Patent Publication Number: US-7711841-B2

Title: Systems and methods for reducing the effects of variations on the playback of streaming media

Description:
RELATED APPLICATIONS 
   This application is related to and claims priority from U.S. Provisional Patent Application Ser. No. 60/777,636 filed Feb. 28, 2006, for “Systems and Methods for Underflow Prevention Under Varying Channel Conditions,” with inventor Sachin G. Deshpande, which is incorporated herein by reference. 

   TECHNICAL FIELD 
   The present invention relates generally to computers and computer-related technology. More specifically, the present invention relates to systems and methods for reducing the effects of variations in available network bandwidth and other network variations on the playback of streaming media. 
   BACKGROUND 
   Streaming media is media that is consumed while it is being delivered. Streaming technology allows a user to download media files (e.g., audio-video files) for immediate playback, thereby avoiding time-consuming downloads of large files. Audio-video streaming over the Internet has become quite popular. 
   A streaming server is a software program that is capable of providing a media file to a client program as a stream of media data. A media file may be an audio file, a video file, or an audio-video file (i.e., a file containing both audio and video). Media data is the data within a media file. 
   A streaming client is a software program that allows a user to play a media file that is being streamed from a streaming server. To play (or play back) a media file refers to converting media data into a user-perceptible form. For example, to play an audio-video file refers to converting video data into moving images and converting the corresponding audio data into audible sounds that may be heard by the user. 
   A streaming client may utilize a streaming buffer, which is an amount of memory that is used by the streaming client to temporarily store media data. In typical operation, a streaming client does not begin playing a streaming media file until the streaming buffer has been filled to a threshold level. As the streaming client plays the streaming media file, it uses up media data in the buffer. However, at the same time, more media data is being downloaded to the buffer. As long as the data can be downloaded as fast as it is used up in playback, the media file will play smoothly. 
   Unfortunately, however, the available bandwidth on the network for streaming media can vary based on the channel conditions. As an example in a wireless network, interference from cordless phones, microwaves, and the like can cause the available bandwidth to degrade. Cross-traffic on the network provides another source contributing to variable available bandwidth for streaming media. 
   Variations in the amount of available network bandwidth can cause problems for streaming media. For example, if the network becomes congested, the rate at which a media file is being streamed may be less (even significantly less) than the rate at which the streaming client is consuming media data from the streaming buffer. If this continues for some period of time, the streaming buffer may become depleted so that there is no media data remaining in the streaming buffer. This condition is sometimes referred to as buffer underflow (or, alternatively, buffer underrun). When buffer underflow occurs, the streaming client temporarily stops playing the media file until it is able to receive and re-buffer enough media data to restart playback. Of course, such interruptions in playback of the media file can be frustrating to the user of the streaming client. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only exemplary embodiments and are, therefore, not to be considered limiting of the invention&#39;s scope, the exemplary embodiments of the invention will be described with additional specificity and detail through use of the accompanying drawings in which: 
       FIG. 1  illustrates an exemplary system in which embodiments may be practiced; 
       FIG. 2  illustrates an embodiment of a method for reducing the effects of variations in network bandwidth on the playback of streaming media; 
       FIG. 3  illustrates another embodiment of a method for reducing the effects of variations in network bandwidth on the playback of streaming media; 
       FIG. 3A  illustrates another embodiment of a method for reducing the effects of variations in network bandwidth on the playback of streaming media; 
       FIG. 4  illustrates an exemplary method for determining the amount of media data that is received from the streaming server and added to the streaming buffer over T time units; 
       FIG. 5  illustrates components that may be provided within a streaming client in order to implement the method of  FIG. 3 ; 
       FIG. 6  shows various simulation results that were obtained during a simulation that was performed of the method of  FIG. 3 ; 
       FIG. 7  shows additional simulation results that were obtained during a simulation that was performed of the method of  FIG. 3 ; 
       FIG. 8  shows additional simulation results that were obtained during a simulation that was performed of the method of  FIG. 3 ; 
       FIG. 9  shows additional simulation results that were obtained during a simulation that was performed of the method of  FIG. 3 ; 
       FIG. 10  shows additional simulation results that were obtained during a simulation that was performed of the method of  FIG. 3 ; and 
       FIG. 11  illustrates various components that may be utilized in a server device and/or a client device. 
   

