Abstract:
A system for assessing a voice communication quality of a communication path between first and second nodes over a network, wherein coded data of voice communication signals are transferred in a stream of packets via the communication path, including: a capturing unit for capturing at the first node at least one packet containing coded data representing non voice signals among the packets of the coded data to be transferred from the first node to the second node; a replacing unit for replacing a part of the coded data representing non voice signals in the captured packet with a predetermined code before the captured packet is transferred from the first node; a retrieval unit for retrieving at the second node said at least one packet containing coded data representing non voice signals; and an assessment unit for assessing the voice communication quality of the communication path.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation of Application PCT/JP2007/065827, filed on Aug. 13, 2007, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     A certain aspect of the embodiments discussed herein relates to a technique of assessing a voice communication quality of a communication path. 
     BACKGROUND 
     Networks are shifting from current circuit switching networks to packet exchange networks, and Next Generation Network (NGN) representing such shift has been established. The NGN is configured to implement voice communication using the network while maintaining voice communication quality equivalent to the quality of the current circuit switching networks and so on. 
     Thus, it is more important to estimate voice communication quality in a next generation network represented by the NGN, and measurement technology for evaluating voice communication quality on a middle path and a terminal of the network is required. That is, technology for testing voice quality degradation used for estimating the voice communication quality from data being transferred is necessary. In particular, technology for analyzing data during operation, i.e., so called active measurement technology is required. 
     Japanese Laid-open Patent Publication No. 2000-332758 and Japanese Laid-open Patent Publication No. 07-250107 disclose technology for estimating voice quality. 
     SUMMARY 
     According to an aspect of an embodiment, a system for assessing a voice communication quality of a communication path between first and second nodes over a network, wherein coded data of voice communication signals are transferred in a stream of packets via the communication path, the system includes: a capturing unit for capturing at the first node at least one packet containing coded data representing non voice signals among the packets of the coded data to be transferred from the first node to the second node; a replacing unit for replacing a part of the coded data representing non voice signals in the captured packet with a predetermined code before the captured packet is transferred from the first node; a retrieval unit for retrieving at the second node said at least one packet containing coded data representing non voice signals transferred from the first node; and an assessment unit for assessing the voice communication quality of the communication path on the basis of detection of the predetermined code in the retrieved packet. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates a network system of the embodiment configured to test voice quality degradation. 
         FIG. 2  is a schematic diagram of an analog signal of the embodiment. 
         FIG. 3  is a schematic diagram of a code of the embodiment. 
         FIG. 4  is a block diagram of a pattern replacement unit of the embodiment. 
         FIG. 5  is a block diagram of a pattern test unit of the embodiment. 
         FIG. 6  is a flowchart of a process performed by the pattern replacement unit of the embodiment. 
         FIG. 7  is a flowchart of a process performed by the pattern test unit of the embodiment. 
         FIG. 8  is a block diagram of a pattern replacement unit of the embodiment. 
         FIG. 9  is a block diagram of a test unit of the embodiment. 
         FIG. 10  is a flowchart of a process performed by the pattern test unit of the embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  illustrates a network system  100  of the embodiment configured to test voice quality degradation. According to the embodiment, a test for a voice quality degradation factor of the network system  100  will be explained hereafter. According to the test method of the embodiment, a factor of voice quality degradation caused by a change of data itself transferred by the network system  100  such as noise inserted in a quality controlled area  115  of the network system  100  can be detected while the network is being practically operated. Incidentally, operations of a pattern replacement unit  101  and pattern test units  102  and  103  will be explained in detail with reference to  FIG. 4  and  FIG. 5 . 
     [Configuration of Network System  100 ] 
     The network system  100  is constituted by the pattern replacement unit  101 , the pattern test units  102  and  103 , a detection identification unit  104 , a terminal adaptor (TA)  106 , a core edge (CE)  107 , gateways  108  and  109 , a terminal adaptor (TA)  110  and terminals  111  and  117 . According to the embodiment, a terminal  105  performs voice communication with the terminals  111  and  117  through a private network  112 , an access network  113 , a core network  114  and a public network  116 . Specifically, the network system  100  is a network system in which the terminal  105  obtains a voice signal as an interface, encodes the voice signal and transfers the voice signal to the terminals  111  and  117  in real time in an IP packet form and so on (VoIP: Voice over Internet Protocol). 
     The network system  100  is a network system configured to detect a problem that occurs in the quality controlled area  115  (the access network  113  and the core network  114 ) by using the pattern replacement unit  101  and the pattern test units  102  and  103 . The network system  100  of the embodiment can detect voice quality degradation caused by noise inserted in the quality controlled area  115  in the network. Further, the network system  100  of the embodiment can also detect voice quality degradation caused by transcoding such that a compression format of data is converted in the quality controlled area  115  in the network. 
