Abstract:
A streaming media system employs dynamic rate adaptation. The method includes a file format compatible with legacy HTTP infrastructure to deliver media over a persistent connection. The method further includes the ability for legacy client media players to dynamically change the encoded delivery rate of the media over a persistent connection. The method provided works transparently with standard HTTP servers, requiring no modification and leverages standard media players embedded in mobile devices for seamless media delivery over wireless networks with high bandwidth fluctuations. A system is also specified for implementing a client and server in accordance with the method.

Description:
BACKGROUND 
     This invention relates in general to streaming media and more specifically to implementing dynamic bit rate adaptation while streaming media on demand. 
     Available bandwidth in the internet can vary widely. For mobile networks, the limited bandwidth and limited coverage, as well as wireless interference can cause large fluctuations in available bandwidth which exacerbate the naturally bursty nature of the internet. When congestion occurs, bandwidth can degrade quickly. For streaming media, which require long lived connections, being able to adapt to the changing bandwidth can be advantageous. This is especially so for streaming which requires large amounts of consistent bandwidth. 
     In general, interruptions in network availability where the usable bandwidth falls below a certain level for any extended period of time can result in very noticeable display artifacts or playback stoppages. Adapting to network conditions is especially important in these cases. The issue with video is that video is typically compressed using predictive differential encoding, where interdependencies between frames complicate bit rate changes. Video file formats also typically contain header information which describe frame encodings and indices; dynamically changing bit rates may cause conflicts with the existing header information. 
     There have been a number of solutions proposed for dealing with these problems. One set of solutions is to use multiple independently encoded files, however, switching between files typically requires interrupting playback, which is undesirable. These solutions also typically require starting again from the beginning of the file, which is very disruptive. Solutions based on the RTSP/RTP transport delivery protocols have the advantage of being frame-based, which eases the switching between streams, but they require that multiple streams be running simultaneously, which is bandwidth and server resource inefficient. Other solutions propose alternate file encoding schemes with layered encodings. Multiple files are used, but each file can be added to previous files to provide higher quality. Rate adaptation is performed by sending fewer layers of the encoding, during congestion. These schemes require much more complex preprocessing of files, and the codecs are not typically supported natively by most devices. For mobile devices with limited resources, this can be a large barrier to entry. 
     More recently, schemes have been proposed which use multiple files, each encoded at a different bit rate, but then the files are divided into segments. Each segment is an independently playable file. The segments provide fixed boundaries from which to switch and restart playback. This solves the problem of having to restart from the beginning, and limits the playback disruption. The granularity is not nearly as fine as with RTSP which may be as low as 1/30th of a second, but rather at the granularity of seconds to tens of seconds. With finer granularity, disruption to users is minimized, however, segment overhead is maximized. In cases where round trip latency between the client and the server is higher than the segment duration, undue overhead is introduced as the rate cannot be adapted that quickly. If caching is employed, cache distribution and synchronization latency may compound these issues. However, coarser granularity limits the utility of the switching scheme. If the available network bandwidth varies at a period less than the segment duration, inability to adapt in a timely manner negates the value of segmentation. 
     Content providers produce content and monetize it through a variety of means (advertising sponsorship, product placement, direct sales, etc.). One of the primary methods for monetizing video content is the periodic insertion of video advertisements, as with television and some internet-based long form video content delivery, as well as through strictly pre-roll and/or post-roll advertisements as with movies and some short form video content delivery. 
     For desktop delivery of media, switching between content and ads is fairly seamless given the high bandwidth provided by broadband connections and the high CPU power of modern desktop PCs. For mobile delivery of media, however, high latency and low bandwidth cellular networks coupled with low CPU power in most handsets can cause long playback disruptions when retrieving separate content and advertisement video files. On-demand transcoding and stitching of advertisements to content is a CPU intensive task which requires dedicated servers. It incurs the cost of maintaining servers and prevents the use of tried and true content delivery networks (CDN). To alleviate this, pre-stitching of advertisements to content is often used to limit costs. However, advertisements are typically rotated periodically with changing ad campaigns. For long form content, changing the ads may require re-stitching extremely large amounts of content and then re-uploading all of that content to a CDN. Network bandwidth is typically a bottleneck and uploading can take a long time; upload can also be costly if network access is paid for by the amount of bandwidth used. With long form content, the ads are typically very small, relative to the size of the feature content. Re-uploading the entire file, including both ad and feature content needlessly incurs the cost of re-uploading the feature content. 
     SUMMARY 
     Methods and apparatus are disclosed for streaming data over a network. The type and rate of streaming are varied based on the network bandwidth available without interrupting the user. Stream data throughput may be maximized in a network-friendly manner during highly variable network conditions. In one embodiment, video media is transcoded into a plurality of different bit rate encodings. The plurality of encodings chopped into a collection of segment files. The segments are sent from a network-aware adaptive streaming (NAAS) server and reassembled at the client and presented to the media player. In one embodiment, the network type such as Wi-Fi, 3G, Edge, etc. is detected and used to determine the range of available data rates. In another embodiment, available bandwidth is determined by segment download rate. During sustained extreme (i.e. poor) network conditions, retransmissions are avoided in order to avoid overwhelming the network. Under good network conditions, the system downloads additional segment files ahead of time from multiple NAAS servers to increase the total throughput. In one embodiment, the client stores playback status on the local device. In another embodiment, the client sends playback status back to the NAAS servers. The playback status is used to keep track of what the user has viewed. In one embodiment, the playback status (referred to as a bookmark herein) is used to allow users to continue playing from where they left off. In one embodiment the user may continue watching from the bookmark point on the same device. In another embodiment, the user may continue watching from the bookmark point on a different device. 
     The disclosed technique may employ a single concatenated file for managing a plurality of encodings for a given piece of source media. In one embodiment, video media is transcoded into a plurality of different bit rate encodings. The plurality of encodings are concatenated into a single file. The concatenated file is concatenated in a manner that allows for all encodings to be played sequentially and continuously. Encoding concatenation is file format specific but those methods should be well known to anyone skilled in the art. Concatenated files are created for a plurality of file formats, to support a plurality of client devices. In another embodiment, the concatenated files may contain non-video data which has been compressed and encrypted using different encoding methods to produce a plurality of encodings. The different compression and encryption methods may require different levels of complexity and different amounts of client resources to reconstruct. Different compression and encryption schemes provide different levels of quality (i.e. higher or lower compression and higher or lower security); they also have different types of framing and format organization, the details of which should be known to those skilled in the art. 
     In one embodiment, the concatenated files contain padding between the individual component encodings. In one embodiment, video files are padded out to integer time boundaries. The padding serves a dual purpose. First, it provides a buffer for stopping the media renderer before the next encoding begins. Second, it simplifies time-based offset calculations. In another embodiment, video files are padded out with interstitial advertisements. Interstitial advertisements provide the same benefits as blank padding, but also include the flexibility to incorporate different advertising revenue models in video delivery. In another embodiment compressed and/or encrypted files are padded out to round numbered byte boundaries. This can help simplify byte-based offset calculations. It also can provide a level of size obfuscation, for security purposes. 
     In one embodiment, the concatenated files are served from standard HTTP servers. In another embodiment, the concatenated files may be served from an optimized caching infrastructure. In another embodiment, the concatenated files may be served from an optimized video streaming server with server side pacing capabilities. In one embodiment, the streaming server maps requests for specific encodings to the proper encoding in the concatenated file. In one embodiment, the streaming server maps requests for specific time-based positions to the proper concatenated file byte offset for a given encoding. In one embodiment, the streaming server delivers concatenated file data using the HTTP chunked transfer encoding, and paces the delivery of each chunk to limit network congestion. In one embodiment, the streaming server includes metadata in each chunk specifying the encoding, time-based position, and concatenated file byte offset information for the chunk. The concatenated files are designed to be usable with existing infrastructure. They do not require special servers for delivery and they do not require decoding for delivery. They also do not require custom rendering engines for displaying the content. An example of a suitable adaptive HTTP streaming server is described in PCT Application No. PCT/US09/60120 filed Oct. 9, 2009 and entitled, Method And Apparatus For Efficient Http Data Streaming. 
     In one embodiment, a rate map index file is used. The rate map index file contains a plurality of entries, each entry containing an index into the concatenated file. Each index contains a plurality of concatenated file byte offsets which are offsets into the concatenated file. Each entry contains a concatenated file byte offset for each encoding in the concatenated file, such that each byte offset maps a position, in the current encoding, to the corresponding position in another encoding within the concatenated file. The offsets may be tuned to different granularity. In one embodiment the rate map indices map out only the start of the encodings. In another embodiment, the rate map indices map out individual frames of a video encoding. In another embodiment, the rate map indices map out groups of frames, beginning with key frames, for a video encoding. In another embodiment, the rate map indices map out the different compression or encryption blocks of a data encoding. The rate map indices are all of fixed size, so that the rate map indices themselves may be easily indexed by a rate map index file byte offset which is an offset into the rate map index file. For example, the index for a given frame F of a given encoding E can be found in the rate map index file at byte (((E*N)+F)*I), where N is the number of frames in each encoding, and I is the size of each index. The number of frames N is preferably consistent for all encodings of a given source video, though may differ from one source video to another. 
     In one embodiment, stitched media files are generated which may be split into a plurality of discrete particles. The particles are used to facilitate dynamic ad rotation. Three particles are used: a header particle, a feature particle, and an ad particle. Header and ad particle pairs are generated such that they may be used interchangeably with a given feature particle. A stitched media file is first generated by stitching feature content to ad content. In one embodiment, the feature content is a single video clip. In another embodiment the feature content is a concatenation of a plurality of video clips. In one embodiment, each video clip is for a single bit rate encoding. In another embodiment, each video clip is a concatenation of a plurality of different bit rate encodings, for the same clip. In one embodiment, the ad content is a single ad clip. In another embodiment the ad content is a concatenation of a plurality of ad clips. In one embodiment, each ad clip is for a single bit rate encoding. In another embodiment, each ad clip is a concatenation of a plurality of different bit rate encodings, for the same clip. The concatenated clips are concatenated in a manner that allows for all encodings to be played sequentially and continuously. Encoding concatenation is file format specific but those methods should be well known to anyone skilled in the art. In one embodiment the particles may be encrypted. The different compression and encryption methods may require different levels of complexity and different amounts of client resources to reconstruct. Different compression and encryption schemes provide different levels of quality (i.e. higher or lower compression and higher or lower security); they also have different types of framing and format organization, the details of which should be known to those skilled in the art. 
     In one embodiment, a particle index file is used. The particle index file contains a plurality of entries, each entry containing a particle index into the particle file. Each particle index contains a stitched media file byte offset which is an offset into the particle file. Each stitched media file byte offset points to the start of the particle. The particle index file also keeps track of the particle versions associated with a specific incarnation of the stitched media file. Separate rate map index files may be used for accessing the data within a particle, as described above. 
     In one embodiment, the native client media player is used as the rendering engine. In another embodiment, a custom rendering engine is used. 
     In one embodiment, a progressive downloader is used to manage a data buffer. A data source feeds the buffered data to the rendering engine. In one embodiment, the downloader uses simple HTTP requests to retrieve data. In another embodiment, the downloader uses HTTP range GETs to retrieve segments of data. In one embodiment, data is retrieved as fast as possible to ensure maximum buffering of data to protect against future network interruption. In another embodiment, the segments are retrieved at paced time intervals to limit load on the network. The paced time intervals are calculated based on the encoding rate, such that the download pace exceeds the encoded data rate. The paced time intervals also take into account average bandwidth estimates, as measured by the downloader. In another embodiment, other legacy data retrieval methods are used, e.g. FTP. 
     In one embodiment, the downloader measures network bandwidth based on the round trip download time for each segment as (S/T), where S is the size of the segment and T is the time elapsed in retrieving the segment. This includes the latency associated with each request. For video media, as available bandwidth decreases and rate adaptation is employed, the total bytes per segment should decrease, as segments should be measured in frames which is time based, and the lower bit rate video will produce fewer bytes per frame. Thus, segment size should only decrease as network congestion occurs, due to dynamic rate adaptation, so the higher relative impact of request latency overhead should increase as congestion occurs which helps to predict rapid downward changes in bandwidth. In one embodiment, the downloader keeps a trailing history of B bandwidth estimates, calculating the average over the last B samples. When a new sample is taken, the Bth oldest sample is dropped and the new sample is included in the average, as illustrated in the following example pseudocode: 
     
