Patent Publication Number: US-2022239601-A1

Title: Background data traffic distribution of media data

Description:
This application claims the benefit of U.S. Provisional Application No. 63/141,580, filed Jan. 26, 2021, the entire contents of which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to transport of encoded media data. 
     BACKGROUND 
     Digital video capabilities can be incorporated into a wide range of devices, including digital televisions, digital direct broadcast systems, wireless broadcast systems, personal digital assistants (PDAs), laptop or desktop computers, digital cameras, digital recording devices, digital media players, video gaming devices, video game consoles, cellular or satellite radio telephones, video teleconferencing devices, and the like. Digital video devices implement video compression techniques, such as those described in the standards defined by MPEG-2, MPEG-4, ITU-T H.263 or ITU-T H.264/MPEG-4, Part 10, Advanced Video Coding (AVC), ITU-T H.265 (also referred to as High Efficiency Video Coding (HEVC)), and extensions of such standards, to transmit and receive digital video information more efficiently. 
     After media data, such as video data, has been encoded, the media data may be packetized for transmission or storage. The media data may be assembled into a media file conforming to any of a variety of standards, such as the International Organization for Standardization (ISO) base media file format and extensions thereof, such as AVC. 
     SUMMARY 
     In general, this disclosure describes techniques for streaming media data using background data transfer. In some instances, background data transfer may be used to distribute content in an efficient manner to customers. That is, media data may be sent to client devices during off-peak hours (e.g., when many users are asleep or otherwise not using their devices). Users of the client devices may then play back media data transferred via background data transfer at a later time. Mobile network operators (MNOs) may offer reduced charging for traffic during off-peak hours. An application provider may make predictions about what content will be consumed by various customers, then push the appropriate content to corresponding client devices (also referred to as “user equipment” or “UE”) during a designated time window, e.g., during off-peak hours. 
     This disclosure describes various techniques related to transferring media data using background data transfer. For example, this disclosure describes techniques related to managing the download process on client devices and the network, how downloads may be triggered, and how cache space on the client devices may be managed. 
     In one example, a method of retrieving media data includes sending, by one or more processors of a client device, a request to retrieve media data according to a background data transfer to a media streaming application function (AF); in response to the request, receiving, by the one or more processors of the client device, an indication of a background data transfer opportunity from the media streaming AF; in response to the indication of the background data transfer opportunity, retrieving, by the one or more processors, the media data according to the background data transfer; and storing, by the one or more processors, the retrieved media data. 
     In another example, a device for retrieving media data, the device comprising: a memory configured to store media data; and one or more processors implemented in circuitry and configured to: send a request to retrieve media data according to a background data transfer to a media streaming application function (AF); in response to the request, receive an indication of a background data transfer opportunity from the media streaming AF; in response to the indication of the background data transfer opportunity, retrieve the media data according to the background data transfer; and store the retrieved media data to the memory. 
     In another example, a computer-readable storage medium has stored thereon instructions that, when executed, cause one or more processors of a client device to: send a request to retrieve media data according to a background data transfer to a media streaming application function (AF); in response to the request, receive an indication of a background data transfer opportunity from the media streaming AF; in response to the indication of the background data transfer opportunity, retrieve the media data according to the background data transfer; and store the retrieved media data to the memory. 
     In another example, a device for retrieving media data includes: means for sending a request to retrieve media data according to background data transfer; means for receiving, in response to the request, an indication of a background data transfer opportunity; means for retrieving, in response to the indication of the background data transfer opportunity, the media data according to the background data transfer; and means for storing the retrieved media data. 
     The details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating an example system that implements techniques for streaming media data over a network. 
         FIG. 2  is a block diagram illustrating an example set of components of a retrieval unit. 
         FIG. 3  is a conceptual diagram illustrating elements of example multimedia content. 
         FIG. 4  is a block diagram illustrating elements of an example video file, which may correspond to a segment of a representation. 
         FIG. 5  is a block diagram illustrating an example system that may perform the techniques of this disclosure. 
         FIG. 6  is a flowchart illustrating an example method for transporting media data using a background data transfer according to the techniques of this disclosure. 
         FIG. 7  is a flowchart illustrating an example method of retrieving media data according to the techniques of this disclosure. 
         FIG. 8  is a flowchart illustrating another example method of retrieving media data according to the techniques of this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In general, this disclosure describes techniques for streaming media data using background data transfer. In some instances, background data transfer may be used to distribute content in an efficient manner to customers. That is, media data may be sent to client devices during off-peak hours (e.g., when many users are asleep or otherwise not using their devices). Users of the client devices may then play back media data transferred via background data transfer at a later time. Mobile network operators (MNOs) may offer reduced charging for traffic during off-peak hours. An application provider may make predictions about what content will be consumed by various customers, and then push the appropriate content to corresponding client devices (also referred to as “user equipment” or “UE”), based on the predictions, during a designated time window, e.g., during off-peak hours. 
     This disclosure describes various techniques related to transferring media data using background data transfer. For example, this disclosure describes techniques related to managing the download process on client devices and the network, how downloads may be triggered, and how cache space on the client devices may be managed. 
     The techniques of this disclosure may be applied to video files conforming to video data encapsulated according to any of ISO base media file format, Scalable Video Coding (SVC) file format, Advanced Video Coding (AVC) file format, Third Generation Partnership Project (3GPP) file format, and/or Multiview Video Coding (MVC) file format, or other similar video file formats. 
     In HTTP streaming, frequently used operations include HEAD, GET, and partial GET. The HEAD operation retrieves a header of a file associated with a given uniform resource locator (URL) or uniform resource name (URN), without retrieving a payload associated with the URL or URN. The GET operation retrieves a whole file associated with a given URL or URN. The partial GET operation receives a byte range as an input parameter and retrieves a continuous number of bytes of a file, where the number of bytes correspond to the received byte range. Thus, movie fragments may be provided for HTTP streaming, because a partial GET operation can get one or more individual movie fragments. In a movie fragment, there can be several track fragments of different tracks. In HTTP streaming, a media presentation may be a structured collection of data that is accessible to the client. The client may request and download media data information to present a streaming service to a user. 
     In the example of streaming 3GPP data using HTTP streaming, there may be multiple representations for video and/or audio data of multimedia content. As explained below, different representations may correspond to different coding characteristics (e.g., different profiles or levels of a video coding standard), different coding standards or extensions of coding standards (such as multiview and/or scalable extensions), or different bitrates. The manifest of such representations may be defined in a Media Presentation Description (MPD) data structure. A media presentation may correspond to a structured collection of data that is accessible to an HTTP streaming client device. The HTTP streaming client device may request and download media data information to present a streaming service to a user of the client device. A media presentation may be described in the MPD data structure, which may include updates of the MPD. 
     A media presentation may contain a sequence of one or more Periods. Each period may extend until the start of the next Period, or until the end of the media presentation, in the case of the last period. Each period may contain one or more representations for the same media content. A representation may be one of a number of alternative encoded versions of audio, video, timed text, or other such data. The representations may differ by encoding types, e.g., by bitrate, resolution, and/or codec for video data and bitrate, language, and/or codec for audio data. The term representation may be used to refer to a section of encoded audio or video data corresponding to a particular period of the multimedia content and encoded in a particular way. 
     Representations of a particular period may be assigned to a group indicated by an attribute in the MPD indicative of an adaptation set to which the representations belong. Representations in the same adaptation set are generally considered alternatives to each other, in that a client device can dynamically and seamlessly switch between these representations, e.g., to perform bandwidth adaptation. For example, each representation of video data for a particular period may be assigned to the same adaptation set, such that any of the representations may be selected for decoding to present media data, such as video data or audio data, of the multimedia content for the corresponding period. The media content within one period may be represented by either one representation from group 0, if present, or the combination of at most one representation from each non-zero group, in some examples. Timing data for each representation of a period may be expressed relative to the start time of the period. 
     A representation may include one or more segments. Each representation may include an initialization segment, or each segment of a representation may be self-initializing. When present, the initialization segment may contain initialization information for accessing the representation. In general, the initialization segment does not contain media data. A segment may be uniquely referenced by an identifier, such as a uniform resource locator (URL), uniform resource name (URN), or uniform resource identifier (URI). The MPD may provide the identifiers for each segment. In some examples, the MPD may also provide byte ranges in the form of a range attribute, which may correspond to the data for a segment within a file accessible by the URL, URN, or URI. 
     Different representations may be selected for substantially simultaneous retrieval for different types of media data. For example, a client device may select an audio representation, a video representation, and a timed text representation from which to retrieve segments. In some examples, the client device may select particular adaptation sets for performing bandwidth adaptation. That is, the client device may select an adaptation set including video representations, an adaptation set including audio representations, and/or an adaptation set including timed text. Alternatively, the client device may select adaptation sets for certain types of media (e.g., video), and directly select representations for other types of media (e.g., audio and/or timed text). 
       FIG. 1  is a block diagram illustrating an example system  10  that implements techniques for streaming media data over a network. In this example, system  10  includes content preparation device  20 , server device  60 , and client device  40 . Client device  40  and server device  60  are communicatively coupled by network  74 , which may comprise the Internet. In some examples, content preparation device  20  and server device  60  may also be coupled by network  74  or another network, or may be directly communicatively coupled. In some examples, content preparation device  20  and server device  60  may comprise the same device. 
