VIDEO PEDESTAL NETWORK

Abstract: An architecture (200) for distributing digital information to subscriber units (202) wherein selection from among multiple digital services is accomplished by transmitting a tuning command from a subscriber unit to an intermediate interface (206). The intermediate interface (206) selects the desired service from a broadband network and transmits it to the subscriber unit (202) over a bandwidth-constrained access line. The bandwidth-constrained access line may be implemented with existing infrastructure, yet the subscriber unit (202) may access a wide variety of digital information available on the broadband network. Universal broadband access is thus provided at low cost. Output bandwidth of broadcast equipment may also be optimized.

Description: VIDEO PEDESTAL NETWORK STATEMENT OF RELATED APPLICATIONS This application is a continuation-in-part of Serial No. 08/451,264, filed May 26, 1995. BACKGROUND OF THE INVENTION The present invention relates generally to delivery of digital information to subscriber premises and more particularly to providing this access without substantial new wiring expense. The delivery of digital video services to the home represents one important aspect of the much publicized "information highway." These services include HDTV, video- onrdemand (VOD) , near-video-on-demand (NVOD) providing staggered program starting times, interactive video services (IVS) , and other digital variants of conventional broadcast services. One of the main obstacles to the introduction of these services is development of the necessary infrastructure for delivering digital video information to the subscribers' premises. Video services, even with the use of modern compression standards such as MPEG-1 and MPEG-2, intrinsically require large bandwidths. Somehow an infrastructure must be constructed to distribute the necessary signals to individual subscriber premises. At a minimum, each subscriber should be able to select from among numerous digital video programs as can be done now with analog broadcast television. Various solutions have been proposed. In one scheme, the digital data are distributed via an asynchronous transfer mode (ATM) network to each subscriber's premises. The physical medium of the ATM network may be implemented in more than one way. One way is to lay optical fiber to each home. Alternatively, fiber may be laid up to the curb, from which point a coaxial cable can relay the ATM cells. The advantages of this technique are the low latency and flexibility of the ATM technology and a large bidirectional bandwidth sufficient to distribute numerous interactive digital video programs. Nonetheless, this approach is practically infeasible today since the cost of laying fiber or coaxial cable to each home is prohibitive. Additionally, the time required to deploy such an infrastructure over a large geographic area makes the scheme even more unattractive. An alternative scheme is the so-called hybrid fiber coax (HFC) scheme. The HFC scheme provides a two-level network. At the higher level, optical fibers are used to distribute digital information to a plurality of Cable Headends or Host Digital Terminals (HDT) . Each Headend or HDT in turn distributes information to multiple hybrid fiber coaxial cables, each of which serves several hundred subscriber units in a bus/loop architecture. The return channel over the coaxial cable is also shared by multiple subscriber units by employing Time Division Multiplexing (TDM) . Again, for those network providers that do not already have such an infrastructure installed, costs are prohibitive because coaxial cable must be brought to each home. Furthermore, the use of TDM coupled with highly limited bandwidth gives rise to a large latency in the return channel. Network security is another drawback of the HFC architecture as several users share a single coaxial cable, a particular concern for interactive services that may require transmission of a subscriber's private information. Other schemes take advantage of the existing telephone network by using ADSL technology to transfer high data rate information, such as video, over existing telephone company twisted pair lines to subscriber premises. Optical fiber may be used to transfer digital information to the telephone company central office or to a curbside interface where the twisted pair lines begin. The latter architecture is commonly referred to as Fiber-To-The-Curb (FTTC) . Alternatively, fiber may be deployed till the basement of a large building, from which point existing twisted pair lines can establish connection with each subscriber. Such an architecture is commonly referred to as Fiber-To-The-Building (FTTB) . The disadvantage of this approach is that ADSL provides insufficient bandwidth. Most current ADSL trials carry only 1.5 or 2 Mb/s over twisted pair. Laboratory demonstrations have shown that in the next few years cost effective solutions that provide up to 25 Mb/s may be possible, but even this would be insufficient to provide a broadcast or NVOD service with an acceptable number of service selections. Approaches which bring fiber to the curb carry the added cost of laying the fiber. Prior art digital data delivery schemes that use relatively narrow bandwidth connections to the subscriber premises require point-to-point sessions between the ultimate server and subscriber unit. This is because the narrow bandwidth link that is closest to the subscriber permits only a point-to-point connection if the desired service quality is expected to be reasonable. These point-to-point sessions waste bandwidth since the server must separately transmit to multiple subscriber units requesting the same program. If the user wishes to switch channels, there is significant extra latency resulting from the need to end the previous point-to- point session before beginning a new one. Furthermore, the network and server hardware needed to accommodate point-to- point sessions is particularly complex and expensive. SUMMARY OF THE INVENTION The invention provides a network architecture for distributing digital data to subscriber units wherein selection from among multiple digital services is accomplished by transmitting a tuning command from a subscriber unit to one or more intermediate interfaces in a series of links interconnecting the subscriber unit and a server. An example of such digital data is digital video and the services could be multiple digital video programs. The network architecture of the present invention is capable of providing public broadband access without the use of very high bandwidth access lines to subscriber premises. Using the information received from the subscriber unit, the intermediate interface selects the desired digital service from a multitude of services available in a broadband link coupled to the interface's input and transmits it to the subscriber unit over a bandwidth-constrained link. The bandwidth-constrained link may be implemented with existing , infrastructure, yet the subscriber unit may readily access a wide variety of digital services available on the broadband network. The present invention thus combines universal broadband access with low cost. In accordance with the invention, a service provider may offer broadcast services over the network in addition to point-to-point interactive services. The broadcast services may be offered without requiring multiple point-to-point sessions from the server to each requesting subscriber unit. Instead, a single copy of a digital stream provided via the broadband link is sufficient to service multiple subscriber units which request it via their intermediate interfaces, thus conserving bandwidth. By contrast, in the prior art point-to- point schemes, the broadband network would be forced to carry a separate point-to-point connection for each requesting subscriber unit. Note that the present invention provides the advantages of forgoing point-to-point connections for broadcast purposes without requiring the construction of the very high bandwidth links necessary to continuously transmit each available program to each subscriber unit. Thus digital broadcast and NVOD services may be provided effectively and at low cost. In accordance with one aspect of the present invention, a conventional telephone network is enhanced to provide universal high bandwidth digital service. Typically, telephone service is provided to subscriber premises via individual access lines extending from the subscriber premises to a neighborhood hub or pedestal. The access lines are private and secure twisted pair lines. The pedestal is fed by a high data-rate trunk line, typically implemented as a fiber optic connection. High bandwidth digital service, including video programming, is transmitted over a twisted pair connection by implementing Asymmetric Digital Subscriber Link (ADSL) modulation and demodulation over the connection which allows for transmission of high speed digital data in a manner that is transparent to existing telephonic traffic, as is well known to those of skill in the art. The network equipment for ADSL modulation and demodulation typically resides in the pedestal. In accordance with this aspect of the present invention, the pedestal is further specially adapted to receive tuning information from a subscriber unit to which it is coupled by an access line. The pedestal uses the tuning information to select the specified digital data from the multitude of service data received over the broadband network at its inputs to be relayed to the subscriber unit. In accordance with the present invention, this architecture could be further extended by including one or more additional interfaces. For example, in addition to the pedestal, an interface could be provided within the telephone company central office. The overall video distribution scheme would then incorporate three interconnected layers. The lowest layer would constitute the individual narrow bandwidth access lines between the pedestals and subscriber premises, the middle layer would constitute an intermediate bandwidth link between the central office and the pedestal, and the top layer would be a broadband network feeding the central office or some other broadband source, e.g., a satellite feed. With this scheme, the tuning function could be shared between the pedestal and the interface within the central office. The pedestal would respond to a tuning command from a subscriber unit by attempting to extract the desired program from the intermediate bandwidth network. If the desired program is not already available via the intermediate bandwidth link, the pedestal signals the central office interface to retrieve the program from the broadband network. Of course, this multilayer architecture could be extended indefinitely over any number of interfaces and layers. The capacity of the intermediate bandwidth link would limit the number of different choices that could be selected simultaneously by subscribers serviced by the same pedestal. For example, the intermediate bandwidth link could carry a fixed number of NVOD channels with each subscriber able to choose one for current viewing. However, not all of the NVOD channels available via the intermediate bandwidth link are likely to be viewed simultaneously. In accordance with the invention, additional digital services may opportunistically exploit intermediate link capacity left unused by current subscriber activity. These additional services could include other broadcast and NVOD channels, VOD service, or interactive services. In accordance with the present invention, this opportunistic bandwidth scheme could also be adapted to operate in the context of a loop or bus architected access line network, such as a hybrid fiber/coaxial (HFC) network, providing digital broadcast, NVOD or other interactive services. In a HFC network a Host Digital Terminal (HDT) or Cable Headend couples a broadband optical network to one or more HFC cables each of which serves multiple subscriber units in a loop/bus architecture. The HDT or Cable Headend can be modified in accordance with the invention to incorporate an interface that passes only that digital service that has been requested by a subscriber unit as opposed to the prior art method of carrying all the streams belonging to a broadcast or NVOD offering. If bandwidth remains on the cable, additional services can opportunistically exploit it. Numerous other combinations of layers and interfaces are possible within the scope of the present invention. For example, the broadband network may be implemented as one or more satellite or MMDS (Multichannel Microwave Distribution System) feeds. A video distribution system in accordance with the present invention may also be easily extended to provide interactive services. Alternatively, information besides video information could be distributed. For example, interactive internet services may be provided over the same network that incorporates the present invention. In accordance with another aspect of the invention, statistical multiplexing is extended to optimize usage of the output bandwidth of broadcast equipment, such as NVOD servers and MPEG multiplexers. This aspect of the invention is independent of network architecture. The invention will be better understood by reference to the following detailed description in connection with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 depicts a typical pedestal-based telephone network. Fig. 2 depicts