       
         
               
               
             
               
               
             
           
               
                   
               
             
             
               
                 integer B_index 
                 // tail position in the circular history buffer 
               
               
                 integer B_total 
                 // sum of all the entries in the history buffer 
               
               
                 integer B_count 
                 // total number of entries in the history buffer 
               
               
                 integer B_new 
                 // newly sampled bandwidth measurement 
               
               
                 integer B_old 
                 // oldest bandwidth sample to be replaced 
               
               
                 integer B_average 
                 // current average bandwidth 
               
               
                 array B_history 
                 // circular history buffer 
               
             
          
           
               
                 B_old = B_history[B_index] 
                 // find the sample to be replaced 
               
               
                 B_history[B_index] = B_new 
                 // replace the sample with the new 
               
               
                   
                 sample 
               
               
                 B_total = B_total − B_old 
                 // remove the old sample from the 
               
               
                   
                 sum 
               
               
                 B_total = B_total + B_new 
                 // add the new sample into the 
               
               
                   
                 sum 
               
               
                 B_average = B_total / B_count 
                 // update the average 
               
               
                 B_index = (B_index + 1) % 
                 // update the buffer index 
               
               
                 B_count 
               
               
                   
               
             
          
         
       
     
     The history size is preferably selected so as not to tax the client device. A longer history will be less sensitive to transient fluctuations, but will be less able to predict rapid decreases in bandwidth. In another embodiment the downloader keeps only a single sample and uses a dampening filter for statistical correlation. 
     
