SYSTEM FOR OPTIMIZING VIDEO ON DEMAND TRANSMISSION BY PARTITIONING VIDEO PROGRAM INTO MULTIPLE SEGMENTS, DECREASING TRANSMISSION RATE FOR SUCCESSIVE SEGMENTS AND REPEATEDLY, SIMULTANEOUSLY TRANSMISSION

[0002] The present invention relates to a method and system for transmitting a program to a plurality of viewers, permitting flexible, nearly on demand viewing by an unlimited number of concurrent viewers, and, more particularly, to a method and system for transmission that minimizes transmission bandwidth, possibly subject to certain constraints.

[0003] Video-On-Demand (VOD) is the on-line version of traditional video-rental services. As with video rental services, each viewer receives a dedicated “copy” of the movie and can view it in a flexible manner, including the ability to pause and resume, rewind, and possibly even fast-forward. With VOD, the “rental” operation is essentially instantaneous, and viewing can begin within seconds of the decision to view.

[0004] The “copy” in VOD is a dedicated video stream. This stream is generated by a video server and sent to the viewer over a communication network. An important advantage of VOD over tape rental is the great flexibility in allocation of resources: the maximum number of concurrent viewers is independent of viewing choices, and is limited only by the server's total streaming capacity. The required bandwidth resources, both in the server and in the communication network, are proportional to the number of concurrent viewers.

[0005] There are important situations in which a large number of people wish to view the same content during the same period of time, albeit not simultaneously. One example is viewing emergency-preparedness instruction in the hours or days prior to the arrival of a major storm. Another example is a newly released “hot” movie that is advertised heavily. Yet another example is a movie whose viewing is assigned as homework, or even a recorded lecture viewed (individually) in class by the students. These pre-recorded instructions, movies and lectures are examples of programs that are to be viewed by many viewers, concurrently but not necessarily simultaneously.

[0006] Although VOD could be used to address such situations, it is both highly desirable and intuitively possible to do better. The desire stems from the fact that even the total (over all programs) number of concurrent viewers may be temporarily much higher than usual, so it would be very costly if not impossible to design the infrastructure (server and communication network) for such peaks. The intuition that something can be done arises from the observation that the many viewers of the “hot” program are viewing the same material concurrently but not simultaneously. The various schemes for doing better than VOD in this situation are called “Near Video On Demand”. The goal of NVOD is to provide an unlimited number of viewers of the same program similar service flexibility to that of VOD at a reasonable cost to the server and communication network. Ideally, this cost is independent of the number of viewers. “Near” is defined to mean commencement of viewing within a reasonable time interval following viewer request, for example one minute in the case of a movie, as well as the ability to pause and resume at any time. Rewind and fast-forward functions are not obligatory.

[0007] There are two categories of NVOD systems: open-loop systems and closed-loop systems. In both systems, the viewers are provided with devices, called herein “clients” because of their relationship with the server, that receive program copies transmitted by the server and display those copies to their respective viewers. In open-loop systems there is no feedback from the viewing client to the server, so neither server transmissions nor routing on the network are affected by viewer actions (other than the possible effect on routing due to a viewer joining a multicast group). Open-loop schemes lend themselves most naturally to broadcast-based networks, such as cable television networks, and even to networks that have only one-way communication, which is the common case in satellite-based information-dissemination networks. Closed-loop systems permit some feedback that allows the server to adjust to client requests throughout the viewing period. Note that the terms “play” and “display” are used interchangeably herein, to refer to the displaying of the received program by a client.

[0008] Recently, several open-loop NVOD schemes have been proposed. These schemes are based on partitioning the program into several segments, on the assumption that every client has a substantial amount of available storage capacity, for example on a hard disk, which can be used to temporarily store the segments. In such schemes, the server's transmission schedule, and the algorithm used by the client to decide whether or not to record any given transmitted segment, jointly ensure that every segment of the movie is stored in the client's recording medium by its viewing time.

[0009] One such scheme is taught by DeBey in U.S. Pat. No. 5,421,031, which is incorporated by reference for all purposes as if fully set forth herein. DeBey's partitioning and scheduling scheme is illustrated in FIG. 1, for the case of segments of equal length. The vertical axis of FIG. 1 is segment number. The horizontal axis of FIG. 1 is the time at which a given segment is broadcast by the server, with the unit of time, as well as the basic time interval, being the duration of one segment. For each segment, the time during which that segment is broadcast by the server is represented by a double-headed arrow. The first segment is broadcast in every time interval, the second segment is broadcast every second time interval, the third segment is broadcast every third time interval, and in general the n-th segment is broadcast every n-th time interval. Note that all segments are transmitted at the same transmission rate of one segment per time interval. In addition, the transmissions continue throughout the time period during which the viewers are permitted to view the program.

[0010] A client that tunes in to the broadcast at the beginning of any time interval receives all the segments promptly enough to display the program with no interruptions. For example, a client that tunes in at the beginning of the seventh time interval, and that actually begins to display the movie to its viewer at the beginning of the eighth time interval, receives and records the seventh copy of the first segment during the seventh time interval, the fourth copy of the second segment during the eighth time interval, the third copy of the third segment during the ninth time interval, the second copy of the fourth segment during the eighth time interval, etc. In this case, the first segment is displayed during the eighth time interval, the second segment is displayed during the ninth time interval, the third segment is displayed during the tenth time interval, the fourth segment is displayed during the eleventh time interval, etc.