a top-level diagram of a video pedestal network in accordance with one embodiment of the present invention. Fig. 3 depicts how a desired program may be filtered from a multi-program stream in accordance with one embodiment of the present invention. Fig. 4 depicts how two desired programs may be filtered from a multi-program stream in accordance with the invention. Fig. 5 depicts data flow to the subscriber within a video pedestal in accordance with one embodiment of the present invention. Fig. 6 depicts data flow from the subscriber within a video pedestal in accordance with one embodiment of the present invention. Fig. 7 depicts a multiple-feed video pedestal architecture based on a cross-bar switch in accordance with one embodiment of the present invention. Fig. 8 depicts a two layer video pedestal network architecture in accordance with one embodiment of the present invention. Fig. 9 depicts statistical multiplexing within a four layer video pedestal network in accordance with one embodiment of the present invention. Fig. 10 depicts a video pedestal network wherein a video pedestal is adapted to incorporate input from a local server in accordance with one embodiment of the present invention. Fig. 11 depicts a modified NVOD server in accordance with one embodiment of the present invention. Fig. 12A depicts a modified NVOD server connected to the video pedestal network architecture of Fig. 8 in accordance with the invention. Fig. 12B depicts a modified NVOD server directly connected to subscriber units in accordance with the invention. Fig. 13A depicts the operation of a prior art NVOD server. Fig. 13B depicts the operation of a modified NVOD server in accordance with one embodiment of the present invention. Fig. 14 depicts an MPEG multiplexer modified in accordance with one embodiment of the present invention. DESCRIPTION OF SPECIFIC EMBODIMENTS The present invention provides a system for distributing digital information from a broadband digital information source to one or more subscriber units. An exemplary digital information distribution system in accordance with the present invention includes a plurality of digital information servers coupled to a plurality of Subscriber Terminal Units (STUs) or subscriber units via a plurality of links. A link provides a mechanism for transferring data from one entity, called the source, to another entity, called the receiver. A link is capable of transferring data in either direction or both directions simultaneously. An entity that is a receiver for data travelling along one direction in a link can be the source for data travelling along the opposite direction in the same link. Likewise, an entity that is a receiver for one link can be a source for another link. For example, if data passes down one link into a receiver and continues into another link, the same entity that was the receiver for the first link becomes the source for the second link. Multiple receivers may be coupled to a common source via the same link. Similarly, multiple sources may be coupled to a common receiver via the same link. An example of these two implementations is a bus or loop architecture. In another architecture, commonly referred to as the star architecture, one link couples only one receiver to one source (of course each entity can be either the receiver or the source or even both at one time since data may travel bi-directionally) . Digital data travels from the information servers, which in this context constitute the broadband digital information source, to the STUs via two or more links and possibly via one or more receivers/sources between the links. All receivers/sources through which data travels before reaching the STU may be referred to as intermediate interfaces or simply interfaces. The digital data provided by the information servers may be in the form of text, graphics, audio or sound, video or still image, or binary computer data. It should be noted that the STU can be viewed as a receiver or source as well as these terms have been used in the discussion above. The function of the STU or the subscriber unit is to receive the desired data from the network and convert it to a form useable by the subscriber, e.g., a video stream may be displayed on a conventional television screen, or a text stream may be printed on paper. The information servers may be general purpose media servers, video servers, real-time audio and video encoders, or any other device from which digital data may be extracted. Links may be implemented in a variety of physical media without restriction. For example, a link may be a satellite channel, an optical fiber channel, a coaxial cable, a telephone twisted pair, or a microwave channel. In accordance with the invention, the subscriber unit may select a particular stream of digital information to be received from many streams of digital information available from a source even if the available streams would not together fit within the bandwidth of the link connected to the subscriber unit. This is done by shifting the selection or tuning function away from the subscriber unit to one or more interfaces between the links or between a link and the source. The tuning function is directly controlled by the subscriber unit. The subscriber unit sends a message to a particular interface selecting a desired stream of digital information. The interface responds to the message by selecting the desired stream from a multitude of streams received at that interface from the source and relaying that stream toward the subscriber unit via one or more links. In a system with multiple links and interfaces between them, the interface to which the message is directed may not itself currently have the desired digital information stream available to it. Such an interface would respond by requesting the desired stream from another interface closer to the broadband source. The tuning function could be distributed over many interfaces in this way. The digital information distribution system of the present invention will be made clearer with reference to a particular example, the modification of a conventional public telephone network to distribute digital video services such as digital broadcasting of standard definition and High Definition Television (HDTV) programs, video-on-demand (VOD) , near-video-on-demand (NVOD) providing staggered program starting times, interactive video services (IVS) , and internet services. Fig. 1 depicts a typical pedestal-based telephone network 100. Telephone network 100 includes a series of access lines 102 connected to individual telephone terminals, a hub or pedestal 104, and a fiber trunk 106 to connect the pedestal to the central office. It should be noted that the pedestal may in fact be within the Central Office itself, i.e., it is not a remote unit. In such a situation, fiber trunk 106 is the core telephone network. If the pedestals are remote units, the connection between the Central Office and the pedestals, fiber trunk 106, could either have a star topology, or it could have a bus/loop topology. Access lines 102 provide the individual customer terminals with private and secure connections to the network. Fiber trunk 106 typically carries data at OC1/DS3 (approximately 45 Mb/s) or OC3 (approximately 155 Mb/s) rates, typically over an optical link. Access lines 102 are typically copper twisted pair , lines having a maximum length less than a mile. The function of pedestal 104 in the downstream direction is to demultiplex telephone data received from fiber trunk 106 and route it to appropriate terminals in customer premises. For upstream operation, pedestal 104 aggregates incoming data received via access lines 102, multiplexes it and sends it over trunk 106. In accordance with one embodiment of the present invention, pedestal 104 is modified so that digital video and other digital services are provided without laying of new access lines. A telephone network so-modified is herein referred to as a "Video Pedestal Network" or "VPN". A pedestal so-modified is referred to as a "Video Pedestal" or "VP". In an exemplary VPN, MPEG-2 data is delivered to the pedestal at a very high rate (e.g., OC3 or higher). Data is carried to subscriber premises via the copper line using Asymmetric Digital Subscriber Link (ADSL) technology as known to those of skill in the art. However, since the bandwidth of the copper line is much less than that of the fiber, not all the incoming data can be transmitted over the copper line. This bandwidth mismatch is resolved by requiring the pedestal to filter out all MPEG-2 packets that are irrelevant to the program selected by a subscriber unit which is itself connected to the copper access line. In other words, the pedestal sends out over each copper line only those MPEG-2 packets that are necessary to enable reception of the selected program channel. The subscriber unit receives only a single program MPEG-2 transport stream (or alteratively, a very small set of MPEG-2 transport streams) at its input. From the above it is clear that the pedestal must somehow know which packets in the incoming stream are to go down through each access line. In accordance with the invention, each subscriber unit tells the VP as to which MPEG- 2 packets it wishes to receive. This is accomplished through a special signaling protocol between the subscriber unit and the VP. The VP generally does not need to know anything about the content of either the incoming or outgoing MPEG-2 stream. It receives information from each subscriber unit as to which packets that subscriber unit wishes to receive and merely filters those packets down through the access line. As a result, the VP architecture is kept very simple. In essence, a part of the channel tuning functionality has been shifted from the subscriber unit, where it would reside for a conventional cable television system or prior art digital VOD and NVOD systems, to the network. The shifting of tuning intelligence into the VP gives the VPN significant advantages over the so-called FTTCO (Fiber-to-the-Central Office) , FTTB and other similar schemes of the prior art, wherein the subscriber unit selects from among digital video data transmitted over the twisted pair line via ADSL. In FTTCO, FTTB and other similar schemes, the subscriber unit only "sees" a small amount of MPEG-2 data, equal to whatever can be transmitted over the copper line from an optical network unit (which couples the fiber to the low bandwidth access line) . Since FTTCO, FTTB and other similar schemes in their current implementation do not perform any tuning in the network, their capabilities are limited to point-to-point services. In contrast, the VPN presents the subscriber unit with a virtual connection to the core network (fiber trunk) , thus enabling it to receive substantially more services, such as broadcast and NVOD, than would be available with FTTCO. It is stressed that the broadcast and NVOD services are in addition to whatever services existing FTTC, FTTB and other existing similar schemes provide. This is because the VPN can be architected to have the same switching capabilities as these prior art networks for providing point-to-point connections. VPN has several advantages over the HFC network architecture as well. While it is true that HFC supports broadcast and NVOD services readily, it does not effectively support point-to-point interactive services. This is due to extremely low bandwidths available for the return channel. The VPN does not suffer from such a disadvantage since it is based on a star topology. Security is also superior in the VPN. Once again, since the VPN uses a star topology for the access part of the network, each customer has a private connection to the network. This is in contrast to the HFC architecture, where each coax is shared by several hundred users in a loop architecture. As a consequence "there is much more security and privacy in the VPN. Note that in the VPN architecture the ADSL latency also adds to the channel acquisition time since part of the tuning is performed in the VP. However, the extra few tens of milliseconds does not represent a significant impact on the typical 500 milliseconds or higher number that subscriber units typically require for locking onto a new video channel. Because VPN as described with reference to Fig. 1 does not require laying of new cables or wires to every individual subscriber's premises, it is possible to launch digital broadcast and NVOD services on a large scale relatively quickly - much more quickly than for distribution systems such as FTTCO, FTTC and FTTB. The important modifications to the telephone network to implement the VPN are in the pedestals. A very significant advantage of the VPN, however, is that it seamlessly evolves to a completely interactive network once service offerings mature. By ensuring that the signaling protocol between the subscriber unit and the VPN is very general it is possible to support all the interactive services right from the beginning. This service evolution feature, combined with the relatively insignificant cost of network construction, makes the VPN architecture of the present invention extremely attractive. The architecture of the subscriber unit that connects to the VPN is no different from that of subscriber units of the prior art, except, of course, for the support required for the tuning signaling protocol between the subscriber unit and the network. As in the case of any other network, the subscriber unit connected