       
         
               
               
             
               
             
           
               
                   
               
             
             
               
                 integer B_new 
                 // newly sampled bandwidth measurement 
               
               
                 integer B_average 
                 // current average bandwidth 
               
               
                 float B_weight 
                 // weight of new samples, between 0 and 1 
               
             
          
           
               
                 B_average = (B_average * (1 − B_weight)) + (B_average * B_weight) 
               
               
                 // update the average 
               
               
                   
               
             
          
         
       
     
     This method requires less memory and fewer calculations. It also allows for exponential drop off in historical weighting. In one embodiment, download progress for a given segment is monitored periodically so that the segment size S of the retrieved data does not impact the rate at which bandwidth measurements are taken. There are numerous methods for estimating bandwidth, as should be known to those skilled in the art; the above are representative of the types of schemes possible but do not encompass an exhaustive list of schemes. Other bandwidth measurement techniques as applicable to the observed traffic patterns may be acceptable for use as well. 
     In one embodiment, bandwidth measurements are used to determine when a change in encoding is required. If the estimated bandwidth falls below a given threshold for the current encoding, for a specified amount of time, then a lower bit rate encoding should be selected. Likewise if the estimated bandwidth rises above a different threshold for the current encoding, for a different specified amount of time, then a higher bit rate encoding may be selected. 
     An offset is calculated into the concatenated file for the new encoding. The offset corresponds to the same current position in the current encoding. In one embodiment, the offset is calculated as a time offset (e.g. 30 seconds in to the first encoding, and 30 seconds in to the second encoding). In another embodiment, the offset is calculated as a frame offset (e.g. 5th frame of the first encoding, to the 5th frame of the second encoding). The offset is then converted into a concatenated file byte offset. In one embodiment, the offset is calculated directly, using a known frame size for a given encoding, as (N*F), where N is the frame number and F is the known frame size. In another embodiment, the offset is looked up in the rate map index file, as described above. In one embodiment, the offset is calculated as the next frame or range to be retrieved by the downloader. In another embodiment, the offset is calculated at some position in the future, to allow for better rendering continuity. In one embodiment, rendering continuity is measured based on scene transitions. In another embodiment, rendering continuity is measured based on motion intensity. The calculated by offset is used by the downloader as the starting point for subsequent media retrieval. 
     In one embodiment, the rendering engine is notified when to start its initial rendering. The rendering engine should request data from the data source, starting at the beginning of the file. When new encodings are selected, rendering time and data source requests are monitored and an optimal switching point is selected. The rendering engine is notified to seek to the new file location, which corresponds to the new encoding. The seek notification is timed so that the rendering engine maintains rendering continuity when seeking to the new position. When an ad is to be displayed, the rendering engine is notified to seek to the stitched media file location corresponding to the desired ad clip in the ad particle. Ad insertion points, ad durations, and ad selection criteria are predefined for the data source. Once the ad has completed rendering, the rendering engine is notified to seek back to the feature content location where rendering left off, prior to ad insertion. 
     In one embodiment, the data source updates its particle index file at each time the buffer is reinitialized. The buffer is reinitialized to maintain stitched media file byte offset continuity. Updating the particle index file allows for new versions of the stitched media file to be specified. The new file may have different header and ad particle, thus initiating ad rotation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of various embodiments of the invention. 
         FIGS. 1 and 2  are block diagrams of systems capable of conducting procedures, in accordance with various embodiments of the invention; 
         FIG. 3  is a diagram of files used to create the single concatenated file, in accordance with an embodiment of the present invention; 
         FIG. 4  is a diagram of a rate map index file used to map concatenated file byte offsets to time offsets, in accordance with an embodiment of the present invention; 
         FIG. 5  is a diagram of a buffer and data source management used, in accordance with an embodiment of the present invention; 
         FIG. 6  is a flow chart showing a method for performing rate adaptation, in accordance with an embodiment of the present invention; 
         FIG. 7  is a block diagram of a second system which is capable of conducting procedures, in accordance with various embodiments of the invention; 
         FIG. 8  is a flow chart showing a third method for performing rate adaptation, in accordance with an embodiment of the present invention; 
         FIG. 9  is a diagram of the files used to create ad stitched files, in accordance with an embodiment of the present invention; 
         FIG. 10  is a diagram of file particles and particle index file used to dynamically rotate ads, in accordance with an embodiment of the present invention; 
         FIG. 11  is a flow chart showing a method for retrieving particles, in accordance with an embodiment of the present invention; 
         FIG. 12  shows an interchange involving a special server interaction; and 
         FIG. 13  shows a segment structure for the interaction of  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1  is a block diagram for one embodiment of the present invention. It shows a client device  11  and a plurality of network-aware adaptive streaming (NAAS) servers  10 . The client device  11  and NAAS servers  10  are both typically computerized devices which include one or more processors, memory, storage (e.g., magnetic or flash memory storage), and input/output circuitry all coupled together by one or more data buses, along with program instructions which are executed by the processor out of the memory to perform certain functions which are described herein. Part or all of the functions may be depicted by corresponding blocks in the drawings, and these should be understood to cover a computerized device programmed to perform the identified function. 
     In one embodiment, the NAAS servers  10  (referred to as servers herein) each contain a copy of the content being delivered to the client  11 . In one embodiment, the servers  10  may be collocated in a single data center. In another embodiment, the servers  10  may be geographically distributed in multiple data centers. In another embodiment, the servers  10  may be physically in the same region, but connected to the client  11  through separate network paths (e.g. through different network service providers). In one embodiment, the servers  10  are situated as part of a content delivery network (CDN). The segment downloader  12  retrieves the media from the servers  10  in segments. In one embodiment, the segments are of fixed size (measured in bytes), resulting in variable duration segments. In another embodiment, the segments are of a fixed duration (measured in rendering time), resulting in variable size segments. In one embodiment, the segments of media are stored as separate files. In another embodiment, the segments of media are stored as a single file, and segments are created by reading the specific subset of data from the file. In one embodiment, the segment downloader  12  retrieves the segment data files using the HTTP protocol. In another embodiment, the segment downloader  12  retrieves segments of data from a single file using HTTP range GETs. 
     While downloading the segments, the segment downloader  12  measures the network bandwidth. If the bandwidth falls below a certain threshold for the current network type then the media player  14  is notified that insufficient bandwidth exists. The threshold is the bandwidth needed to download the smallest playable chunk, e.g., all frames between a pair of I-frames, in the current bitrate during the time to play out the buffered content. In one embodiment, the segment downloader  12  sends TCP acknowledgements for non-received data to prevent TCP retransmissions and limit network congestion. In another embodiment, the segment downloader  12  resets the TCP connection to limit network congestion. This is known as “squelching” and is accomplished by sending an acknowledgment for the sequence number of the last TCP-segment received from the server, indicating that everything until that segment has been received regardless of whether the intervening data were actually received. This would prevent any retransmissions from the server. The downloader also chooses the lowest acceptable bitrate. 
     In another embodiment, there is a special server modification in which the server responds to a special http GET command with a mime-header labeled “SWITCH” to cancel the last segment request and request at a lower bitrate. This request may also used to send the acknowledgment to squelch retransmissions. The switch command is used to propagate the squelch upwards through the application and enable the server to switch to a new lower bitrate at the next segment indicated in the body of the request. The server could choose to advance to a more recent segment in the response as indicated via a SWITCH response header. The interchange and segment structure are described below with reference to  FIGS. 12 and 13 . 
     In another embodiment, the segment downloader  12  continues to download data in the hope of restarting playback at some point in the future. The downloader is assumed to be at the lowest bitrate and is moving the window forwards using the gratuitous ACK method (and squelch and switch methods described earlier). 
     If the bandwidth rises above a certain threshold for the current network type then the segment downloader  12  will begin to issue multiple parallel requests for sequential segments. In one embodiment, all requests are sent to the same server  10 . In another embodiment, the requests are spread across multiple servers  10 . Spreading the load across multiple servers allows for network path diversity and server load distribution. In one embodiment, server load balancing is performed explicitly by the client  11 . In another embodiment, server load balancing is performed by the network, either through DNS load balancing or server load balancing within the data center. 
     Data retrieved by the segment downloader  12  is passed to a stream assembler  13 . The stream assembler  13  reassembles the segments, and parses out the video frames and provides them to a media player  14 . In one embodiment, the stream assembler  13  is also responsible for decrypting the data provided by the segment downloader  12 . As the stream assembler  13  provides data to the media player  14 , it keeps track of the current position of the media stream. In one embodiment the media stream position is derived from the frames requested by the media player  14 . In one embodiment, the stream position is adjusted for the size of the media player&#39;s  14  playback buffer. In one embodiment, the stream position is saved locally as a bookmark. In another embodiment, the stream position is provided by the stream assembler  13  to the segment downloader  12 , so that a bookmark may be set on the server  10 . The server  10  stores the bookmark as per-user/per-media metadata in the server database. 
     When the media player  14  starts, it may either request that rendering begin at the start of the content, or it may request that rendering begin at the last known bookmark position. In the latter case, the segment downloader  12  retrieves the bookmark metadata from the server  10 , calculates the necessary offsets and begins downloading segments from that point. 