     Content preparation device  20 , in the example of  FIG. 1 , comprises audio source  22  and video source  24 . Audio source  22  may comprise, for example, a microphone that produces electrical signals representative of captured audio data to be encoded by audio encoder  26 . Alternatively, audio source  22  may comprise a storage medium storing previously recorded audio data, an audio data generator such as a computerized synthesizer, or any other source of audio data. Video source  24  may comprise a video camera that produces video data to be encoded by video encoder  28 , a storage medium encoded with previously recorded video data, a video data generation unit such as a computer graphics source, or any other source of video data. Content preparation device  20  is not necessarily communicatively coupled to server device  60  in all examples, but may store multimedia content to a separate medium that is read by server device  60 . 
     Raw audio and video data may comprise analog or digital data. Analog data may be digitized before being encoded by audio encoder  26  and/or video encoder  28 . Audio source  22  may obtain audio data from a speaking participant while the speaking participant is speaking, and video source  24  may simultaneously obtain video data of the speaking participant. In other examples, audio source  22  may comprise a computer-readable storage medium comprising stored audio data, and video source  24  may comprise a computer-readable storage medium comprising stored video data. In this manner, the techniques described in this disclosure may be applied to live, streaming, real-time audio and video data or to archived, pre-recorded audio and video data. 
     Audio frames that correspond to video frames are generally audio frames containing audio data that was captured (or generated) by audio source  22  contemporaneously with video data captured (or generated) by video source  24  that is contained within the video frames. For example, while a speaking participant generally produces audio data by speaking, audio source  22  captures the audio data, and video source  24  captures video data of the speaking participant at the same time, that is, while audio source  22  is capturing the audio data. Hence, an audio frame may temporally correspond to one or more particular video frames. Accordingly, an audio frame corresponding to a video frame generally corresponds to a situation in which audio data and video data were captured at the same time and for which an audio frame and a video frame comprise, respectively, the audio data and the video data that were captured at the same time. 
     In some examples, audio encoder  26  may encode a timestamp in each encoded audio frame that represents a time at which the audio data for the encoded audio frame was recorded, and similarly, video encoder  28  may encode a timestamp in each encoded video frame that represents a time at which the video data for an encoded video frame was recorded. In such examples, an audio frame corresponding to a video frame may comprise an audio frame comprising a timestamp and a video frame comprising the same timestamp. Content preparation device  20  may include an internal clock from which audio encoder  26  and/or video encoder  28  may generate the timestamps, or that audio source  22  and video source  24  may use to associate audio and video data, respectively, with a timestamp. 
     In some examples, audio source  22  may send data to audio encoder  26  corresponding to a time at which audio data was recorded, and video source  24  may send data to video encoder  28  corresponding to a time at which video data was recorded. In some examples, audio encoder  26  may encode a sequence identifier in encoded audio data to indicate a relative temporal ordering of encoded audio data but without necessarily indicating an absolute time at which the audio data was recorded, and similarly, video encoder  28  may also use sequence identifiers to indicate a relative temporal ordering of encoded video data. Similarly, in some examples, a sequence identifier may be mapped or otherwise correlated with a timestamp. 
     Audio encoder  26  generally produces a stream of encoded audio data, while video encoder  28  produces a stream of encoded video data. Each individual stream of data (whether audio or video) may be referred to as an elementary stream. An elementary stream is a single, digitally coded (possibly compressed) component of a representation. For example, the coded video or audio part of the representation can be an elementary stream. An elementary stream may be converted into a packetized elementary stream (PES) before being encapsulated within a video file. Within the same representation, a stream ID may be used to distinguish the PES-packets belonging to one elementary stream from the other. The basic unit of data of an elementary stream is a packetized elementary stream (PES) packet. Thus, coded video data generally corresponds to elementary video streams. Similarly, audio data corresponds to one or more respective elementary streams. 
     Many video coding standards, such as ITU-T H.264/AVC, the ITU-T H.265 High Efficiency Video Coding (HEVC) standard, and the ITU-T H.266 Versatile Video Coding (VVC) standard, define the syntax, semantics, and decoding process for error-free bitstreams, any of which conform to a certain profile or level. Video coding standards typically do not specify the encoder, but the encoder is tasked with guaranteeing that the generated bitstreams are standard-compliant for a decoder. In the context of video coding standards, a “profile” corresponds to a subset of algorithms, features, or tools and constraints that apply to them. As defined by the H.264 standard, for example, a “profile” is a subset of the entire bitstream syntax that is specified by the H.264 standard. A “level” corresponds to the limitations of the decoder resource consumption, such as, for example, decoder memory and computation, which are related to the resolution of the pictures, bit rate, and block processing rate. A profile may be signaled with a profile_idc (profile indicator) value, while a level may be signaled with a level_idc (level indicator) value. 
     The H.264 standard, for example, recognizes that, within the bounds imposed by the syntax of a given profile, it is still possible to require a large variation in the performance of encoders and decoders depending upon the values taken by syntax elements in the bitstream such as the specified size of the decoded pictures. The H.264 standard further recognizes that, in many applications, it is neither practical nor economical to implement a decoder capable of dealing with all hypothetical uses of the syntax within a particular profile. Accordingly, the H.264 standard defines a “level” as a specified set of constraints imposed on values of the syntax elements in the bitstream. These constraints may be simple limits on values. Alternatively, these constraints may take the form of constraints on arithmetic combinations of values (e.g., picture width multiplied by picture height multiplied by number of pictures decoded per second). The H.264 standard further provides that individual implementations may support a different level for each supported profile. 
     A decoder conforming to a profile ordinarily supports all the features defined in the profile. For example, as a coding feature, B-picture coding is not supported in the baseline profile of H.264/AVC but is supported in other profiles of H.264/AVC. A decoder conforming to a level should be capable of decoding any bitstream that does not require resources beyond the limitations defined in the level. Definitions of profiles and levels may be helpful for interpretability. For example, during video transmission, a pair of profile and level definitions may be negotiated and agreed for a whole transmission session. More specifically, in H.264/AVC, a level may define limitations on the number of macroblocks that need to be processed, decoded picture buffer (DPB) size, coded picture buffer (CPB) size, vertical motion vector range, maximum number of motion vectors per two consecutive MBs, and whether a B-block can have sub-macroblock partitions less than 8×8 pixels. In this manner, a decoder may determine whether the decoder is capable of properly decoding the bitstream. 
     In the example of  FIG. 1 , encapsulation unit  30  of content preparation device  20  receives elementary streams comprising coded video data from video encoder  28  and elementary streams comprising coded audio data from audio encoder  26 . In some examples, video encoder  28  and audio encoder  26  may each include packetizers for forming PES packets from encoded data. In other examples, video encoder  28  and audio encoder  26  may each interface with respective packetizers for forming PES packets from encoded data. In still other examples, encapsulation unit  30  may include packetizers for forming PES packets from encoded audio and video data. 
     Video encoder  28  may encode video data of multimedia content in a variety of ways, to produce different representations of the multimedia content at various bitrates and with various characteristics, such as pixel resolutions, frame rates, conformance to various coding standards, conformance to various profiles and/or levels of profiles for various coding standards, representations having one or multiple views (e.g., for two-dimensional or three-dimensional playback), or other such characteristics. A representation, as used in this disclosure, may comprise one of audio data, video data, text data (e.g., for closed captions), or other such data. The representation may include an elementary stream, such as an audio elementary stream or a video elementary stream. Each PES packet may include a stream_id that identifies the elementary stream to which the PES packet belongs. Encapsulation unit  30  is responsible for assembling elementary streams into video files (e.g., segments) of various representations. 
     Encapsulation unit  30  receives PES packets for elementary streams of a representation from audio encoder  26  and video encoder  28  and forms corresponding network abstraction layer (NAL) units from the PES packets. Coded video segments may be organized into NAL units, which provide a “network-friendly” video representation addressing applications such as video telephony, storage, broadcast, or streaming. NAL units can be categorized to Video Coding Layer (VCL) NAL units and non-VCL NAL units. VCL units may contain the core compression engine and may include block, macroblock, and/or slice level data. Other NAL units may be non-VCL NAL units. In some examples, a coded picture in one time instance, normally presented as a primary coded picture, may be contained in an access unit, which may include one or more NAL units. 
     Non-VCL NAL units may include parameter set NAL units and SEI NAL units, among others. Parameter sets may contain sequence-level header information (in sequence parameter sets (SPS)) and the infrequently changing picture-level header information (in picture parameter sets (PPS)). With parameter sets (e.g., PPS and SPS), infrequently changing information need not to be repeated for each sequence or picture; hence, coding efficiency may be improved. Furthermore, the use of parameter sets may enable out-of-band transmission of the important header information, avoiding the need for redundant transmissions for error resilience. In out-of-band transmission examples, parameter set NAL units may be transmitted on a different channel than other NAL units, such as SEI NAL units. 
     Supplemental Enhancement Information (SEI) may contain information that is not necessary for decoding the coded pictures samples from VCL NAL units, but may assist in processes related to decoding, display, error resilience, and other purposes. SEI messages may be contained in non-VCL NAL units. SEI messages are the normative part of some standard specifications, and thus are not always mandatory for standard compliant decoder implementation. SEI messages may be sequence level SEI messages or picture level SEI messages. Some sequence level information may be contained in SEI messages, such as scalability information SEI messages in the example of SVC and view scalability information SEI messages in MVC. These example SEI messages may convey information on, e.g., extraction of operation points and characteristics of the operation points. In addition, encapsulation unit  30  may form a manifest file, such as a media presentation descriptor (MPD) that describes characteristics of the representations. Encapsulation unit  30  may format the MPD according to extensible markup language (XML). 
