Patent Publication Number: US-7586852-B2

Title: Devices, software and methods for measuring packet loss burstiness to determine quality of voice data transmission through a network

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 09/735,429, filed Dec. 12, 2000, now U.S. Pat. No. 6,930,982. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to the field of voice data transmission through a network, and more specifically to devices, software, and methods for measuring a burstiness of packet loss in determining a quality of voice for network telephony. 
     2. Description of the Related Art 
     Voice transmissions, such as telephone calls, are increasingly made over packet switched networks. Unlike traditional telephone lines, networks break down the voice into data, arrange the data in distinct packets, and send the packets across the network. The voice is reconstructed at the other end of the voice call for the listener. 
     Sometimes packets can be lost. This can happen when packets are dropped from the network, or are delivered too late to be incorporated in the reconstructed voice sequence. In that case, the packets are absent, but are generally called lost packets in the art. 
     When a packet is lost, that means that a portion of the voice message does not reach the listener. In its place, the listener hears a small interruption, or the voice is syncopated. Thus, losing packets degrades the quality of the voice call, and thus of the telephone service. 
     There have been efforts to rectify at least the problem of low call quality. For example, when a single packet is lost, its data can be reconstructed from its immediately previous data. So, even though its information may be actually lost, the user will not hear an interruption. This is called packet loss concealment. 
     Packet loss concealment is harder to perform, however, when many packets are lost in a large group. They are harder to reconstruct, because for many the immediately previous data is not available. 
     In addition, there have been efforts to monitor the quality of service, so that other corrective measures can be taken. These efforts include quantifying the quality of service, in other words, determining how poor the service is at any given time. Once service is determined to be poor, other corrective measures can be taken. 
     Quality of service is quantified by measuring other parameters in addition to packet loss. These other parameters are delay, echo, codec degradation, etc. All the parameters are taken together to determine a total voice quality statistic, or figure of merit. 
     Packet loss is traditionally measured in the prior art as a rate R according to Equation (1):
 
 R =(number of lost packets)/(total number of packets)  Equation (1)
 