[0011] DeBey's scheme imposes certain burdens on the server and on the clients. With N segments, the mean transmission bandwidth, in units of segments transmitted per time interval, is approximately ln(N); but the actual transmission bandwidth varies widely. For example, in prime-numbered time intervals after the first time interval, only two segments are broadcast, vs. e.g. six segments during the twelfth time interval. The client must be able to record the received segments fast enough to keep up with the peak aggregate transmission rate. Furthermore, the client must have enough storage capacity to store all recorded segments that are received too soon to play.

[0012] There is thus a widely recognized need for, and it would be highly advantageous to have, a NVOD method that imposes less of a burden on the resources available to the server and to the clients.

[0013] According to the present invention there is provided, in a system wherein a server transmits a program having a certain duration, the program being received by at least one client, a method for scheduling the transmission of the program, including the steps of: (a) partitioning the program into a plurality of sequential segments; (b) selecting a transmission rate for each segment, the transmission rate that is selected for a first the segment being faster than the transmission rate that is selected for any other the segment; and (c) transmitting the segments, by the server, each segment being transmitted at the transmission rate of the each segment.

[0014] According to the present invention there is provided a system for transmitting a program to a plurality of viewers, including: (a) a software module including a plurality of instructions for scheduling a transmission of the program by: (i) partitioning the program into a plurality of sequential segments, and (ii) selecting a transmission rate for each segment, the transmission rate that is selected for a first the segment being faster than the transmission rate that is selected for any other the segment; (b) a processor for executing the instructions; (c) a server for transmitting each segment at the respective transmission rate; and (d) for each viewer, a client for receiving the transmitted segments, recording the received segments and playing the recorded segments in the sequence.

[0015] According to the present invention there is provided, in a system wherein a server repeatedly transmits a program that is partitioned into a plurality of segments and wherein a client receives and records the segments and displays the program, the transmitting, receiving and recording of the segments being effected according to a schedule, a method for displaying the program intermittently, including the steps of: (a) transmitting, along with the segments, metadata describing the schedule, by the server; (b) pausing the display of the program, by the client; (c) resuming the display of the program, by the client, subsequent to the pausing; and (d) during the pausing, continuing to record at least a portion of the segments then received, by the client.

[0016] In its most basic embodiment, the present invention is a modification of the NVOD scheme of DeBey that smoothes out the instantaneous transmission bandwidth to always be close to the mean transmission bandwidth. This is accomplished by broadcasting all the segments concurrently, starting at the first time interval, but taking n time intervals to broadcast the n-th segment. In other words, if the first segment is transmitted at a transmission rate of T bits per unit time, then the n-th segment is transmitted at a transmission rate of T/n bits per unit time. This is illustrated in FIG. 2. Note that all transmissions end at the same times as in FIG. 1, but start at different times. Specifically, all the first transmissions of all the segments start at the beginning of the first time interval; and subsequently, as soon as the transmission of any segment ends, that segment is immediately retransmitted.

[0017] The simultaneous commencement shown in FIG. 2 is illustrative, and applies primarily to the basic embodiment illustrated therein. More generally, because the segments are transmitted repetitively and concurrently throughout the period during which the program is being offered to the viewers, the starting times of the transmission of different segments may be chosen at will.

[0018] The most basic embodiment of the present invention relieves the burden on the server, but not necessarily the burden on the clients. For example, a client that tunes in must receive and record all the data being transmitted until such time as it has finished recording a given segment and can cease recording the data for that segment. Thus, this client must be capable of receiving and recording data at a rate equal to the server's aggregate transmission rate. Furthermore, towards the middle of the program, the client's storage medium must have sufficient capacity to store a significant portion of the program even if the client discards the data of a segment once it is displayed. These stringent requirements may be relaxed if the start of the recording by a client of later segments is delayed long enough for earlier segments to be recorded, displayed and possibly even discarded, thus freeing up client resources to accommodate the later segments. Clearly, this implies an increase in the aggregate transmission rate, because a segment must be transmitted in its entirely during the time in which it is being recorded by the client; so that shortening this time without changing segment size mandates an increase in its transmission rate. In fact, the optimum overall system design is a tradeoff between minimizing the aggregate transmission rate and meeting client constraints such as limited recording rate and limited storage capacity. The ability to optimally design the system to optimize certain aspects of performance while adhering to constraints on others is a salient feature of the current invention. Algorithms for minimizing the aggregate transmission bandwidth subject to these constraints are presented in the Appendix.

[0019] More generally, the scope of the present invention includes NVOD methods and systems in which the first segment is transmitted at a higher transmission rate than the subsequent segments. Preferably, transmission rates of successive program segments are such that the latest time, measured from the time of a client's viewing request, at which the client may begin to record a segment such that the recording of the segment is completed before that segment must be displayed, is no earlier for a later program segment than for an earlier one. It should be noted that this transmission rate of a segment is defined in terms of the overall transmission rate of the segment as a whole. For example, a server may use time division multiplexing to transmit all the segments concurrently on a single channel, by partitioning the segments into subsegments, interleaving the subsegments and transmitting all the subsegments at the same rate. Consider, for example, in the context of the most basic embodiment of the present invention, a case in which 100-second segments are partitioned into 1-second subsegments of B bits each, that are transmitted on a 1 gigabit-per-second channel. The duration of the transmission of each subsegment is B×10⁻⁹seconds, no matter which segment is the source of the subsegment; but the 100 subsegments of the first segment are transmitted over the course of 100 seconds, for an overall transmission rate of B bits per second, whereas the 100 subsegments of the second segment are transmitted over the course of 200 seconds, for an overall transmission rate of B/2 bits per second.