to the VPN has a network interface module appropriate to the characteristics of the access line connecting the subscriber unit to the network, e.g., an ADSL network interface module will be required for a twisted pair copper access line. Fig. 2 depicts a top-level diagram of a VPN 200 of one embodiment of the present invention. VPN 200 includes a representative subscriber unit 202, an ADSL access network 204 having a star topology, a VP 206 s , a high bandwidth core network 208, a series of servers and/or encoders 209, and a network manager 210. VP 206 interfaces core network 208 with access network 204. Core network 208 is a high bandwidth digital network, or series of networks, that interfaces with the content servers 209. Core network 208 could be based on fiber, satellite, or microwave, or any combination of these technologies. Access network 204 is the final segment of the delivery system. To save cost access network 204 is preferably implemented on existing medium such as telephone wires (copper twisted pair) . Of course, it is not a requirement for the present invention that the access lines be copper twisted pair. For example, if no existing access lines are available as would be the case if the network provider is an entity other than a telephone company, any other physical medium that is appropriate to the needs and constraints of the network provider may be used, e.g., coaxial cable. Subscriber unit 202 interfaces with access network 204 through a network interface module (not shown) . As described above VP 206 interfaces access network 204 with core network 208. In essence, VP 206 shields subscriber unit 202 from the core network architecture, thus making it possible to use the same subscriber unit with different core delivery systems (satellite, fiber, microwave) . Of course, the part of VP 206 that interfaces with core network 208 will be dependent on the characteristics of the latter. The VP also has an interface with network manager 210 for purposes of control and monitoring. The input to VP 206 is preferably a high bandwidth MPEG-2 stream appropriately adapted to the physical medium of core network 208. For example, if core network 208 is built using optical fiber, MPEG-2 data could be carried over ATM cells using AAL5 as known to those of skill in the art. Alternatively, if core network 208 is a satellite channel, MPEG-2 data would typically be directly modulated using Quadrature-Phase Shift Keying (QPSK) , again as known to those of skill in the art. Any physical layer could be used to implement core network 208 in accordance with the present invention. VP 206 has multiple physical outputs, each of which is a twisted pair telephone line. First consider the case where only one program is required to be sent down over each twisted pair connection. Current ADSL technology limits the bandwidth of each telephone line to approximately 6-8 Mb/s, which is much narrower than the bandwidth of the core network. The principal function of VP 206 is to receive from each subscriber unit information as to which one program that subscriber unit wishes to decode, filter from the input stream the appropriate MPEG-2 packets corresponding to that program, and transmit them to the subscriber unit. The process of filtering MPEG-2 packets and transmitting them over the access network is performed without violating any MPEG-2 requirements, and without introducing significant jitter in the MPEG-2 packets. Fig. 3 depicts how a desired program may be filtered from a multi-program stream in accordance with the invention. The top portion of Fig. 3 depicts a high bandwidth MPEG-2 stream 302 coming into VP 206. This bitstream contains packets belonging to many different programs including a single program whose packets are denoted by slant-hatched lines. The bottom portion shows a stream 304 of filtered packets belonging to this single program leaving VP 206. The output rate is much lower than the input rate which results in "stretching" of the packets on the time line (since the packets in the input and output streams are all of the same size, i.e., 188 bytes as specified by MPEG-2). Note that the order of packets in the filtered program is not altered. In reality, packets of disparate origin must be carried over the access line at the same time. For example, in one embodiment that is compliant with the European Digital Video Broadcasting (DVB) standard, both program data and Service Information (SI) (which carries the electronic program guide) data-will typically be simultaneously delivered to the subscriber unit as known to those of skill in the art. Furthermore, with future advances"in ADSL technology, it will become feasible to increase the capacity of the twisted pair medium to the point that it will become possible to support multiple video programs over a single telephone line. For example, with 25 Mb/s capacity it should be possible to carry 5 or 6 services. This can be used to serve multiple subscriber units within one home simultaneously over a single twisted pair access line. Alternatively, if the access line is implemented using a coaxial cable whose bandwidth is higher than that of twisted pair, it may be required to carry more than one service over this line in order to support multiple subscriber units in the home. Fig. 4 depicts filtering of MPEG-2 packets belonging to two different programs. The top portion of Fig. 4 depicts a high data rate MPEG-2 packet stream 402 entering VP 206. The bottom portion of Fig. 4 depicts a stream 404 carrying packets belonging to two different programs. In Fig. 4, one program (denoted by shaded packets) has a rate equal to one eighth of the total incoming rate, while the other program (denoted by slant-hatched packets) has a rate equal to one fourth of the total rate. These ratios are of course only examples. The outgoing dual-program rate equals the sum of the rates of the two filtered programs, i.e., it is 3/8 times the incoming rate. The packets in the outgoing stream are in the same order as they were in the incoming stream. By ensuring that the outgoing rate is exactly equal to the sum of all the filtered program rates, it is possible to ensure that the received packet time stamp values in the subscriber unit are accurate. Fig. 5 depicts data flow to the subscriber within VP 206 in accordance with one embodiment of the present invention. VP 206 includes a network adapter 502, and then for each access line, an ADSL modulator 504, and a line buffer 506. There is also an MPEG-2 Packet ID (PID) Lookup Table (PLUT) 508 and filter 510 for each outgoing access line. The concept of PID is well known to those of skill in the art. PLUT 508 contains a list of all PIDs that are to be transmitted over the access line and is constructed based on information received from the subscriber unit. Using PLUT 508, PID Filter 510 selects the appropriate PIDs from the multitude of PIDs contained in the entering stream and sends them to line buffer 506, which is a simple FIFO. Line buffer 506 is allowed to fill up until it is half-full before packets are removed. The rate at which buffer 506 is emptied equals the sum of the rates of all the programs being transmitted over the access line. For example, if 2 programs are filtered, with rates of 3 and 6 Mb/s respectively, line buffer 506 is emptied at a rate of 9 Mb/s. The access line data rate is preferably adjustable, in real time, up to its maximum value. If the access line data rate is fixed and greater than the rate of the outgoing MPEG-2 stream then it may be necessary to appropriately "pad" the outgoing stream with "null" or "dummy" packets to fill up the total available bandwidth. A side benefit is that VP 206 can also be used as a "de-jittering" device, i.e., it can be used to eliminate any jitter introduced by core network 204. This eliminates the need for an extra buffer within the subscriber unit and also improves the timing recovery performance of the MPEG-2 decoder in the subscriber unit. The de-jittering is done in line buffer 506. Its size therefore depends on the extent of jitter in the incoming stream. If there is no jitter in the incoming bitstream, then this buffer need only be a few packets deep. However, if core network 204 does introduce jitter (e.g., if it is a switched ATM network), then the size of line buffer 506 can be set equal to twice the maximum jitter times the maximum rate on the access line. For example, assume that the core network can introduce a maximum delay of +/- 2 milliseconds. If the maximum data rate on the ADSL access line is 20 Mb/s, line buffer 506 can be specified to have a size of 2*2(milliseconds)*20(Mb/s) = 80,000 bits. This scheme assumes that VP 206 is aware of the data rate of the outgoing streams on each access line 204. This is ensured by having the subscriber units convey the stream rate to the VP, .in addition to conveying the MPEG-2 PID information. The subscriber units themselves know the rates from the Program Specific Information (PSI) stream that is part of the MPEG-2 data stream, as will be easily understood by those of skill in the art. The "rate descriptor" in the PSI stream is specified by MPEG-2 to be an optional field. However, it is clear from the above statement that in this embodiment, the rate descriptor is mandatory in the PSI. In the case of MPEG-2 services that are of variable rate, the maximum possible rate of the service is carried in the rate descriptor field of the PSI. Such services are relayed by the VP to the subscriber unit at their maximum rate values. To cover the most general case, the subscriber unit signals not only the PID and rate information to the VP, but also all other information that uniquely identifies the required stream in the core network. For example, in one embodiment that is compliant with the European Standard for Digital Video Broadcasting (DVB) , certain fields in the Service Information (SI) data are available, such as "original_network_ID", "transport_stream_ID" and "service_ID" that uniquely identify a particular service stream in a multitude of data streams. In such a system, the subscriber unit could communicate all this information to the VP in order to uniquely identify the digital stream. PID Filter 510 in Figure 5 would then be a more sophisticated filter since it must process all the additional identifier fields in addition to the MPEG-2 PID values. The SI data stream is of course only one example of how program selection data could be distributed to subscriber units. Other protocols could be employed using the core and/or access networks. Alternatively, program selection data could be distributed via another medium such as floppy disk or CD-ROM. There is however data that VP 206 cannot get from the subscriber units and must instead obtain from network manager 210. Alternatively, this data may be "hard-wired" into the VP if it can be ensured at start of deployment that the data will never or seldom change in the future. As an example, in a network following the DVB standard, this data includes the list of PIDs that comprise the SI data and the stream rate of the SI data. This information is required to enable VP 206 to provide the SI stream to any subscriber unit whenever the latter requests it. The SI stream may be treated as simply another program and sent over the access line whenever any subscriber unit requests it. Alternatively, the SI stream may be transparently routed over all the access lines. This way, any change in the SI data is automatically available to the subscriber units. This also simplifies the architecture of VP 206 since it need not monitor and determine when the SI stream has " been updated and therefore needs to be routed to the subscriber units. The disadvantage of this scheme is that some bandwidth over the access lines will always be consumed by the SI data. This is also however a drawback to the prior art video distribution schemes. The SI data may be structured so as to minimize bandwidth usage. Network adaptor 502 recovers MPEG-2 packets from the core network delivery medium. For example, if core network 208 is based on ATM, network adaptor 502 would perform the appropriate ATM Segmentation and Re-assembly (SAR) functions. Alternatively, if the core network were a satellite channel, network adaptor 502 would include the appropriate channel decoder, e.g. a QPSK demodulator and Forward Error Correction (FEC) decoder. As discussed above, VP 206 requires rate and MPEG-2 PID values for the filtered program from the subscriber units. This is accomplished by sending the information in the control plane via the return channel on the access line as will be understood by those of skill in the art. However, the same return channel may carry signaling data for a service provider further upstream than VP 206. For example, in a VOD appli- cation a subscriber unit may send a so-called VCR "trick mode" (pause/fast forward/etc.) command to the server. VP 206 recognizes which data in the upstream channel is addressed to it, and which is to be sent upstream. This function is accomplished within the signaling protocol used between VP 206 and the subscriber units. Fig. 6 depicts return channel data flow within video pedestal 206 in accordance with one embodiment of the present invention. Each return channel within video pedestal 206 includes a check address block 602 coupled to a PID filter setup unit 604 that controls the contents of PID LUT 508. Return signal data intended for VP 206 is identified within check address block 602 and