     In  FIG. 2  is a block diagram  100  for one embodiment of the present invention. It shows a client device  102  and media server  110 . The client device  102  and media server  110  are both typically computerized devices which include one or more processors, memory, storage (e.g., magnetic or flash memory storage), and input/output circuitry all coupled together by one or more data buses, along with program instructions which are executed by the processor out of the memory to perform certain functions which are described herein. Part or all of the functions may be depicted by corresponding blocks in the drawings, and these should be understood to cover a computerized device programmed to perform the identified function. 
     The media server  110  uses a standard HTTP server  112  to deliver data. The concatenated files are stored on a storage device  114 . The storage may be local or remote and may use any of a number of storage technologies, as should be known to those skilled in the art. The concatenated files are generated by a file encoder  116 . The file encoder  116  is responsible for transcoding source media files into a plurality of encodings, where each encoding uses a different bit rate. In one embodiment, default encoding parameters are provided in a configuration file. In another embodiment, default encoding parameters are provided at invocation. In one embodiment, individual source files may override default encoding parameters via an accompanying configuration file. In another embodiment, individual source files may override default encoding parameters using parameters provided at invocation. The file encoder  116  then concatenates the plurality of encodings into a single concatenated file. The individual encodings are of compatible formats for concatenation, the constraints of which should be known to those skilled in the art. 
     In one embodiment the file encoder  116  may be invoked manually. In another embodiment, the file encoder  116  may be asynchronously invoked programmatically, when new source media is available. In another embodiment, the file encoder  116  may be invoked periodically to check if new source media is available. In one embodiment, the file encoder  116  logs transcoding and concatenation to a file or database. The client  102  may be notified asynchronously of the availability of new files. 
     The file encoder  116  is also responsible for generating the rate map index files for each concatenated file. During the transcoding and concatenation processes, the file encoder  116  has all the information necessary to generate the rate map index files. The transcoding configurations contain information on the granularity and units for index information. The rate map index files are written to the storage device  114  with the concatenated media files. 
     In one embodiment, the encodings are concatenated in descending order of bit rate. This scheme provides highest quality for environments which expect few network interruptions. In another embodiment, the encodings are concatenated in ascending order of bit rate. This scheme adopts a slow start paradigm for challenged environments which have higher probability of network interruption. In another embodiment, the encodings are concatenated in an order based on the expected encoding transitions. In one embodiment, multiple concatenation orders may be used, creating a plurality of concatenated files for a given format, for a given source media. The physical interface type of the client device may be used as a predictor of network quality. In one embodiment, a mobile client device with both Wi-Fi and cellular capabilities may attach different network quality expectations to the two interfaces and select different concatenated files, when retrieving data over a given interface. The concatenated files will contain the same encodings, just in a different order, therefore switching between concatenated files requires only a basic remapping of offsets. The selection of a given concatenated file is preferably based on which initial encoding is desired. 
     In one embodiment, the client  102  contains a downloader  104 . The downloader  104  is responsible for interacting with the media server  110  to retrieve data required by a data source  118 . This includes encoded media data as well as file index data. In one embodiment, the downloader  104  uses HTTP range GETs to directly access the encoded media data. The HTTP range GETs also allow the downloader  104  to perform data retrieval pacing. Pacing allows the client  102  to limit its network bandwidth usage. The data retrieved is passed to the data source  118 . In one embodiment, the downloader  104  uses HTTP range GETs to directly access the rate map index file. The HTTP range GETs allow the downloader  104  to access only the rate map index data required, preventing the retrieval of unnecessary data. In another embodiment, the downloader  104  uses HTTP GETs with query strings, wherein the query string specifies the rate map index desired and the HTTP server  112  uses that information to retrieve the desired information from the rate map index file. There are numerous methods for integrating application level support with standard HTTP servers (e.g. CGI scripts, java servlets, Ruby-on-Rails applications, PHP script, etc.) as should be known to those skilled in the art. 
     The data source  118  uses rate map index data to manage media data prefetching, when switching encodings. The data source  118  stores media data in a media buffer  106 . In one embodiment, the media buffer  106  is implemented as a circular buffer to limit memory consumption. A circular buffer is typically a small buffer relative to the size of the data it will hold, i.e. smaller than the data it is intended to hold. It is logically circular in that once the end of the buffer is reached, subsequent data is written to the front of the buffer, overwriting the oldest data. It is useful for devices that have limited memory, and do not have the capacity to store the entire concatenated file, however, it increases the complexity of managing the media buffer  106 . In another embodiment, the media buffer  106  is implemented as a full flat buffer to ease offset calculations. With a full flat buffer, the data is stored, in its entirety, in the media buffer  106 . This simplifies buffer management, as offsets are exact, rather than modulo the buffer size as with a circular buffer, however, it requires that the client device have enough storage space to hold the entire file, which may not be the case for some devices (e.g. mobile phones). In one embodiment, the data retrieved by the downloader  104  may be encrypted. Before the data source  118  adds the data to the media buffer  106 , the data is decrypted. A decryption buffer  120  is used to store encrypted information until it is decrypted and can be transferred to the media buffer  106 . 
     The downloader  104  is also responsible for calculating average available bandwidth. In one embodiment, the downloader  104  uses HTTP range GETs to limit retrieval sizes, and periodically calculates the available bandwidth based on download time and size of data retrieved. This information is also passed to the data source  118 . The data source  118  uses the bandwidth information to determine when to switch encodings. When the data source  118  determines that a change in encoding is necessary, it determines the switch over time and notifies the downloader  104  to retrieve the rate map index information for that switch over time. Once the data source has the rate map index information, and there is room available in the media buffer  106 , it notifies the downloader  104  to begin downloading media data from the new offset. 
     In one embodiment, the client  102  relies on the client&#39;s native media player  108  as a rendering engine. The client requests media data from the data source  118 . In one embodiment, the data source  118  acts as any other data source in the system (e.g. a local file). In another embodiment, the data source  118  may be implemented as a local network proxy (e.g. an HTTP proxy server). Implementation of data proxies, data sources, and device resources in general should be known to those skilled in the art. The media is retrieved from the media buffer  106  and returned to the native media player  108 . When a change in encoding is required and the necessary media data is available in the media buffer  106 , the data source  118  notifies the native media player  108  and issues a seek operation. The seek results in the native media player  108  issuing a new request to the data source  118  for data at the new offset. The data source  118  switches the read position of the media buffer  106  to the location of the new media encoding data. The data is then returned from the media buffer  106  to the native media player  108 . 
     In  FIG. 3  is a diagram  200  of files  202 ,  204 ,  206 , and  208  used by the file encoder  116 , in one embodiment of the present invention, to create the concatenated media file  210 . The source media file  202  is transcoded into a plurality of encodings  204 ,  206 ,  208 , where each successive encoding is done at a lower bit rate than the previous (e.g. encoding  206  is a lower bit rate than encoding  204 , and encoding  208  is a lower bit rate than encoding  206 , etc). The differences in target bit rates are preferably large enough that the video compression schemes can actually achieve a difference in encoded bit rate, but not so large that transition between bit rates is overly disruptive to the viewer. For a given resolution and frame rate, which are preferably consistent through each encoding, there is typically a minimum achievable bit rate, below which the video cannot be reasonably compressed. In one embodiment, a source video may be encoded at resolution of 320×240, at a frame rate of 15 frames per second, and with three target bit rates of 500 kbps, 350 kbps, and 200 kbps. 
     The concatenated file  210  includes concatenations of the individual encodings  204 ,  206 , and  208  in sequence, without interleaving. Between each encoding, padding  212  may be inserted. The padding  212  is use to simplify offset calculations, and to provide a buffer zone for the data source  118  to issue stop commands to the native media player  108 . In one embodiment, the padding  212  may take the form of a video advertisement (typically between 5 and 30 seconds). In another embodiment, the padding  212  may take the form of a static banner advertisement which is displayed for a certain amount of time (typically between 5 and 30 seconds). In another embodiment, the padding  212  may take the form of a blank video (i.e. black screen) which is displayed for a certain amount of time (typically between 5 and 30 seconds). 
     In  FIG. 4  is a block diagram  300  of a rate map index file  304 , created by the file encoder  116 , in one embodiment of the present invention. Each of the rate map indices  306  is of uniform size, and packed without padding into the rate map index file  304 . The rate map indices  308  for each encoding are packed contiguously and in order, with the rate map indices of the encodings packed in the same order as the encodings are packed in the concatenated file  210 . 
     In  FIG. 5  is a block diagram  400  of the buffer management performed by the data source  118 , in one embodiment of the present invention. Data arrives from the downloader  104  and is placed into the media buffer  106 . In one embodiment, the encoded data is encrypted, and is first placed in the decryption buffer  120 . Once the encrypted data is decrypted, it is moved to the media buffer  106 . When an encoding switch is executed, the downloader  104  continues to deliver data  404  for the current encoding until the switch over time (which may be in the future) is reached. Additional buffer space  406  is reserved to accommodate the remaining data for the current encoding. In one embodiment, the data  408  for the new encoding is prefetched from the server and placed in the media buffer, beyond the reserved space  406 . When the switch over is signaled by the data source  118  to the native media player  108 , data will begin being sourced from the new encoding data  408 . 
     In one embodiment, switching encodings to one of lower bit rate is initiated when the average bandwidth falls below the current encoding&#39;s bit rate, and the buffer occupancy of the media buffer  106  falls below the playback threshold: 
     