[0020] It also is clear that a client may ignore, and not record, transmitted copies of any particular segment, until the transmission of the last such copy that is transmitted in its entirety before the segment must be displayed. To facilitate this, each transmitted copy of the segment includes metadata that describe various aspects of the segment, including, for example, the segment number, the size of the segment, the transmission rate of the segment, and a temporal value related to the time interval between the start of the transmission of this copy of the segment and the start of the transmission of the next copy of the segment. An example of this temporal value is the length of this time interval itself. Similarly, in an embodiment of the present invention in which the segments are partitioned into subsegments, each copy of a subsegment includes metadata that describe various aspects of the subsegment, including, for example, the number of the segment to which the subsegment belongs, the sequence number of the subsegment within its segment, the size of the subsegment, and a temporal value related to the time interval between the start of the transmission of this copy of the subsegment and the start of the transmission of the next copy of the subsegment.

[0021] When a segment is not partitioned into subsegments, the preferred embodiments of the current invention nonetheless include metadata that provides sufficient identification so as to permit the recording of the segment's data to begin without waiting for the beginning of the segment. Similarly, when a segment is partitioned into subsegments, the metadata permits the subsegments to be recorded by the client in any order and to subsequently be assembled to form the original segment. Thus, the recording of a given segment by a client may commence essentially as late as one transmission time of the entire segment prior to the earliest time at which that segment may have to be displayed. The time to transmit the entire segment is, in turn, essentially equal to the size of the segment divided by the rate at which the segment is transmitted. Similarly, as soon as a segment is displayed, that segment may be deleted from the recording medium to free up the storage space occupied by that segment.

[0022] In one preferred embodiment of the present invention, the subsegments of at least one program segment are used to compute a larger number of such subsegments, such that the original segment can be derived from any sufficiently large subset of the subsegments; and all the subsegments are transmitted such that the time between successive transmissions of the same subsegment remains unchanged. This increases the transmission rate of the segment, but permits the use of error correcting codes to receive the segment successfully even in the event of communication problems.

[0023] In one application of the present invention, in order to allow a viewer to tune in to a broadcast of an entire live program after the program has started but before the program has ended, the basic embodiment of the present invention is initiated simultaneously with the live broadcast, with the repeated transmission of each segment initiated after the live transmission of that segment.

[0024] In some embodiments of the present invention, only one copy of each segment is stored at the server. In other embodiments of the present invention, multiple copies of the program data are stored at the server, to increase the efficiency of storage access when transmitting the program.

[0025] Optionally, the segments are encrypted and/or compressed by the server prior to transmission, and are decrypted and/or decompressed by each client prior to display.

[0026] A system of the present invention includes a software module that embodies the algorithm of the present invention, a processor for executing the instructions of the software module, and a server that includes both a data storage area for storing the program segments and a mechanism for transmitting the program segments according to the transmission rates assigned by the algorithm. Preferably, the software module and the processor also are included in the server; although it also is possible, within the scope of the present invention, for the server to obtain a rate allocation table or a transmission schedule from another device that includes the software module and the processor of the present invention and that operates off-line. The system also includes, for each viewer, a client for receiving, recording and displaying the segments, and a distribution network for broadcasting the segments to the clients.

[0027] The scope of the present invention also includes a method for pausing and resuming the display of the program by a client, whether the NVOD transmission rates are selected according to the teachings of the present invention or according to prior art schemes such as DeBey's. Essentially, the client stops displaying the program, but continues to record incoming segments as though the program were being displayed at the point, in the program, where the display was temporarily stopped. To conserve the client's storage space, metadata are transmitted along with the segments to indicate when the segments will be transmitted again, and segments that will be transmitted and recorded again before they need to be displayed, if displaying were to be resumed immediately, are discarded.

[0028] The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

[0029] FIG. 1 is a graphical representation of a prior art NVOD scheduling scheme;

[0030] FIG. 2 is a graphical representation of the NVOD scheduling scheme of the present invention;

[0031] FIG. 3 shows eight snapshots of the execution of an algorithm for minimizing the aggregate server transmission bandwidth subject to constraints on client storage capacity and recording rate;

[0032] FIG. 4 is a schematic diagram of a system of the present invention.

[0033] The present invention is of a NVOD method and system for transmitting a program to a plurality of viewers for on-demand viewing. Specifically, the present invention can be used by the viewers to view the program continuously, without undue burden on system resources.

[0034] The principles and operation of NVOD according to the present invention may be better understood with reference to the drawings and the accompanying description.

[0035] The Appendix presents an analysis of the present invention, and preferred methods for assigning transmission rates to program segments according to the present invention, for the special case of a program partitioned into segments of equal length. Nevertheless, the scope of the present invention includes the use of segments of unequal length.

