channel k. First we give the rules for this case. After that, we consider the cased in which the destination point p is located after the channel point c i .
         Case 1 ( FIG. 4 ): S k =S k+1 . This is the case in which the size of the current segment is equal to the size of the next segment. It is abbreviated as (x, x) case.
           If pc k  is in the buffer, then p is feasible; otherwise, the later nearest feasible point is the point q such that qc k  are in the buffer with the smallest q value.   
           Case 2 ( FIG. 5 ): S k+1 =2*S k  and S k+2 =S k+1 . This is the case in which the size of the current segment is half of the size of the next segment and the next two segments are of equal size. It is abbreviated as (x, 2x, 2x) case.
           case 2.1 c k =C k+ . This is case in which the broadcasting position of channel k is the same as channel k+1, or they are left aligned.   
           If pc k  and b k+1 c k+1  are in the buffer, then p is feasible: otherwise, if b k+1 c k+1  is in the buffer, the later nearest feasible point is the point q such that qc k  are in the buffer with the smallest q value; if b k+1 c k+1  is not in the buffer, the later nearest feasible point is the point q such that qc k+1  are in the buffer with the smallest q value;   case 2.2 c k  ≠ C k+1 . This is case in which the broadcasting position of channel k is not the same as channel k+1, or they are left aligned.   If pc k  is in the buffer, then p is feasible; otherwise, the later nearest feasible point is the point q such that qc k  are in the buffer with the smallest q value.       

   Case 3 ( FIG. 6 ): S k+1 =2* S k+1  and S k+2 =2* S k+1 . This is the case in which the size of the current segment is half of the next segment and the size of next segment is in turn half of the size of its next segment. It is abbreviated as (x, 2x, 4x) case.
         case 3.1 c k =C k+1 =c k+2 . This is case in which the broadcasting position of channels k, k+1 and k+2 are the same, or they are left aligned.
           If pc k , b k+1 c k+1  and b k+2 c k+2  are in the buffer, then p is feasible; otherwise, if b k+1 c k+1  and b k+2 c k+2  are in the buffer, the later nearest feasible point is the point q such that qc k  are in the buffer with the smallest q value; if b k+2 c k+2  is in the buffer and b k+1 c k+1  is not all in the buffer, the later nearest feasible point is in the point q such that qc k+1  is in the buffer with the smallest q value: if b k+1 c k+1  is not in the buffer, the later nearest feasible point is the point q such that qc k+2  is in the buffer with the smallest q value:   
           case 3.2 c k =C k+1  ≠ c k+2 . This is case in which the broadcasting position of channels k, k+1 are the same, but not the same as that of channel k+2.   If pc k , b k+1 c k+1  are in the buffer, then p is feasible; otherwise, if b k+1 c k+1  is in the buffer, the later nearest feasible point is the point q such that qc k  is in the buffer with the smallest q value; if b k+1 c k+1  is not all in the buffer, the later nearest feasible point is the point q such that qc k+1  is in the buffer with the smallest q value: if b k+1 c k+1  is not in the buffer, the later nearest feasible point is the point q such that qc k+1  is in the buffer with the smallest q value.   case 3.3 c k ≈C k+1 ≈c k+2 . This is case in which the broadcasting position of channels k+1, k+2 are the same, but not the same as that of channel k.   If pc k , b k+2 c k+2  are in the buffer, then p is feasible; otherwise, if b k+2 c k+2  is in the buffer, the later nearest feasible point is the point q such that qc k  is in the buffer with the smallest q value; if b k+2 c k+2  is not all in the buffer, the later nearest feasible point is the point q such that qc k+2  is in the buffer with the smallest q value.   case 3.4 c k  ≈C k+1  ≈c k+2 . This is case in which the broadcasting position of channels k, k+1 and k+2 are all different, or they are all right-aligned.       