     Equation (1) is used to determine voice quality. By using Equation (1), however, the prior art makes a fundamental assumption about the nature of packet loss. The assumption is that packet loss is distributed uniformly over the duration telephone call. 
     The assumption of the prior art does not take into account the true nature of packet loss. Packet loss is not distributed uniformly over the duration telephone call. Instead, it tends to come in bursts, or groups. Worse, some bursts last longer than others. Yet the present systems do not detect that, which is a deficiency. 
     The deficiency of the prior art is illustrated with reference to  FIG. 1 . Three waveforms A, B, C, are given for three sample voice data transmissions, all of the same time duration. The waveforms A, B, C illustrate packets PR as they are received, prior to any data reconstruction for packet loss concealment. 
     Each of the three waveforms A, B, C, reflects a sequence as reconstructed. Each sequence leaves blanks PL for absent packets PL. Packets are absent either because they are lost, or simply arrive too late to be incorporated in the play out. 
     In all three waveforms A, B, C, one fifth, or 20% of the packets are not received. These are indicated as lost packets PL. So, for all three waveforms A, B, C, Equation (1) returns R=20%. This yields the same voice call quality grade for all three. In the example of  FIG. 1 , that grade is “FAIR”. 
     It will be appreciated that the lost packets PL are grouped differently in each of waveforms A, B, C. In waveform A, the lost packets PL are distributed uniformly over the examined duration of the voice call. More particularly, every fifth packet PL is missing. In waveform B, the lost packets PL occur in somewhat bigger bursts. As drawn, every ninth and tenth packet PL are missing. In waveform C, all 10 packets are lost in one big burst. 
     In reality, for waveform B, the perception of the voice call quality will be somewhat worse than in waveform A, while the perception for waveform C will be by far the worst. That is because it will be harder to reconstruct the missing packets, since they occur in bursts. As such, waveform B deserves a worse grade than waveform A. For the same reason, waveform C deserves an even worse grade. That is because packet loss concealment is the hardest to perform for waveform C. Many of the immediately previous data is not available, and the voice will be perceived as lost. 
     The problem, however, is that the differentiation in eventual quality of the reconstruction is not predicted by the grading system. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention overcomes these problems and limitations of the prior art. 
     Generally, the present invention provides devices, software, and methods for measuring a burstiness of packet loss episodes, in determining a quality of a voice transmission through a network. More specifically, the invention teaches to determine a burstiness statistic, for quantifying how the lost packets are distributed with respect to the received packets within the sequence. The burstiness statistic is optionally used to determine a figure of merit, which in turn can be used to give a grade for predicting how well a packet loss concealment scheme will work. 
     The invention finds applications in broadcasting through a network (such as internet radio) and network telephony. 
     The invention will become more readily apparent from the following Detailed Description, which proceeds with reference to the drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates three possible waveforms of received packets for a voice data transmission, and quality grades assigned to these waveforms by systems of the prior art. 
         FIG. 2  is a block diagram of a transmitting device made according to an embodiment of the present invention. 
         FIG. 3  is a block diagram of a receiving device made according to an embodiment of the present invention. 
         FIG. 4  is a flowchart illustrating methods according to embodiments of the present invention. 
         FIG. 5  is a flowchart illustrating methods for performing a box of the flowchart of  FIG. 4 . 
         FIG. 6  illustrates the three possible waveforms of  FIG. 1 , and voice transmission quality grades assigned to these waveforms by embodiments of  FIG. 5 . 
         FIG. 7  is a flowchart illustrating methods for performing a box of the flowchart of  FIG. 4 . 
         FIG. 8  illustrates the three possible waveforms of  FIG. 1 , and voice transmission quality grades assigned to these waveforms by embodiments of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     As has been mentioned, the present invention provides devices, software, and methods for measuring an additional statistic for quality of voice in Voice over Internet Protocol (VoIP) transmissions. The additional statistic measures a burstiness, grouping, or clustering in the pattern of lost packets. The invention is now described in more detail. 
     Referring to  FIG. 2 , a transmitting device  200  made according to an embodiment of the invention is described. Device  200  is for transmitting voice packets through a network  120 . Device  200  preferably transmits the voice packets to a network switch  140 , such as Gateway-A  140 , of the network  120 . 
     One or more of the components of device  200  can be implemented in combination with each other, consistently with components of this description. For example, device  200  can be implemented as part of a larger Digital Signal Processing (DSP) architecture. 
     In the embodiment of  FIG. 2 , device  200  includes an encoder  210  for encoding a frame of voice data into a voice packet. Encoder  210  includes a processor  212 , which is also referred to as Central Processing Unit (CPU)  212 , and a memory  214 . The processor  212  is adapted to perform the method of the invention. Preferably it is so adapted by running a program  216  made according to the invention, which resides on memory  214 . 
     In addition, device  200  optionally includes a transmit buffer  230  adapted to receive inputs from the encoder  210 , for storing the encoded frames prior to transmitting. 
     Device  200  includes a network interface (not shown separately) for interfacing with network  120 . The network interface can be implemented as a stand-alone feature, or in conjunction with another component, such as transmit buffer  230 . 
     Referring to  FIG. 3 , a receiving device  300  according to the invention is described. One or more of the components of device  300  can be implemented in combination with each other, consistently with components of this description. For example, device  300  can be implemented as part of a larger Digital Signal Processing (DSP) architecture. 
     In the embodiment of  FIG. 3 , device  300  includes a jitter buffer  310 . This stores a number of frames as they are received from the network  120 . The jitter buffer thus prevents an anomaly that could be experienced, if frames were played out exactly when they are received. Due to the nature of transmission through the network  120 , they can be received in bunches, with gaps between the bunches. Jitter buffer  310  permits playing them out at a regular pace, notwithstanding when exactly they are received. 
     Device  300  also includes a decoder  370 . Decoder  370  includes a processor  372 , which is also referred to as Central Processing Unit (CPU)  372 , and a memory  374 . The processor  372  is adapted to perform the method of the invention. Preferably it is so adapted by running a program  376  made according to the invention, which resides on memory  374 . 
     Device  300  can also include other components, such as a Digital to Analog Converter (DAC)  380 . This converts the decoded voice data into an analog signal, which can be input in a speaker  390 . 
     Device  300  includes a network interface (not shown separately) for interfacing with network  120 . The network interface can be implemented as a stand-alone feature, or in conjunction with another component, such as jitter buffer  310 . Device  300  is deemed to interface with a network switch  160 , such as Gateway-B  160 , of network  120 . 
     As an example, a device that may incorporate aspects of the present invention would be an Internet Protocol (IP) telephone. Its transmitting side could include device  200  of  FIG. 2 , while its receiving side could include device  300  of  FIG. 3 . 
     A device of the invention need not be limited to two-way voice communication. Devices, software and methods for one way transmitting, such as broadcasting, are also included. 
     The invention can be performed at the conclusion of a transmission, for example to calibrate a system. That would be using all the packets of the voice transmission, which can be 10,000 for a telephone call. Alternately the invention can be performed dynamically during the voice transmission by either participant. In that case, it can look at a segment of a waveform or sequence. 