     The pattern replacement unit  101  replaces a portion of a non-voice code to be sent to the quality controlled area  115  with a particular pattern. Then, the pattern test units  102  and  103  detect the particular pattern in the quality controlled area  115 . The detection identification unit  104  senses an involved factor of voice quality degradation from whether or not the particular pattern has been detected. The particular pattern of the embodiment is a code pattern that the pattern replacement unit  101  and the pattern test units  102  and  103  mutually hold, and is a code pattern that has been determined in advance. Further, the particular pattern is a code sequence having same weights of a plurality of code series. 
     Thus, the network system  100  can test a factor of voice quality degradation of data flowing through the quality controlled area  115  in the network system  100  even while the network is being practically operated. According to the test method of the embodiment, voice quality degradation caused by inserted noise that changes data itself can be detected as well as other voice quality degradation caused by a data delay or a packet loss. 
     The pattern replacement unit  101  is provided at the CE 107 , i.e., a point corresponding to an entrance to the quality controlled area  115  (the access network  113  and the core network  114 ). The pattern replacement unit  101  replaces a portion of a non-voice code with a particular pattern for sensing a problem. The non-voice code described here is a code corresponding to background noise included in a code that the terminal  105  produces by receiving a voice signal as an interface and encoding the received voice signal. Codec technology of the embodiment is, e.g., G. 711 recommended by ITU-T. Thus, an analog voice signal received by the terminal  105  is encoded in accordance with G. 711. The TA  106  encodes and decodes (codec) the analog voice signal. The terminals  105 ,  111  and  117  may be configured to encode and decode the analog voice signal into a digital voice signal. The encoding technology according to G. 711 recommended by ITU-T is a system for implementing a data rate of 64 kbps on a telephone line by using a PCM (pulse code modulation) system. Specifically, the analog signal is sampled at 8 kHz (sampling), and every sample is represented in 8 bits. 
     The pattern replacement unit  101  takes in a code  300  transferred by the network system  100  through the CE  107  every frame of 20 msec including 160 samples. Then, the pattern replacement unit  101  analyzes a voice characteristic of the code  300 , and detects a non-voice frame. The voice characteristic indicates data classifying every frame of the code  300  into a voice frame and a non-voice frame. Specifically, the voice characteristic is formed by a number of zero-crossings and an amplitude level. The number of zero-crossings represents how many times a voice waveform crosses a zero level in a certain period of time. The amplitude level is an average amplitude value of each of data  310 - 318  in a certain period of time. The pattern replacement unit  101  identifies a frame of a data amplitude level being equal to or greater than a certain value and of a number of zero-crossings being equal to or smaller than a certain number in a certain period of time as a voice frame. Then, the pattern replacement unit  101  identifies a frame other than the voice frame as a non-voice frame. 
     Further, the pattern replacement unit  101  analyzes a characteristic of a frame identified as a non-voice frame. The pattern replacement unit  101  detects a background noise level from the characteristic analysis. Then, the pattern replacement unit  101  replaces a portion of a level range lower than or around the detected background noise level of the detected non-voice frame with a particular pattern. The level described here is an amplitude value when one sample of an analog signal is encoded to 8-bit data in accordance to G. 711. The less significant digit and the more significant digit of the 8-bit data indicate portions of the smaller and greater amplitude values, respectively. 
     The pattern replacement unit  101  does not perform a particular pattern replacement process for a silent frame. The silent frame is a frame such that frame data includes all zero bits or at most bits indicating a faint sound of a value equal to or lower than a certain threshold. A reason why is that replacement of a portion of the silent frame with the particular pattern affects subjective sound quality a lot. Thus, as a portion of the silent frame is not replaced with the particular pattern, the pattern replacement unit  101  can prevent noise from being inserted by using a masking effect (refer to Wegel, R. L. and Lane, C. E.: Phys. Rev., 23, pp. 266-285 (1924)). 
     According to the test method of the embodiment, the pattern replacement unit  101  replaces a portion of a non-voice code with the particular pattern. Thus, an amount of data that flows through the network system  100  does not increase. Hence, according to the test method of the embodiment, a traffic increase in the network caused by the test for voice quality degradation can be removed. That is, the test method of the embodiment enables the voice quality degradation to be tested without causing ill effects such as a traffic increase in the network and resultant congestion. 
     The pattern test units  102  and  103  are provided at the GWs  108  and  109 , respectively. The pattern test units  102  and  103  take in the code  300  transferred by the network system  100  through the GWs  108  and  109 , respectively, every frame of 20 msec including 160 samples. The pattern test units  102  and  103  choose a non-voice frame as a frame to be tested. A reason why is that the pattern replacement unit  101  performs a replacement process with the particular pattern for a non-voice frame. The pattern test units  102  and  103  detect a background noise level from a characteristic analysis of a frame identified as a non-voice frame. The pattern test units  102  and  103  refer to the background noise level, and examine whether or not the particular pattern exists in a frame to be tested of a level lower than or around the background noise level. The pattern test units  102  and  103  can thereby efficiently detect the particular pattern. A reason why is that a replacement position of the particular pattern is of a level equal to or lower than the background noise level of the frame to be tested. 