[0036] The analysis and the methods presented in the Appendix are directed towards three system parameters, the aggregate server transmission bandwidth R_t, the peak client recording rate R_rand the required client storage capacity S_max. Specifically, the algorithms presented in the Appendix are for minimizing R_tsubject to the constraints of particular values of R_rand S_max. Note that the terms “aggregate transmission rate” and “aggregate transmission bandwidth” are used interchangeably herein. The program is partitioned into L sequential segments indexed by an index m. The time at which the client begins recording segment m is denoted by s(m). The time at which a viewer begins to view segment m is denoted by v(m). In particular, v(1) is equal to the permissible delay from the time of a viewer's request to view a program until the viewer's client begins displaying the program. Recording by the client begins immediately upon the viewing request. The rate at which a particular segment m is transmitted by the server is denoted by r_t(m). In order to guarantee that all the data of segment m is present at the client when the segment is displayed, s(m)<v(m)−(size of segment m)/r_t(m). As noted above, the principle of the present invention, in its most general form, is to use r_t(m>1)<r_t(1). Also as noted above, it is preferable to assign r_t(m) such that s(m₂)≧s(m₁) if and only if m₂>m₁, i.e., the times at which the recording of successive segments begins form a monotonically non-decreasing sequence. In the basic embodiment, r_t(m) is proportional to 1/m. The algorithms presented in the Appendix minimize R_t, subject to the constraints, by systematically increasing s(m) of the basic embodiment (thereby increasing r_t(m)) to ensure that the amount of storage in the client's recording medium occupied by recorded segments never exceeds S_maxand/or that the rate at which the client records incoming segments never exceeds R_r. These algorithms also can be used by one skilled in the art to minimize any one of the client storage requirement, or the client's peak recording rate, subject to constraints on R_tand the other parameters.

[0037] Referring again to the drawings, frames A-H of FIG. 3, which is identical to FIG. 1 of the Appendix, show eight stages of an algorithm that minimizes R_tsubject to particular fixed maximum values of S_maxand R_r. The frames represent snapshots of the progress of a transmission-rate assignment algorithm of the present invention. Each snapshot depicts the recording rate r_r(v) and the amount of storage used by a client, S(v), as a function of the elapsed time v since the client requested the viewing of the program, as calculated by the algorithm at eight different stages in the progress of the algorithm. The axes are intentionally uncalibrated but are linear, and the point at which both storage and recording rate drop to zero is the end of the viewing of the program. The transmission-rate assignments themselves are not shown. It can be seen that the assignment algorithm considers viewing times from earliest to latest, so successive snapshots differ primarily at the higher values of the viewing time v. FIG. 3A depicts the unconstrained minimum transmission rate solution (the most basic embodiment of the present invention): r_r(v) is fixed until v=v(1), the time at which viewing of the first segment commences, because s(m)=0 for all m and all segments are being recorded; and S(v) increases linearly during this time interval. After the display of the program begins, the recording of segments gradually comes to an end, with the highest-rate (earliest) segments being dropped first. S(v) is the integral with respect to time of recording rate minus the fixed viewing rate. In FIG. 3A, S(v) reaches a peak and then falls off gradually. FIGS. 3B, 3C and 3D show how the algorithm sweeps across viewing time, forcing the recording rate r_r(v) down to the permissible upper limit. In so doing, the recording rate r_r(v) for subsequent segments may actually increase, but storage never increases. The sudden rise in the recording-rate plot reveals the viewing time that is being addressed by the algorithm. In FIGS. 3E through 3G, storage becomes the limiting factor and the recording rate r_r(v) falls below its permissible maximum. In FIG. 3H, the algorithm enters a regime in which both recording rate and storage consumption are below their upper limits at all times, and the algorithm stops.

[0038] Preferably, segments that are transmitted concurrently by the server are multiplexed on a common channel by a conventional multiplexing method such as time division multiplexing, frequency division multiplexing or code division multiplexing. As noted above, if time division multiplexing is used (and optionally if a different multiplexing method is used), each segment is partitioned into sequential subsegments. These subsegments need not be transmitted in their sequential order, although it is preferable that the transmission order of the subsegments be a cyclic permutation of the sequential order of the subsegments, and it is most preferable that the transmission order of the subsegments be identical to the sequential order of the subsegments. Also as noted above, each subsegment include metadata that indicates the sequential order of that subsegment within the corresponding segment, so that a client can start receiving a stream of subsegments in the middle of the sequence, and need not wait for the transmission of the sequence to start again from the beginning of the sequence.

[0039] Although the descriptions, herein and in the Appendix, of the present invention, all refer to a situation wherein a client does not store any portion of the program prior to the viewing request, the present invention also applies to the case in which the client does store a portion of the program in advance of viewing. Specifically, if the client stores the first t seconds of the program in advance, the present invention can be applied to the remainder of the program, excluding the first t seconds, with the first subsequent segment labeled as segment number one. The viewing time v still is measured from the time at which the viewing request is made, but v(1) is equal to the permissible delay from viewing request until the commencement of the display of the program plus t. The viewing times of later segments then differ from v(1) by the same amount as when no portion of the program is stored at the client prior to the viewing request.

[0040] FIG. 4 is a schematic block diagram of a system 10 of the present invention. The high level components of system 10 include a server 12, a distribution network 30 and several clients 40.

[0041] Server 12 stores the program and transmits the program to distribution network 30 according to the NVOD transmission rate assignments of the present invention. Server 10 includes a processor 24; one or more storage/memory units indicated collectively by reference numeral 14; a set of input/output devices, such as a keyboard, a floppy disk drive, a modem and a video monitor, represented collectively by I/O block 26; and a mechanism for transmitting the program to clients 40 via distribution network 30, represented by a transmitter 28. Memory 14 includes an instruction storage area 16 and a data storage area 22. Within instruction storage area 16 is a software module 18 including a set of instructions which, when executed by processor 24, enable processor 24 to schedule the transmission of the program in accordance with the teachings of the present invention.