forwarded to setup unit 604 so that PID LUT 508 required for filtering of incoming MPEG-2 packets may be updated appropriately. Return signal data intended for upstream components is instead directed to an upstream multiplexer 606 that aggregates all the signaling packets to be sent upstream. A program selection monitor 608 is optionally provided to log subscriber selections for billing and/or survey purposes. At power-up, subscriber unit 202 requests VP 206 to send SI data (if the SI data is always present on the access line, this step is omitted) . Subscriber unit 202 then informs VP 206 as to which MPEG-2 PIDs to filter in order to receive a specific service. It will be appreciated that numerous variations of and extensions to the basic VPN architecture are possible within the scope of the present invention. VPN implementation details will depend on a number of factors including settlement patterns and the nature of the existing infrastructure and communication protocols. One simple topological variation in the basic VPN scheme is obtained by placing the VP at the telephone company Central Office (CO) , instead of at a hub in each neighborhood. An advantage of this configuration is lower cost since core network 208 is now terminated at the CO. Access network 204 is still twisted pair based on ADSL, however, the average length of each access line is much longer, of the order of 2 or 3 miles. One disadvantage is that current ADSL technology severely limits bandwidth available on telephone cables of these lengths. This configuration works very well in conjunction with so-called "scalable" ADSL modems as are envisioned. A scalable modem can operate over variable distances and support different bandwidths. For example, over a 2 or 3 mile distance, the supported bandwidth'may be limited to 8 or 9 Mb/s; however, as the line length is dropped, the bandwidth increases. Thus, for example, at a distance of 3000 feet or less, the maximum bandwidth would be 25 Mb/s or higher with scalable modems. The service can be started using the VP in the CO and with limited bandwidth to the subscriber unit. However, as services proliferate, the VP can be moved closer to subscriber premises thus increasing the available bandwidth, but without any modifications to the existing subscriber units themselves. Another group of variations on the basic VPN architecture involves interfacing multiple core networks to VP 206. In one alternative embodiment, multiple OC3 (155 Mb/sec) or OC12 trunk lines (approximately 622 Mb/sec) are fed to the same VP 206. VP 206 is then configured to filter any program from any of these incoming feeds. With this type of configuration, VP 206 can potentially have more available bandwidth than an entire coaxial cable used in the HFC system. Another similar configuration employs multiple satellite feeds to VP 206. This configuration is particularly useful in the context of apartment buildings with multiple satellite receiver antennas, each tuned to a different satellite. The sum total of all available transponder bandwidths could easily be of the order of 1000 Mb/s, with each satellite transponder carrying a bandwidth between 30 and 80 Mb/s. In the multi-satellite feed case, VP 206 includes a network adaptor for each incoming transponder signal. There may be feeds from as many as, e.g., 50 different transponders each with an average bandwidth of 30 Mb/s. If it is desirable to limit the number of feeds coming into VP 206, the MPEG-2 streams from individual transponders can be combined within an MPEG-2 re-multiplexer prior to input to VP 206. Fig. 7 depicts a multiple-feed VP architecture based , on a cross-bar switch in accordance with one embodiment of the present invention. A multi-feed VP 702 is implemented using a simple crossbar switched connection. The inputs to multi-feed VP 702 are a set of input feeds 704 corresponding to, e.g., input trunk lines and/or satellite receiver outputs. The outputs of multi-feed VP 702 are a set of access lines 706. Each access line 706 receives its data from a filter/modulator 708 which performs the functions discussed in reference to Fig. 5. A series of switches 710 connects each input feed 704 to an access line 706. Only one of the switches 710 is on for any access line 706 at any time. The corresponding MPEG-2 stream is filtered and delivered to the output filter/modulator 708. As previously described, the subscriber unit to VP signaling protocol is expanded accordingly to implement "feed-selection" in VP 702. The subscriber unit additionally informs VP 702 as to which input feed to connect to. It does this, for example in the context of a DVB compliant network, by providing VP 702 with the complete set of identification parameters for the required MPEG-2 stream ("original_network_ID", "transport_ID", and "service_ID") . The subscriber unit receives this information as a part of network information within the SI data as is known to those of skill in the art. VP 702 interprets this information to determine which input feed 704 to select for the specific subscriber unit. Again, as mentioned previously, the mapping between the information provided by the subscriber unit and the input feed may be static, i.e. hard-wired in VP 702, or it may be under the control of a network manager which can change it dynamically. Another variation concerns filtering of desired program material which need not be limited to the MPEG-2 layer based on the unique identifier values of MPEG-2 packets. The concept of extending the tuning upstream into the network may be implemented in other ways as well. For example, in one embodiment in which the MPEG-2 packets are transported to the subscriber unit over ATM cells, the filtering of digital data in the pedestal may be performed by selecting the appropriate ATM cells. Since ATM cells are uniquely identifiable by their Virtual Path Identifier (VPI) and Virtual Connection Identifier (VCI) values, the VP merely selects the ATM cells with the appropriate VPI/VCI values from the incoming multitude of ATM cells. In this case the subscriber unit has to signal the correct values of the VPI/VCI instead of MPEG-2 stream identifier values to the VP. Alternatively, the VP may have the intelligence to map the MPEG-2 stream identifiers values received from the subscriber unit to the correct VPI/VCI values. In either case, the task is simplified if each MPEG- 2 service has a unique VPI/VCI value associated with it. For example, in one embodiment which is a DVB compliant system, the Service Information data standardized by DVB is further extended to also include VPI/VCI values for each service. This information is received by the subscriber unit and conveyed to the VP. If it is not possible to ensure that each MPEG-2 service has a unique VPI/VCI then the structure of the VP becomes more complex since it now has to monitor the MPEG-2 PID values buried inside the incoming ATM cells to determine which cells to relay to the subscriber unit. This may be even more complicated by the fact that MPEG-2 packets belonging to different services may be packed together inside the same ATM cell. The advantage of such an embodiment, of course, is that the entire MPEG-2 layer of the transport protocol is independent of the network architecture. For example, this makes the task of generating Service Information independent of network architecture. Other embodiments that do not rely on MPEG-2 can also take advantage of this invention, as can be appreciated by those of skill in the art. Exemplary embodiments of the present invention include the appropriate network architecture with adequate signaling protocol support, as described above, a mechanism to uniquely identify each digital stream available from the source, and a mechanism to convey the stream identifier information to the entity that is desirous of selecting any specific stream from the network or the source. In the specific case of a DVB compliant MPEG-2 based system, the stream identifier information is carried in the SI stream and is made available to all entities that wish to select a specific digital stream from the network. A non-MPEG-2 system could use other means to convey the same information that is contained in the SI stream of a DVB system, for example, through a floppy disk. The basic concept of shifting tuning into the network in accordance with the present invention may be extended to implement a hierarchical VPN architecture, with multiple VP's regulating the flow of data through the network. Any component such as a VP or a subscriber unit may transmit a signal upstream, i.e., toward the broadband digital information source, to request a specific digital service from amongst the multitude of digital services that are available in the network. Of course, it is obvious from the discussion so far that all such components should have the ability to uniquely identify the digital stream they want to receive. In one embodiment such information is transmitted in the downstream direction, i.e., away from the broadband digital information source, by the server or other entity like network manager. Specifically, the SI stream in a DVB compliant architecture carries this information. Thus, all components that are desirous of selecting streams from a location upstream in the network should be capable of decoding and interpreting the SI stream. In the simplest implementation, a signal requesting a specific digital service may be allowed to travel upstream along all available paths until it reaches the servers wherein it is terminated. All VPs traversed by the signal set their stream filters to pass the desired stream downstream. Of course, if any VP already has its filters appropriately set, it need not respond explicitly to the message. One potential disadvantage of such an implementation is the possibility of "flooding" in the upstream channel since all signals are blindly broadcast along all available upstream paths. This problem is eliminated by building additional , intelligence into the VPs as follows. Each VP determines if it is already -passing the requested stream through to the output. If yes, it terminates the signal. If no, the VP sets its stream filters appropriately and then determines along which upstream path amongst its multiple input feeds should the signal be propagated and sends the signal along that path. Determination of the suitable upstream path is done by maintaining a list of identifiers for all streams that are available in each input feed and comparing this list with the stream identifier information in the received signal. For example, in a DVB compliant architecture, the list may contain the "original_network_ID's" of all the networks that are connected to a particular input feed. The "original_network_ID" of the requested stream is then matched with the appropriate input feed. In more complex networks additional information like "transport_ID" and "service_Id" may also be required to be matched. The list of stream identifiers for each input feed may be hard-wired into the VP if it known before hand that they will not be altered. Alternatively, an external entity like a network manager may update the list dynamically. In accordance with the invention, bandwidth available within a hierarchical VPN may be opportunistically exploited by supplementary services, video or otherwise by employing "statistical multiplexing" as herein described. Fig. 8 depicts an exemplary two layer VPN 800 suitable for implementing statistical multiplexing in accordance with one embodiment of the present invention. VPN 800 includes a core VP 802, an access VP 804, a representative subscriber unit 806. Feeds 808 are the input to core VP 802. An OC48 line 810 interconnects core VP 802 and access VP 804. Consider the use of VPN 800 to implement an NVOD service. For the purposes of this example it is assumed that OC48 line 810 can deliver approximately 480 channels of video programming. (The actual number of channels that can be delivered over any band-limited network is a function of the data rate allocated to each channel as well as the total capacity of the network.) This is sufficient for an NVOD , service consisting of 20 movies, each of 2 hours duration, and with a stagger interval of 5 minutes. Any additional service would require more capacity on the connection between the core VP 802 and access VP 804. Statistically speaking, it is improbable that every one of the 480 available channels will be viewed by some subscriber served by access VP 804 at all times. At any one moment, some of the channels would not have any subscriber tuned to them, and their bandwidth would essentially be wasted. In accordance with the statistical multiplexing feature of the present invention, these channels could instead be used to provide additional services. For example, if a particular user did not want to watch any of the movies on the NVOD service, he/she could be offered some other service (e.g., interactive home shopping or VOD or even a program from another broadcast/ NVOD service) on one of the NVOD channels not currently being viewed by anybody. These "opportunistic" services directly contribute to the revenue stream of the service provider. Since the underlying model here is statistical, there will be occasions when situations arise so that statistical multiplexing could result in degradation of service quality. For example, consider the case when a user decides to take a break from watching a movie knowing that she/he can return to the