     Then, the pattern test units  102  and  103  send existence data indicating whether or not the particular pattern exists to the detection identification unit  104 . The detection identification unit  104  detects a problem and estimates where the problem occurs from the existence data. Specifically, in a case where the non-voice frame for which the pattern replacement unit  101  has performed the replacement process includes a particular pattern, the detection identification unit  104  identifies no occurrence of voice quality degradation. Meanwhile, in a case where the particular pattern does not exist in the non-voice frame for which the pattern replacement unit  101  has performed the replacement process, the detection identification unit  104  identifies an occurrence of voice quality degradation. The detection identification unit  104  can make the above identification by comparing data of a replacement-processed non-voice frame that the detection identification unit  104  has ready beforehand and the existence data. That is, in a case where the existence data of a non-voice frame expected to include the particular pattern indicates “no particular pattern”, the detection identification unit  104  identifies an occurrence of voice quality degradation. Thus, the detection identification unit  104  obtains existence data of a particular pattern  319  from the pattern test units  102  and  103 , and can test existence of an occurrence of voice quality degradation of data that flows through the quality controlled area  115  in the network system  100 . Further, the detection identification unit  104  can estimate a position of the occurrence of the voice quality degradation by analyzing where the pattern test units  102  and  103  are provided. 
     [Analog Signal  200 , Code  300 ] 
       FIG. 2  illustrates a schematic diagram of an analog signal  200  of the embodiment. The terminal  105  sends the code  300  into which the analog signal  200  is digitized to the terminals  111  and  117 . The code  300  into which the analog signal  200  is digitized is to be tested with respect to the voice quality degradation. 
     The TA  106  of the embodiment illustrated in  FIG. 1  encodes the analog signal  200  into the code  300 . The TA  106  of the embodiment samples the analog signal  200  with a desired frequency. Specifically, the TA  106  samples the analog signal  200  as 21-38. Then, the TA  106  quantizes and thereby encodes amplitude values of 21-38 into data  301 - 318 , respectively. The TA  106  of the embodiment sends the code to the network as frames of 160 samples each. That is, one frame to be sent includes 160 samples. 
       FIG. 3  illustrates a schematic diagram of the code  300  corresponding to the analog signal  200 . The code  300  is formed by the data  301 - 318 . Although one frame includes 160 samples as described above,  FIG. 3  typically illustrates nine samples per one frame (data  301 - 309  and data  310 - 318 ). The data  301  is an amplitude value of a quantized and digitally encoded sampling value 21. Further, the data  302  is an encoded  22 . The data (300+n) is similarly an encoded (20+n) (n is a natural number of 1-18). Encoding technology of the embodiment is G. 711. G. 711 is a waveform encoding method of analog voice data recommended by ITU-T. The sampling rate (sampling speed) is 8 kHz. The data  301 - 318  are in 8 bits each. The data  301  of the sampling  21  is, e.g., “10101111”. The data  302  of the sampling  22  is “00110001”. The data  303  of the sampling  23  is “10111000”. The other following data  304 - 318  are in 8 bits each. The data  301 - 318  are in 8 bits each. First bits of the data  301 - 318  represent a portion of a great amplitude of the samplings  21 - 38 , respectively. Second, third and other bits are data indicating portions of small amplitudes in the above order. One frame is formed by 160 data (samples) of the data  301 - 309 . The network system  100  of the embodiment transfers data every frame (160 samples). 
     Two kinds of quantization methods, μ-Law and A-Law, are specified. The methods, μ-Law and A-Law, are one of encoding laws for converting an analog signal into a digital signal by using PCM (pulse code modulation) each. Further, G. 711 is one of simplest methods for waveform encoding called PCM (pulse code modulation). An amplitude value of each of samples sampled at 8 kHz is quantized into a discrete value of 256 (8 bit) levels. At that moment, as amplitude distribution of a voice signal illustrates exponential distribution, quantized bits can be made appear equally frequently to one another by logarithmic conversion so that distortion can be minimized. That is, a value of a small amplitude is quantized with a fine step width, and a value of a great amplitude is quantized with a coarse step width. According to G. 711, two kinds of equations, μ-law and A-law, are adopted as logarithmic conversion rules. According to G. 711, one of the conversion rules (e.g., μ-Law) is used so that the code  300  is produced. 
     According to the embodiment, the pattern replacement unit  101  replaces a portion of an area in the code  300  which corresponds to the non-voice signal  202  (the frame including the data  310 - 318 ) with the particular pattern  319 . As illustrated in  FIG. 3 , the particular pattern  319  is “010110 . . . 011”. A position where the pattern replacement unit  101  replaces the particular pattern  319  can be fixed depending on the background noise level. The pattern replacement unit  101  can, e.g., render a portion of bit weight equal to or less than 320. 