     Encapsulation unit  30  may provide data for one or more representations of multimedia content, along with the manifest file (e.g., the MPD) to output interface  32 . Output interface  32  may comprise a network interface or an interface for writing to a storage medium, such as a universal serial bus (USB) interface, a CD or DVD writer or burner, an interface to magnetic or flash storage media, or other interfaces for storing or transmitting media data. Encapsulation unit  30  may provide data of each of the representations of multimedia content to output interface  32 , which may send the data to server device  60  via network transmission or storage media. In the example of  FIG. 1 , server device  60  includes storage medium  62  that stores various multimedia contents  64 , each including a respective manifest file  66  and one or more representations  68 A- 68 N (representations  68 ). In some examples, output interface  32  may also send data directly to network  74 . 
     In some examples, representations  68  may be separated into adaptation sets. That is, various subsets of representations  68  may include respective common sets of characteristics, such as codec, profile and level, resolution, number of views, file format for segments, text type information that may identify a language or other characteristics of text to be displayed with the representation and/or audio data to be decoded and presented, e.g., by speakers, camera angle information that may describe a camera angle or real-world camera perspective of a scene for representations in the adaptation set, rating information that describes content suitability for particular audiences, or the like. 
     Manifest file  66  may include data indicative of the subsets of representations  68  corresponding to particular adaptation sets, as well as common characteristics for the adaptation sets. Manifest file  66  may also include data representative of individual characteristics, such as bitrates, for individual representations of adaptation sets. In this manner, an adaptation set may provide for simplified network bandwidth adaptation. Representations in an adaptation set may be indicated using child elements of an adaptation set element of manifest file  66 . 
     Server device  60  includes request processing unit  70  and network interface  72 . In some examples, server device  60  may include a plurality of network interfaces. Furthermore, any or all of the features of server device  60  may be implemented on other devices of a content delivery network, such as routers, bridges, proxy devices, switches, or other devices. In some examples, intermediate devices of a content delivery network may cache data of multimedia content  64 , and include components that conform substantially to those of server device  60 . In general, network interface  72  is configured to send and receive data via network  74 . 
     Request processing unit  70  is configured to receive network requests from client devices, such as client device  40 , for data of storage medium  62 . For example, request processing unit  70  may implement hypertext transfer protocol (HTTP) version 1.1, as described in RFC 2616, “Hypertext Transfer Protocol—HTTP/1.1,” by R. Fielding et al, Network Working Group, IETF, June 1999. That is, request processing unit  70  may be configured to receive HTTP GET or partial GET requests and provide data of multimedia content  64  in response to the requests. The requests may specify a segment of one of representations  68 , e.g., using a URL of the segment. In some examples, the requests may also specify one or more byte ranges of the segment, thus comprising partial GET requests. Request processing unit  70  may further be configured to service HTTP HEAD requests to provide header data of a segment of one of representations  68 . In any case, request processing unit  70  may be configured to process the requests to provide requested data to a requesting device, such as client device  40 . 
     Additionally or alternatively, request processing unit  70  may be configured to deliver media data via a broadcast or multicast protocol, such as eMBMS. Content preparation device  20  may create DASH segments and/or sub-segments in substantially the same way as described, but server device  60  may deliver these segments or sub-segments using eMBMS or another broadcast or multicast network transport protocol. For example, request processing unit  70  may be configured to receive a multicast group join request from client device  40 . That is, server device  60  may advertise an Internet protocol (IP) address associated with a multicast group to client devices, including client device  40 , associated with particular media content (e.g., a broadcast of a live event). Client device  40 , in turn, may submit a request to join the multicast group. This request may be propagated throughout network  74 , e.g., routers making up network  74 , such that the routers are caused to direct traffic destined for the IP address associated with the multicast group to subscribing client devices, such as client device  40 . 
     As illustrated in the example of  FIG. 1 , multimedia content  64  includes manifest file  66 , which may correspond to a media presentation description (MPD). Manifest file  66  may contain descriptions of different alternative representations  68  (e.g., video services with different qualities) and the description may include, e.g., codec information, a profile value, a level value, a bitrate, and other descriptive characteristics of representations  68 . Client device  40  may retrieve the MPD of a media presentation to determine how to access segments of representations  68 . 
     In particular, retrieval unit  52  may retrieve configuration data (not shown) of client device  40  to determine decoding capabilities of video decoder  48  and rendering capabilities of video output  44 . The configuration data may also include any or all of a language preference selected by a user of client device  40 , one or more camera perspectives corresponding to depth preferences set by the user of client device  40 , and/or a rating preference selected by the user of client device  40 . Retrieval unit  52  may comprise, for example, a web browser or a media client configured to submit HTTP GET and partial GET requests. Retrieval unit  52  may correspond to software instructions executed by one or more processors or processing units (not shown) of client device  40 . In some examples, all or portions of the functionality described with respect to retrieval unit  52  may be implemented in hardware, or a combination of hardware, software, and/or firmware, where requisite hardware may be provided to execute instructions for software or firmware. 
     Retrieval unit  52  may compare the decoding and rendering capabilities of client device  40  to characteristics of representations  68  indicated by information of manifest file  66 . Retrieval unit  52  may initially retrieve at least a portion of manifest file  66  to determine characteristics of representations  68 . For example, retrieval unit  52  may request a portion of manifest file  66  that describes characteristics of one or more adaptation sets. Retrieval unit  52  may select a subset of representations  68  (e.g., an adaptation set) having characteristics that can be satisfied by the coding and rendering capabilities of client device  40 . Retrieval unit  52  may then determine bitrates for representations in the adaptation set, determine a currently available amount of network bandwidth, and retrieve segments from one of the representations having a bitrate that can be satisfied by the network bandwidth. 
     In general, higher bitrate representations may yield higher quality video playback, while lower bitrate representations may provide sufficient quality video playback when available network bandwidth decreases. Accordingly, when available network bandwidth is relatively high, retrieval unit  52  may retrieve data from relatively high bitrate representations, whereas when available network bandwidth is low, retrieval unit  52  may retrieve data from relatively low bitrate representations. In this manner, client device  40  may stream multimedia data over network  74  while also adapting to changing network bandwidth availability of network  74 . 
     Additionally or alternatively, retrieval unit  52  may be configured to receive data in accordance with a broadcast or multicast network protocol, such as eMBMS or IP multicast. In such examples, retrieval unit  52  may submit a request to join a multicast network group associated with particular media content. After joining the multicast group, retrieval unit  52  may receive data of the multicast group without further requests issued to server device  60  or content preparation device  20 . Retrieval unit  52  may submit a request to leave the multicast group when data of the multicast group is no longer needed, e.g., to stop playback or to change channels to a different multicast group. 
     Network interface  54  may receive and provide data of segments of a selected representation to retrieval unit  52 , which may in turn provide the segments to decapsulation unit  50 . Decapsulation unit  50  may decapsulate elements of a video file into constituent PES streams, depacketize the PES streams to retrieve encoded data, and send the encoded data to either audio decoder  46  or video decoder  48 , depending on whether the encoded data is part of an audio or video stream, e.g., as indicated by PES packet headers of the stream. Audio decoder  46  decodes encoded audio data and sends the decoded audio data to audio output  42 , while video decoder  48  decodes encoded video data and sends the decoded video data, which may include a plurality of views of a stream, to video output  44 . 
     Video encoder  28 , video decoder  48 , audio encoder  26 , audio decoder  46 , encapsulation unit  30 , retrieval unit  52 , and decapsulation unit  50  each may be implemented as any of a variety of suitable processing circuitry, as applicable, such as one or more microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), discrete logic circuitry, software, hardware, firmware or any combinations thereof. Each of video encoder  28  and video decoder  48  may be included in one or more encoders or decoders, either of which may be integrated as part of a combined video encoder/decoder (CODEC). Likewise, each of audio encoder  26  and audio decoder  46  may be included in one or more encoders or decoders, either of which may be integrated as part of a combined CODEC. An apparatus including video encoder  28 , video decoder  48 , audio encoder  26 , audio decoder  46 , encapsulation unit  30 , retrieval unit  52 , and/or decapsulation unit  50  may comprise an integrated circuit, a microprocessor, and/or a wireless communication device, such as a cellular telephone. 
     Client device  40 , server device  60 , and/or content preparation device  20  may be configured to operate in accordance with the techniques of this disclosure. For purposes of example, this disclosure describes these techniques with respect to client device  40  and server device  60 . However, it should be understood that content preparation device  20  may be configured to perform these techniques, instead of (or in addition to) server device  60 . 
     Encapsulation unit  30  may form NAL units comprising a header that identifies a program to which the NAL unit belongs, as well as a payload, e.g., audio data, video data, or data that describes the transport or program stream to which the NAL unit corresponds. For example, in H.264/AVC, a NAL unit includes a 1-byte header and a payload of varying size. A NAL unit including video data in its payload may comprise various granularity levels of video data. For example, a NAL unit may comprise a block of video data, a plurality of blocks, a slice of video data, or an entire picture of video data. Encapsulation unit  30  may receive encoded video data from video encoder  28  in the form of PES packets of elementary streams. Encapsulation unit  30  may associate each elementary stream with a corresponding program. 