       
         
               
               
             
               
             
               
               
             
               
               
             
               
             
               
               
             
               
               
             
               
               
             
               
             
           
               
                   
               
             
             
               
                 int bandwidth_avg 
                 // average available network bandwidth 
               
               
                 int video_bit_rate 
                 // current video encoding bit rate 
               
               
                 int buffer_occupancy 
                 // seconds of video currently in the buffer 
               
               
                 int playback_thresh 
                 // seconds of video buffered before playback starts 
               
             
          
           
               
                 if bandwidth_avg &lt; video_bit_rate &amp;&amp; buffer_occupancy &lt; 
               
               
                 playback_thresh 
               
             
          
           
               
                   
                 for each encoding sorted by bit rate in descending order 
               
             
          
           
               
                   
                 if encoding.bit_rate &lt; bandwidth_avg &amp;&amp; encoding.bit_rate != 
               
             
          
           
               
                 video_bit_rate 
               
             
          
           
               
                   
                 change encoding 
               
               
                   
                 break 
               
             
          
           
               
                   
                 end 
               
             
          
           
               
                   
                 end 
               
             
          
           
               
                 end 
               
               
                   
               
             
          
         
       
     
     In this scheme, the average network bandwidth is unable to sustain the video playout rate and a playback stoppage is imminent once the buffer runs out. This scheme requires relatively few calculations to determine when to switch encodings, however, it also has relatively low capability for predicting when a stoppage will occur. The encoding to switch to is the next lowest bit rate encoding whose bit rate is less than the average network bandwidth. Switching encodings to one of higher bit rate is initiated when the buffer occupancy of the media buffer  106  has reached its capacity and the average bandwidth exceeds the encoding bit rate of another encoding: 
     
       
         
               
               
             
               
             
               
               
             
               
               
             
               
             
               
               
             
               
               
             
               
               
             
               
             
           
               
                   
               
             
             
               
                 int bandwidth_avg 
                 // average available network bandwidth 
               
               
                 int video_bit_rate 
                 // current video encoding bit rate 
               
               
                 int buffer_occupancy 
                 // seconds of video currently in the buffer 
               
               
                 int buffer_capacity 
                 // seconds of video the buffer can hold 
               
             
          
           
               
                 if bandwidth_avg &gt; video_bit_rate &amp;&amp; buffer_occupancy &gt; 
               
               
                 buffer_capacity 
               
             
          
           
               
                   
                 for each encoding sorted by bit rate in descending order 
               
             
          
           
               
                   
                 if encoding.bit_rate &lt; bandwidth_avg &amp;&amp; encoding.bit_rate != 
               
             
          
           
               
                 video_bit_rate 
               
             
          
           
               
                   
                 change encoding 
               
               
                   
                 break 
               
             
          
           
               
                   
                 end 
               
             
          
           
               
                   
                 end 
               
             
          
           
               
                 end 
               
               
                   
               
             
          
         
       
     
     The encoding to switch to is the highest bit rate encoding whose bit rate is less than the average network bandwidth. This is an optimistic approach which assumes no further degradation in bit rate. This scheme works well when connected to a reliable, high bandwidth network. It waits until the last minute to change rate, without predicting when a rate range may be necessary. 
     In another embodiment, a rate predictive scheme is used, where the current average bandwidth estimate represents the incoming data rate for the media buffer  106 , and the current video bit rate represents the outgoing data rate for the media buffer  106 . The historical bandwidth samples are used as a predictor of future bandwidth availability and future incoming data rates for the media buffer  106 . The alternate video bit rates available are used to vary the possible future outgoing data rates for the media buffer  106 . The rate switching scheme uses the future incoming and outgoing data rates to estimate the future occupancy of the media buffer  106 . A threshold is set for optimal buffer occupancy. The optimal buffer occupancy is selected to minimize the probability of under-running the native media player  108 , while also limiting device resource usage (i.e. limiting the storage requirement of the media buffer  106 ). 
     