   DETAILED DESCRIPTION 
   A method for reducing effects of network and other variations on playback of media being streamed from a streaming server is disclosed. In accordance with the method, a lower limit may be defined for a level of a streaming buffer. A media stream may be received from a streaming media server as a stream of media data. The media stream may be played back from the streaming buffer at a playback rate. The level of the streaming buffer may be monitored during the playback of the media file to determine whether the level of the streaming buffer is below the lower limit. If it is determined that the level of the streaming buffer is below the lower limit, the playback rate may be set at a value that is less than an intended playback rate for the media stream. The value of the set playback rate may depend on the level of the streaming buffer. 
   Defining the lower limit for the level of the streaming buffer may involve defining a slow playback condition. The slow playback condition may depend on the level of the streaming buffer at time t, how much media data was added to the streaming buffer between time t and time t+T, a target level for the streaming buffer, and a margin for the streaming buffer. An exemplary formula for the slow playback condition will be discussed below. 
   Monitoring the level of the streaming buffer to determine whether the level of the streaming buffer is below the lower limit may involve determining the level of the streaming buffer at time t, determining how much media data is added to the streaming buffer between time t and time t+T, and evaluating the slow playback condition. Setting the playback rate at a value that is less than the intended playback rate may involve multiplying the intended playback rate by a scaling factor that is less than one. The scaling factor may depend on the level of the streaming buffer at time t, how much media data was added to the streaming buffer between time t and time t+T, and a target level for the streaming buffer. The scaling factor may also depend on a minimum scale factor and an estimated amount of data that will be received in timestamp units at the receiver in the next l th  time segment. Some exemplary formulas for calculating the scaling factor will be discussed below. 
   An upper limit may be defined for the level of the streaming buffer. The level of the streaming buffer may be monitored during the playback of the media file to determine whether the level of the streaming buffer is above the upper limit. If it is determined that the level of the streaming buffer is above the upper limit, the playback rate may be set at a value that is greater than an intended playback rate for the media stream. The value of the playback rate may depend on the level of the streaming buffer. 
   Defining the upper limit for the level of the streaming buffer may involve defining a fast playback condition. The fast playback condition may depend on the level of the streaming buffer at time t, how much media data was added to the streaming buffer between time t and time t+T, a target level for the streaming buffer, and other factors. An upper margin may be provided for the streaming buffer. Some exemplary formulas for the fast playback condition will be discussed below. 
   Monitoring the level of the streaming buffer to determine whether the level of the streaming buffer is above the upper limit may involve determining the level of the streaming buffer at time t, determining how much media data is added to the streaming buffer between time t and time t+T, and evaluating the fast playback condition. Setting the playback rate at a value that is greater than the intended playback rate may involve multiplying the intended playback rate by a scaling factor that is greater than one. The scaling factor may depend on the level of the streaming buffer at time t, how much media data was added to the streaming buffer between time t and time t+T, a target level for the streaming buffer, and other factors. Some exemplary formulas for calculating the scaling factor will be discussed below. 
   The level of the streaming buffer may be monitored periodically. The method may involve ensuring that the playback rate does not decrease below a defined minimum value. The method may involve ensuring that the playback rate does not increase above a defined maximum value. The method may be implemented by a streaming client that is in wireless communication with the streaming server. 
   A non-transitory computer readable storage medium comprising executable instructions for implementing a method for reducing effects of network and other variations in network bandwidth on playback of media being streamed from a streaming server is also disclosed. In accordance with the method, a lower limit may be defined for a level of a streaming buffer. A media stream may be received from a streaming media server as a stream of media data. The media stream may be played back from the streaming buffer at a playback rate. The level of the streaming buffer may be monitored during the playback of the media file to determine whether the level of the streaming buffer is below the lower limit. If it is determined that the level of the streaming buffer is below the lower limit, the playback rate may be set at a value that is less than an intended playback rate for the media stream. The value of the set playback rate may depend on the level of the streaming buffer. 
   A client device that is configured to reduce effects of network and other variations on playback of media that is being streamed from a streaming server is also disclosed. The client device may include a processor and memory in electronic communication with the processor. Instructions may be stored in the memory. The instructions may be executable to define a lower limit for a level of a streaming buffer. The instructions may also be executable to receive media from a streaming media server as a stream of media data. The instructions may also be executable to play back the media stream from the streaming buffer at a playback rate. The instructions may also be executable to monitor the level of the streaming buffer during the playback of the media stream to determine whether the level of the streaming buffer is below the lower limit. If it is determined that the level of the streaming buffer is below the lower limit, the instructions may also be executable to set the playback rate at a value that is less than an intended playback rate for the media stream. The value of the playback rate may depend on the level of the streaming buffer. 