[0042] Using the appropriate input device 26, source code of software module 18, in a suitable high level language, for implementing the scheduling algorithms of the present invention and for scheduling the actual transmission of segments of the program in accordance with the output of one of the algorithms, is loaded into instruction storage area 16. Selecting a suitable language for the instructions of software module 18 is easily done by one ordinarily skilled in the art. The language selected should be compatible with the hardware of server 12, including processor 24, and with the operating system of server 12. Examples of suitable languages include but are not limited to compiled languages such as FORTRAN, C and C++. If a compiled language is selected, a suitable compiler is loaded into instruction storage area 16. Following the instructions of the compiler, processor 24 turns the source code into machine-language instructions, which also are stored in instruction storage area 16 and which constitute a portion of software module 18. Using the appropriate input device 26, the program is loaded into data storage area 22. Following the machine-language instructions of software module 18, processor 24 partitions the stored program into segments, schedules the transmission of the segments and directs transmitter 28 to transmit the segments according to the schedule.

[0043] Distribution network 30 carries the transmitted segments to clients 40. Distribution network 30 is illustrated in FIG. 4 as a satellite network, including at least one satellite 32, an uplink channel 34 from server 12 to satellite 32 and multiple downlink channels 36 from satellite 32 to clients 40. Alternatively, distribution network 30 is a cable television distribution network, a network based on telephone lines, a network based on optical fibers, a special-purpose network, or any suitable local-, metropolitan- or wide-area network. Preferably, distribution network 30 supports the transmission of information packets to multiple recipients.

[0044] Each client 40 includes a receiver unit 46 for receiving the transmitted segments (for example, from downlink channel 36); a recording medium, represented in FIG. 4 as a hard disk 44, for storing received segments until those segments are displayed; a video display unit 42 for displaying the recorded segments consecutively in their sequential order; and a microprocessor-based control unit 48 for controlling the overall operation of client 40. Receiver unit 46 is chosen to be appropriate to distribution network 30. For example, if distribution network 30 is a satellite network, as shown, then receiver unit 46 is a satellite transmission receiver, and if distribution network 30 is a cable television distribution network, then receiver unit 46 is a modem. Client 40 receives the transmitted data, selects the data that should be recorded on recording medium 44, reorders the data to reconstruct the original program sequence, decompresses the data if necessary, and displays the data on video display 42. Additionally, client 40 is responsive to viewer commands (play, pause, stop, rewind) directed to control unit 48 via a conventional input device (not shown) such as a keyboard or a remote control device. Control unit 48 makes its decisions based on the viewer commands it receives, on the viewing time v, on the contents of recording medium 44, and on the scheduling information that is embedded in the received data or that is supplied separately. After displaying a segment, client 40 either discards that segment or continues to save the segment in recording medium 44 in support of rewind. Client 40 may be an autonomous unit such as a suitably configured personal computer or television set. Alternatively, components 44, 46 and 48 of client 40 may be embodied in a stand-alone unit such as a television set-top box, for use with a television set that serves as video display 42.

[0045] Because the clocks of server 12 and client 40 may operate at slightly different rates, and because server 12 transmits the same data to all clients 40, client 40 preferably is capable of synchronizing to the server clock.

[0046] In addition to the segments and the metadata, the information transmitted by server 12 to clients 40 optionally includes request/billing information that enables a viewer to select a program from a menu of offered programs and that enables the provider to record and charge for the transaction. This information may also include a “password” that server 12 transmits to a client 40 to enable that client 40 to display a particular program.

[0047] Typically, client 40 uses a secondary storage device such as a hard disk as its main recording medium 44. Client 40 may also have a substantial amount of primary (random access) memory. Preferably, client 40 uses the available primary memory in order to buffer certain program segments until their playback time, thus obviating the need to store these program segments in main recording medium 44 and read these program segments from main recording medium 44. This is particularly useful for the earliest segments of the program, because these segments are received and played at a time in which the client recording rate is at its maximum and hard disk bandwidth is stressed. In this preferred embodiment, the main memory that is available for this purpose is used to store the newly received segments whose playback time is the earliest. If received data belongs to an earlier (in the program and thus in terms of playback time) segment than one whose data is already being buffered in the primary memory, the data of the latter is evacuated to main recording medium 44 in favor of the newly received data. In this embodiment, the transmission-rate allocation algorithm is modified to reflect the fact that certain early segments of the program are never recorded in main recording medium 44 or read from main recording medium 44. This permits the aggregate transmission bandwidth from server 12 to be reduced if that aggregate transmission bandwidth was originally determined by the constraint on recording rate. The peak storage requirement at client 40 is unaffected by the choice of storage medium for early program segments.

[0048] As noted above, the scope of the present invention includes the partition of the program into segments of unequal length. Appropriate modifications must be made to the algorithms described in the Appendix to account for segments of unequal length. Each such segment is characterized by a size (in bytes) and the time (in seconds since the start of playing of the program) at which the display of the segment must begin. The transmission rate assigned to a segment is obtained by dividing the size of the segment by the difference in time between the time at which display of the segment begins and the time at which the recording of the segment begins: r_t(m) size(m)/(v(m)−s(m)). Optionally, the sequence of points in time at which the algorithm is applied are the (irregularly spaced) points in time at which the displays of the different segments begin.