same point in the movie by tuning to another channel which has the same movie delayed by the appropriate time. In the meantime, however, there is a possibility that particular channel was not transmitted since nobody wanted to watch it, and its slot in the network is occupied by an opportunistic service. In the worst case, all such "freed" NVOD slots could have been grabbed by opportunistic services with the result that this particular user would not be able to receive her/his movie at the end of her/his break. Preferably, an appropriate scheduling and policing mechanism ameliorates this problem. In one embodiment, the , VPN ensures that there is always a minimum number of free channels at any time and does not offer all the available channels to opportunistic services. Referring again to Fig." 8, the network(s) to the left of core VP 802 carry very high bandwidth data. There may be one or more core networks feeding core VP. All the NVOD and other broadcast channels are available at the input to core VP 802. The SI data for the NVOD and broadcast service is always routed through to the subscriber units. Using the received SI data (which contains the Electronic Program Guide - EPG) , each user selects a desired channel for viewing. As in the basic VPN architecture depicted in Fig. 3, the appropriate information regarding the selected channel (stream ID, rate and PID list, etc.) is signaled from the subscriber unit to access VP 804 to enable the latter to appropriately tune its filters and route the program through to the user. In the absence of statistical multiplexing, this is all the signaling required between the subscriber unit and the network. Since all the broadcast and NVOD channels are available at the input of access VP 804, the latter appropriately sets its filters and subscriber unit 806 receives the selected program. However, when statistical multiplexing is employed, the requested program may or may not be available at the input to access VP 804. If it is available at the input to access VP 804 (which would be the case if some other user had already requested that program) , then the access VP needs only set the filters appropriately and provide the requested program to the user. However, if the program is not available at the input to access VP 804 (which would be the case if no other user had requested that program until that point, or if all the sessions with that program had been terminated (e.g., the movie had ended) and hence that channel had been offered to other opportunistic services), then access VP 804, in addition to setting its filters appropriately, also bounces the request to core VP 802. Core VP 802 then adjusts its filters to select the requested program and multiplexes it down the connection to access VP 804. In one embodiment, for simplicity of implementation, the request is always bounced from access VP 804 to core VP 802, regardless of whether or not the program is already being filtered by the latter for transmission to access VP 804. This minimizes the complexity of the software in access VP 804, and reduces the latency in acquiring the channel. Of course, core VP 802 must then check if it needs to respond to the received signal. The general nature of this signaling protocol (i.e., when it is terminated and when it is propagated upstream) has been described above. Core VP 802 maintains a list of which channels are currently being used and which are not. The list of channels not being used is the pool of available slots for the opportunistic services. These lists are dynamically maintained based on information received from the subscriber units. In an alternative embodiment, the concept of statistical multiplexing is generalized to a network with more than two layers of VPs. Fig. 9 depicts statistical multiplexing within a four layer VPN 900 in accordance with one embodiment of the present invention. VPN 900 includes a service provider 902, a subscriber unit 904 and four VPs 906, 908, 910, and 912 therebetween. Multiple input and output connections are shown for each VP. In the configuration of Fig. 9, a request from subscriber unit 904 for a particular program bounces back sequentially from right to left until it reaches a VP which is already filtering the requested program through to its output. Each VP to the right of this VP, i.e., downstream from it, adjusts its filters appropriately and once this process is complete, the subscriber unit begins receiving the requested program. The VPs in the network can be understood to be "pseudo-servers" for the NVOD services. When a user requests a program, the signal traverses upstream, i.e., toward the broadband digital information source, until it reaches the VP where the program is available. That VP then is the "pseudo-server" for that request. Note that the pseudo-server , for the next request for the same program from another user could be a -different VP, depending on the location of that user in the network. The network designer must consider the effects of latency as the number f VPs increases in the chain. In accordance with the invention, statistical multiplexing may be applied to introduce local programming at a neighborhood VP. Fig. 10 depicts a VPN 1000 wherein a core VP 1002 is adapted to incorporate input from a local server 1004 in accordance with one embodiment of the present invention. VPN 1000 includes core VP 1002, an access VP 1006, a representative subscriber unit 1008, an OC48 interconnection 1010 between core VP 1002 and access VP 1006, and access lines 1012 representing the outputs of access VP 1006. Core VP 1002 includes multiple feeds 1014 including a feed from a local server 1004. Server 1004 could be used, for example, to opportunistically inject programming and/or commercials relevant to a neighborhood or community. Any subscriber unit in the neighborhood can access this server. In accordance with the invention, statistical multiplexing may also be applied in the context of a shared physical medium, e.g., a loop or bus architected access network such as an HFC network. This configuration is particularly useful to the operators of cable TV systems or others who already have such a network in place. Every subscriber unit connected to the loop access network potentially has access to all the data being transmitted. Typically, encryption is used to protect data from unauthorized use. The prior art scheme for providing NVOD services over HFC is to simply broadcast all the programming over the cable, consuming as much bandwidth as is required but limited, of course, by the total available bandwidth. Each subscriber unit can access all these programs, however it can decode only what it is authorized to. By introducing a VP at the headend (which is the source of all the services in the HFC cable) , access to the data transmitted over the HFC is restricted to only those services that have been requested by the subscriber , units. Services that have not been requested are not transmitted on the cable. In accordance with the invention, capacity left unused due to unrequested services can then be used for opportunistic services, 'including accessing programs from a local server in the headend. In accordance with the present invention, standard HFC spectrum allocations may be modified to minimize latency when selecting programming. Typical HFC implementations allocate only a small portion of the spectrum (of the order of 25 MHz near the low end) for upstream signaling from the subscriber units. Due to the limited bandwidth here, which is shared between all the users, the signaling latency is typically very large, and would cause delays in selecting programming. In accordance with the invention, the upstream signaling bandwidth is increased to reduce this latency. Since statistical multiplexing increases the efficiency of the downstream allocation, the spectrum reallocated for upstream signaling does not have an appreciable impact. In an alternative embodiment of the present invention, the VP in an HFC environment implements a logical star topology over the existing physical loop topology. Assuming that there are at least as many channels (herein a channel is defined as the minimum network capacity that is needed for delivering any one of the total available services) available on the cable as there are subscriber units connected to it, a specific channel is dedicated to each subscriber unit, and the subscriber unit tunes this channel to the appropriate service available at the VP in the headend. This architecture is structurally similar to the telephone network- based VPN discussed in reference to Figs. 2-8. As compared to the telephone network, however, the cable implementation has restricted capacity. In a telephone network VPN used for broadcast and/or NVOD, any number of subscriber units may be connected to a VP and offered services without any drop in quality as long as the trunk capacity feeding the VP is sufficient to carry all the programming. Of course, the size of the VP will increase with the number of users. However, for the logical star HFC architecture, once , the number of subscriber units connected to the cable equals the cable capacity, i.e., the number of channels available over the cable, the network cannot add any more users, even with the same NVOD service offerings. The logical star HFC architecture, is thus most useful in the context of high bandwidth implementations which rely heavily on fiber optics for distribution. In accordance with one embodiment of the present invention, statistical multiplexing is extended to optimizing use of the output bandwidth of an NVOD server. The output bandwidth that is made available by this optimization then becomes available for other services. For the purposes of this discussion, an NVOD server is a device that stores digital information in the form of multiple digital information streams or content streams. A content stream is a stream of data such as video programming and/or other data such as computer software, audio data, stock market quotes, etc. Since the content streams are not restricted to video, an NVOD server may be understood to be a digital information stream server. A given content stream is transmitted over multiple channels such that the start time for the content stream on each channel is staggered relative to other channels by a predetermined amount of time. A subscriber requesting a given content stream must wait until the next such starting time and tune to the channel having that starting time for the requested content stream. Fig. 11 depicts a simplified representation of an NVOD server 1100 modified in accordance with the invention. As will be explained below, an NVOD server modified in this fashion will be referred to herein as an on-demand NVOD server or OD-NVOD server. OD-NVOD server 1100 incorporates NVOD program material storage 1102, opportunistic service material storage 1104, a scheduler 1106 which implements scheduling functions described below, a network adaptation unit 1108 which actually transmits the correct program material on the requested channel, and a request processor 1110 which handles requests from subscriber units or downstream VPs. An input 1112 is also provided to receive external sources of , opportunistic services. The structure depicted in Fig. 11 is illustrative only and the functional units depicted could be implemented in either hardware or software. Fig. 12A depicts OD-NVtiD server 1100 coupled to the hierarchical VPN of Fig. 8. Statistical multiplexing may be applied to the output of an NVOD server whether it is used with a VPN or is coupled to subscriber units in some other way. Fig. 12B depicts NVOD server 1100 coupled to a series of subscriber units 1204 by a cable or HFC distribution system 1206 without use of a VPN. A very low bandwidth return channel is provided to return tuning information from the subscriber units. Fig. 13A depicts the operation of a prior art NVOD server. Prior art NVOD servers store a number of content streams and transmit them at staggered start times. For example, a movie of 2 hours duration may be streamed out on 24 channels with a five minute staggered interval between the start times on different channels. The prior art NVOD server incorporates a scheduler which automatically commences transmission of the movie at an appropriate time. This way a user interested in the movie has the flexibility to watch the movie at his/her convenience. In accordance with the invention, NVOD server 1100 may be modified to incorporate statistical multiplexing capabilities to optimize usage of its output bandwidth. If 24 channels are broadcasting the same movie at staggered intervals, it is likely that at least some of these channels are wasted since no user is tuned toward them. In accordance with the invention, instead of simply streaming out the same content data on multiple channels, the intelligent NVOD server now streams out the content on only as many channels as have been demanded. The remaining channels allocated to that content are then available for offering opportunistic services. The information for these services may either be stored internally to the server or be available externally. An NVOD server modified in this fashion is thus referred to herein as an "on-demand" NVOD server, or OD-NVOD server. Software-based scheduler 1106 maintains relevant schedule information for each output channel. However, the OD-NVOD server does not control its output in response to the schedule alone. Instead, the