     Encapsulation unit  30  may also assemble access units from a plurality of NAL units. In general, an access unit may comprise one or more NAL units for representing a frame of video data, as well as audio data corresponding to the frame when such audio data is available. An access unit generally includes all NAL units for one output time instance, e.g., all audio and video data for one time instance. For example, if each view has a frame rate of 20 frames per second (fps), then each time instance may correspond to a time interval of 0.05 seconds. During this time interval, the specific frames for all views of the same access unit (the same time instance) may be rendered simultaneously. In one example, an access unit may comprise a coded picture in one time instance, which may be presented as a primary coded picture. 
     Accordingly, an access unit may comprise all audio and video frames of a common temporal instance, e.g., all views corresponding to time X. This disclosure also refers to an encoded picture of a particular view as a “view component.” That is, a view component may comprise an encoded picture (or frame) for a particular view at a particular time. Accordingly, an access unit may be defined as comprising all view components of a common temporal instance. The decoding order of access units need not necessarily be the same as the output or display order. 
     A media presentation may include a media presentation description (MPD), which may contain descriptions of different alternative representations (e.g., video services with different qualities) and the description may include, e.g., codec information, a profile value, and a level value. An MPD is one example of a manifest file, such as manifest file  66 . Client device  40  may retrieve the MPD of a media presentation to determine how to access movie fragments of various presentations. Movie fragments may be located in movie fragment boxes (moof boxes) of video files. 
     Manifest file  66  (which may comprise, for example, an MPD) may advertise availability of segments of representations  68 . That is, the MPD may include information indicating the wall-clock time at which a first segment of one of representations  68  becomes available, as well as information indicating the durations of segments within representations  68 . In this manner, retrieval unit  52  of client device  40  may determine when each segment is available, based on the starting time as well as the durations of the segments preceding a particular segment. 
     After encapsulation unit  30  has assembled NAL units and/or access units into a video file based on received data, encapsulation unit  30  passes the video file to output interface  32  for output. In some examples, encapsulation unit  30  may store the video file locally or send the video file to a remote server via output interface  32 , rather than sending the video file directly to client device  40 . Output interface  32  may comprise, for example, a transmitter, a transceiver, a device for writing data to a computer-readable medium such as, for example, an optical drive, a magnetic media drive (e.g., floppy drive), a universal serial bus (USB) port, a network interface, or other output interface. Output interface  32  outputs the video file to a computer-readable medium, such as, for example, a transmission signal, a magnetic medium, an optical medium, a memory, a flash drive, or other computer-readable medium. 
     Network interface  54  may receive a NAL unit or access unit via network  74  and provide the NAL unit or access unit to decapsulation unit  50 , via retrieval unit  52 . Decapsulation unit  50  may decapsulate a elements of a video file into constituent PES streams, depacketize the PES streams to retrieve encoded data, and send the encoded data to either audio decoder  46  or video decoder  48 , depending on whether the encoded data is part of an audio or video stream, e.g., as indicated by PES packet headers of the stream. Audio decoder  46  decodes encoded audio data and sends the decoded audio data to audio output  42 , while video decoder  48  decodes encoded video data and sends the decoded video data, which may include a plurality of views of a stream, to video output  44 . 
     Content preparation device  20  and/or server device  60  may represent application provider devices, while client device  40  may represent user equipment (UE). Network  74  may represent a Fifth Generation (5G) mobile network. In general, content preparation device  20  and/or server device  60  may create background data transfer (BDT) configuration, and client device  40  may determine to download media data using a background data transfer. Client device  40  (e.g., retrieval unit  52 ) may execute a media session handler (MSH) and a media player application or streaming application (e.g., a DASH application or plug-in to a web browser). In accordance with the techniques of this disclosure, retrieval unit  52  may request to perform a background data transfer to retrieve media data from, e.g., server device  60 , receive data representing a BDT opportunity for a particular time, and then perform the background data transfer at the time indicated for the BDT opportunity. 
       FIG. 2  is a block diagram illustrating an example set of components of retrieval unit  52  of  FIG. 1  in greater detail. In this example, retrieval unit  52  includes media session handler (MSH) unit  100  and media application  112 . 
     In this example, MSH unit  100  further includes reception unit  106 , cache  104 , and proxy server unit  102 . In this example, reception unit  106  is configured to receive data via a communication standard, such as 3GPP, 5G, or the like. In some examples, reception unit  106  may receive media data via a file delivery protocol, e.g., according to File Delivery over Unidirectional Transport (FLUTE), described in T. Paila et al., “FLUTE—File Delivery over Unidirectional Transport,” Network Working Group, RFC 6726, Nov. 2012, available at tools.ietf.org/html/rfc6726. That is, reception unit  106  may receive files via broadcast from, e.g., server device  60 , which may act as a broadcast/multicast service center (BM-SC). 
     As MSH unit  100  receives data for media data (e.g., files), MSH unit  100  may store the received data in cache  104 . Cache  104  may comprise a computer-readable storage medium, such as flash memory, a hard disk, RAM, or any other suitable storage medium. 
     Proxy server unit  102  may act as a server for providing media data from cache  104  to media application  112 . For example, proxy server unit  102  may provide a MPD file or other manifest file to media application  112  or an intermediate application, such as a DASH client. Proxy server unit  102  may advertise availability times for segments in the MPD file, as well as hyperlinks from which the segments can be retrieved. These hyperlinks may include a localhost address prefix corresponding to client device  40  (e.g., 127.0.0.1 for IPv4). In this manner, media application  112  or an intermediate application may request segments from proxy server unit  102  using, e.g., HTTP GET or partial GET requests. For example, for a segment available from link http://127.0.0.1/rep1/seg3, media application  112  may construct an HTTP GET request that includes a request for http://127.0.0.1/rep1/seg3, and submit the request to proxy server unit  102 . Proxy server unit  102  may retrieve requested media data from cache  104  and provide the data to media application  112  in response to such requests. 
     In accordance with the techniques of this disclosure, media application  112  may correspond to a media or streaming application that interacts with MSH unit  100  to retrieve media data according to a background data transfer. In the example shown in  FIG. 2 , MSH unit  100  may retrieve the media data according to the background data transfer to retrieve and store media data, e.g., in cache  104 . 
     In other examples, MSH unit  100  may alert media application  112  of a BDT opportunity, and media application  112  may retrieve the media data according to the background data transfer. 
     Assuming for purposes of example that MSH unit  100  retrieves the media data from, e.g., server device  60 , according to the background data transfer, MSH unit  100  may generally determine a time at which to retrieve the media data. For example, MSH unit  100  may receive data representing an off-peak designated time window during which to retrieve the media data. For example, media application  112  may initially send a request to MSH unit  100  indicating that particular media data is to be requested and transferred according to a background data transfer. MSH unit  100  may then send data of the request to a 5G Media Streaming downlink (5GMSd) Application Function (AF) or other media streaming application function, e.g., executed by server device  60  or another unit of network  74 . 
     The 5GMSd AF may respond to MSH unit  100  with a notification of a background data transfer opportunity. The notification may include data defining the off-peak designated time window. Thus, MSH unit  100  (or in some examples, media application  112 ) may retrieve the media data during the off-peak designated time window. For examples in which media application  112  is to retrieve the media data, MSH unit  100  may send the data defining the off-peak designated time window from the notification to media application  112 . 
     In the example of  FIG. 2 , MSH unit  100  may store the media data retrieved according to the background data transfer in cache  104 . MSH unit  100  may store this media data in cache  104  until a later time, e.g., a time at which the user desires to observe playback of the media data. In some examples, the media data may be locked until a later time, such that MSH unit  100  may prevent access to the media data until a subsequent time. For example, the media data may correspond to an as-yet unreleased movie. Thus, MSH unit  100  may retrieve the media data in advance of a release date of the media data, and prevent access to the retrieved media data until a date and time of release. The release date and time may be specified in the indication of the background data transfer opportunity in some examples. 
       FIG. 3  is a conceptual diagram illustrating elements of example multimedia content  120 . Multimedia content  120  may correspond to multimedia content  64  ( FIG. 1 ), or another multimedia content stored in storage medium  62 . In the example of  FIG. 3 , multimedia content  120  includes media presentation description (MPD)  122  and a plurality of representations  124 A- 124 N (representations  124 ). Representation  124 A includes optional header data  126  and segments  128 A- 128 N (segments  128 ), while representation  124 N includes optional header data  130  and segments  132 A- 132 N (segments  132 ). The letter N is used to designate the last movie fragment in each of representations  124  as a matter of convenience. In some examples, there may be different numbers of movie fragments between representations  124 . 
     MPD  122  may comprise a data structure separate from representations  124 . MPD  122  may correspond to manifest file  66  of  FIG. 1 . Likewise, representations  124  may correspond to representations  68  of  FIG. 1 . In general, MPD  122  may include data that generally describes characteristics of representations  124 , such as coding and rendering characteristics, adaptation sets, a profile to which MPD  122  corresponds, text type information, camera angle information, rating information, trick mode information (e.g., information indicative of representations that include temporal sub-sequences), and/or information for retrieving remote periods (e.g., for targeted advertisement insertion into media content during playback). 