   Various embodiments of the invention are now described with reference to the Figures, where like reference numbers indicate identical or functionally similar elements. The embodiments of the present invention, as generally described and illustrated in the Figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of several exemplary embodiments of the present invention, as represented in the Figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of the embodiments of the invention. 
   The word “exemplary” is used exclusively herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. 
   Many features of the embodiments disclosed herein may be implemented as computer software, electronic hardware, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various components will be described generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention. 
   Where the described functionality is implemented as computer software, such software may include any type of computer instruction or computer executable code located within a memory device and/or transmitted as electronic signals over a system bus or network. Software that implements the functionality associated with components described herein may comprise a single instruction, or many instructions, and may be distributed over several different code segments, among different programs, and across several memory devices. 
   As used herein, the terms “an embodiment”, “embodiment”, “embodiments”, “the embodiment”, “the embodiments”, “one or more embodiments”, “some embodiments”, “certain embodiments”, “one embodiment, “another embodiment” and the like mean “one or more (but not necessarily all) embodiments of the disclosed invention(s)”, unless expressly specified otherwise. 
   The term “determining” (and grammatical variants thereof) is used in an extremely broad sense. The term “determining” encompasses a wide variety of actions and therefore “determining” can include calculating, computing, processing, deriving, investigating, looking up (e.g., looking up in a table, a database or another data structure), ascertaining and the like. Also, “determining” can include receiving (e.g., receiving information), accessing (e.g., accessing data in a memory) and the like. Also, “determining” can include resolving, selecting, choosing, establishing, and the like. 
   The phrase “based on” does not mean “based only on,” unless expressly specified otherwise. In other words, the phrase “based on” describes both “based only on” and “based at least on.” 
     FIG. 1  illustrates a system  100  in which embodiments may be practiced. The system  100  includes a server device  102  and a client device  104 . The server device  102  and the client device  104  may be in electronic communication over a computer network (not shown). 
   The server device  102  includes a streaming server  106 . As discussed above, the streaming server  106  is a software program that is capable of providing a media file  108  (or, alternatively, a live stream  107  from a camera, encoder, etc.) to a client program as a stream of media data. The streaming server  106  may be a specialized program for performing audio/video streaming tasks. Alternatively, the streaming server  106  may be a conventional web server. 
   The client device  104  includes a streaming client  110 . As discussed above, the streaming client  110  is a software program that allows a user to play a media file  108  that is being streamed from the streaming server  106 . The streaming client  110  may be a standalone application, or it may be integrated into another application. For example, the streaming client  110  may be a plug-in for a web browser. 
   During typical operation, a user of the client device  104  provides input to the streaming client  110  which initiates playback of a media file  108 . In response to this user input, the streaming client  110  sends a request to the streaming server  106  to begin streaming the desired media file  108 . In response to this request, the streaming server  106  sends the media file  108  to the streaming client  110  as a data stream. As media data is received by the streaming client  110 , it is stored in a streaming buffer  112 . Once the streaming buffer  112  is filled to a defined level, the streaming client  110  begins playing back the media file  108  from the streaming buffer  112 . Typically, playback of the media file  108  begins before the entire media file  108  is downloaded to the client device  104 . 
     FIG. 2  illustrates an embodiment of a method  200  for reducing the effects of variations in network bandwidth on the playback of streaming media. The depicted method  200  may be implemented by the streaming client  110  in the system  100  that is shown in  FIG. 1 . 
   In accordance with the depicted method  200 , a target range for the level of the streaming buffer  112  is defined  202 . In this context, the “level” of the streaming buffer  112  refers to the extent to which the streaming buffer  112  is filled with media data. In the depicted embodiment, the target range includes an upper limit and a lower limit for the streaming buffer  112 . As will be made clear from the discussion that follows, it is intended that the level of the streaming buffer  112  will stay within the target range during the playback of a streaming media file  108 . However, if the level of the streaming buffer  112  varies outside this range, then the rate at which the media file  108  is played back (which is known as the “playback rate,” “playout rate,” “playback speed,” and/or “playout speed”) is adjusted. 
   Before continuing with the description of the method  200  of  FIG. 2 , a brief explanation of what it means to adjust the playback rate of a media file  108  will be provided. Typically, a media file  108  has a certain playback rate associated with it. This playback rate will be referred to as the intended playback rate. The playback rate of an audio-video file may be expressed in frames/second. If the intended playback rate of a particular audio-video file is N frames/second, this means that N frames of the audio-video file are supposed to be displayed each second. This may be referred to as “normal playback.” Decreasing the playback rate below the intended playback rate results in fewer than N frames/second being displayed. This may be referred to as “slow playback.” Conversely, increasing the playback rate above the intended playback rate results in more than N frames/second being displayed. This may be referred to as “fast playback.” In general audio and video may have different values for N for normal playback. For example, N=30 frames per second for video and N=44.1 kilo-samples per second for audio. 