     It is readily apparent that the present invention can be implemented by one or more devices that include logic circuitry. It can also be implemented by a device that includes a dedicated processor system, that can include a microcontroller or a microprocessor. 
     The invention additionally provides methods, which are described below. Moreover, the invention provides apparatus that performs, or assists in performing the methods of the invention. This apparatus may be specially constructed for the required purposes, or it may comprise a general-purpose computer selectively activated or reconfigured by a computer program stored in the computer. The methods and algorithms presented herein are not necessarily inherently related to any particular computer or other apparatus. In particular, various general-purpose machines may be used with programs in accordance with the teachings herein, or it may prove more convenient to construct more specialized apparatus to perform the required method steps. The required structure for a variety of these machines will appear from this description. 
     Useful machines or articles for performing the operations of the present invention include general-purpose digital computers or other similar devices. In all cases, there should be borne in mind the distinction between the method of operating a computer and the method of computation itself. The present invention relates also to method steps for operating a computer and for processing electrical or other physical signals to generate other desired physical signals. 
     The invention additionally provides a program, and a method of operation of the program. The program is most advantageously implemented as a program for a computing machine, such as a general purpose computer, a special purpose computer, a microprocessor, etc. 
     The invention also provides a storage medium that has the program of the invention stored thereon. The storage medium is a computer-readable medium, such as a memory, and is read by the computing machine mentioned above. 
     A program is generally defined as a sequence of steps leading to a desired result. These steps, also known as instructions, are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated or processed. When stored, they can be stored in any computer-readable medium. It is convenient at times, principally for reasons of common usage, to refer to these signals as bits, data bits, samples, values, elements, symbols, characters, images, terms, numbers, or the like. It should be borne in mind, however, that all of these and similar terms are associated with the appropriate physical quantities, and that these terms are merely convenient labels applied to these physical quantities. 
     This detailed description is presented largely in terms of flowcharts, display images, algorithms, and symbolic representations of operations of data bits within a computer readable medium, such as a memory. Such descriptions and representations are the type of convenient labels used by those skilled in programming and/or the data processing arts to effectively convey the substance of their work to others skilled in the art. A person skilled in the art of programming can use this description to readily generate specific instructions for implementing a program according to the present invention. For the sake of economy, however, flowcharts used to describe methods of the invention are not repeated in this document for describing software according to the invention. 
     Often, for the sake of convenience only, it is preferred to implement and describe a program as various interconnected distinct software modules or features, collectively also known as software. This is not necessary, however, and there may be cases where modules are equivalently aggregated into a single program with unclear boundaries. In any event, the software modules or features of the present invention can be implemented by themselves, or in combination with others. Even though it is said that the program can be stored in a computer-readable medium, it should be clear to a person skilled in the art that it need not be a single memory, or even a single machine. Various portions, modules or features of it can reside in separate memories, or even separate machines. The separate machines may be connected directly, or through a network, such as a local access network (LAN), or a global network, such as the Internet. 
     In the present case, methods of the invention are implemented by machine operations. In other words, embodiments of the program of the invention are made such that they perform methods of the invention that are described in this document. These can be optionally performed in conjunction with one or more human operators performing some, but not all of them. As per the above, the users need not be collocated with each other, but each only with a machine that houses a portion of the program. Alternately, some of these machines can operate automatically, without users and/or independently from each other. 
     Methods of the invention are now described. 
     Referring now to  FIG. 4 , a flowchart  400  is used to illustrate methods according to embodiments of the invention. 
     According to a box  410 , packets are received that contain voice data. Voice data are those data that represent sound, such as voice. The packets are received from a network, such as network  120  of  FIG. 3 . 
     According to a next box  420 , an intended sequence of the voice data is determined. This is preferably performed by examining sequence data in the received packets. 
     According to a next box  430 , the received packets are arranged in the sequence. This may leave blanks in the sequence. 
     According to a next box  440 , lost packets PL are inferred to be in the blanks of the sequence. 
     According to a next box  450 , a burstiness statistic is computed, for quantifying how the lost packets are distributed with respect to the received packets within the sequence. It will be perfectly apparent to a person skilled in the art that a burstiness statistic according to the invention is equivalently defined if it quantifies how the received packets are distributed with respect to the lost packets within the sequence. 
     A person skilled in the art will be able to determine a large number of burstiness statistics that can be computed according to the invention. By way of example, and not of limitation, two methods according to the invention are described later in this document, each of which yields at least two possible burstiness statistics. 
     According to an optional next box  460 , a figure of merit is determined for the sequence from any one or more of the burstiness statistics computed according to the invention. The figure of merit can be an equipment impairment factor IE, which in turn can use other parameters. 
     The figure of merit is used to determine the overall grade given to the sequence. Optionally and preferably, it is also computed from the average packet loss rate R. This can be performed either by a table lookup, or using a polynomial function. The table or the coefficients of the polynomial can be determined using subjective or objective speech quality test results. 
     Referring now to  FIG. 5 , a flowchart  500  is used for illustrating performing box  450  of  FIG. 4  according to an embodiment of the invention. 
     According to a box  510 , a duration is counted in the sequence of a packet loss episode. This is in the form of a number of lost packets PL, which is also known as duration number DPL. A packet loss episode is an episode of contiguously occurring lost packets PL. Equivalently, a duration number can be a number of contiguously occurring received packets PR. It is preferred that more than one duration numbers of the same type are counted. 
     According to an optional next box  520 , the longest packet loss episode is determined. This is preferably performed by determining a maximum, or largest one, of the duration numbers DPL. 
     According to an optional next box  530 , the packet loss episodes (NPL) are counted. Equivalently, the complementary packet received instances can be counted. While the complementary number may differ by one from NPL, the large numbers involved will make the difference insubstantial. 
     According to an optional next box  540 , a number of packets is counted. In the preferred embodiment, the packets that are counted are the lost packets PL, yielding a variable that is denoted by B. 
     According to an optional next box  550 , the average packet loss duration (ADU) is computed. Computation is performed preferably according to Equation (2):
 