     Header data  126 , when present, may describe characteristics of segments  128 , e.g., temporal locations of random access points (RAPs, also referred to as stream access points (SAPs)), which of segments  128  includes random access points, byte offsets to random access points within segments  128 , uniform resource locators (URLs) of segments  128 , or other aspects of segments  128 . Header data  130 , when present, may describe similar characteristics for segments  132 . Additionally or alternatively, such characteristics may be fully included within MPD  122 . 
     Segments  128 ,  132  include one or more coded video samples, each of which may include frames or slices of video data. Each of the coded video samples of segments  128  may have similar characteristics, e.g., height, width, and bandwidth requirements. Such characteristics may be described by data of MPD  122 , though such data is not illustrated in the example of  FIG. 3 . MPD  122  may include characteristics as described by the 3GPP Specification, with the addition of any or all of the signaled information described in this disclosure. 
     Each of segments  128 ,  132  may be associated with a unique uniform resource locator (URL). Thus, each of segments  128 ,  132  may be independently retrievable using a streaming network protocol, such as DASH. In this manner, a destination device, such as client device  40 , may use an HTTP GET request to retrieve segments  128  or  132 . In some examples, client device  40  may use HTTP partial GET requests to retrieve specific byte ranges of segments  128  or  132 . 
       FIG. 4  is a block diagram illustrating elements of an example video file  150 , which may correspond to a segment of a representation, such as one of segments  128 ,  132  of  FIG. 3 . Each of segments  128 ,  132  may include data that conforms substantially to the arrangement of data illustrated in the example of  FIG. 4 . Video file  150  may be said to encapsulate a segment. As described above, video files in accordance with the ISO base media file format and extensions thereof store data in a series of objects, referred to as “boxes.” In the example of  FIG. 4 , video file  150  includes file type (FTYP) box  152 , movie (MOOV) box  154 , segment index (sidx) boxes  162 , movie fragment (MOOF) boxes  164 , and movie fragment random access (MFRA) box  166 . Although  FIG. 4  represents an example of a video file, it should be understood that other media files may include other types of media data (e.g., audio data, timed text data, or the like) that is structured similarly to the data of video file  150 , in accordance with the ISO base media file format and its extensions. 
     File type (FTYP) box  152  generally describes a file type for video file  150 . File type box  152  may include data that identifies a specification that describes a best use for video file  150 . File type box  152  may alternatively be placed before MOOV box  154 , movie fragment boxes  164 , and/or MFRA box  166 . 
     In some examples, a Segment, such as video file  150 , may include an MPD update box (not shown) before FTYP box  152 . The MPD update box may include information indicating that an MPD corresponding to a representation including video file  150  is to be updated, along with information for updating the MPD. For example, the MPD update box may provide a URI or URL for a resource to be used to update the MPD. As another example, the MPD update box may include data for updating the MPD. In some examples, the MPD update box may immediately follow a segment type (STYP) box (not shown) of video file  150 , where the STYP box may define a segment type for video file  150 . 
     MOOV box  154 , in the example of  FIG. 4 , includes movie header (MVHD) box  156 , track (TRAK) box  158 , and one or more movie extends (MVEX) boxes  160 . In general, MVHD box  156  may describe general characteristics of video file  150 . For example, MVHD box  156  may include data that describes when video file  150  was originally created, when video file  150  was last modified, a timescale for video file  150 , a duration of playback for video file  150 , or other data that generally describes video file  150 . 
     TRAK box  158  may include data for a track of video file  150 . TRAK box  158  may include a track header (TKHD) box that describes characteristics of the track corresponding to TRAK box  158 . In some examples, TRAK box  158  may include coded video pictures, while in other examples, the coded video pictures of the track may be included in movie fragment boxes  164 , which may be referenced by data of TRAK box  158  and/or sidx boxes  162 . 
     In some examples, video file  150  may include more than one track. Accordingly, MOOV box  154  may include a number of TRAK boxes equal to the number of tracks in video file  150 . TRAK box  158  may describe characteristics of a corresponding track of video file  150 . For example, TRAK box  158  may describe temporal and/or spatial information for the corresponding track. A TRAK box similar to TRAK box  158  of MOOV box  154  may describe characteristics of a parameter set track, when encapsulation unit  30  ( FIG. 3 ) includes a parameter set track in a video file, such as video file  150 . Encapsulation unit  30  may signal the presence of sequence level SEI messages in the parameter set track within the TRAK box describing the parameter set track. 
     MVEX boxes  160  may describe characteristics of corresponding movie fragment boxes  164 , e.g., to signal that video file  150  includes movie fragment boxes  164 , in addition to video data included within MOOV box  154 , if any. In the context of streaming video data, coded video pictures may be included in movie fragment boxes  164  rather than in MOOV box  154 . Accordingly, all coded video samples may be included in movie fragment boxes  164 , rather than in MOOV box  154 . 
     MOOV box  154  may include a number of MVEX boxes  160  equal to the number of movie fragment boxes  164  in video file  150 . Each of MVEX boxes  160  may describe characteristics of a corresponding one of movie fragment boxes  164 . For example, each MVEX box may include a movie extends header box (MEHD) box that describes a temporal duration for the corresponding one of movie fragment boxes  164 . 
     As noted above, encapsulation unit  30  may store a sequence data set in a video sample that does not include actual coded video data. A video sample may generally correspond to an access unit, which is a representation of a coded picture at a specific time instance. In the context of AVC, the coded picture include one or more VCL NAL units, which contain the information to construct all the pixels of the access unit and other associated non-VCL NAL units, such as SEI messages. Accordingly, encapsulation unit  30  may include a sequence data set, which may include sequence level SEI messages, in one of movie fragment boxes  164 . Encapsulation unit  30  may further signal the presence of a sequence data set and/or sequence level SEI messages as being present in one of movie fragment boxes  164  within the one of MVEX boxes  160  corresponding to the one of movie fragment boxes  164 . 
     SIDX boxes  162  are optional elements of video file  150 . That is, video files conforming to the 3GPP file format, or other such file formats, do not necessarily include SIDX boxes  162 . In accordance with the example of the 3GPP file format, a SIDX box may be used to identify a sub-segment of a segment (e.g., a segment contained within video file  150 ). The 3GPP file format defines a sub-segment as “a self-contained set of one or more consecutive movie fragment boxes with corresponding Media Data box(es) and a Media Data Box containing data referenced by a Movie Fragment Box must follow that Movie Fragment box and precede the next Movie Fragment box containing information about the same track.” The 3GPP file format also indicates that a SIDX box “contains a sequence of references to subsegments of the (sub)segment documented by the box. The referenced subsegments are contiguous in presentation time. Similarly, the bytes referred to by a Segment Index box are always contiguous within the segment. The referenced size gives the count of the number of bytes in the material referenced.” 
     SIDX boxes  162  generally provide information representative of one or more sub-segments of a segment included in video file  150 . For instance, such information may include playback times at which sub-segments begin and/or end, byte offsets for the sub-segments, whether the sub-segments include (e.g., start with) a stream access point (SAP), a type for the SAP (e.g., whether the SAP is an instantaneous decoder refresh (IDR) picture, a clean random access (CRA) picture, a broken link access (BLA) picture, or the like), a position of the SAP (in terms of playback time and/or byte offset) in the sub-segment, and the like. 
     Movie fragment boxes  164  may include one or more coded video pictures. In some examples, movie fragment boxes  164  may include one or more groups of pictures (GOPs), each of which may include a number of coded video pictures, e.g., frames or pictures. In addition, as described above, movie fragment boxes  164  may include sequence data sets in some examples. Each of movie fragment boxes  164  may include a movie fragment header box (MFHD, not shown in  FIG. 4 ). The MFHD box may describe characteristics of the corresponding movie fragment, such as a sequence number for the movie fragment. Movie fragment boxes  164  may be included in order of sequence number in video file  150 . 
     MFRA box  166  may describe random access points within movie fragment boxes  164  of video file  150 . This may assist with performing trick modes, such as performing seeks to particular temporal locations (i.e., playback times) within a segment encapsulated by video file  150 . MFRA box  166  is generally optional and need not be included in video files, in some examples. Likewise, a client device, such as client device  40 , does not necessarily need to reference MFRA box  166  to correctly decode and display video data of video file  150 . MFRA box  166  may include a number of track fragment random access (TFRA) boxes (not shown) equal to the number of tracks of video file  150 , or in some examples, equal to the number of media tracks (e.g., non-hint tracks) of video file  150 . 
     In some examples, movie fragment boxes  164  may include one or more stream access points (SAPs), such as IDR pictures. Likewise, MFRA box  166  may provide indications of locations within video file  150  of the SAPs. Accordingly, a temporal sub-sequence of video file  150  may be formed from SAPs of video file  150 . The temporal sub-sequence may also include other pictures, such as P-frames and/or B-frames that depend from SAPs. Frames and/or slices of the temporal sub-sequence may be arranged within the segments such that frames/slices of the temporal sub-sequence that depend on other frames/slices of the sub-sequence can be properly decoded. For example, in the hierarchical arrangement of data, data used for prediction for other data may also be included in the temporal sub-sequence. 