OD-NVOD server is responsive to requests for programming as received by request processor 1110. A video program is not transmitted at the scheduled time unless there is a specific request for it. This request could originate from a subscriber (i.e., the channel carrying the video program is selected) or from a network operator that processes subscriber requests. In the VPN context, the request may be received via one or more VPs. Often, a content stream will be requested after its scheduled start time. The OD-NVOD preferably handles such a request by initiating transmission of the program not at the beginning of the program, but at the point at which the program would have been had the program started at its scheduled start time. Of course, the exact point in the program at which the request is received may be difficult to determine precisely and thus may be approximated. Additionally, it may be advantageous to begin streaming the data from a convenient MPEG synchronization point close to the requested point to enable rapid decoder synchronization. Fig. 13B depicts the operation of the NVOD server in response to user requests for a particular program. Consider the activity on channel 3. The scheduled start time for this channel is t = 10 minutes. However, if no subscriber has requested this channel, the OD-NVOD will not begin streaming data at t = 10 minutes. Assume now that a subscriber tunes to this channel at t = 24 minutes. Since the program is not currently being transmitted by the NVOD server, the request for the program bounces back from the subscriber to the server through the VPN. The OD-NVOD server upon receiving the request begins transmitting the program on channel 3. However, transmission of the program commences not at the beginning but rather at a point 14 minutes into the program (this is the point where the program would have been at the time the program is placed on the output (i.e., 24 minutes) had it indeed started playing at t = 10 minutes. Note that for the purposes of this discussion, the , term "channel" simply refers to an allocated portion of the NVOD server output capacity or network transmission capacity necessary to transmit a single video program in real time. Thus, a channel could represent an MPEG packet stream, an ATM virtual connection, or a spectral allocation. For example, if the output interface of the server is ATM-based, each channel is mapped to a specific VPI/VCI at the ATM layer. The VPI/VCI fields in the ATM cells are used to differentiate this channel from any other channel when transmitted over a common network. The actual VPI/VCI mapped to a particular channel may change dynamically, and it is the responsibility of some other entity, to ensure that the settop units are updated properly to allow tuning. In fact, it may well be the case that the same channel is mapped to different VPI/VCI values before the movie ends. For example, referring again to Fig. 13B, channel 4 may initially be mapped to a specific VPI/VCI value, however, when its transmission is resumed at approximately t = 112 minutes after being turned-off at t = 87 minutes, it may be assigned a different VPI/VCI. This may happen because the original VPI/VCI assigned was assigned to another service during the interval when this channel was turned off. Those of skill in the art will appreciate that some latency will be introduced between when the request is received and when transmission of the program is initiated. To simplify the discussion, this latency is ignored here. Further efficiency in bandwidth usage is obtained by ceasing transmission of programs once they are no longer being viewed by any subscriber. Referring again to Fig. 13B, at t=85 minutes, the OD-NVOD server learns that the last user has tuned off channel 4 and responds by ceasing transmission of channel 4, again making this channel available for opportunistic services. For a network as in Fig. 12B where the OD-NVOD is in direct communication with the settop units, request processor 1110 maintains for each channel a list of settop units that have requested the program transmitted over that channel. When a settop unit ceases to monitor a program (or is powered off) , it sends a cancellation request to the OD-NVOD server. , The OD-NVOD server responds to the cancellation request by deleting that settop unit from the list of requesters. Once the list of requesters for a particular channel is empty, the OD-NVOD server ceases transmission on that channel. The OD- NVOD server may check if the list is empty either periodically or whenever a cancellation request is received. In a hierarchical VPN, the process of turning off a channel is somewhat more complex. Each VP determines if a specific channel available at its input is also being passed through downstream, i.e., if it is being filtered by that VP. If a particular channel is not being filtered it implies that no subscriber downstream from that VP is tuned to that channel, and consequently that channel is not required at the input of this VP. The VP then signals to the next VP in the upstream direction from which it is receiving that channel to halt transmission of that stream. (The process of determining which VP is the source of a particular stream is performed by matching various stream identifier fields, e.g., "original_network_ID" in the case of a DVB compliant system, as described above.) The upstream VP responds to the signal by halting further transmission of the channel, and consequently, the portion of the bandwidth between the two VPs that was previously occupied by the channel is now available to other opportunistic services. Each VP in the network, beginning with the VPs which are directly connected to the settop units, repeats the above described process of determining whether any channel is not currently in use, and if so signaling a cancellation request to the next upstream VP to halt transmission of that channel. This checking process may be prompted by the receipt of a cancellation request from a downstream VP or STU or may occur periodically. The process stops at the VP which finds at least one user downstream from it that is monitoring the channel, i.e., is tuned to the channel. Ultimately, if no subscriber is tuned to a particular channel, the process bounces through all the VPs to the OD-NVOD server which turns off that channel. To implement this turn-off function, the OD-NVOD server maintains a list of VPs to which it is coupled which have requested a particular channel. When a VP sends the OD- NVOD server a cancellation request, the OD-NVOD server deletes the VP from the requester list. Once the requester list for a particular channel is empty, the OD-NVOD server ceases transmission on that channel. The OD-NVOD server may check the list to see if it is empty upon receipt of a cancellation request or periodically. Preferably, the VP or OD-NVOD server waits a pre-determined length of time after the last user has tuned off from a channel before concluding that no subscriber is utilizing that channel. This wait is useful to avoid false alarms due to accidental channel changes or so-called "channel surfing." OD-NVOD output bandwidth that is left free because no subscriber requires it, is available for opportunistic services. Many such services are possible including promotional material, digital information requested by a particular subscriber, and other programming. This programming material may be stored within the OD-NVOD server or be available to it. In one embodiment, there are two classes of subscribers, privileged and non-privileged. Accordingly, OD- NVOD server 1100 maintains in conjunction with scheduler 1106 a database that indicates which subscribers are privileged and which subscribers are non-privileged. In an alternative embodiment, the database may be maintained by an entity external to the OD-NVOD server, e.g., a Subscriber Management System - or SMS - as is known to those of skill in the art. Scheduler 1106 honors requests for NVOD material only when they originate with privileged subscribers. Opportunistic programming material is available to all subscribers but is of course only available on a particular channel when NVOD material is not currently being transmitted to fulfill a request by a privileged subscriber. Alternatively, in the hierarchical VPN case, one or more VPs in the chain between the subscriber and the OD-NVOD server perform the function of rejecting NVOD program material requests from non-privileged ■. subscribers. The advantages of the above-described digital video distribution scheme incorporating statistical multiplexing of OD-NVOD output along with differentiation of two classes of subscribers will be made apparent with reference to two specific examples of revenue generating services. In both examples, the privileged subscribers have paid for access to NVOD program materials, e.g., movies. In the first example, if no privileged subscriber is tuned to a particular NVOD channel, the OD-NVOD server will not stream out the movie designated for this channel. Instead, the OD-NVOD server will transmit other data that is freely available to all customers as available from opportunistic program storage 1104 or input 1112. For example, a stream containing promotional material could be broadcast on this channel. In this case, when a non-privileged subscriber tunes to the channel, a request for the promotional material will be automatically generated and he or she will see the promotional material instead of a blank screen as would be provided in prior art systems. The economic benefit to the service provider is clear since channels that would typically not be available to a non-privileged customer can now be used to generate revenues by providing him/her with promotional material. Of course, as soon as any privileged subscriber tunes to this channel, the OD-NVOD server receives a request and switches from the promotional content to the movie stream. The non-privileged subscriber then loses her/his picture and may instead see a blank screen. Thus, the OD-NVOD server can be used more profitably than prior art NVOD servers. In the second example, the non-privileged subscriber may take advantage of unused NVOD channels to download content, e.g., computer data, from the OD-NVOD server as available from opportunistic program storage 1104 or input 1112. Of course, there is the possibility that if a privileged subscriber tunes to the channel before all the data has been downloaded, the latter may lose her/his data (this is because the NVOD material has higher priority) . The advantage of this scheme over the scheme of the first example is that a non-privileged subscriber need not watch the promotional material broadcast by the service provider, but can instead receive services of her/his choice for the duration the channel is available. Of course, in such a situation, it is possible that one non-privileged subscriber may monopolize usage of the opportunistic channel. In an alternative embodiment, there are different levels of priority among non-privileged subscribers requesting opportunistic services, and a request from a higher priority subscriber displaces one from a lower priority subscriber. The techniques described above for optimizing usage of NVOD server output bandwidth may be further generalized to other broadcast equipment such as an MPEG multiplexer as would typically be installed at a cable headend. Fig. 14 illustrates an MPEG multiplexer 1400 configured in accordance with the invention. Multiplexer 1400 is connected to a scheduler 1402 and a request processor 1406 as well as a network adaptation unit 1408. Multiplexer 1400 receives as input several movies and at least one commercial stream as well as a stream containing other services and combines them into a multiplexed stream to transmit to network adaptation unit 1408 for relaying to subscribers. In the prior art, a scheduler controlling the multiplexer would periodically remove one of the movie feeds from the multiplexed stream and substitute the commercial stream. Alternatively, this could occur under external control. At the appropriate time, the movie is resubstituted for the commercial stream. In accordance with the invention, statistical multiplexing is applied to enable "on-demand" transmission of content. As before, scheduler 1402 may maintain an internal database indicating when the commercial stream is to be substituted for a particular movie feed. However, scheduler 1402 does not insert a movie feed in the multiplexed stream unless request processor 1406 receives at least one request from a subscriber unit for that particular feed. Instead a commercial stream is inserted instead. Once a request is received, the movie stream is switched in using the MPEG PID previously- used by the commercial stream. Alternatively, some other service requested by a user could be inserted instead. In one embodiment, only privileged subscriber units are entitled to request and receive the movie feed but all subscriber units may receive the commercial stream. While the above is a complete description of the preferred embodiments of the invention, various alternatives, modifications, and equivalents may be used. It should be evident that the present invention is equally applicable by making appropriate modifications to the embodiments described above. Therefore, the above description should not be taken as limiting the scope of the invention which is defined by the metes and bounds of the appended claims.