 ADU=B/NPL   Equation (2)
 
     According to an optional next box  560 , a variance of ADU is computed. 
     Referring to  FIG. 6 , the result of the embodiments of the invention can be appreciated in full.  FIG. 6  shows the three waveforms of  FIG. 1 , except grading takes place according to the methods of  FIG. 4  and  FIG. 5 , not the prior art. 
     In waveform A, the lost packets PL are the least grouped, and thus the burstiness is the least. Each gap is a packet loss episode PL. Equation (2) yields ADU=1 as a measure of the amount of burstiness. The grade “GOOD” can thus be assigned, since it can be anticipated that packet loss concealment can accommodate completely the little burstiness. This is even though the average packet loss rate P is 20%. 
     In waveform B, the lost packets PL are grouped or clustered a little more, and thus there is somewhat higher burstiness. Equation (2) yields ADU=2 as a measure of the amount of burstiness. Indeed, duration number DP would be DPLB=2 for all gaps of lost packets PL. The grade “FAIR” can thus be assigned, since it can be anticipated that packet loss concealment can accommodate mostly, but not completely, this amount of burstiness. 
     In waveform C, the lost packets PL are the most grouped, and thus the burstiness is the highest. Equation (2) yields ADU=10 as a measure of the amount of burstiness. Indeed, duration number DP would be DPLC=10 for the single gap. The grade “POOR” can thus be assigned, since these numbers, combined with an average packet loss rate of R=20%, can be used to anticipate that the known packet loss concealment techniques will not succeed in the face of such large gaps. 
     It will be appreciated that the grades of  FIG. 6  are more discerning than those of  FIG. 1 , and work better to predict the success of a packet loss concealment scheme. 
     Referring now to  FIG. 7 , a flowchart  700  is used for illustrating performing box  450  of  FIG. 4  according to another embodiment of the invention. In the embodiment of  FIG. 7 , the burstiness statistic is computed by counting transitions between received packets PR and lost packets PL, using a Markov chain type model for the sequence. For a given average loss rate R, the more the transitions, the smaller the duration of each packet loss episode is inferred to be. The less each packet loss episode lasts, the less the burstiness, and a higher grade can be assigned. 
     More specifically, according to a box  710 , good states are defined in the sequence where there are at least some received packets PR. In addition, bad states are defined in the sequence where there are lost packets PL. 
     According to a next box  720 , the transitions are counted between the good states in the bad states. In the preferred embodiment, the transitions that are counted are from the bad states to the good states. That variable is denoted by U. 
     According to a next box  730 , a number of packets is counted. In the preferred embodiment, the packets that are counted are the lost packets, yielding a variable that is denoted by B. 
     According to next box  740 , the counted number of transitions is divided by the counted number of packets. This yields a variable that is denoted by Q, according to Equation (3):
 