   If pc k  is in the buffer, then p is feasible; otherwise, the later nearest feasible point is the point q such that qc k  are in the buffer with the smallest q value. 
   If the destination point p is located after the channel point c i , the next segment to the one that contains the desired destination is called the current segment. We assume it is segment k. In this situation, pc k  means pe k−1  and b k c k . After this modification, we can use the above rules. 
     FIG. 7  shows an example how to determine the feasible point in this case. Assume the destination point is p, which is later than the broadcasting point c 7 . So the current segment should be k=8. Since the segment size of current segment is half next segment and next two segments are of equal size, this is the case 2.1 of the above rule. As stated above, we use pc k  to represent pe 7  and b 8 c 8 . According to the rule 2.1, p is a feasible point in case (a). In case (b), p is not a feasible point and the nearest feasible point is d because b k+1 c k+1  is in the buffer and d is the smallest value such that dc k  is in the buffer. 
   The feasible point calculations, described above, allow the user to specify an arbitrary destination within the video-on-demand data stream. The system automatically adjusts the destination to a nearby feasible point, if the point requested by the user is not feasible. Thus, when the destination is not in the buffer, only those destination frames that are being broadcast at each channel are available immediately. The feasible point calculation module assesses what is possible and selects the nearest frame among those available as the adjusted destination if the requested destination cannot be met. If the feasible point calculation module determines that the requested frame is feasible, we call this an allowed destination. If the feasible point calculation module determined that the requested frame cannot be met without discontinuity, it substitutes a different frame that can be met. We call this the adjusted destination. 
   One desirable property of the destination, either allowed or adjusted, is that once the client resumes normal play from this destination, the client is able to playback to the end of the video without experiencing interruptions, provided there are no further VCR actions. 
   In developing an actual embodiment of the invention, the system designer may wish to specify the number of buffers used at each client. To implement a robust VCR-oriented scheme, two conditions should be preferably met:
         (1) If the destination segment is not in the buffer, and we still require that we can always select the frame being broadcast at this channel as the destination, the segments sized must be equal to the size of the next segment in order to guarantee that the client can playback the next segment when it finishes playing the current segment.   (2) The sizes of all segments beginning from the latter of these two equal segments must satisfy the continuity condition that guarantees smooth playback of the remainder of the video will be possible, assuming the client can download from m channels simultaneously.       

   Equation 1 below presents the function f(n) that will satisfy the above two conditions. The equation assumes that a client can download from m channels simultaneously. 
               f   ⁡     (   n   )       =     2     n   -     [     n   m     ]                 Equation   ⁢           ⁢   1             
 
   As an example, we give below the series generated by f(n) with m=5: 
   1, 2, 4, 8, 16, 16, 32, 64, 128, 256, 256, 512, 1024, 2048, 4096, . . . 
   In designing a system, the value of m (simultaneously available channels) places a minimum requirement on the bandwidth needed at each client. Considering the heterogeneity of the client, we like m to be small. On the other hand, in order to pre-fetch nearby segments of the current playback segment, clients download both the next segment for forward VCR action and the previous segment for backward VCR action. These segments are fetched in addition to the current segment. Thus in the presently preferred embodiment the value of m is preferably 3 or greater. If fewer than 3 channels are available simultaneously, then the system will have to sacrifice in supporting forward or backward VCR functionality. Thus in the preceding example, we have used m=3 to illustrate both forward and backward VCR functionality. Of course, in a practical implementation, the value selected for m can be greater than 3 while still preserving the benefit of supporting both forward and backward action. 
   Given that the series dictated by Equation 1 is monotonically increasing and is bounded by f(u)=f(K), we determine the buffer requirement for a client to be the size of the largest segment minus one size unit. Thus the buffer requirement may be expressed as a function of f(n) as follows: 
             Buffer   =           f   ⁡     (   u   )       ⁢   xL         ∑     n   =   1     K     ⁢           ⁢     f   ⁡     (   n   )           .             Equation   ⁢           ⁢   2             
 
   In operation, when a client downloads video segments from the broadcast channels, we shall assume that at the outset no VCR functions are invoked and the video is to proceed at the beginning of the program. At the beginning, three loaders wait until the boundary of the first segment is detected. This waiting time is the startup latency and the maximum waiting time will be the time required to playback the first segment. The first loader begins to download from the first channel and contents of its buffer are played. The other two loaders are assigned to the second and third channels, respectively. Depending on the relative positions of each channel, the loader may download from the second channel at the beginning or at the end of the first segment. Similarly, the loader may download from the third channel at the beginning or end of the second segment. When the contents of the buffers are played they are immediately discarded. Whenever a loader finishes downloading the assigned channel, it is assigned to the next immediate unassigned channel. It begins downloading from the channel when the length of the contents in its buffer is less than the assigned channel segment size. This guarantees that downloading of a segment is delayed as much as possible to save buffer space, but it also guarantees smooth playback of that segment. 
   The foregoing system is thus able to provide smooth, continuous playback while supporting all popular VCR functions. While the invention has been described in its presently preferred embodiments, it will be understood that the invention is capable of modification without departing from the spirit of the invention as set forth in the appended claims. For example, the number of buffers employed at a client location can vary. Also, the number of loaders used to service the buffer population can also vary. While a one-to-one correspondence between buffers and loaders is presently preferred, some implementations may be able to provide adequate throughput with fewer loaders than buffers.