Claims: WHAT IS CLAIMED IS; 1. A system for supplying desired digital information to a receiver comprising: a broadband digital information source; a series of three or more links between said broadband digital information source and said receiver, wherein digital information supplied from said broadband digital information source travels via each link in said series to said receiver; a first interface between a first link of said series coupled to said receiver, and a second link of said series; a second interface between said second link and a third link of said series; and wherein said first interface receives via said first link a message selecting a desired stream of digital information and responds to said message by: determining if said desired stream of digital information is currently available to said first interface via said second link, if said desired stream of digital information is currently available via said second link, forwarding said desired stream to said receiver via said first link; and if said desired stream of digital information is currently unavailable at an input of said first interface, forwarding a second message from said first interface to said second interface via said second link, requesting that said desired digital stream be placed on said second link if available to said second interface via said third link. 2. The system of claim 1 wherein said receiver comprises a further interface to a further series of links to a subscriber unit. 3. The system of claim 1 wherein said receiver comprises a subscriber unit. 4. The system of claim 1 wherein said broadband digital information source comprises an information server. 5. The system of claim 1 wherein said digital information supplied by said broadband digital information source comprises video information. 6. The system of claim 1 wherein said digital information supplied by said broadband digital information source comprises audio information. 7. The system of claim 1 wherein said digital information supplied by said broadband digital information source comprises interactive service information. 8. The system of claim 1 wherein said digital information supplied by said broadband digital information source comprises textual information. 9. The system of claim 1 wherein said digital information supplied by said broadband digital information source comprises graphical information. 10. The system of claim 1 wherein said digital information supplied by said broadband digital information source comprises binary computer data. 11. The system of claim 1 wherein said receiver unit receives information from said broadband information source identifying available streams of digital information and corresponding codes to be included within said message to identify said desired stream. 12. The system of claim 2 wherein a first link of said series of links that is directly coupled to said subscriber unit comprises a twisted pair telephone line and an interface between said first link and a next link toward said broadband digital information source comprises a pedestal. 42 13. A system for broadcasting digital information to a plurality of receivers comprising: an interface coupled to said plurality of receivers and to a broadband communications network, said interface comprising: means for receiving a first message from a first one of said plurality of receivers, said first message including a first request for a desired stream of information; means for responding to said first message in real- time by retrieving said desired stream from said broadband communications network and forwarding said desired stream to said first one of said receivers, said desired stream thereafter being available at an input of said interface; means for receiving a second message from a second one of said plurality of receivers, said second message including a second request for said desired stream of information, said second message being received after said first message; and means for responding to said second message by forwarding said desired stream of information as available at said input of said interface to said second one of said receivers. 14. The system of claim 13 wherein said means for responding to said first message comprises: means for determining if said desired stream is already available on said broadband communication network; means for, if said desired stream is unavailable, forwarding said first request through said broadband communication network to an information source that responds by placing said desired stream on said broadband communication network. 15. The system of claim 13 wherein said means for responding to said first message comprises: means for determining if said desired stream is already available on said broadband communication network; means for, if said desired stream is unavailable, forwarding said first request through said broadband communication network to a further interface that responds by retrieving said desired stream from a further broadband communication network. 16. The system of claim 13 wherein said plurality of receivers are a plurality of subscriber units. 17. The system of claim 16 wherein said interface comprises a program selection monitor that monitors and logs program request messages received from said subscriber units. 18. In a system for broadcasting digital information comprising a broadband digital information source, a plurality of receivers, a first interface coupled to said plurality of receivers, a series of one or more links between said first interface and said broadband digital information source, and a series of one or more interfaces interconnecting said links, a method for providing a receiver with a desired stream of digital information from said broadband digital source: detecting at said first interface a message from a requesting one of said receivers requesting said desired stream of digital information; checking if said desired stream of digital information is currently provided to said first interface via said series of links; if said desired stream is currently provided, relaying said desired stream from said first interface to said requesting receiver; if said desired stream is not currently provided, a) forwarding a request for said desired stream through said series of interfaces toward said broadband source until one of said interfaces has said desired stream currently available at an input and responds to said request by forwarding said desired stream to said first interface; b) relaying said desired stream from said first interface to said requesting receiver; and c) marking said desired stream as being currently provided to said first interface and available for , relaying to further requesting receivers. 19. The method of claim 18 wherein said requesting receiver comprises an interface to a further series of one or more links to a subscriber unit, said message has been originated by said subscriber unit and transmitted to said requesting receiver via said further series of one or more links, and said method further comprises the step of: relaying said desired stream from said requesting receiver to said subscriber unit via a further series of one or more links. 20. In a public switched telephone network comprising a broadband network interconnected to a plurality of subscriber units by a plurality of twisted pair lines, a pedestal for interfacing between said broadband network and said twisted pair lines, said pedestal comprising: means for exchanging digital information with said broadband network; means for exchanging digital information with said subscriber units via said twisted pair lines; means for receiving messages from said subscriber units, said messages identifying streams of digital information desired to be received at said subscriber units; and means for extracting said desired streams of digital information from said broadband network using information contained in said messages received from said subscriber units and relaying said desired streams to said subscriber units. 21. The pedestal of claim 20 wherein said streams of digital information comprise streams of packets and said extracting means comprise means for identifying packets belonging to said desired streams. 22. The pedestal of claim 21 wherein said packets comprise MPEG packets. 23. The pedestal of claim 21 wherein said packets , comprise ATM cells. 24. The pedestal of claim 20 wherein said means for exchanging information with said broadband network comprises an ATM interface. 25. The pedestal of claim 20 wherein said means for exchanging information with said broadband network comprises a fiber optic interface. 26. The pedestal of claim 20 wherein said means for exchanging digital information with said subscriber units comprises a plurality of ADSL modems. 27. The pedestal of claim 20 wherein said digital information comprises video information. 28. The pedestal of claim 20 further comprising means for logging said messages to monitor program selections. 29. The system of claim 13 wherein said interface is coupled to said receivers via a plurality of twisted pair access lines. 30. The system of claim 13 wherein said interface is coupled to said receivers via a shared physical medium with a unique channel being reserved for communication with each receiver. 31. An interface between a satellite broadcast system and a plurality of subscriber units, said interface comprising: a plurality of access lines connecting said interface to individual ones of said plurality of subscriber units; a plurality of satellite broadcast receivers, each , said receiver operative to demodulate a satellite broadcast signal and-extract one or more MPEG transport streams; means for receiving messages from requesting ones of said subscriber units including identifiers identifying particular MPEG transport streams received by said plurality of receivers; and means for responding to said messages by filtering said particular MPEG transport streams from all MPEG transport streams received by said receiver and relaying said particular MPEG transport streams to requesting ones of said subscriber units. 32. A system for supplying desired digital information to a receiver comprising: a broadband digital information source; a series of two or more links between said broadband digital information source and said receiver, wherein digital information supplied from said broadband digital information source travels via each link in said series to said receiver, said digital information comprising MPEG packets; and an interface between at least one pair of adjoining links in said series; and wherein said receiver sends a message to said interface selecting a desired MPEG transport stream of digital information and said interface responds to said message by selecting said desired MPEG transport stream from multiple streams of digital information already received at an input of said interface from said digital information source via one or more of said series of links and relaying said desired stream to said receiver via a remaining one or more of said series of links. 33. The system of claim 32 wherein said interface is further coupled to a further broadband digital information source via a further series of links, further streams of digital information being available from said further broadband digital information source for relaying to said receiver. 34. The system of claim 32 wherein said receiver receives from said interface a service information stream that includes identifiers of available MPEG transport streams, said identifiers being usable within said message to specify said desired MPEG transport stream. 35. A system for supplying desired digital information to a receiver comprising: a broadband digital information source; a series of two or more links between said broadband digital information source and said receiver, wherein digital information supplied from said broadband digital information source travels via each link in said series to said receiver, a last link of said series to said receiver being an ADSL connection via a twisted pair line; and an interface between at least one pair of adjoining links in said series; and wherein said receiver sends a message to said interface selecting a desired stream of digital information and said interface responds to said message by selecting said desired stream from multiple streams of digital information already received at an input of said interface from said digital information source via one or more of said series of links and relaying said desired stream to said receiver via a remaining one or more of said series of links. 