 Q=U/B   Equation (3)
 
     A number of different embodiments are now described with reference to boxes  750  and  760 . As will be appreciated, execution can proceed from box  740 , to box  750 , to box  760 , to box  770 . Alternately, the order of box  750  and box  760  could be reversed. In addition, one of box  750  and box  760  can be omitted. 
     Referring now to box  750 , the total packets T are counted. In addition, the average loss ratio R is computed according to equation (1), which is repeated below:
 
 R =(number of lost packets)/(total number of packets)= B/T   Equation (1)
 
     Referring now to box  760 , the complementary transitions are counted, i.e. those from the good states to the bad states. This yields a variable D. In addition, the received packets PR are counted, which yields a variable G. Moreover, a variable P is computed from Equation (4) below. It will be appreciated that variable P is complementary to variable Q.
 
 P=D/G   Equation (4)
 
     According to an optional next box  770 , a normalized burstiness statistic Qn is computed according to Equation (5) below:
 
 Qn=Q/( 1 −P )  Equation (5)
 
     It should also be appreciated that the normalized burstiness statistic of the equation (5) is chosen so as to amplify the computation of variable Q. 
     Now it will be appreciated why one of box  750 , and  760  can be omitted. The above equations can be solved in the way as to avoid at least one of these boxes. For example, a corollary of these equations is equation (6) below:
 
 R=P /( P+Q )  Equation (6)
 
     Referring to  FIG. 8 , the result of the embodiments of the invention can be appreciated in full.  FIG. 8  shows the three waveforms of  FIG. 1 , except grading takes place according to the methods of  FIG. 4  and  FIG. 7 , not the prior art. The waveforms show the transitions between the good and the bad states (i.e. from PL to PR, and vice versa) as arrows. Transitions at the beginning and the end of the waveform are not included, as they can be arbitrary. 
     In waveform A, the lost packets PL are the least grouped. Equations (3) and (5) yield Q=1, and Qn=1.33. The grade “GOOD” can thus be assigned, for the same rationale as in  FIG. 6 . 
     In waveform B, the lost packets PL are clustered a little more. Equations (3) and (5) yield Q=0.5, and Qn=0.57. The grade “FAIR” can thus be assigned, for the same rationale as in  FIG. 6 . 
     In waveform C, the lost packets PL are the most grouped. Equations (3) and (5) yield Q=0.125, and Qn=0.125. The grade “POOR” can thus be assigned, for the same rationale as in  FIG. 6 . 
     Again, it will be appreciated that the grades of  FIG. 8  are more discerning than those of  FIG. 1 , and work better to predict the success of a packet loss concealment scheme. 
     The above description for a Markov chain type model can be extended for the implementations of  FIG. 6 . In addition, more burstiness parameters can be computed with a more general Markov chain type model. For example, one may use an M-state Markov chain model, having two states. The first state−1 is “good”, state m corresponds a packet loss episode of length M−1, and so on. One will recognize that the examples worked above where for M=2, but that is not limiting. In fact, extending to a higher M yields another M−1 burstiness statistics that may be used as per the above. 
     A person skilled in the art will be able to practice the present invention in view of the description present in this document, which is to be taken as a whole. Numerous details have been set forth in order to provide a more thorough understanding of the invention. In other instances, well-known features have not been described in detail in order not to obscure unnecessarily the invention. 
     While the invention has been disclosed in its preferred form, the specific embodiments thereof as disclosed and illustrated herein are not to be considered in a limiting sense. Indeed, it should be readily apparent to those skilled in the art in view of the present description that the invention can be modified in numerous ways. The inventor regards the subject matter of the invention to include all combinations and subcombinations of the various elements, features, functions and/or properties disclosed herein. 
     The following claims define certain combinations and subcombinations, which are regarded as novel and non-obvious. Additional claims for other combinations and subcombinations of features, functions, elements and/or properties may be presented in this or a related document.