   Returning to the method  200  of  FIG. 2 , as the streaming client  110  is playing  204  a media file  108  that is being streamed from the streaming server  106 , the level of the streaming buffer  112  is monitored  206 . In some embodiments, the level of the streaming buffer  112  may be monitored periodically. In this context, the term “periodically” means occurring from time to time, and not necessarily at regular intervals. In other embodiments, the level of the streaming buffer  112  may be monitored continuously. As long as it is determined  206  that the level of the streaming buffer  112  is within the defined target range, then the streaming client  110  sets  208  the playback rate at a value that corresponds to normal playback. In other words, the playback rate is set equal to the intended playback rate for the media file  108 . 
   However, if it is determined  206  that the level of the streaming buffer  112  is below the lower limit of the target range, then the streaming client  110  sets  210  the playback rate for slow playback. In other words, the playback rate is set equal to a value that is less than the intended playback rate for the media file  108 . The specific value of the playback rate depends on the level of the streaming buffer  112 , and/or the data that was received in the previous few time segments, and/or the data that is estimated to be received in future time segments. Typically, the lower the level of the streaming buffer  112 , the slower the playback rate. 
   Similarly, if it is determined  206  that the level of the streaming buffer  112  is above the upper limit of the target range, then the streaming client  110  sets  212  the playback rate for fast playback. In other words, the playback rate is set equal to a value that is greater than the intended playback rate for the media file  108 . Again, the specific value of the playback rate depends on the level of streaming buffer  112 , and/or the data that was received in the previous few time segments, and/or the data that is estimated to be received in future time segments. Typically, the higher the level of the streaming buffer  112 , the faster the playback rate. 
   The streaming client  110  may repeatedly monitor  206  the level of the streaming buffer  112  and adjust the playback rate of the media file  108  as discussed above until it is determined  214  that playback of the media file  108  has ended. When this occurs, the method  200  ends. 
   A brief discussion of various alternatives to the method  200  of  FIG. 2  will now be provided. In accordance with one alternative embodiment, if the size of the streaming buffer  112  can grow, then the streaming client  110  may be configured so that fast playback is not utilized under any circumstances. In such an embodiment, instead of defining a target range for the level of the streaming buffer  112 , the streaming client  110  may only define a lower limit. If the level of the streaming buffer  112  falls below the lower limit, then the streaming client  110  may set the playback rate at a value that is less than the intended playback rate for the media file  108  (i.e., slow playback). Otherwise, in such an embodiment the streaming client  110  would set the playback rate equal to the intended playback rate for the media file  108  (i.e., normal playback), regardless of how high the level of the streaming buffer  112  becomes. 
   In accordance with another alternative embodiment, a minimum value and/or a maximum value may be defined for the playback rate. In such an embodiment, the streaming client  110  may be configured to ensure that the playback rate does not decrease below the defined minimum value, or increase above the defined maximum value. In other words, the streaming client  110  may be configured so that it does not ever play back a media file  108  at a playback rate that is less than the defined minimum value for the playback rate, regardless of how low the level of the streaming buffer  112  becomes. Similarly, the streaming client  110  may be configured so that it does not ever play back a media file  108  at a playback rate that is greater than the defined maximum value of the playback rate, regardless of how high the level of the streaming buffer  112  becomes. In some embodiments, these values may be set based on user perception of an “acceptable” slow/fast playback rate and/or based on the type of content being played back. 
     FIG. 3  illustrates another embodiment of a method  300  for reducing the effects of variations in network bandwidth on the playback of streaming media. This method  300  can be thought of as a more specific implementation of the method  200  that is shown in  FIG. 2 . 
   In the method  300  of  FIG. 3 , adjusting the playback rate of a media file  108  involves multiplying the intended playback rate by a scaling factor. If the scaling factor is greater than one, this corresponds to fast playback. The larger the scaling factor, the faster the playback rate. Conversely, if the scaling factor is less than one, this corresponds to slow playback. The smaller the scaling factor, the slower the playback rate. 
   The method  300  that is depicted in  FIG. 3  begins by determining  302  the amount of media data that is stored in the streaming buffer  112  (i.e., the level of the streaming buffer  112 ). The point in time at which this determination is made will be referred to as time t. The amount of media data that is stored in the streaming buffer  112  at time t will be referred to as B(t). The term B(t) is expressed in time units. 
   The amount of media data that is received from the streaming server  106  and added to the streaming buffer  112  over the next T time units (i.e., from time t until time t+T) is also determined  304 . This value will be referred to as (tli−tfi). The term (tli−tfi) is also expressed in time units. 
   At the end of the T time units, the streaming client  110  makes a determination about whether it should utilize slow playback, fast playback, or normal playback. Initially, the streaming client  110  evaluates  306  a condition which, if satisfied, means that slow playback should be utilized. This condition, which will be referred to as the slow playback condition, is expressed in equation 1 below:
 