     Then, the pattern test units  102  and  103  detect the particular pattern  319  replaced by the pattern replacement unit  101 . The detection identification unit  104  obtains the particular pattern  319  from the pattern test units  102  and  103 , and identifies whether or not voice quality degradation exists in the code  300 . 
     [Configuration of Pattern Replacement Unit  101 ] 
       FIG. 4  illustrates a block diagram of the pattern replacement unit  101  of the embodiment illustrated in  FIG. 1 . Operation of the pattern replacement unit  101  will be explained below in detail with reference to  FIG. 4 . 
     The pattern replacement unit  101  is constituted by a voice analysis unit  401 , an object frame selection unit  402 , a particular pattern replacement unit  403  and a detection use pattern saving unit  404 . 
     The voice analysis unit  401  takes in the code  300  illustrated in  FIG. 3  from the CE  107  illustrated in  FIG. 1  every frame of 20 msec including 160 samples. Then, the voice analysis unit  401  analyzes a voice characteristic of the code  300  that has been taken in. The voice characteristic is data that classifies every frame of the code  300  into a voice frame and a non-voice frame. Specifically, the voice characteristic is formed by a number of zero-crossings and an amplitude level. The number of zero-crossings represents how many times a voice waveform crosses a zero level in a certain period of time. The amplitude level is an average amplitude value of each of data  310 - 318  in a certain period of time. The voice analysis unit  401  identifies a frame of a data amplitude level being equal to or greater than a certain value and of a number of zero-crossings being equal to or smaller than a certain number in a certain period of time as a voice frame. In order to prevent portions of small amplitudes corresponding to a rise and a fall in voice from being clipped at a start and an end of a voice frame interval, respectively, the voice analysis unit  401  brings the start of the voice frame interval forward and puts the end of the voice frame interval off so as to severally give margins for identifying voice and non-voice frames. 
     The object frame selection unit  402  selects the non-voice frame that the voice analysis unit  401  has detected as a frame to be replaced with the particular pattern  319 . Why the object frame selection unit  402  does not cause a voice frame to be a frame to be replaced with the particular pattern  319  is for preventing the particular pattern  319  from causing noise. Meanwhile, a non-voice frame is in general background noise data. Thus, a partial change of a non-voice frame can hardly be sensed as voice quality degradation. 
     The particular pattern replacement unit  403  replaces a portion of the non-voice frame that the object frame selection unit  402  has selected with the particular pattern  319 . The particular pattern replacement unit  403  of the embodiment replaces a seventh bit of the non-voice frame with the particular pattern  319  (see  FIG. 3 ). The seventh bit of the non-voice frame means a code sequence corresponding to the seventh row from the MSB (Most Significant Bit) of the G. 711 code (data  310 - 318 ) of the non-voice frame illustrated in  FIG. 3 . That is, the data  310 - 318  are arranged in chronological order for the code  319  illustrated in  FIG. 3 . And the particular pattern replacement unit  403  replaces the seventh row corresponding to a portion of a small amplitude of the non-voice signal  202  with the particular pattern  319 . 
     The voice analysis unit  401  performs an amplitude analysis of a frame identified as a non-voice frame, and determines a background noise level  320 . The amplitude analysis is a process by means of the voice analysis unit  401  for averaging amplitude values of non-voice frames for a certain period of time so as to cause the average value to be the background noise level. The particular pattern  319  is saved in the detection use pattern saving unit  404 . The particular pattern replacement unit  403  reads the particular pattern  319  from the detection use pattern saving unit  404 . Then, the particular pattern replacement unit  403  replaces a portion of a non-voice frame with the particular pattern  319  with reference to the background noise level  320 . That is, the particular pattern replacement unit  403  replaces the seventh row of the non-voice frame with the particular pattern  319  (“010110 . . . 011”). As a replacement position of the particular pattern  319  is equal to or lower than the background level, the particular pattern replacement unit  403  refers to the background noise level  320 . Further, the particular pattern replacement unit  403  does not perform a replacement process of the particular pattern  319  for a non-voice frame of volume imperceptible for a person. 
     [Processing Procedure of Pattern Replacement Unit  101 ] 
       FIG. 6  illustrates a flowchart of a process performed by the pattern replacement unit  101 . Further,  FIG. 4  illustrates a configuration of the pattern replacement unit  101 . The process flowchart illustrated in  FIG. 6  will be explained below with reference to the configuration of the pattern replacement unit  101  illustrated in  FIG. 4 . 
     The voice analysis unit  401  takes in the code  300  illustrated in  FIG. 3  every frame (160 samples) (step S 601 ). 
     The voice analysis unit  401  performs a voice analysis process on the code  300  from the voice characteristic of the code  300  that has been taken in (step S 602 ). 
     The voice analysis unit  401  identifies whether the frame that has been taken in is a voice frame or a non-voice frame in order from a result of the voice analysis process (step S 603 ). The voice analysis unit  401  identifies a frame of an amplitude level of the data forming the code  300  being equal to or greater than a certain value and of a number of zero-crossings being equal to or smaller than a certain number in a certain period of time as a voice frame, and identifies a frame other than that as a non-voice frame. 