[0049] In its basic form, the present invention assumes that the entire program is available to server 12 at the outset, e.g. in a file. Thus, the transmission of all segments can take place in an effectively concurrent manner, and a client 40 can begin recording any segment at any time. There are, however, situations wherein it is desired to permit the viewing in NVOD mode to commence even before the original (live) event has been completed. For example, it is desired to permit the viewing of a one-hour news program to begin at any time after its actual beginning, not only after it is over. The basic NVOD scheme does not work in this case, because the recording of (particularly late) segments by client 40 must begin before the corresponding real event took place.

[0050] In order to permit delayed viewing to begin even while the live event is still taking place, the basic scheme is extended as follows:

[0051] The live event is transmitted once, as it takes place, at the nominal video rate. The transmission of the first live segment also triggers the beginning of the (infinitely repetitive) NVOD schedule of the basic scheme. Segments that have yet to happen are simply not transmitted. The two constituent transmissions are integrated to form a single data stream. This stream, in addition to the data itself, contains information that permits client 40 to know when a given block of data will be transmitted again, or equivalent information that enables client 40 to decide whether client 40 must record the current instance of a given piece of data. A joining client 40 receives both the remainder of the “live stream” and the basic NVOD transmissions. This client 40 records, from both sources, data that will not be received again before the earliest time at which this client 40 may need such data for viewing. As soon as the live event is over, the “live stream” terminates and system 10 again operates in the basic NVOD mode.

[0052] The aggregate transmission rate is initially lower than with the basic NVOD scheme, because there is hardly any data to transmit aside from the “live” stream. As the live event progresses, however, the rate increases and eventually reaches the full NVOD rate plus the live video rate. Once the live event is over, the aggregate transmission rate drops to the NVOD rate. In a most preferred embodiment of the present invention, the amount of additional transmission bandwidth is reduced by increasing the individual NVOD transmission rates, of program segments that are already available because the corresponding part of the live event has already occurred, during the initial portions of the live event and gradually reducing the NVOD transmission rates to below the original NVOD transmission rate toward the later parts of the live event. The exact details vary between implementations and can be worked out by one ordinarily skilled in the art.

[0053] It is well known that communication channels are imperfect due to a variety of reasons. In order to overcome errors, it is common to use error correcting codes. These entail the generation of redundant information that is derived from the original information, such that a sufficiently large subset of the total (redundant plus original) information suffices for the reconstruction of the original information. The aggregate transmission rate is increased in a manner commensurate with the degree of redundancy. For example, if the amount of data transmitted for each segment is increased by a factor of k+r)/k, then the transmission rate must be increased by the same factor. In some preferred embodiments of the present invention, such codes are constructed by dividing each program segment into subsegments of equal sizes, and deriving redundant subsegments from those subsegments. This technique is advantageous over the more common one, wherein redundant information is appended to each subsegment and serves to overcome errors within that segment. Specifically, the advantage of the technique of the present invention is that the technique of the present invention is more resilient than the more common technique to bursts of errors, because the redundant information is transmitted distantly (in time) from the original information. Thus, a burst of errors destroys small portions of different segments rather than a large portion of a single segment.

[0054] Error correcting codes usually entail the computation of redundant information from the original information and the transmission of the original information as well as the redundant information. A different flavor of codes entails computation of information from the original information such that the total number of bits is larger than the original number of bits, and such the original information can be derived from a sufficiently large subset of the fall set of bits. In both cases, repetitive transmission of a segment entails the repeated transmission of the total information derived from the original information. So doing, including the special case of no redundant information, requires that client 40 continue to be active even when pausing. As is described in the Appendix, while pausing, client 40 continues to record portions of segments while discarding other portions of those segments. The reason is that, although the fraction of any given segment that must still be recorded before the segment can be viewed does not change with time while client 40 is pausing, the portion (content) of the segment, that will be transmitted between the current time and the time at which the segment will be viewed if playing is resumed at the current time, changes. For example, if a pausing client 40 has recorded the first half of a segment at the time of pausing, this client 40 still needs the second half of that segment. If this client 40 pauses for a time equal to the transmission time of one quarter of that segment and does not record during that time, the missing fraction remains unchanged. However, because transmission has progressed, the fraction of the segment that will be transmitted between resumption of playing and the viewing time of this segment will comprise the fourth quarter of the segment followed by the first quarter of the segment. The third quarter of the segment will thus be missing.

[0055] For the reasons just explained, it is also clear that, because the viewing time is frozen while pausing, whereas transmission by the server continues, certain already recorded portions of some segments are certain to be transmitted again prior to the earliest time at which these segments may have to be displayed subsequent to the resumption of viewing. These portions may be discarded by the client. In order to enable the client to decide whether to discard these portions, the client stores (at all times) the actual (real) time at which any given portion is recorded, as well as the metadata identifying when that portion will next be transmitted.

[0056] It is sometimes desirable to cease the recording when pausing. This may be due to a need to use the client computer for other purposes or due to a need to interrupt the communication. In order to permit this, one preferred embodiment of the present invention uses a code that entails the generation of a sequence of subsegments derived from the original segment such that the sequence is much longer than the number of subsegments that are required for reconstruction of the original segment. Furthermore, any sufficiently large subset of the subsegments suffices for this reconstruction. Thus, as long as the transmission time of the entire sequence of subsegments is sufficiently longer than the pausing time, the subsegments recorded by client 40 prior to pausing and the subsegments recorded by client 40 after resuming will all be different, and reconstruction will be possible. An example of such a code is the well-known Reed-Solomon code. Another suitable type of codes is Tornado codes.