36. A system for supplying desired digital information to a receiver comprising: a broadband digital information source; a series of two or more links between said broadband digital information source and said receiver, wherein digital information supplied from said broadband digital information source travels via each link in said series to said receiver, at least one of said links being a switched ATM network that carries said digital information in the form of ATM cells; and an interface between at least one pair of adjoining links in said series; and wherein said receiver sends a message to said interface selecting a desired stream of digital information, said message including identifier information specifying said desired stream and said interface * responds to said message by 1) using identifier information to segregate cells belonging to said desired stream from multiple streams of digital information already received at an input of said interface from said digital information source via one or more of said series of links, and 2) relaying said desired stream to said receiver via a remaining one or more of said series of links. 37. The system of claim 36 wherein said identifier information comprises a Virtual Path Identifier (VPI) and a Virtual Connection Identifier (VCI) of cells belonging to said stream. 38. The system of claim 36 wherein said interface employs ADSL to communicate with said receivers via said plurality of twisted pair access lines. 39. In a digital communication system including a plurality of subscriber units, a digital information transmission system coupled to said plurality of subscriber units by a communication network, comprising: means for transmitting a digital information stream of a predetermined duration to one or more of said subscriber units via said communication network over a predetermined channel beginning at a predetermined time; and scheduling means coupled to said transmitting means for if a request for said digital information stream is received prior to said predetermined time, initiating transmission of said digital information stream at said predetermined time starting at a beginning of said digital information stream over said predetermined channel; and if said request for said digital information stream is received after said predetermined time, initiating transmission of said digital information stream at a point in said digital information stream determined relative to said predetermined time so that said digital information stream ends said predetermined duration * after said predetermined time. 40. The system of claim 39 wherein said request is received from a subscriber unit. 41. The system of claim 40 wherein said plurality of subscriber units includes privileged and non-privileged subscriber units and said scheduling means accepts requests for said digital information stream only from privileged subscriber units. 42. The system of claim 39 wherein said scheduling means further comprises means for: if no request for said digital information stream is received, accepting requests for an alternative digital information stream on said channel, and using said transmitting means to transmit information relating to said alternative digital information stream over said channel. 43. The system of claim 42 wherein said scheduling means further comprises means for: upon receipt of a request for said digital information stream within said predetermined duration after said predetermined time, terminating transmission of information relating to said alternative digital information stream. 44. The system of claim 39 wherein said scheduling means further comprises means for: if no request for said digital information stream is received, initiating transmission of an alternative digital information stream over said channel from said predetermined time until said request for said digital information stream is , received. 45. The system of claim 44 wherein said scheduling means further comprises means for: upon receipt of a request for said digital information stream, terminating transmission of said alternative digital information stream. 46. The system of claim 39 wherein said communication network is a hierarchical network interconnecting said digital information transmission system and said plurality of subscriber units. 47. The system of claim 39 wherein said communication network is a shared communication medium interconnecting said digital information transmission system and said plurality of subscriber units. 48. The system of claim 39 wherein said transmitting means is an MPEG multiplexer. 49. The system of claim 40 wherein said scheduling means further comprises: means for, once transmission of said digital information stream has been initiated, receiving messages from further subscriber units indicating that said digital information stream is being monitored and receiving further messages from subscriber units indicating that said digital information stream is no longer being monitored, and upon a determination that no subscriber unit of said plurality is currently monitoring said digital information stream, thereafter terminating transmission of said digital information stream and accepting requests for transmitting alternative services over said channel. 1 50. The system of claim 39 wherein said channel is 2 an MPEG multiplexed packet stream allocation. 1 51. The system of claim 39 wherein said channel is 2 , a frequency allocation. 1 52. The system of claim 39 wherein said channel is 2 an ATM virtual connection. 1 53. In a digital information transmission system 2 coupled to a plurality of subscriber units via a communication 3 network, said subscriber units including privileged and non- 4 privileged subscriber units, a system for allocating access to 5 a channel among said subscriber units, said system comprising: 6 means for transmitting a content stream from a 7 plurality of available content streams over said channel, said 8 available content streams including general access content 9 streams and limited access content streams; 10 means for receiving a request specifying a desired 11 content stream from one of said plurality of subscriber units; 12 prioritization means, coupled to said receiving 13 means and transmitting means, for initiating transmission of a 14 limited access content stream over said channel only if a 15 request for said limited access stream is received from a 16 privileged subscriber unit, and for initiating access to a 17 general access content stream only if no request from a 18 privileged subscriber unit for a limited access stream is 19 currently being fulfilled. 20 54. The system of claim 53 wherein said 21 prioritization means further comprises: 22 means for, once transmission of said limited access 23 content stream has been initiated, receiving messages from , 24 further privileged subscriber units indicating that said 25 limited access content stream is being monitored and receiving .26 further messages from privileged subscriber units indicating 27 that said is no longer being monitored, and upon a 28 determination that no privileged subscriber unit is currently monitoring said limited access content stream, thereafter terminating transmission of said limited access content stream and accepting requests for transmitting general access content streams over said channel. 55. The system of claim 53 wherein said channel is an MPEG multiplexed packet stream allocation. 56. The system of claim 53 wherein said channel is a frequency allocation. 57. The system of claim 53 wherein said communications network comprises a hierarchical network interconnecting said video transmission system and said plurality of subscriber units. 58. The system of claim 53 wherein said transmitting means is an MPEG multiplexer. 59. In a digital information transmission system coupled to a plurality of subscriber units via a communications network, a digital information stream server having a channel for communication to said subscriber units, said system comprising: means for storing a plurality of content streams, said content streams including general access content streams and limited access content streams; means for transmitting a content stream from said plurality of said content streams over said channel; means for receiving a request specifying a desired content stream from one of said plurality of subscriber units; and prioritization means, coupled to said receiving means and transmitting means, for initiating transmission of a limited access content stream over said channel only if a request for said limited access stream is received, and for initiating access to a general access content stream only if no request for a limited access stream is currently being fulfilled. 60. A digital information distribution system comprising: a digital information stream server comprising: means for storing a digital information stream of predetermined duration; network adaptation means for transmitting digital information onto a first communication network on a predetermined channel; request receiving means for receiving requests for said digital information stream from said first communication network; scheduling means for directing said digital information stream to said network adaptation means for transmission over said first communication network on said predetermined channel at a predetermined time, if a request for said digital information stream is received by said request receiving means; opportunistic programming means for directing digital information to said network adaptation means for transmission over said first communication network only if said digital information stream is not being transmitted; a network interface coupled to said first communication network for connecting said first communication network to a plurality of subscriber units via a second communication network, said network interface comprising: request receiving means for receiving requests originating from said subscriber units, said subscriber units including privileged subscriber units and non-privileged subscriber units; request relay means for relaying only requests from privileged subscriber units for said digital information stream to said digital information stream server; and means for relaying said digital information stream from said first communication network to said requesting ones of said privileged subscriber units via said second communication network. 61. The system of claim 60 wherein said scheduling means further comprises means for: if a request for said digital information stream is received prior to a predetermined time, initiating transmission of said digital information stream starting at a beginning of said digital information stream over said predetermined channel at said predetermined time; and if said request for said digital information stream is received after said predetermined time, initiating transmission of said digital information stream at a point in said digital information stream determined relative to said predetermined time so that said digital information stream ends said predetermined duration after said predetermined time 62. The system of claim 60 wherein said first communication network is a hierarchical network. 63. The system of claim 60 wherein said second communication network is a hierarchical network. 64. The system of claim 60 wherein said network interface further comprises: cancellation request means for receiving via said second communication network messages from privileged subscriber units indicating that they are no longer monitoring said digital information stream; and means for, when no privileged subscriber unit is monitoring said digital information stream through said network interface, transmitting a cancellation request on said first communication network and halting transmission of said digital information stream via said second communication network. 65. The system of claim 60 wherein said opportunistic programming means transmits digital information via said channel only upon request of a subscriber unit when said digital information stream is not being transmitted. 66. The system of claim 60 wherein said , opportunistic programming means transmits digital information via said channel whenever said digital information stream is not being transmitted. 67. The system of claim 60 wherein said digital information transmitted by said opportunistic programming means comprises an alternative video program. 68. The system of claim 60 wherein said digital information transmitted by said opportunistic programming means comprises computer data.

Inventors: LEVESQUE GILBERT, RAO R PADMANABHA, SICCARDO PAOLO L, LEVESQUE, GILBERT, RAO, R., PADMANABHA, SICCARDO, PAOLO, L.
Applicants: HYUNDAI ELECTRONICS AMERICA, LEVESQUE GILBERT, RAO R PADMANABHA, SICCARDO PAOLO L, HYUNDAI ELECTRONICS AMERICA, INC., LEVESQUE, GILBERT, RAO, R., PADMANABHA, SICCARDO, PAOLO, L.
Application year: 1996
Main classification: H04L5
IPC codes: H04H1, H04H20, H04N5, H04N7