     If the voice analysis unit  401  identifies the frame that has been taken in as a non-voice frame (step S 603  YES), the particular pattern replacement unit  403  fixes a position of the non-voice frame to be replaced with the particular pattern  319  (particular pattern replacement position) (step S 604 ). The particular pattern replacement unit  403  fixes the particular pattern replacement position with reference to the background noise level  320 . On this occasion, the voice analysis unit  401  determines the background noise level  320 . The particular pattern replacement unit  403  causes a code pattern in a level range equal to or lower than the background noise level to be replaced with the particular pattern. The code pattern is a combination of bits of samples illustrating a same amplitude level in a frame formed by a plurality of samples. In other words, the code pattern is a chain of codes having a same bit weight in a frame formed by a plurality of samples. A portion of the code pattern to be replaced is formed by a combination of the codes in a level range equal to or lower than the background noise level, and the one formed with respect to the amplitude level is an example. 
     The voice analysis unit  401  calculates an average value of amplitude values of a non-voice frame for a certain period of time, and causes the average value to be the background noise level. Further, the voice analysis unit  401  may be configured to calculate an average value of amplitude values of a past non-voice frame and to cause the average value to be the background noise level. The past non-voice frame means a non-voice frame existing chronologically ahead of the non-voice frame to be replaced with the particular pattern  319 . Further, the past non-voice frame is in other words a non-voice frame that the voice analysis unit  401  receives, and a non-voice frame that the voice analysis unit  401  receives before receiving the non-voice frame to be partially replaced with the particular pattern  319 . Further, the position of the non-voice frame to be replaced with the particular pattern  319  by the particular pattern replacement unit  403  (particular pattern replacement position) is a position of a code belonging to a level equal to or lower than the background noise level. The particular pattern replacement unit  403  replaces a code pattern that is a portion of a non-voice frame selected by the object frame selection unit  402  and of a level range equal to or lower than the background noise level (step S 605 ). 
     Further, if the voice analysis unit  401  identifies the frame as not being a non-voice frame (step S 603  NO), the pattern replacement unit  101  ends the replacement process. 
     [Configuration of Pattern Test Units  102  and  103 ] 
       FIG. 5  illustrates a block diagram of the pattern test unit  102  of the embodiment. The pattern test unit  102  is constituted by a voice analysis unit  501 , an object frame selection unit  502 , a pattern detection unit  503  and a detection use pattern saving unit  504 . Operation of the pattern test unit  102  will be explained below in detail with reference to  FIG. 5 . The pattern test unit  103  has a same configuration and performs a same operation process as the pattern test unit  102 . 
     The voice analysis unit  501  takes in the code  300  illustrated in  FIG. 3  from the GW  108  illustrated in  FIG. 1  every frame of 20 msec including 160 samples. Then, the voice analysis unit  501  identifies whether the frame that has been taken in is a voice frame or a non-voice frame in order. Specifically, the voice analysis unit  501  analyzes a voice characteristic of the code  300 , and classifies every frame into a voice frame and a non-voice frame. In order to prevent portions of small amplitudes corresponding to a rise and a fall in voice from being clipped at a start and an end of a voice frame interval, respectively, the voice analysis unit  501  also brings the start of the voice frame interval forward and puts the end of the voice frame interval off so as to severally give margins for identifying voice and non-voice frames. 
     The object frame selection unit  502  selects the non-voice frame identified by the voice analysis unit  501  as a frame to be replaced with the particular pattern  319 . That is, an object to be tested by the pattern test unit  503  is a non-voice frame. The pattern test unit  503  does not test a voice frame. 
     The pattern detection unit  503  identifies whether or not the particular pattern  319  exists in the non-voice frame selected by the object frame selection unit  502 . The particular pattern to be detected by the pattern detection unit  503  is saved in the detection use pattern saving unit  504 . The pattern detection unit  503  reads the particular pattern saved in the detection use pattern saving unit  504 , and identifies whether or not the particular pattern exists in the non-voice frame while referring to the particular pattern. On this occasion, the voice analysis unit  501  performs an amplitude value analysis of the non-voice frame, and determines a background noise level. The pattern detection unit  503  identifies whether or not the particular pattern exists in the non-voice frame with reference to the background noise level. That is, the pattern detection unit  503  identifies whether or not the particular pattern exists in the level range equal to or lower than the background noise level. Thus, as the pattern detection unit  503  does not search a level range higher than the background noise level for the particular pattern, the particular pattern detection can be made more efficient. 
     The pattern detection unit  503  provides the detection identification unit  104  with pattern replacement existence data indicating whether or not the particular pattern exists in the tested non-voice frame. 