       FIG. 5  is a block diagram illustrating an example system  180  that may perform the techniques of this disclosure. In this example, system  180  includes content service provider  182  (which may correspond to content preparation device  20  of  FIG. 1 ), content delivery network  184  (which may include server device  60  of  FIG. 1 ), mobile network operator (MNO)  190  (which may be included in network  74  of  FIG. 1 ), and client device  200  (which may correspond to client device  40  of  FIG. 1 ). In the example of  FIG. 5 , MNO  190  includes cache management unit  192  and access network unit  194 , and client device  200  includes native application or browser  202  (which may include a web browser, a web browser plug-in, and/or other media player application or media streaming application), and 3GPP standard units  204 , including UE-based cache and management unit  206  and connectivity unit  208 . 
     In this example, native application or browser  202  may act as a streaming application or media player application (e.g., corresponding to media application  112  of  FIG. 2 , and may further include a DASH client), and 3GPP standard units  204  may act as a media session handler (MSH). Client device  200  may include an application programming interface (API), such as an M6 API, between native application or browser  202  and 3GPP standard units  204 . The M6 API may be extended to include a new configuration element representing background data transfer, such as “_backgroundTraffic” or “_backgroundDownload.” The API may cover both downlink and uplink commands and data transfer. The M6 API may include the API calls of registerBDT( ) or registerDownload( ) and registerUplink( ) which registers a request for a downlink/uplink background data transfer. Parameters may include a list of files, file sizes, a desired time, and/or a flag that indicates whether the MSH or the application will do the download. 
     The M6 API may also include a notifyBDTOpportunity( ) API call. The MSH (e.g., of 3GPP standard units  204 ) may use this callback function to notify media player application of native application or browser  202  of the opportunity to perform the download. Parameters may include the total traffic volume allocated for this session, the bitrate allocated for the session, and the time window to perform the download. 
     The M6 API may further include a notifyBDTComplete( ) API call. If the register request indicates that the MSH is to perform the download, the MSH may use this call to notify the media player application that the download is complete. Parameters may include the location of the downloaded content, the size of the downloaded content, and the cache duration for the content. 
     In some examples, the MSH may receive a special link to perform the download, for enhanced security. Also for security, the content may be subject to an additional encryption step, with a special key that is only available to the media player application. Furthermore, for extra security, the application provider may distribute a group key to all applications that will perform a BDT download. 
     The MSH may allow leasing of cache space in some examples. The application provider may lease a certain amount of disk space on the UEs for caching BDT downloaded content. The amount of space may differ between UEs, but the amount may be discoverable by the media player application. 
     There may also be various BDT policy features. For example, the application provider may define multiple policies and tag them with one or more feature tags. The feature tags may be used to differentiate media quality, e.g., 4 K vs. FHD vs. HD. The 5G Media Streaming downlink (5GMSd) Application Function (AF) may keep track of consumption quotas and downgrade to a lower policy if a quota is exceeded. 
       FIG. 6  is a flowchart illustrating an example method for transporting media data using a background data transfer according to the techniques of this disclosure. The method of  FIG. 6  is explained with respect to the elements of  FIGS. 1 and 2 , although it should be understood that other devices, such as the devices of  FIG. 5 , may also be configured to perform the techniques of this disclosure. 
     In some examples according to the techniques of this disclosure, content preparation device  20  and/or server device  60  may provision background data transfer (BDT) configuration with a 5G Media Streaming downlink (5GMSd) Application Function (AF). Provisioning such a configuration may include providing, to the 5GMSd AF, information about overall data volume for media data, a list of user equipment (UEs), a data budget per UE, one or more geographical areas, and/or the like ( 220 ). The 5GMSd AF may contact a device that provides a policy and charging function (PCF) to create a new BDT policy ( 222 ). The PCF device may reply to a unified data repository (UDR) with a BDT reference ID for the policy ( 224 ). The 5GMSd AF may then confirm that a successful BDT policy has been created ( 226 ) to the application provider. 
     Client device  40  may execute a media player application and a media session handler (MSH). The media player application may provide data to the MSH about background data transfer needs and register a background data transfer request ( 228 ). For example, the media player application may provide a list of files, their corresponding sizes, and a desired availability time to the MSH. The media player application may, in various examples, request that the MSH perform the download using the background data transfer, or the media player application may request notification about a download opportunity and perform the download itself. The MSH may provide a location of the download to the media player application after the MSH performs the download, if the MSH itself performs the download. 
     The MSH may register a request with the 5GMSd AF for a BDT download opportunity ( 230 ). The MSH may then provide an application provider identifier or domain name and a UE identifier (such as a generic public subscription identifier (GPSI)). The 5GMSd AF may inform the MSH when a BDT download opportunity is available ( 232 ). The 5GMSd AF may also verify the existence of an appropriate BDT policy for that application provider and UE. The 5GMSd AF may query a unified data repository (UDR) directly to verify the existence of the BDT policy ( 234 ). If the BDT policy is found, the 5GMSd AF may identify the BDT reference ID, time window, data limits per UE, total data, and the like. The MSH may then perform the download or trigger the media player application to perform the download. The MSH may also receive data representing remaining quotas for download. 
     In particular, in one example, the MSH sends a notification to the media player application that a background data transfer opportunity is available ( 236 A). In response, the media player application retrieves media data content from the application provider directly ( 238 A). In another example, the MSH itself retrieves the media data and then sends a notification to the media player application when the media data retrieval has completed (fully or partially) ( 236 B). In response, the media player application retrieves the media data from the MSH ( 238 B). 
     In this manner, the method of  FIG. 6  represents an example of a method including sending, by one or more processors of a client device, a request to retrieve media data according to a background data transfer to a media streaming application function (AF); in response to the request, receiving, by the one or more processors of the client device, an indication of a background data transfer opportunity from the media streaming AF; in response to the indication of the background data transfer opportunity, retrieving, by the one or more processors, the media data according to the background data transfer; and storing, by the one or more processors, the retrieved media data. 
     As discussed above, this disclosure describes a framework that may be used to perform background data transfer for, e.g., 5G media delivery. This framework may seamlessly integrate with existing 5G media streaming architecture. These techniques may also allow an MNO to keep control of data volume and download windows. These techniques may also be secure and provide opportunistic retrieval of media content. 
     Application providers and MNOs may be likely to encourage use of these techniques to reduce costs and offload traffic to less-busy time windows. These techniques may be implemented as part of a media session handler service, which could be part of a protocol stack of a modem. These techniques may also be incorporated into the 5G standard. 
       FIG. 7  is a flowchart illustrating an example method of retrieving media data according to the techniques of this disclosure. The method of  FIG. 7  is explained with respect to client device  40  of  FIG. 1 . Other devices may be configured to perform this or a similar method, such as client device  200  of  FIG. 5 . Retrieval unit  52  of client device  40  of  FIG. 1  may include both a media application and a media session handler (MSH), e.g., as shown in  FIG. 2 . The media application and the MSH of retrieval unit  52  of client device  40  of  FIG. 1  may perform the various elements of  FIG. 7  discussed below. 
     Initially, the media application may request a background data transfer ( 250 ), e.g., for a particular media presentation. The media application may send the request to the MSH. In response, the MSH may register the background data transfer request with the 5GMSd AF ( 252 ). The MSH may subsequently receive a notification of a background data transfer opportunity from the 5GMSd AF ( 254 ). The notification may include data representing a time at which media data of the media presentation can be retrieved according to the background data transfer. 
     In the example of  FIG. 7 , the MSH may send data for the background data transfer opportunity to the media application ( 256 ). The data may indicate, for example, the time at which the media data of the media presentation can be retrieved according to the background data transfer. The media application may receive the background data transfer opportunity data ( 258 ) and then retrieve the media data according to the background data transfer ( 260 ). For example, the media application may retrieve the media data at the indicated time. The indicated time may correspond to an off-peak designated time window. 
     In this manner, the method of  FIG. 7  represents an example of a method including sending, by one or more processors of a client device, a request to retrieve media data according to a background data transfer to a media streaming application function (AF); in response to the request, receiving, by the one or more processors of the client device, an indication of a background data transfer opportunity from the media streaming AF; in response to the indication of the background data transfer opportunity, retrieving, by the one or more processors, the media data according to the background data transfer; and storing, by the one or more processors, the retrieved media data. 
       FIG. 8  is a flowchart illustrating another example method of retrieving media data according to the techniques of this disclosure. The method of  FIG. 8  is explained with respect to client device  40  of  FIG. 1 . Other devices may be configured to perform this or a similar method, such as client device  200  of  FIG. 5 . Retrieval unit  52  of client device  40  of  FIG. 1  may include both a media application and a media session handler (MSH), e.g., as shown in  FIG. 2 . The media application and the MSH of retrieval unit  52  of client device  40  of  FIG. 1  may perform the various elements of  FIG. 8  discussed below. 
     Initially, the media application may request a background data transfer ( 280 ), e.g., for a particular media presentation. The media application may send the request to the MSH. In response, the MSH may register the background data transfer request with the 5GMSd AF ( 282 ). The MSH may subsequently receive a notification of a background data transfer opportunity from the 5GMSd AF ( 284 ). The notification may include data representing a time at which media data of the media presentation can be retrieved according to the background data transfer. 