       
         
               
               
             
               
             
               
               
             
               
               
             
               
             
               
               
             
               
               
             
               
               
             
               
             
           
               
                   
               
             
             
               
                 int bandwidth_cur 
                 // current estimated network bandwidth 
               
               
                 int video_bit_rate 
                 // current video encoding bit rate 
               
               
                 array bandwidth_hist 
                 // historical bandwidth measurements 
               
               
                 array encoding_rates 
                 // bit rates of other available encodings 
               
               
                 int buffer_occupancy 
                 // seconds of video currently in the buffer 
               
               
                 int buffer_capacity 
                 // seconds of video the buffer can hold 
               
               
                 int buffer_optimal_hi 
                 // high threshold for optimal number of seconds 
               
               
                   
                 // of video to keep in the buffer 
               
               
                 int buffer_optimal_lo 
                 // low threshold for optimal number of seconds 
               
               
                   
                 // of video to keep in the buffer 
               
               
                 int prediction_period 
                 // seconds into the future to predict occupancy 
               
             
          
           
               
                 int acceleration 
               
               
                 int incoming 
               
               
                 int outgoing 
               
               
                 int predicted_occupancy 
               
               
                 acceleration = calculate_rate_of_change(bandwidth_hist) 
               
               
                 incoming = (bandwidth_cur + (acceleration / 2)) * prediction_period 
               
               
                 outgoing = video_bit_rate * prediction_period 
               
               
                 predicted_occupancy = buffer_occupancy + incoming − outgoing 
               
               
                 if predicted_occupancy &lt; buffer_optimal_lo | | predicted_occupancy &gt; 
               
               
                 buffer_optimal_hi 
               
             
          
           
               
                   
                 for each encoding_rate in encoding_rates sorted from highest to 
               
               
                   
                 lowest 
               
             
          
           
               
                   
                 outgoing = encoding_rate * prediction_period 
               
               
                   
                 predicted_occupancy = buffer_occupancy + incoming − 
               
               
                   
                 outgoing 
               
               
                   
                 if predicted_occupancy &gt; buffer_optimal_lo &amp;&amp; 
               
             
          
           
               
                 predicted_occupancy &lt; buffer_optimal_hi 
               
             
          
           
               
                   
                 change encoding 
               
               
                   
                 break 
               
             
          
           
               
                   
                 end 
               
             
          
           
               
                   
                 end 
               
             
          
           
               
                 end 
               
               
                   
               
             
          
         
       
     
     The algorithm shown above uses a basic linear prediction scheme for estimating future bandwidth. In one embodiment a linear bandwidth prediction scheme is used. This type of scheme requires less complexity to implement and can be used to smooth out samples with high jitter, however it provides coarse granularity for predicting changes in bandwidth. In another embodiment, a higher degree interpolation may be used to better simulate the changes in available bandwidth. This type of scheme requires more computational resources to implements, but provides a finer granularity for detecting changes in bandwidth. There are a number of algorithms for using historical data to approximate rate of change and should be known to those skilled in the art. 
     Once the bandwidth has been estimated, the maximum number of bits of data which could be received is calculated. In one embodiment, the maximum number of bits received may be reduced by a constant factor to provide bandwidth overhead and to limit the impact on network resources. The predicted future buffer occupancy for the current encoded bit rate is calculated. If the occupancy falls within the thresholds for optimal buffer usage, then nothing is done. If the occupancy falls outside the thresholds for optimal buffer usage, then predictions are performed for the other available encoding bit rates. The algorithm shown above checks to see if one of the alternate encodings can achieve the desired buffer occupancy. In one embodiment, an alternate encoding is selected only if it can achieve the desired buffer occupancy. In another embodiment, an alternate encoding is selected if it provides a closer match to the desired buffer occupancy. 
     
       
         
               
             
               
               
             
               
             
               
               
             
               
             
               
               
             
               
             
               
               
             
               
             
               
               
             
               
             
           
               
                   
               
             
             
               
                 if current_bit_rate_predicted_occupancy &lt; buffer_optimal_lo 
               
             
          
           
               
                   
                 current_bit_rate_distance = buffer_optimal_lo − 
               
             
          
           
               
                 current_bit_rate_predicted_occupancy 
               
               
                 else if current_bit_rate_predicted_occupancy &gt; buffer_optimal_hi 
               
             
          
           
               
                   
                 current_bit_rate_distance = 
               
             
          
           
               
                 current_bit_rate_predicted_occupancy − buffer_optimal_hi 
               
               
                 end 
               
               
                 if new_bit_rate_predicted_occupancy &lt; buffer_optimal_lo 
               
             
          
           
               
                   
                 new_bit_rate_distance = buffer_optimal_lo − 
               
               
                   
                 new_bit_rate_predicted_occupancy 
               
             
          
           
               
                 else if new_bit_rate_predicted_occupancy &gt; buffer_optimal_hi 
               
             
          
           
               
                   
                 new_bit_rate_distance = new_bit_rate_predicted_occupancy − 
               
             
          
           
               
                 buffer_optimal_hi 
               
               
                 end 
               
               
                 if new_bit_rate_distance &lt; current_bit_rate_distance 
               
             
          
           
               
                   
                 change encoding 
               
             
          
           
               
                 end 
               
               
                   
               
             
          
         
       
     