     [Process Procedure of Pattern Test Unit  102 ] 
       FIG. 7  illustrates a flowchart of a process performed by the pattern test unit  102  of the embodiment. The pattern test unit  102  detects the particular pattern  319  in the non-voice frame. The pattern test unit  103  performs a same process as the pattern test unit  102 . Further,  FIG. 5  illustrates a configuration of the pattern test unit  102 . The process flowchart illustrated in  FIG. 7  will be explained below with reference to the configuration of the pattern replacement unit  101  illustrated in  FIG. 5 . 
     The voice analysis unit  501  takes in the code  300  from the GW  108  every frame of 20 msec including 160 samples (step S 701 ). 
     The voice analysis unit  501  performs a voice analysis process on the code  300  from the voice characteristic of the code  300  that has been taken in (step S 702 ). 
     The voice analysis unit  501  identifies whether the frame that has been taken in is a voice frame or a non-voice frame in order from a result of the voice analysis process (step S 703 ). 
     If the voice analysis unit  501  identifies the frame that has been taken in as a non-voice frame (step S 703  YES), the object frame selection unit  502  selects the non-voice frame as an object to be tested with respect to the particular pattern. Then, the pattern test unit  503  obtains the background noise level from the voice analysis unit  501 , and determines with reference to the background noise level a level range in which the particular pattern of the non-voice frame is tested (step S 704 ). Specifically, the level range in which the particular pattern is tested described above is a portion of the code pattern in the non-voice frame of a level equal to or lower than the background noise level. The level described here is an amplitude value when one sample of an analog signal is encoded to 8-bit data in accordance to G. The less significant digit and the more significant digit of the 8-bit data represent portions of the smaller and greater amplitude values, respectively. 
     The pattern detection unit  503  identifies whether or not the particular pattern  319  exists in a level range equal to or lower than the background noise level in the non-voice frame selected by the object frame selection unit  502  (step S 705 ). The particular pattern to be detected by the pattern detection unit  503  is saved in the detection use pattern saving unit  504 . The pattern detection unit  503  reads the particular pattern saved in the detection use pattern saving unit  504 , and identifies whether or not the particular pattern exists in the level range equal to or lower than the background noise level in the non-voice frame while referring to this particular pattern. 
     Upon identifying the particular pattern as existing in the non-voice frame (S 705  YES), the pattern detection unit  503  sends pattern replacement existence data indicating that the particular pattern exists in the non-voice frame to the detection identification unit  104 . What is described above means that the particular pattern with which the pattern replacement unit  101  has replaced a portion of the non-voice frame has not changed and has flown in the quality controlled area  115 , and illustrates that degradation of the sound quality of the code  300  has not occurred. 
     Upon identifying the particular pattern as not existing in the non-voice frame (step S 705  NO), the pattern detection unit  503  sends pattern replacement absence data indicating that no particular pattern exists in a non-silent frame to the detection identification unit  104 . What is described above means that the particular pattern with which the pattern replacement unit  101  has replaced a portion of the non-voice frame has changed and has flown in the quality controlled area  115 , and illustrates that a change such as degradation of the sound quality of the code  300  has occurred. 
     The pattern detection unit  503  provides the detection identification unit  104  with pattern replacement existence/absence data (indicating the pattern replacement existence data and the pattern replacement absence data) indicating whether or not the particular pattern exists in the tested non-voice frame. 
     Further, if the voice analysis unit  501  identifies the frame that has been taken in as a voice frame at the step S 703  (step S 703  NO), the pattern test unit  101  does not perform the replacement process of the particular pattern  319 . 
     [Configuration of Pattern Replacement Unit  800 ] 
       FIG. 8  illustrates a block diagram of a pattern replacement unit  800  of the embodiment. For a series of frames to be replaced and identified as non-voice frames, the pattern replacement unit  800  changes a position of each of the non-voice frames replaced with the particular pattern in accordance with a certain rule. For a series of the non-voice frames, the pattern replacement unit  800  of the embodiment causes a position at which an amplitude level periodically changes to be the particular pattern replacement position. 
     The pattern replacement unit  800  is constituted by a voice analysis unit  801 , an object frame selection unit  802 , a particular pattern replacement unit  803  and a detection use pattern saving unit  804 . Further, the particular pattern replacement unit  803  has a pattern position fixing unit  805 . The pattern position fixing unit  805  has a function of fixing particular pattern replacement positions of a series of non-voice frames in accordance with a certain rule. 
     The voice analysis unit  801  takes in the code  300  every frame of 20 msec including 160 samples. Where the voice analysis unit  801  takes in the frames is the CE  107  (CE: core edge) illustrated in  FIG. 1  and so on. As a matter of course, a network device other than the core edge will do. 
