SERVICE RATE CHANGE METHOD AND
CROSS REFERENCE TO RELATED
 This application claims benefit of U.S. Provisional Patent Application Serial No. 60/127,335 (attorney docket number 044), which was filed on Apr. 1, 1999 and is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION  1. Field of the Invention
 The present invention relates to an information distribution system such as a video-on-demand (VOD) system. More particularly, the present invention relates to a method and apparatus for adapting at least a service level in response an amount of bandwidth available within the information distribution system.
 2. Description of the Background Art
 In an information distribution system, such as video on demand (VOD) system, an information provider (e.g., a head-end in a cable television system) must control of the distribution of requested information to ensure that requests for information, such as video information, are satisfied in an orderly manner.
 Unfortunately, in an information distribution system serving a large number of subscribers, the bandwidth of the various components forming the information distribution system may, at times, be insufficient to satisfy each subscriber request. Present information distribution systems, such as VOD systems, respond to bandwidth constraints by denying subscriber information requests or providing the requested information in a substantially degraded manner. In a VOD system, this degradation may take the form of poor temporal alignment between video and audio tracks (i.e., poor "lip-sync"), missing information due to "dropped" video or audio frames and/or excessive latency in responding to subscriber interactions. These and other qualitative degradations are common in present systems.
 Therefore, it is seen to be desirable to provide a method and apparatus for ensuring that a bandwidth constrained system provides information having at least a minimal qualitative level. Moreover, it is seen to be desirable to provide such functionality within the context of a video on demand system.
SUMMARY OF THE INVENTION
 The disadvantages heretofore associated with the prior art are overcome by the present invention of a method and apparatus for allocating bandwidth within a bandwidth constrained interactive information distribution system. The system determined if a requested information stream may be provided to a requesting subscriber at an appropriate bandwidth level (i.e., appropriate bitrate providing full quality), a minimal bandwidth level (i.e., a reduced bitrate providing minimally acceptable quality) or not at all. Each information stream and any ancillary streams may be stored twice by the system, once at an appropriate encoded bitrate and once at a minimal encoded bitrate.
 Specifically, in an information distribution system comprising provider equipment and subscriber equipment, said provider equipment providing information to said subscriber equipment via a forward channel, said subscriber equipment requesting said information via a back channel, a method according to the invention comprises the steps of: determining whether said information distribution system has sufficient bandwidth available to provide information requested by a subscriber; providing, in the event of appropriate bandwidth availability, said requested information to said subscriber using said appropriate bandwidth; and providing, in the event of minimum bandwidth availability, said requested information to said subscriber using said minimum bandwidth.
BRIEF DESCRIPTION OF THE DRAWINGS
 The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:
 FIG. 1 depicts a high level block diagram of an interactive information distribution system; and
 FIG. 2 depicts a flow diagram of a method for allocating bandwidth suitable for use in the information distribution system of FIG. 1.
 To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.
 The present invention will be primarily described within the context of a video on demand (VOD) systems providing content encoded according to the various Moving Pictures Experts Group (MPEG) standards are known. For example, a first standard known as MPEG-1 refers to ISO/IEC standards 11172, which is incorporated herein by reference in its entirety. A second standard known as MPEG-2 refers to ISO/IEC standards 13818, which is incorporated herein by reference in its entirety. Additionally, a compressed digital video system is described in the Advanced Television Systems Committee (ATSC) digital television standard document A/53, incorporated herein by reference. It will be appreciated by those skilled in the art that the teachings of the present invention may be advantageously applied to other information distribution systems where components or functional elements within the system are bandwidth constrained.
 FIG. 1 depicts a high level block diagram of an interactive information distribution system. Specifically, FIG. 1 depicts a high level block diagram of an interactive information distribution system 100 containing the present invention. The system 100 contains service provider equipment 102, a communications network 104 and subscriber equipment 106n, where n is an integer greater than zero.
 The service provider equipment 102 contains an information server 108 which is typically a parallel processing computer containing at least one central processing unit 110 and associated memory 112. The server interacts with at least one data storage device 114-1 (e.g., a disk drive array) that generally stores the subscriber information (e.g., video data) that will be recalled and downloaded to the subscriber.