( B ( t )+ tli−tfi )&lt;( B   Target   +T−B   L   margin )  (1)
 
   In equation 1, the term B Target  is a target level for the streaming buffer  112 , expressed in time units. The term B L   margin  is a margin for the streaming buffer  112 , also expressed in time units. 
   If the slow playback condition is satisfied, then the streaming client  110  adjusts  308  the scaling factor to a value that is less than one (i.e., slow playback). The specific value of the scaling factor is calculated  308  according to equation 2 below (the term sf is the scaling factor):
 
 sf =( B ( t )+ tli−tfi )/( B   Target   +T )  (2)
 
   If the slow playback condition is not satisfied, then the streaming client  110  evaluates  310  another condition which, if satisfied, means that fast playback should be utilized. This condition, which will be referred to as the fast playback condition, is expressed in equation 3 below:
 
( B ( t )+ tli−tfi )&gt;( B   Target   +T )  (3)
 
   If the fast playback condition is satisfied, then the streaming client  110  adjusts  312  the scaling factor to a value that is greater than one (i.e., fast playback). The specific value of the scaling factor is calculated  312  according to equation 4 below (again, the term sf is the scaling factor):
 
 sf =( B ( t )+ tli−tfi−B   Target )/( T )  (4)
 
   If neither the fast playback condition nor the slow playback condition are satisfied, this means that normal playback should be utilized. Thus, the streaming client  110  sets  314  the value of the scaling factor equal to one. Once the scaling factor has been determined, then the intended playback rate is multiplied by the scaling factor that is calculated. The method  300  is then repeated. This continues until it is determined  316  that playback of the media file  108  has ended, at which point the method  300  ends. 
   As mentioned, the method  300  of  FIG. 3  may be thought of as a more specific implementation of the method  200  that is shown in  FIG. 2 . A brief explanation will now be provided about how the method  300  of  FIG. 3  implements the method  200  of  FIG. 2 . 
   The method  200  of  FIG. 2  involves defining  202  a target range for the level of the streaming buffer  112 . The target range includes a defined lower limit and a defined upper limit. In the method  300  of  FIG. 3 , this is accomplished by defining a slow playback condition and a fast playback condition. 
   The method  200  of  FIG. 2  also involves monitoring  206  the level of the streaming buffer  112  to determine whether it is within the target range. In the method  300  of  FIG. 3 , this is accomplished by determining  302  B(t), determining  304  (tli−tfi), evaluating  306  the slow playback condition, and evaluating  308  the fast playback condition. 
   The method  200  of  FIG. 2  also involves setting  208  the playback rate of the media file  108  at a value that corresponds to normal playback if the level of the streaming buffer  112  is within the target range. In the method  300  of  FIG. 3 , the level of the streaming buffer  112  is within the target range if neither the slow playback condition nor the fast playback condition are satisfied. If neither the slow playback condition nor the fast playback condition are satisfied, the scaling factor is set  314  equal to one. 
   The method  200  of  FIG. 2  also involves playing back  208  the media file  108  at a playback rate that corresponds to slow playback if the level of the streaming buffer  112  is below the lower limit of the target range. In the method  300  of  FIG. 3 , the level of the streaming buffer  112  is below the lower limit of the target range if the slow playback condition is satisfied. If the slow playback condition is satisfied, the scaling factor is calculated using an equation that is defined for slow playback (i.e., equation 2), and the intended playback rate is multiplied by the scaling factor that is calculated. 
   The method  200  of  FIG. 2  also involves playing back  208  the media file  108  at a playback rate that corresponds to fast playback if the level of the streaming buffer  112  is above the upper limit of the target range. In the method  300  of  FIG. 3 , the level of the streaming buffer  112  is above the upper limit of the target range if the fast playback condition is satisfied. If the fast playback condition is satisfied, the scaling factor is calculated using an equation that is defined for fast playback (i.e., equation 4), and the intended playback rate is multiplied by the scaling factor that is calculated. 
   A brief discussion of various alternatives to the method  300  of  FIG. 3  will now be provided. In the method  300  shown in  FIG. 3 , a decision is made every T time units about whether to utilize normal playback, slow playback, or fast playback. In an alternative embodiment, the value of T may be adjusted. For example, the value of T may be set relatively high when significant fluctuations in available network bandwidth are not anticipated, and relatively low when such fluctuations are expected. In some embodiments, the value for T may be varied and may not be constant. 
   In accordance with another alternative embodiment, a small constant buffer margin may be added to the fast playback condition. In particular, the fast playback condition may be expressed as:
 
( B ( t )+ tli−tfi )&gt;( B   Target   +T+B   U   margin )  (5)
 
   In equation 5, the term B U   margin  is a margin for the streaming buffer  112 , expressed in time units. 
   If the fast playback condition is as shown in equation 5, then the specific value of the scaling factor may be calculated using equation 6:
 
 sf= ( B ( t )+ tli−tfi )/( B   Target   +n*T )  (6)
 
   In equation 6, the term n is an integer that is greater than or equal to zero. As will be discussed in greater detail below, the alternative embodiment that is represented by equations 5 and 6 may allow a smoother transition to normal playback than the method  300  of  FIG. 3 . 
   In accordance with another alternative embodiment, the amount of media data that is to be received over the next T time units may be estimated (e.g. based on the amount of media data received in the past few durations of T time units) and this estimate may be used to calculate the scaling factor (sf) using equation 7:
 
 sf =( B ( t )+ tli−tfi+tE )/( B   Target +2 *T )  (7)
 