     Then, the voice analysis unit  801  analyzes a voice characteristic of the code that has been taken in. The voice characteristic is data that identifies a voice frame and a non-voice frame of the code. Specifically, the voice characteristic is formed by a number of zero-crossings and an amplitude level. The number of zero-crossings represents how many times a voice waveform crosses a zero level in a certain period of time. The amplitude level is an average amplitude value of each of data  310 - 318  in a certain period of time. The voice analysis unit  801  identifies a frame of a data amplitude level being equal to or greater than a certain value and of a number of zero-crossings being equal to or smaller than a certain number in a certain period of time as a voice frame. Further, the voice analysis unit  801  identifies background noise from the amplitude level and the number of zero-crossings which are the voice characteristic of the data. 
     In order to prevent portions of small amplitudes corresponding to a rise and a fall in voice from being clipped at a start and an end of a voice frame interval, respectively, the voice analysis unit  801  brings the start of the voice frame interval forward and puts the end of the voice frame interval off so as to severally give margins for identifying voice and non-voice frames. 
     The voice analysis unit  801  identifies whether the frame that has been taken in is a voice frame or a non-voice frame in order from the analysis of the voice characteristic. The voice characteristic is data that identifies a voice frame and a non-voice frame of the code  300 . Specifically, the voice characteristic is formed by a number of zero-crossings and an amplitude level. The number of zero-crossings represents how many times a voice waveform of a zero-crossing wave crosses a zero level. The voice waveform of the zero-crossing wave is a waveform having no amplitude component. The amplitude level is an amplitude value of each of data  310 - 318 . The voice analysis unit  801  identifies, in a certain period of time, a frame of a data amplitude level being equal to or greater than a certain value and taken in a certain period of time since the number of zero-crossings becomes equal to or greater than a certain number until the number of zero-crossings becomes equal to or smaller than a certain number as a voice frame. In order to prevent portions of small amplitudes corresponding to a rise and a fall in voice from being clipped at a start and an end of a voice frame interval, respectively, the voice analysis unit  801  brings the start of the voice frame interval forward and puts the end of the voice frame interval off so as to severally give margins for identifying voice and non-voice frames. 
     The object frame selection unit  802  selects the non-voice frame detected by the voice analysis unit  801  as a frame to be replaced with the particular pattern  319 . Why the object frame selection unit  802  does not cause a voice frame to be a frame to be replaced with the particular pattern  319  is for preventing the particular pattern from causing noise. A non-voice frame is in general background noise data. Thus, a partial change of a non-voice frame can hardly be sensed as new voice quality degradation. 
     [Pattern Position Fixing Unit  805 ] 
     The particular pattern replacement unit  803  replaces a portion of the non-voice frame selected by the object frame selection unit  802  with the particular pattern  319 . The pattern position fixing unit  805  fixes a position of the non-voice frame to be replaced with the particular pattern. That is, the pattern position fixing unit  805  fixes particular pattern replacement positions of a plurality of non-voice frames in accordance with a given rule. The above given rule is, e.g., the amplitude levels at the particular pattern replacement positions are periodically different in a series of the non-voice frames to be replaced. That is, the particular pattern replacement positions of the respective non-voice frames have a periodic correlation. 
     A process by means of the pattern position fixing unit  805  for fixing particular pattern replacement positions in a plurality of non-voice frames will be explained here. The function processed by the pattern position fixing unit  805  is a new function that the pattern replacement unit  800  has in comparison with the pattern replacement unit  101 . The pattern position fixing unit  805  obtains a background noise level from the voice analysis unit  801 . Then, the voice analysis unit  801  performs an amplitude analysis of a frame identified as a non-voice frame, and determines the background noise level  320 . The amplitude analysis is a process by means of the voice analysis unit  801  for averaging amplitude values of non-voice frames for a certain period of time so as to cause the average value to be the background noise level. The pattern position fixing unit  805  causes a code pattern in a level range equal to or lower than the background noise level to be replaced with the particular pattern. The code pattern is a combination of bits of respective samples illustrating a same amplitude level in a frame formed by a plurality of samples. A portion of the code pattern to be replaced is formed by a combination of the codes in a level range equal to or lower than the background noise level, and the one formed with respect to the amplitude level is an example. If, e.g., the code  300  illustrated in  FIG. 3  is taken as an example, the pattern position fixing unit  805  fixes a replacement position of the particular pattern  319  “010110 . . . 011”. The replacement position of the particular pattern  319  of the embodiment is a code pattern position of a level in the level range equal to or lower than the background noise level and closest to an average value of the level range. 
     Then, for a plurality of non-voice frames that the particular pattern replacement unit  803  obtains in chronological order, the pattern position fixing unit  805  fixes a position to be replaced with the particular pattern (particular pattern replacement position) in accordance with a certain rule. That is, the pattern position fixing unit  805  fixes the particular pattern replacement position of a non-voice frame while referring to the particular pattern replacement position of a past non-voice frame. 
     The particular pattern is saved in the detection use pattern saving unit  804 . The particular pattern replacement unit  803  reads the particular pattern from the detection use pattern saving unit  804 . Then, the particular pattern replacement unit  803  replaces the position of the code pattern fixed by the pattern position fixing unit  805  with the particular pattern. Further, the particular pattern replacement unit  803  does not perform a particular pattern replacement process for a silent frame of volume imperceptible for a person. The silent frame is a frame such that frame data includes all zero bits or at most bits indicating a faint sound of a value equal to or lower than a certain threshold. 