[0057] In its basic form, system 10 entails transmission of data by server 12, such that all the data is received by all clients 40, and each client 40 selects the relevant portions of the data for recording in its memory or disk buffers. This is referred to as “broadcast and select”, and is appropriate for situations wherein the transmission medium is a broadcast channel (e.g., satellite and cable television) and wherein the permissible transmission rate on a single channel equals or exceeds the aggregate transmission rate required for providing the NVOD service and wherein each client 40 is capable of receiving data into memory at a rate equal to this aggregate rate. This is not always the case. Nevertheless, the present invention is applicable to such situations.

[0058] One important configuration in which this problem occurs involves the use of telephone lines to reach the client location. These “digital subscriber loops” are inherently point-to-point, with different (physical or logical) lines going to each client 40, and the permissible data rate over each such line is lower than the aggregate NVOD transmission rate. One preferred embodiment of the method of the present invention for this situation is to execute the selection function at the distant (from client 40) end of the telephone line. This end of the line is referred to herein as the “server side” of the line. By so doing, server 40 still transmits at the same rate as before, but the data rate over the private line of any given client 40 is only the recording rate, which varies with time and is initially higher than the video rate, but is substantially lower than the NVOD transmission rate. In order for this scheme to work, client 40 must advise its “agent” at the “server side” of its request to start viewing and of any pausing and resumption. The agent, located on the server side of the private communication line, acts as a filter on behalf of client 40, receiving all transmitted data but refraining from transmitting to client 40 over the private line at least some of the segments that do not need to be recorded by client 40, e.g., segments that have already been viewed by client 40. Indeed, this agent must be able to receive (but not necessarily store on disk) at the full NVOD transmission rate. Another way to think of this is that the filtration that otherwise would occur in client 40, resulting in the recording of only relevant portions of the received data, is carried out at the server-end of the private line, thereby reducing the data rate on the private line from R_tto a lower value, possibly as low as r_r(v).

[0059] Another important configuration entails the use of cable television or satellite, such that the entire NVOD transmission can be carried over a single channel and the modem of client 40 can listen to all transmissions, but the permissible data rate between the modem and control unit 48 is lower than the channel rate. With current cable modems made by Motorola, for example, the data rate over a single cable channel is 30 Mbit/sec, but the connection between the cable modem and the client computer is a 10 Mbit/sec Ethernet connection. One preferred embodiment of the present invention for dealing with such a situation entails the use of multiple destination addresses, as follows:

[0060] Each program segment is transmitted at the rate determined by the NVOD transmission-rate allocation scheme to at least one of the destination addresses. Each client 40, depending on the viewing time of that client 40, accepts data that is destined to a particular subset of the destination addresses. Because certain segments may be transmitted to more than one destination address and are thus transmitted concurrently more than once, this technique increases the aggregate transmission rate. The segments can be grouped in various ways for this purpose, representing a trade-off between transmission rate, the data rate that a client 40 must receive, and the number of destination addresses for which a client 40 must be able to receive data. Because each transmitted data packet is generally received by some subset of clients 40, this kind of transmission is referred to as multicast.

[0061] In another preferred embodiment of the present invention for dealing with this situation, the microcontroller that is part of the cable modem is programmed to act as the server-side agent that was referred to in the example of digital subscriber loops, passing on to client 40 only those segments that client 40 should record.

[0062] Yet another important situation occurs when the permissible data rate over a single channel is lower than the aggregate NVOD transmission rate. A preferred embodiment of the present invention in this case entails the transmission of any given segment over at least one channel. At any given time, any given client 40 receives the data transmitted over a subset of the channels. This subset depends on the allocation of segments to channels and on the point in the program that is being viewed at that time. For a given channel data rate, the assignment of segments to channels represents a trade-off between the aggregate transmission rate and the number of channels from which a client 40 must receive data concurrently.

[0063] In one preferred embodiment of the present invention, any piece of data is stored in data storage area 22 of server 12 exactly once, and data are read for transmission as required by the transmission rates of the various program segments. This storage scheme minimizes storage requirements in server 12; but if data storage area 22 is implemented as a hard disk, this storage scheme requires numerous seek operations in order to bring the disk reading head to the location of the data that are to be transmitted. In another embodiment of the present invention, wherein each segment is partitioned into subsegments and the concurrent transmission of the segments at their respective transmission rates is carried out by appropriately interleaving the transmission of their subsegments over a common channel, the program data are recorded sequentially on disk in the order in which the interleaved subsegments are transmitted over the channel. The recorded sequence is of sufficient length, on the order of the viewing time of the entire program. This substantially increases storage requirements in data storage area 22 of server 12, but requires no seek operations and thus substantially increases the hard disk's effective data rate.

[0064] As noted above, in order for client 40 to decide which data client 40 should record and to correctly reassemble the program, client 40 must know the location within the program of arriving data and whether this data will be received again prior to the earliest possible time at which this client may have to play it for viewing. Again as noted above, in one preferred embodiment of the present invention, each program segment is partitioned into subsegments. Additionally, each transmitted subsegment contains a unique sequence number that identifies the sequence order of that subsegment in the program, as well as the elapsed time from the current transmission of that subsegment until that subsegment will be transmitted again. In another preferred embodiment of the present invention, each subsegment is tagged with its sequence number in the program and also contains that size (in bytes) and transmission rate assigned to the segment to which that subsegment belongs. In yet another embodiment, a subsegment is tagged with its sequence number and server 12 periodically transmits metadata advising clients 40 of the transmission rate assigned to each segment.