     In the scheme above, a new encoding is selected if it is deemed better than the previous one, by being closer to the desired buffer occupancy. In another embodiment, additional weight may be given to having higher rather than lower occupancy, though this may also be achieved by setting the high threshold higher. 
     In  FIG. 6  is a flow chart  500  describing the process of retrieving data and switching encodings, in accordance with one embodiment of the present invention. When a rendering request is issued for a given media, the native media player  108  notifies the data source  118  which in turn instructs the downloader  104  to start retrieving data in step  502 . In step  504 , the downloader  104  begins retrieving the concatenated file from the beginning. In one embodiment, the downloader  104  issues an HTTP range GET request to the HTTP server  112  for the concatenated file. In another embodiment, the downloader  104  issues an HTTP GET request with a query string specifying a range of data to retrieve. In one embodiment the range is specified in time. In another embodiment, the range is specified in frames, which directly correlate to time through a known fixed frame rate. 
     Download begins from the start of file so that file header information may be retrieved. It is assumed that sufficient bandwidth is available, as historical bandwidth data may not be available or current. In one embodiment, the concatenated media file is selected such that the first encoding matches the expected bandwidth availability for the network interface. In another embodiment, a query string parameter specifying the preferred initial encoding is added to the HTTP request and the server selects the a concatenated media file whose first encoding most closely matches the requested encoding. 
     While the data is being downloaded, the downloader  104  also estimates average bandwidth, in step  506 , by periodically checking to see how much data has been downloaded and calculating a download rate. The bandwidth estimate samples are saved in a circular bandwidth history buffer. In step  508 , the downloaded data and bandwidth estimate are passed to the data source  118 . The downloader  104  continues back to step  504  to download the next segment of data. In one embodiment, the downloader  104  pauses before issuing the next HTTP range GET request, in order to pace the requests and limit bandwidth usage. The data source  118  processes the data and bandwidth estimates separately and in parallel. The data processing begins in step  510 , while the bandwidth processing begins in step  520 . 
     In step  510 , the data source  118  checks to see if the downloaded data is encrypted. If it is encrypted it first writes the data to the decryption buffer  120  then decrypts the data, in step  512 . In one embodiment, software-based decryption is performed. In another embodiment, hardware assisted decryption is performed. Once the data is decrypted, or if the downloaded data was not encrypted, the unencrypted data is copied to the media buffer  106 , in step  514 . In step  516 , the data source  118  checks to see if the native media player  108  has already been started, or if it needs to be started, or if it needs to seek to a new position after an encoding switch. Playback will not be started unless a sufficient amount of data has been pre-buffered in the media buffer  106 , to prevent under-running the native media player  108 . If the native media player  108  has not been started, and the current media buffer  106  occupancy exceeds the initial buffer requirement threshold, then the native media player  108  is signaled to start playing, in step  518 . If the native media player  108  has started, and an encoding change is pending, and the new encoding data has been prefetched, then the native media player  108  is signaled to seek to the new file position, in step  518 . Once the native media player  108  has been signaled, or if no change is required by the native media player  108 , processing proceeds to step  532  where the data source  118  goes to sleep until the next range of data is delivered by the downloader  104 . 
     In step  520 , the data source  118  checks the current bandwidth estimate and the current media buffer  106  occupancy. In step  522 , the data source  118 , uses the bandwidth and buffer occupancy information to determine if a change is encoding is desirable. In one embodiment, if the available bandwidth is less than the encoded bit rate and the media buffer  106  contains less than the initial buffer requirement amount a change in encoding to one of lower bit rate is desired. If the available bandwidth is greater than the encoded bit rate and the media buffer  106  has reached its maximum capacity a change in encoding to one of higher bit rate is desired. In another embodiment, if the predicted future buffer occupancy is outside the bounds of the desired buffer occupancy and one of the alternate encodings&#39; bit rate would provide a future buffer occupancy closer to the desired buffer occupancy, then a change to the alternate encoding which should provide a future buffer occupancy closer to the desired buffer occupancy is desired. 
     If no bit rate changes are desired, then there is nothing to do and the data source  118  proceeds to step  532  and waits for the next bandwidth update. If a bit rate change is desired, then, in step  524 , a new encoding is selected along with a switch over time. In one embodiment, the switch over time is selected as the next key frame. In another embodiment, the switch over time is selected in the future to account for round trip latency in requesting the new data. The encoding with the highest bit rate that is lower than the available bandwidth estimate is chosen as the new encoding, assuming another encoding exists that meets the criteria. In one embodiment, data for the new encoding is retrieved directly from the HTTP server  112 , by issuing a new HTTP GET request containing a query string specifying the new range for which to begin retrieving data and the data source  118  proceeds directly to step  532  where it waits for the downloader to signal that the prefetch data has been retrieved. In another embodiment, the data source  118  calculates an offset into the rate map index file and asks the downloader  104  to retrieve the rate map index information. 
     In step  526 , the downloader  104  issues the HTTP range GET to the HTTP server  112  for the rate map index file information. In step  528 , the downloader  104  passes the rate map index information back to the data source  118 . The data source  118  determines the concatenated file byte offset for the first frame to be prefetched from the new encoding. In step  530 , the data source  118  instructs the downloader  104  to start retrieving data for the new encoding from the concatenated file byte offset. The downloader  104  proceeds to step  504 , where the common download infrastructure starts retrieving data from the new offset. The data source  118  proceeds to step  532  where it waits for the downloader to signal that the prefetch data has been retrieved. 
     In  FIG. 7  is a block diagram  700  for another embodiment of the present invention. It shows the client device  102  and media server  110  from block diagram  100  with three component changes. The standard HTTP server  112  has been replaced with a custom adaptive HTTP streaming server  712 , and the downloader  104  and data source  118  have been replaced with a simplified downloader  704  and a simplified data source  718  which do not require bandwidth estimation capabilities. An example of a suitable adaptive HTTP streaming server  712  is described in PCT Application No. PCT/US09/60120 filed Oct. 9, 2009 and entitled, Method And Apparatus For Efficient Http Data Streaming. 
     The streaming server  712  communicates with the client  102  via the standard HTTP protocol. The streaming server accepts query strings specifying an initial encoding. The streaming server selects a concatenated file with a first encoding that matches as closely as possible the requested encoding. The data is sent to the client in a paced manner to limit the bandwidth used by the server  110  and the client  102 . The streaming server  712  monitors TCP window fullness to estimate client bandwidth. As bandwidth decreases, TCP back pressure will cause the server-side TCP window to fill up. When this begins to occur, the streaming server  712  will detect congestion and switch encodings. The HTTP data is sent using the transfer encoding type chunked. At the beginning of each HTTP chunk is a header specifying the encoding, time-based position and concatenated file byte offset for the data within that chunk. Use of HTTP chunking and methods for packing headers into a chunk should be known to those skilled in the art. The downloader  704  extracts the data and the encoding information from the HTTP chunks and pass them to the data source  718 . The data source  718  places the data either in the media buffer  106  or in the temporary decryption buffer  120 , as before. The data source  718  also checks the encoding information for the chunk and checks it against the previous encoding information. If it matches, then no encoding change has occurred. If it doesn&#39;t match, then the offset information is used to notify the native player  108  to seek to the new position, corresponding to the new encoding data received. 
     In  FIG. 8  is a flow chart  800  describing a process of retrieving data and switching encodings, in accordance with another embodiment of the present invention. When a rendering request is issued for a given media, the native media player  108  notifies the data source  718  which in turn instructs the downloader  704  to start retrieving data in step  802 . The downloader  704  begins retrieving the concatenated file from the beginning. It issues an HTTP GET request to the adaptive HTTP streaming server  712  for the entire concatenated file. Download begins from the start of file so that file header information may be retrieved. The streaming server  718  selects the file to use in step  822 . The HTTP GET request contains a query string specifying the encoding to be retrieved. In one embodiment, the encoding is omitted on the initial request and a default concatenated file is chosen. In another embodiment, a concatenated file is chosen such that the first encoding in the concatenated file matches the requested encoding. If a concatenated file whose first encoding matches the requested encoding cannot be found, a default file is chosen. In one embodiment, the HTTP GET request also specifies a start position. In one embodiment the start position is specified in time. In another embodiment, the start position is specified in frames, which directly correlate to time through a known fixed frame rate. 
     In step  824 , the streaming server  712  creates the header containing the encoding, time-based position, and rate map index information for the current segment of data and sends the first HTTP chunk containing the header and data to the client  102 . In step  804 , the downloader  704  parses the HTTP chunk extracting the file data as well as the encoding and rate map index information. In step  808 , the downloaded data and encoding and rate map index information are passed to the data source  718 . The downloader  704  continues back to step  804  to wait for the next HTTP chunk. The data source  718  begins processing the data and encoding and rate map index information in step  810 . The streaming server  712  processing continues in parallel in step  826 . 