     In the example of  FIG. 8 , the MSH may then retrieve the media data according to the background data transfer ( 286 ). For example, the MSH may retrieve the media data at the time indicated in the notification. The indicated time may correspond to an off-peak designated time window. After retrieving some or all of the media data for the media presentation, the MSH may send data indicating that the media data has been retrieved and is available to the media application ( 288 ). 
     The media application may receive the indication of the media data being available from the MSH ( 290 ). In response, at some time later, the media application may retrieve the media data from the MSH ( 292 ). 
     In this manner, the method of  FIG. 8  represents an example of a method including sending, by one or more processors of a client device, a request to retrieve media data according to a background data transfer to a media streaming application function (AF); in response to the request, receiving, by the one or more processors of the client device, an indication of a background data transfer opportunity from the media streaming AF; in response to the indication of the background data transfer opportunity, retrieving, by the one or more processors, the media data according to the background data transfer; and storing, by the one or more processors, the retrieved media data. 
     Various examples of the techniques of this disclosure are summarized in the following clauses: 
     Clause 1: A method of retrieving media data, the method comprising: sending a request to retrieve media data using background data transfer; in response to the request, receiving an indication of a background data transfer opportunity; in response to the indication of the background data transfer opportunity, retrieving the media data using the background data transfer; and storing the retrieved media data. 
     Clause 2: The method of clause 1, wherein retrieving the media data using the background data transfer comprises retrieving the media data during an off-peak designated time window. 
     Clause 3: The method of clause 2, wherein the indication of the background data transfer opportunity comprises data defining the off-peak designated time window. 
     Clause 4: The method of any of clauses 1-3, wherein sending the request to retrieve media data using the background data transfer comprises sending, by a media session handler executed by a client device, the request to retrieve the media data using the background data transfer to a 5G Media Streaming downlink (5GMSd) Application Function (AF). 
     Clause 5: The method of any of clauses 1-4, wherein receiving the indication of the background data transfer opportunity comprises receiving, by a media session handler executed by a client device, a notification of the background data transfer opportunity. 
     Clause 6: The method of clause 5, further comprising sending, by the media session handler, data representative of the background data transfer to a media player application executed by the client device, wherein retrieving the media data comprises retrieving, by the media player application, the media data using the background data transfer. 
     Clause 7: The method of clause 5, wherein retrieving the media data using the background data transfer comprises retrieving, by the media session handler, the media data using the background data transfer, the method further comprising: sending, by the media session handler, data indicating that the media data has been retrieved to a media player application executed by the client device; and sending, by the media session handler, the retrieved data to the media player application. 
     Clause 8: The method of any of clauses 1-7, wherein sending the request comprises sending at least one of a list of one or more files of the media data to be retrieved, sizes of the one or more files, or a desired availability time for the background data transfer. 
     Clause 9: A device for retrieving media data, the device comprising one or more means for performing the method of any of clauses 1-8. 
     Clause 10: The device of clause 9, wherein the one or more means comprise one or more processors implemented in circuitry. 
     Clause 11: A computer-readable storage medium having stored thereon instructions that, when executed, cause a processor to perform the method of any of clauses 1-8. 
     Clause 12: A device for retrieving media data, the device comprising: means for sending a request to retrieve media data using background data transfer; means for receiving, in response to the request, an indication of a background data transfer opportunity; means for retrieving, in response to the indication of the background data transfer opportunity, the media data using the background data transfer; and means for storing the retrieved media data. 
     Clause 13: A method of retrieving media data, the method comprising: sending, by one or more processors of a client device, a request to retrieve media data according to a background data transfer to a media streaming application function (AF); in response to the request, receiving, by the one or more processors of the client device, an indication of a background data transfer opportunity from the media streaming AF; in response to the indication of the background data transfer opportunity, retrieving, by the one or more processors, the media data according to the background data transfer; and storing, by the one or more processors, the retrieved media data. 
     Clause 14: The method of clause 13, wherein retrieving the media data according to the background data transfer comprises: determining an off-peak designated time window; and retrieving the media data during the off-peak designated time window. 
     Clause 15: The method of clause 14, wherein determining the off-peak designated time window comprises determining the off-peak designated time window from data defining the off-peak designated time window included in the indication of the background data transfer opportunity. 
     Clause 16: The method of clause 13, wherein sending the request to retrieve media data according to the background data transfer comprises sending, by a media session handler (MSH) executed by the one or more processors of the client device, the request to retrieve the media data according to the background data transfer to a 5G Media Streaming downlink (5GMSd) Application Function (AF). 
     Clause 17: The method of clause 13, wherein receiving the indication of the background data transfer opportunity comprises receiving, by a media session handler (MSH) executed by the one or more processors of the client device, a notification of the background data transfer opportunity. 
     Clause 18: The method of clause 17, further comprising sending, by the MSH, data representative of the background data transfer to a media player application executed by the one or more processors of the client device, wherein retrieving the media data comprises retrieving, by the media player application executed by the one or more processors of the client device, the media data according to the background data transfer. 
     Clause 19: The method of clause 17, wherein retrieving the media data according to the background data transfer comprises retrieving, by the MSH executed by the one or more processors of the client device, the media data according to the background data transfer, the method further comprising: sending, by the MSH executed by the one or more processors of the client device, data indicating that the media data has been retrieved to a media player application executed by the one or more processors of the client device; and sending, by the MSH executed by the one or more processors of the client device, the retrieved data to the media player application. 
     Clause 20: The method of clause 13, further comprising forming the request to include at least one of a list of one or more files of the media data to be retrieved, sizes of the one or more files, or a desired availability time for the background data transfer. 
     Clause 21: A device for retrieving media data, the device comprising: a memory configured to store media data; and one or more processors implemented in circuitry and configured to: send a request to retrieve media data according to a background data transfer to a media streaming application function (AF); in response to the request, receive an indication of a background data transfer opportunity from the media streaming AF; in response to the indication of the background data transfer opportunity, retrieve the media data according to the background data transfer; and store the retrieved media data to the memory. 
     Clause 22: The device of clause 21, wherein to retrieve the media data according to the background data transfer, the one or more processors are configured to: determine an off-peak designated time window; and retrieve the media data during the off-peak designated time window. 
     Clause 23: The device of clause 22, wherein the one or more processors are configured to determine the off-peak designated time window from data defining the off-peak designated time window included in the indication of the background data transfer opportunity. 
     Clause 24: The device of clause 21, wherein to send the request to retrieve media data according to the background data transfer, the one or more processors are configured to execute a media session handler (MSH) configured to send the request to retrieve the media data according to the background data transfer to a 5G Media Streaming downlink (5GMSd) Application Function (AF). 
     Clause 25: The device of clause 21, wherein to receive the indication of the background data transfer opportunity, the one or more processors are configured to execute a media session handler (MSH) configured to receive a notification of the background data transfer opportunity. 
     Clause 26: The device of clause 25, wherein the MSH is further configured to send data representative of the background data transfer to a media player application executed by the one or more processors, and wherein to retrieve the media data, the media player application is configured to retrieve the media data according to the background data transfer. 
     Clause 27: The device of clause 25, wherein to retrieve the media data according to the background data transfer, the MSH is configured to retrieve the media data according to the background data transfer, and wherein the MSH is further configured to: send data indicating that the media data has been retrieved to a media player application executed by the one or more processors of the client device; and send the retrieved data to the media player application. 
     Clause 28: The device of clause 21, wherein the one or more processors are further configured to form the request to include at least one of a list of one or more files of the media data to be retrieved, sizes of the one or more files, or a desired availability time for the background data transfer. 
     Clause 29: A computer-readable storage medium having stored thereon instructions that, when executed, cause one or more processors of a client device to: send a request to retrieve media data according to a background data transfer to a media streaming application function (AF); in response to the request, receive an indication of a background data transfer opportunity from the media streaming AF; in response to the indication of the background data transfer opportunity, retrieve the media data according to the background data transfer; and store the retrieved media data to the memory. 
     Clause 30: The computer-readable storage medium of clause 29, wherein the instructions that cause the processor to retrieve the media data according to the background data transfer comprise instructions that cause the processor to: determine an off-peak designated time window; and retrieve the media data during the off-peak designated time window. 
     Clause 31: The computer-readable storage medium of clause 30, wherein the instructions that cause the processor to determine the off-peak designated time window comprise instructions that cause the processor to determine the off-peak designated time window from data defining the off-peak designated time window included in the indication of the background data transfer opportunity. 
     Clause 32: The computer-readable storage medium of clause 29, wherein the instructions that cause the processor to send the request to retrieve media data according to the background data transfer comprise instructions that cause the processor to execute a media session handler (MSH) to send the request to retrieve the media data according to the background data transfer to a 5G Media Streaming downlink (5GMSd) Application Function (AF). 
     Clause 33: The computer-readable storage medium of clause 29, wherein the instructions that cause the processor to receive the indication of the background data transfer opportunity comprise instructions that cause the processor to execute a media session handler (MSH) to receive a notification of the background data transfer opportunity. 
     Clause 34: The computer-readable storage medium of clause 33, further comprising instructions that cause the processor to execute the MSH to send data representative of the background data transfer to a media player application executed by the one or more processors of the client device, wherein the instructions that cause the processor to retrieve the media data comprise instructions that cause the processor to execute the media player application to retrieve the media data according to the background data transfer. 