[0065] When the “long” error-correcting code sequences are used, the time until next transmission of a particular subsegment is irrelevant. Instead, if the degree of redundancy is such that the frequency with which subsegments of a segment are transmitted is greater by a factor of (k+r)/k relative to the frequency in the absence of error correction, the preferred embodiment of the present invention is as follows: each subsegment contains its sequence number in the program and the time until the next transmission of the subsegment (of the same segment) that is k+r later than itself in the code generated for this segment. In the event that the end of the code sequence is reached, counting continues with the first subsegment in the code sequence for this segment. Another embodiment of the present invention entails the transmission of the sequence number, the segment size (size of k+r subsegments) and the transmission rate allocated to each segment.

[0066] Instead of discarding program segments that have been viewed, one preferred embodiment of the present invention retains these segments in recording medium 44 of client 40 in order to permit re-viewing of portions that have been viewed. In one variant of this preferred embodiment of the present invention, segments are only discarded when the storage space devoted to storing the segments in recording medium 44 would otherwise exceed the specified limit (note that the storage space is unused at the beginning and at the end of the display of the program, so there are long periods during which there is extra space). Also in this variant, the order in which segments are discarded is the same as their order in the program. In another variant of this preferred embodiment of the present invention, a minimum required rewind distance is specified and the corresponding amount of storage space in recording medium 44 is reserved for this purpose. The transmission scheme is designed based on an available storage space in recording medium 44 that is equal to the specified available storage space minus the reserved space. The discarding policy is the same as in the first variant.

[0067] System 10 optionally includes means of encrypting the transmitted data or a part thereof such that a client 40 must receive authorization from the service provider in order to be able to view a given program. This authorization may be in the form of a password, and may be transmitted through the same channel as the program or provided by other means. According to one embodiment of the present invention, server 12 transmits, with each program, information pertaining to the charge for viewing that program, and the viewer uses a smart card (cash card or credit card) to pay, with the payment occurring at the client site, to enable client 40 (which is trusted by server 12) to present the program to the viewer.

[0068] The embodiments presented above of the NVOD transmission are based on accommodating the least capable client 40. Thus, all clients 40 can successfully participate. However, there are situations in which accommodating the less capable clients 40 (e.g., clients 40 that have much smaller storage space than the other clients 40) would result in an excessive transmission rate. The alternative is reduction in video rate (more lossy compression and lower quality), which may be unacceptable to the capable clients 40. To alleviate this problem, an extension of the NVOD system and method of the present invention entails the use of progressive encoding of the program segments. According to this well-known method in the art of data compression, a fraction of a compressed segment suffices for its reconstruction, albeit at a reduced quality. In one embodiment of the present invention, every segment of the program is compressed according to a progressive compression scheme and is partitioned into two pieces such that the first piece of any given segment suffices for the reconstruction of that segment, albeit at a lower quality. The first pieces of all segments are then treated as one program, and the second pieces of all segments are treated as a second program. The first pieces are assigned transmission rates so as to satisfy the resource constraints of the “poor” clients 40 while minimizing aggregate transmission bandwidth. Concurrently, the “program” containing the second pieces of every segment is assigned transmission rates so as to satisfy the constraints of the “rich” clients 40, having first subtracted from those the resources taken up by the first “program”. (This idea can be generalized by those skilled in the art to more than two levels of clients 40 and qualities.) The exact allocation of resources and size between the first and second pieces of segments is determined by constraints of the compression scheme and also represents a trade-off between the quality received by the less capable clients 40 and the (higher) quality received by the more capable clients 40.

[0069] The pausing algorithm of the present invention is described schematically as follows in the Appendix:

[0070] As noted above, v is the time at which the display of the respective segment commences, relative to when display of the program was requested by the viewer.

[0071] Although the algorithms in the Appendix, including the pausing algorithm, are defined in terms of segments, it will be clear to those skilled in the art how to implement the pausing algorithm in the context of subsegments as described above. For example, the necessity of continued recording during the pause is discussed above in terms of a segment, half of which is recorded by client 40 at the time of pausing.

[0072] While client 40 pauses, client 40 behaves as though frozen in time, so that if client 40 requests display of a program of duration T at time t₀, starts displaying the program at time t₁, pauses at time t₂(t₂−t₁<T), and resumes display at time t₃, the data stored by that client 40 in recording medium 44 at time t₃, when display resumes, is the same as the data that would be stored at time t₃by a client that requested display at time t₀+(t₃−t₂), started displaying at time t₁+(t₃−t₂) and did not pause. In particular, when display resumes, display commencement times v of segments yet to be displayed are with reference to t₀+(t₃−t₂) rather than to t₀.

[0073] The metadata transmitted by server 12 includes, for each segment or for each subsegment, information about when (in terms of clock time, not in terms of display commencement time v) that segment or subsegment will next be transmitted. Client 40 records, for each recorded segment or subsegment, the time at which that segment or subsegment was recorded, and the time at which that segment or subsegment next will be transmitted. This enables client 40 to conserve storage space in medium 44: as needed, client 40 discards segments or subsegments that will be transmitted and recorded again in time to be displayed at their respective display commencement times v referred to t₀+(t₃−t₂).

[0074] While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.

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