   In equation 7, the term tE is the amount of media data that is estimated to be received over the next T time units, i.e., during time [t+T, t+2T). A very conservative method may set tE=0. 
     FIG. 3A  illustrates another embodiment of a method  300 A for reducing the effects of variations in network bandwidth on the playback of streaming media. In accordance with the method  300 A of  FIG. 3A , the amount of data received in timestamp units in various past time segments is monitored  302 A. In the discussion that follows, the term T indicates a time segment, the term t indicates the current time, and the term t k   R  indicates the data received in timestamp units at the receiver (the streaming client  110 ) in the previous k th  time segment, i.e., during the time [t−kT,t−(k−1)T). 
   The streaming client  110  evaluates  304 A a condition which, if satisfied, means that slow playback should be utilized. This condition, which will be referred to as the slow playback condition, is expressed in equation 8 below:
 
( B ( t )+ t   0   R )&lt;( B   Target   +T−B   L   margin )  (8)
 
   In equation 8, the term B(t) refers to the amount of data stored in the buffer  112  in time units at time t. The term B Target  refers to the target buffer level in time units. The term B L   margin  is a small buffer margin in time units to prevent AMP due to very short term variation in the buffer levels. 
   If the slow playback condition is satisfied, then the streaming client  110  adjusts  306 A the scaling factor to a value that is less than one (i.e., slow playback). The specific value of the scaling factor is calculated  306 A according to equation 9 below (the term sf is the scaling factor): 
   
     
       
         
           
             
               
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   In equation 9, the term E(t l   R ) indicates the estimated amount of data that will be received in timestamp units at the receiver (the streaming client  110 ) in the next l th  time segment, i.e., during the time [t−lT,t−(l−1)T), where l=−1, . . . , −(N−1). The term sf min  refers to the minimum scale factor. 
   If it is determined  304 A that the slow playback condition is not satisfied, then the streaming client  110  evaluates  308 A another condition which, if satisfied, means that fast playback should be utilized. This condition, which will be referred to as the fast playback condition, is expressed in equation 10 below:
 
( B ( t )+ t   0   R )&gt;( B   Target   +T+B   U   margin )  (10)
 
   In equation 10, the term B U   margin  is a small buffer margin in time units, which may be set based on the value for B Target  and the size of the buffer  112 . 
   If it is determined  308 A that the fast playback condition is satisfied, then the streaming client  110  adjusts  310 A the scaling factor to a value that is greater than one (i.e., fast playback). The specific value of the scaling factor is calculated  310 A according to equation 11 below (the term sf is the scaling factor): 
   
     
       
         
           
             
               
                 sf 
                 = 
                 
                   min 
                   ( 
                   
                     
                       
                         
                           B 
                           ⁡ 
                           
                             ( 
                             t 
                             ) 
                           
                         
                         + 
                         
                           t 
                           0 
                           R 
                         
                         - 
                         
                           B 
                           Target 
                         
                       
                       T 
                     
                     , 
                     
                       sf 
                       max 
                     
                   
                   ) 
                 
               
             
             
               
                 ( 
                 11 
                 ) 
               
             
           
         
       
     
   
   In equation 11, the term sf max  is the maximum scale factor. Equation 11 does not use an upper buffer margin. Alternatively, the value of the scaling factor may be calculated according to equation 12, which uses an upper buffer margin. 
   
     
       
         
           
             
               
                 sf 
                 = 
                 
                   min 
                   ⁡ 
                   
                     ( 
                     
                       
                         
                           
                             B 
                             ⁡ 
                             
                               ( 
                               t 
                               ) 
                             
                           
                           + 
                           
                             t 
                             0 
                             R 
                           
                         
                         
                           
                             B 
                             Target 
                           
                           + 
                           nT 
                         
                       
                       , 
                       
                         sf 
                         max 
                       
                     
                     ) 
                   
                 
               
             
             
               
                 ( 
                 11 
                 ) 
               
             
           
         
       
     
   