     The data processing steps  810  through  818  are identical to those of steps  510  through  518  from process  500  discussed above. In step  810 , the data source  718  checks to see if the downloaded data is encrypted. If it is encrypted it first writes the data to the decryption buffer  120  then decrypts the data, in step  812 . In one embodiment, software-based decryption is performed. In another embodiment, hardware assisted decryption is performed. In one embodiment, the decryptor is initialized with the rate map index information supplied by the streaming server  712  for the current data, since many decryption schemes are data byte offset dependent, as should be known to those skilled in the art. Once the data is decrypted, or if the downloaded data was not encrypted, the unencrypted data is copied to the media buffer  106 , in step  814 . In step  816 , the data source  718  checks to see if the native media player  108  has already been started, or if it needs to be started, or if it needs to seek to a new position after an encoding switch. An encoding switch is determined by comparing the encoding information provided by the streaming server  712  with the current data, to the encoding information provided by the streaming server  712  for the previous data. If the encodings differ, then a server initiated encoding switch has occurred. Playback will not be started unless a sufficient amount of data has been pre-buffered in the media buffer  106 , to prevent under-running the native media player  108 . If the native media player  108  has not been started, and the current media buffer  106  occupancy exceeds the initial buffer requirement threshold, then the native media player  108  is signaled to start playing, in step  818 . If the native media player  108  has started, and an encoding change is pending, then the native media player  108  is signaled to seek to the new file position, in step  818 . Once the native media player  108  has been signaled, or if no change is required by the native media player  108 , processing proceeds to step  820  where the data source  718  goes to sleep until the next chunk of data is delivered by the downloader  704 . 
     In step  826 , the streaming server  712  checks to see if an encoding change is desired. In one embodiment, the streaming server  712  estimates the bandwidth available by measuring the amount of data accepted to the TCP window. If a non-blocking write is issued for an amount of data greater than the TCP window size, data equal to the amount of space left in the TCP window will be accepted. The current window occupancy can be estimated as (T−W), where T is the TCP window capacity and W is the amount of data written. The streaming server  712  maintains a history of window occupancy. The change in available bandwidth may be calculated as the difference what the streaming server  712  attempted to send, and what is still left in the TCP window, over the period. If the estimated available bandwidth falls below a certain threshold, or climbs above an alternate threshold, a change in encoding is desired. 
     If no change in encoding is desired, processing continues back to step  824 , where the next HTTP chunk is sent to the client  102 . In one embodiment, the streaming server  711  pauses before sending the next HTTP chunk, in order to pace the requests and limit bandwidth usage. The streaming server  712  knows the bit rate of the given encoding, and pauses for a time equal to (D/R*M) where D is the size of the file data segment, R is the bit rate of the current encoding and M is a multiplier greater than one used to decrease the time between sends to prevent under-running the client  102 . Otherwise, if an encoding change is desired, then processing continues to step  828 . In step  828 , a new encoding is selected. In one embodiment, the streaming server  712  sequentially selects new rates based on bit rate. This is an optimistic algorithm for environments that expect low variations in available bit rate. In such an environment, stepping down to the next lowest bit rate or stepping up to the next highest bit rate provides the least noticeable change both to network bandwidth and user perception. In another embodiment, the streaming server  712  selects the encoding whose bit rate is closest to the bandwidth estimation. 
       FIG. 9  is a diagram  900  of files  210 ,  904 ,  906 , and  908  used to create ad stitched media files  910  and  912 . In one embodiment, the feature content is a concatenated file  210 , suitable for use with the dynamic rate adaptation methods of embodiments of the present invention. The feature content is padded with padding  902 , which is separate from the concatenation padding  212 . The padding  902  serves two purposes: to equalize the audio and video track durations and to provide a consistent baseline for stitching. When stitching a first video to a second video, because of the compression schemes used, the last few frames of the first video and the first few frames of the second video may be altered to achieve the best possible compression. By padding out the feature content  210  with a neutral padding  902 , stitching to this baseline should not cause frame distortion in the second video. These techniques should be known to those skilled in the art. In one embodiment, the ads  904 ,  906 , and  908  may all be of the same duration. In another embodiment, the ads  904 ,  906 , and  908  may all be of different durations. Even if ads  904 ,  906 , and  908  are of the same duration, their file sizes will most likely differ due to variability in compression for the actual content. In one embodiment, the ads  904 ,  906 , and  908  may be concatenated files, suitable for use with the dynamic rate adaptation methods of embodiments of the present invention. The stitching media files  910  and  912  are created by stitching the feature content  210 , with its padding  902 , to one or more ads. Stitched media file  910  shows the feature content  210 , with its padding  902 , stitched to ad  904 . Stitched media file  912  shows the feature content  210 , with its padding  902 , stitched to ads  906  and  908 . 
       FIG. 10  is a diagram  1000  of file particles  1002 ,  1004 ,  1006 ,  1008 , and  1010  and particle index files  1012  and  1014 , which are created from the stitched media files  910  and  912 . Dynamic header particles  1002  and  1006  consist of header information from the stitched media files  910  and  912 , respectively. Static particle  1010  consists of the feature content  210  plus padding  902 , without any header information. Dynamic ad particles  1004  and  1008  consist of the stitched ads from the stitched media files  910  and  912 , respectively. The particles  1002 ,  1004 ,  1006 ,  1008 , and  1010  are created by dividing the stitched media files  910  and  912  at the exact stitched media file byte offset of the first frame of the feature content  210  and the first frame of the first ad ( 904  or  906 ) stitched to the feature content  210 , respectively. The particle index files  1012  and  1014  contain file name information for locating each particle, particle version information for determining if the particles have changed, and clip offset information to determine the stitched media file byte offset of the clip. 
     The header information contained in the header particle ( 1002  or  1006 ) contains mapping information for frames in the stitched media file. The mapping information contains stitched media file byte offset information specific to the container format of the stitched media file, as should be known to those skilled in the art. The stitched media file byte offset of the first frame of the feature content  210  will be different, depending upon how much header information is in the file. Given a stitched media file with header  1002  length H and feature content particle  1010  length F (including padding  902 ), the first frame of the feature content  1010  will begin at a stitched media byte offset of H, and the ads  1004  will begin at an offset H+F. However, if different ads are stitched to the feature content  1010 , then the new header  1006  length H′ may be different from the previous header length H, causing the first frame of the feature content to begin at a stitched media byte offset of H′, and the ads  1008  to begin at offset H′+F. These offsets allow the data source  118  to reuse the same feature content particle  1010 , while changing the ad particle (e.g. from  1004  to  1008 ) as long as it knows the proper offsets and file locations. 
       FIG. 11  is a flow chart  1100  showing a method for retrieving stitched media file particles for use in ad rotation. This procedure may be implemented as part of the initialization step  502  in procedure  500 . When the user requests a video in step  1102 , rather than immediately proceeding to step  504  to download the feature content  210 , the downloader  104  first retrieves the most current particle index file (e.g.,  1012  or  1014 ). In step  1104 , the downloader  104  checks to see if the header version is different from any cached version. If the header version has not changed, then processing proceeds to step  1112  and process  500  is initiated at step  504  for downloading the feature content particle  1010 . If the header version is different from the cached version, or if no cached version exists, processing proceeds to step  1108  where the new header particle (e.g.,  1002  or  1006 ) is downloaded and passed to the data source  118 . The data source  118  replaces any previous header particle information in the media buffer  106 , with the new header particle information. The data source  118  makes note of the offset values from the header particle (e.g.,  1002  or  1006 ) for use in managing the circular media buffer  106 . Once the headers are downloaded two separate download processes are initiated. Both download processes follow the procedure  500 , starting at step  504 . From step  1112 , download of the feature content particle  1010  is initiated. From step  1114 , download of the ad particle (e.g.,  1004  or  1008 ) is initiated. 
     The procedure for displaying ads is similar to that of changing rates. A list of ad insertion points, based on time offsets in the feature content  210 , are provided to the data source  118 . When an ad is to be displayed, the data source  118  signals the native client media player  108 , to seek to the position in the stitched media file where the ad resides. Once the ad has finished playing, the data source  118  signals the native client media player  108 , to seek to the position in the stitched media file where the feature content  1010  left off. 
       FIGS. 12 and 13  illustrate the above-mentioned special server modification, in which the server responds to a special http GET command with a mime-header labeled “SWITCH” to cancel the last segment request and request at a lower bitrate. This request may also used to send the acknowledgment to squelch retransmissions. The switch command is used to propagate the squelch upwards through the application and enable the server to switch to a new lower bitrate at the next segment indicated in the body of the request. The server could choose to advance to a more recent segment in the response as indicated via a SWITCH response header. The interchange is shown in  FIG. 12 .  FIG. 13  shows the structure of the segment including the segment number that indicates to the client the position of the segment in the stream and that corresponds to the data in the segment. 
     In the description herein for embodiments of the present invention, numerous specific details are provided, such as examples of components and/or methods, to provide a thorough understanding of embodiments of the present invention. One skilled in the relevant art will recognize, however, that an embodiment of the invention can be practiced without one or more of the specific details, or with other apparatus, systems, assemblies, methods, components, materials, parts, and/or the like. In other instances, well-known structures, materials, or operations are not specifically shown or described in detail to avoid obscuring aspects of embodiments of the present invention. 
     Although the above description includes numerous specifics in the interest of a fully enabling teaching, it will be appreciated that the present invention can be realized in a variety of other manners and encompasses all implementations falling within the scope of the claims herein.