     Clause 35: The computer-readable storage medium of clause 33, wherein the instructions that cause the processor to retrieve the media data according to the background data transfer comprise instructions that cause the processor to execute the MSH to retrieve the media data according to the background data transfer, further comprising instructions that cause the processor to: execute the MSH to send data indicating that the media data has been retrieved to a media player application executed by the one or more processors of the client device; and execute the MSH to send the retrieved data to the media player application. 
     Clause 36: The computer-readable storage medium of clause 29, further comprising forming the request to include at least one of a list of one or more files of the media data to be retrieved, sizes of the one or more files, or a desired availability time for the background data transfer. 
     Clause 37: A device for retrieving media data, the device comprising: means for sending a request to retrieve media data according to background data transfer; means for receiving, in response to the request, an indication of a background data transfer opportunity; means for retrieving, in response to the indication of the background data transfer opportunity, the media data according to the background data transfer; and means for storing the retrieved media data. 
     Clause 38: A method of retrieving media data, the method comprising: sending, by one or more processors of a client device, a request to retrieve media data according to a background data transfer to a media streaming application function (AF); in response to the request, receiving, by the one or more processors of the client device, an indication of a background data transfer opportunity from the media streaming AF; in response to the indication of the background data transfer opportunity, retrieving, by the one or more processors, the media data according to the background data transfer; and storing, by the one or more processors, the retrieved media data. 
     Clause 39: The method of clause 38, wherein retrieving the media data according to the background data transfer comprises: determining an off-peak designated time window; and retrieving the media data during the off-peak designated time window. 
     Clause 40: The method of clause 39, wherein determining the off-peak designated time window comprises determining the off-peak designated time window from data defining the off-peak designated time window included in the indication of the background data transfer opportunity. 
     Clause 41: The method of any of clauses 38-40, wherein sending the request to retrieve media data according to the background data transfer comprises sending, by a media session handler (MSH) executed by the one or more processors of the client device, the request to retrieve the media data according to the background data transfer to a 5G Media Streaming downlink (5GMSd) Application Function (AF). 
     Clause 42: The method of any of clauses 38-41, wherein receiving the indication of the background data transfer opportunity comprises receiving, by a media session handler (MSH) executed by the one or more processors of the client device, a notification of the background data transfer opportunity. 
     Clause 43: The method of clause 42, further comprising sending, by the MSH, data representative of the background data transfer to a media player application executed by the one or more processors of the client device, wherein retrieving the media data comprises retrieving, by the media player application executed by the one or more processors of the client device, the media data according to the background data transfer. 
     Clause 44: The method of clause 42, wherein retrieving the media data according to the background data transfer comprises retrieving, by the MSH executed by the one or more processors of the client device, the media data according to the background data transfer, the method further comprising: sending, by the MSH executed by the one or more processors of the client device, data indicating that the media data has been retrieved to a media player application executed by the one or more processors of the client device; and sending, by the MSH executed by the one or more processors of the client device, the retrieved data to the media player application. 
     Clause 45: The method of any of clauses 38-44, further comprising forming the request to include at least one of a list of one or more files of the media data to be retrieved, sizes of the one or more files, or a desired availability time for the background data transfer. 
     Clause 46: A device for retrieving media data, the device comprising: a memory configured to store media data; and one or more processors implemented in circuitry and configured to: send a request to retrieve media data according to a background data transfer to a media streaming application function (AF); in response to the request, receive an indication of a background data transfer opportunity from the media streaming AF; in response to the indication of the background data transfer opportunity, retrieve the media data according to the background data transfer; and store the retrieved media data to the memory. 
     Clause 47: The device of clause 46, wherein to retrieve the media data according to the background data transfer, the one or more processors are configured to: determine an off-peak designated time window; and retrieve the media data during the off-peak designated time window. 
     Clause 48: The device of clause 47, wherein the one or more processors are configured to determine the off-peak designated time window from data defining the off-peak designated time window included in the indication of the background data transfer opportunity. 
     Clause 49: The device of any of clauses 46-48, wherein to send the request to retrieve media data according to the background data transfer, the one or more processors are configured to execute a media session handler (MSH) configured to send the request to retrieve the media data according to the background data transfer to a 5G Media Streaming downlink (5GMSd) Application Function (AF). 
     Clause 50: The device of any of clauses 38-49, wherein to receive the indication of the background data transfer opportunity, the one or more processors are configured to execute a media session handler (MSH) configured to receive a notification of the background data transfer opportunity. 
     Clause 51: The device of clause 50, wherein the MSH is further configured to send data representative of the background data transfer to a media player application executed by the one or more processors, and wherein to retrieve the media data, the media player application is configured to retrieve the media data according to the background data transfer. 
     Clause 52: The device of clause 50, wherein to retrieve the media data according to the background data transfer, the MSH is configured to retrieve the media data according to the background data transfer, and wherein the MSH is further configured to: send data indicating that the media data has been retrieved to a media player application executed by the one or more processors of the client device; and send the retrieved data to the media player application. 
     Clause 53: The device of any of clauses 38-52, wherein the one or more processors are further configured to form the request to include at least one of a list of one or more files of the media data to be retrieved, sizes of the one or more files, or a desired availability time for the background data transfer. 
     Clause 54: A computer-readable storage medium having stored thereon instructions that, when executed, cause one or more processors of a client device to: send a request to retrieve media data according to a background data transfer to a media streaming application function (AF); in response to the request, receive an indication of a background data transfer opportunity from the media streaming AF; in response to the indication of the background data transfer opportunity, retrieve the media data according to the background data transfer; and store the retrieved media data to the memory. 
     Clause 55: The computer-readable storage medium of clause 54, wherein the instructions that cause the processor to retrieve the media data according to the background data transfer comprise instructions that cause the processor to: determine an off-peak designated time window; and retrieve the media data during the off-peak designated time window. 
     Clause 56: The computer-readable storage medium of clause 55, wherein the instructions that cause the processor to determine the off-peak designated time window comprise instructions that cause the processor to determine the off-peak designated time window from data defining the off-peak designated time window included in the indication of the background data transfer opportunity. 
     Clause 57: The computer-readable storage medium of any of clauses 54-56, wherein the instructions that cause the processor to send the request to retrieve media data according to the background data transfer comprise instructions that cause the processor to execute a media session handler (MSH) to send the request to retrieve the media data according to the background data transfer to a 5G Media Streaming downlink (5GMSd) Application Function (AF). 
     Clause 58: The computer-readable storage medium of any of clauses 54-57, wherein the instructions that cause the processor to receive the indication of the background data transfer opportunity comprise instructions that cause the processor to execute a media session handler (MSH) to receive a notification of the background data transfer opportunity. 
     Clause 59: The computer-readable storage medium of clause 58, further comprising instructions that cause the processor to execute the MSH to send data representative of the background data transfer to a media player application executed by the one or more processors of the client device, wherein the instructions that cause the processor to retrieve the media data comprise instructions that cause the processor to execute the media player application to retrieve the media data according to the background data transfer. 
     Clause 60: The computer-readable storage medium of clause 58, wherein the instructions that cause the processor to retrieve the media data according to the background data transfer comprise instructions that cause the processor to execute the MSH to retrieve the media data according to the background data transfer, further comprising instructions that cause the processor to: execute the MSH to send data indicating that the media data has been retrieved to a media player application executed by the one or more processors of the client device; and execute the MSH to send the retrieved data to the media player application. 
     Clause 61: The computer-readable storage medium of any of clauses 54-60, further comprising forming the request to include at least one of a list of one or more files of the media data to be retrieved, sizes of the one or more files, or a desired availability time for the background data transfer. 
     In one or more examples, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media, which corresponds to a tangible medium such as data storage media, or communication media including any medium that facilitates transfer of a computer program from one place to another, e.g., according to a communication protocol. In this manner, computer-readable media generally may correspond to (1) tangible computer-readable storage media which is non-transitory or (2) a communication medium such as a signal or carrier wave. Data storage media may be any available media that can be accessed by one or more computers or one or more processors to retrieve instructions, code, and/or data structures for implementation of the techniques described in this disclosure. A computer program product may include a computer-readable medium. 
     By way of example, and not limitation, such computer-readable storage media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage, or other magnetic storage devices, flash memory, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer. Also, any connection is properly termed a computer-readable medium. For example, if instructions are transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. It should be understood, however, that computer-readable storage media and data storage media do not include connections, carrier waves, signals, or other transitory media, but are instead directed to non-transitory, tangible storage media. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and Blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. 
     Instructions may be executed by one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable logic arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processor,” as used herein may refer to any of the foregoing structure or any other structure suitable for implementation of the techniques described herein. In addition, in some aspects, the functionality described herein may be provided within dedicated hardware and/or software modules configured for encoding and decoding, or incorporated in a combined codec. Also, the techniques could be fully implemented in one or more circuits or logic elements. 
     The techniques of this disclosure may be implemented in a wide variety of devices or apparatuses, including a wireless handset, an integrated circuit (IC) or a set of ICs (e.g., a chip set). Various components, modules, or units are described in this disclosure to emphasize functional aspects of devices configured to perform the disclosed techniques, but do not necessarily require realization by different hardware units. Rather, as described above, various units may be combined in a codec hardware unit or provided by a collection of interoperative hardware units, including one or more processors as described above, in conjunction with suitable software and/or firmware. 
     Various examples have been described. These and other examples are within the scope of the following claims.