   If neither the fast playback condition nor the slow playback condition are satisfied, this means that normal playback should be utilized. Thus, the streaming client  110  sets  312 A the value of the scaling factor equal to one. Once the scaling factor has been determined, then the intended playback rate is multiplied by the scaling factor that is calculated. 
   In some embodiments a hysteresis type of behavior can be applied during the decision process where the slow playback may be continued (if currently doing slow playback) until the buffer level comes back to at or slightly above the target buffer level. 
   As discussed above, the method  300  that is shown in  FIG. 3  involves determining  304  the amount of media data that is received from the streaming server  106  and added to the streaming buffer  112  over the next T time units (i.e., from time t until time t+T).  FIG. 4  illustrates an exemplary method  400  for performing this step. 
   In accordance with the depicted method  400 , the streaming client  110  examines  402  the timestamp of each packet that it receives from the streaming server  106  between time t and time t+T. The streaming client  110  identifies  404  the earliest timestamp that is associated with a packet that is received during this time. The earliest timestamp will be referred to as tfi. The streaming client  110  also identifies  406  the latest timestamp that is associated with a packet that is received during this time. The latest timestamp will be referred to as tli. 
   For purposes of identifying  404  the earliest timestamp (tfi) and identifying  406  the latest timestamp (tli), the streaming client  110  may rearrange  408  any packets that are received out of order, ignore  410  packets that are based on previous re-transmission requests (for lost packets) or that are received very late, and account  412  for packets that are not received. In addition, the timestamps may be converted  414  to normal play time on the timeline. 
   Once the earliest timestamp (tfi) and the latest timestamp (tli) are identified  404 ,  406  the amount of media data that is received from the streaming server  106  and added to the streaming buffer  112  from time t until time t+T may be determined by calculating  416  the difference between tli and tfi. This value may then be utilized as discussed above in connection with  FIG. 3 . 
     FIG. 5  illustrates components that may be provided within a streaming client  510  in order to implement the method  300  of  FIG. 3 . A data reception monitoring component  514 , an adaptive playback decision component  516 , a scaling factor computation component  518 , and a playback component  520  are provided. 
   In accordance with the depicted embodiment, the data reception monitoring component  514  determines  302  the amount of media data that is stored in the streaming buffer  112  at a particular point in time (time t). The data reception monitoring component  514  also determines  304  the amount of media data that is received from the streaming server  106  and added to the streaming buffer  112  over the previous and/or next time segments of T time units. The data reception monitoring component  514  may also estimate the amount of data that will be received over future time segments, e.g., based on the past and/or current data reception history. The data reception monitoring component  514  may provide this information  522  to the adaptive playback decision component  516 . 
   The adaptive playback decision component  516  uses the information  522  that it receives from the data reception monitoring component  514  to make a decision  524  about whether slow playback, fast playback, or normal playback should be utilized. In particular, the adaptive playback decision component  516  evaluates  306  the slow playback condition (e.g., equation 1 above). If the slow playback condition is satisfied, then the decision  524  is to utilize slow playback. If the slow playback condition is not satisfied, then the adaptive playback decision component  516  evaluates  308  the fast playback condition (e.g., equation 3 above). If the fast playback condition is satisfied, then the decision  524  is to utilize fast playback. If neither the slow playback condition nor the fast playback condition are satisfied, then the decision  524  is to utilize normal playback. The adaptive playback decision component  516  provides its decision  524  to the scaling factor computation component  518 . 
   The scaling factor computation component  518  calculates the scaling factor  526  based on the decision  524  that it receives from the adaptive playback decision component  516 . If the decision  524  is to utilize slow playback, then the scaling factor computation component  518  calculates the scaling factor  526  using an equation that is defined for slow playback (e.g., equation 2 above). If the decision  524  is to utilize fast playback, then the scaling factor computation component  518  calculates the scaling factor  526  using an equation that is defined for fast playback (e.g., equation 6 above). If the decision  524  is to utilize normal playback, then the scaling factor computation component  518  sets the scaling factor  526  equal to one. 
   The scaling factor computation component  518  provides the scaling factor  526  that it calculates to the playback component  520 . The playback component  520  plays back the media file  108  at a playback rate that is determined by multiplying the intended playback rate for the media file  108  by the scaling factor  526 . 
   Of course, the components that are illustrated in  FIG. 5  are exemplary only. The functionality that is performed by distinct components in  FIG. 5  may, in an alternative embodiment, be performed by a single component. Also, the functionality that is performed by a single component in  FIG. 5  may be performed by multiple components. 
   A simulation of the method  300  shown in  FIG. 3  was performed. For the simulation, an NS2 network simulator was used to simulate a wireless channel with bandwidth variation. The NS2 scenario consisted of four wireless network nodes, which will be referred to as node  0 , node  1 , node  2 , and node  3 . It was simulated that a CBR source (UDP) on node  0  streamed a media file to node  1 . Streaming started at t=1 second and stopped at t=121 seconds. The CBR source had a bit-rate of 3 Mbps and a packet size of 1000 bytes. A small amount of cross-traffic was simulated between the other two nodes (i.e., nodes  2  and  3 ). In particular, it was simulated that node  2  sent a 1 MB file to node  3 , using FTP, starting at t=60 seconds. 
     FIG. 6  shows the level of the streaming buffer level at node  1  during the simulation when using normal playback only. For purposes of comparison,  FIG. 6  also shows the level of the streaming buffer at node  1  during the simulation when using the approach described above in connection with  FIG. 3 . As can be seen, in this simulation using normal playback only resulted in buffer underflow, which lasted from t=62 seconds to t=66 seconds. However, the underflow was prevented in this simulation by using the method  300  of  FIG. 3 .  FIG. 7  shows the values of the scaling factor that were calculated during the simulation. 
     FIG. 8  shows the level of the streaming buffer at node  1  during the simulation when a known adaptive playout technique was used. This known adaptive playout technique utilizes a constant scale factor. A constant scale factor of 1.33 was used during the simulation. For purposes of comparison,  FIG. 8  also shows the level of the streaming buffer at node  1  during the simulation when using the approach described above in connection with  FIG. 3 . As can be seen, using a constant scale factor for adaptive playout resulted in a buffer underflow during the simulation, which occurred from t=64 seconds to t=66 seconds. Furthermore, the constant scale factor in this case also resulted in oscillations (between t=68 seconds and t=120 seconds) which can be undesirable. As discussed above, the simulation of the method  300  shown in  FIG. 3  did not result in buffer underflow. 
     FIGS. 9-10  show simulation results for the alternative embodiment in which a small constant buffer margin is added to the fast playback condition. This was discussed above in connection with equations 5 and 6.  FIG. 9  shows the level of the streaming buffer at node  1  during the simulation when the buffer margin was not used (i.e., equation 3 was used for the fast playback condition, and equation 4 was used to calculate the scaling factor for fast playback).  FIG. 9  also shows the level of the streaming buffer at node  1  during the simulation when the buffer margin was used (i.e., equation 5 was used for the fast playback condition, and equation 6 was used to calculate the scaling factor for fast playback). As can be seen, using the buffer margin resulted in a smoother transition from fast playback to normal playback (and vice versa) as compared to a more abrupt transition when the buffer margin was not used. Also, using the buffer margin resulted in an increased streaming buffer level (from t=68 to t=88 seconds). 
     FIG. 10  shows the scaling factors that were computed during the simulation when the buffer margin was used.  FIG. 10  also shows the scaling factors that were computed during the simulation when the buffer margin was not used. The graphs in  FIG. 10  show values for the scaling factor that are greater than one (which correspond to fast playback). 
     FIG. 11  illustrates various components that may be utilized in a server device  1202  and/or a client device  1204 . The illustrated components may be located within the same physical structure or in separate housings or structures. 
   The server device  1202  and/or client device  1204  includes a processor  1203  and memory  1205 . The processor  1203  controls the operation of the server device  1202  and/or client device  1204  and may be embodied as a microprocessor, a microcontroller, a digital signal processor (DSP) or other device known in the art. The processor  1203  typically performs logical and arithmetic operations based on program instructions stored within the memory  1205 . 
   The server device  1202  and/or client device  1204  typically also includes one or more communication interfaces  1207  for communicating with other electronic devices. The communication interfaces  1207  may be based on wired communication technology, wireless communication technology, or both. Examples of different types of communication interfaces  1207  include a serial port, a parallel port, a Universal Serial Bus (USB), an Ethernet adapter, an IEEE 13124 bus interface, a small computer system interface (SCSI) bus interface, an infrared (IR) communication port, a Bluetooth wireless communication adapter, and so forth. 
   The server device  1202  and/or client device  1204  typically also includes one or more input devices  1209  and one or more output devices  1211 . Examples of different kinds of input devices  1209  include a keyboard, mouse, microphone, remote control device, button, joystick, trackball, touchpad, lightpen, etc. Examples of different kinds of output devices  1211  include a speaker, printer, etc. One specific type of output device which is typically included in a computer system is a display device  1213 . Display devices  1213  used with embodiments disclosed herein may utilize any suitable image projection technology, such as a cathode ray tube (CRT), liquid crystal display (LCD), light-emitting diode (LED), gas plasma, electroluminescence, or the like. A display controller  1215  may also be provided, for converting data stored in the memory  1205  into text, graphics, and/or moving images (as appropriate) shown on the display device  1213 . 
   Of course,  FIG. 11  illustrates only one possible configuration of a server device  1202  and/or client device  1204 . Various other architectures and components may be utilized. 
   Information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof. 
   The various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention. 
   The various illustrative logical blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array signal (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
   The steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASIC may reside in a user terminal. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal. 
   The methods disclosed herein comprise one or more steps or actions for achieving the described method. The method steps and/or actions may be interchanged with one another without departing from the scope of the present invention. In other words, unless a specific order of steps or actions is required for proper operation of the embodiment, the order and/or use of specific steps and/or actions may be modified without departing from the scope of the present invention. 
   While specific embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise configuration and components disclosed herein. Various modifications, changes, and variations which will be apparent to those skilled in the art may be made in the arrangement, operation, and details of the methods and systems of the present invention disclosed herein without departing from the spirit and scope of the invention.