     [Configuration of Pattern Test Unit  900 ] 
       FIG. 9  illustrates a block diagram of a pattern test unit  900  of the embodiment. 
     The pattern test unit  900  can detect a level of inserted noise. The pattern test unit  900  detects the particular pattern replaced by the pattern replacement unit  800 . Then, the pattern test unit  900  can calculate amplitude data of a level of the inserted noise from detection positions of respective particular patterns in the non-voice frames which are tested in order. 
     The pattern test unit  900  is constituted by a voice analysis unit  901 , an object frame selection unit  902 , a pattern detection unit  903  and a detection use pattern saving unit  904 . The pattern detection unit  903  has a level estimation unit  905 . The level estimation unit  905  estimates amplitude data of noise inserted in the network from data with respect to whether or not particular patterns exist in a plurality of non-voice frames obtained in chronological order. 
     The voice analysis unit  901  takes in the code  300  every frame of 20 msec including 160 samples. Then, the voice analysis unit  901  identifies whether the frame that has been taken in is a voice frame or a non-voice frame in order. 
     The object frame selection unit  902  selects the non-voice frame identified by the voice analysis unit  901  as a detection object of the particular pattern. 
     The voice analysis unit  901  analyzes an amplitude value of the non-voice frame and determines a background noise level. 
     The pattern detection unit  903  obtains the background noise level from the voice analysis unit  901 , and determines a level range in which the particular pattern of the non-voice frame is tested with reference to the background noise level. Further, the particular pattern to be detected by the pattern detection unit  903  is saved in the detection use pattern saving unit  904 . 
     Thus, the pattern detection unit  903  reads the particular pattern saved in the detection use pattern saving unit  904 , and identifies whether or not the particular pattern exists in the non-voice frame while referring to the background noise level and the particular pattern that has been read. 
     [Level Estimation Unit  905 ] 
     Then, the level estimation unit  905  identifies regularity of the particular pattern detection positions of the non-voice frames obtained in order, and estimates amplitude data of noise inserted in the network. As to the pattern position fixing unit  805  of the pattern replacement unit  800 , the particular pattern replacement unit  803  fixes a position to be replaced with the particular pattern for a series of non-voice frames in accordance with a given rule. Thus, the level estimation unit  905  identifies whether or not detection positions of the particular pattern in a plurality of the non-voice frames obtained in chronological order follow the given rule. Moreover, upon identifying the particular pattern detection position as not following the given rule, the level estimation unit  905  estimates amplitude data of inserted noise from the particular pattern detection position that does not follow the given rule. 
     For a series of non-voice frames, e.g., the pattern position fixing unit  805  fixes particular pattern replacement positions at which the amplitude levels are periodically different. Upon identifying the amplitude levels of the particular pattern replacement positions in a series of non-voice frames as not following the given rule, the level estimation unit  905  estimates amplitude data of the inserted noise from to what extent periodicity of the amplitude level is disturbed, i.e., an average value of an amplitude level of a particular pattern that should have been detected (an amplitude level of a particular pattern that the pattern detection unit  903  has not detected owing to noise). 
     Upon identifying a particular pattern as existing in a non-voice frame, the pattern detection unit  903  sends pattern replacement existence data indicating that the particular pattern exists in the object non-voice frame to the detection identification unit. If the pattern detection unit  903  identifies a particular pattern as not existing in the non-voice frame, the pattern detection unit  903  sends pattern replacement absence data indicating that no particular pattern exists in the object non-voice frame to the detection identification unit. Further, the pattern detection unit  903  also sends amplitude estimation data of inserted noise calculated by the level estimation unit  905  to the detection identification unit. If there is no inserted noise, the amplitude data of the inserted noise indicates “nothing”, e.g., “0”. 
       FIG. 10  illustrates a flowchart of a process performed by the pattern test unit  900  of the embodiment. 
     The voice analysis unit  901  takes in the code  300  every frame of 20 msec including 160 samples, and cuts a frame out (step S 1001 ). The voice analysis unit  901  analyzes a voice characteristic of the frame that has been taken in (step S 1002 ). 
     The voice analysis unit  901  identifies whether the frame that has been taken in is a voice frame or a non-voice frame in order from the analysis of the voice characteristic (step S 1003 ). 
     If the voice analysis unit  901  identifies the frame that has been taken in as a non-voice frame (step S 1003  YES), the object frame selection unit  902  selects the non-voice frame as an object to be tested with respect to the particular pattern. Then, the pattern test unit  903  obtains the background noise level from the voice analysis unit  901 , and determines with reference to the background noise level a level range in which the particular pattern of the non-voice frame is tested (step S 1004 ). The level range in which the particular pattern is tested is a portion of the code pattern in the non-voice frame for which the level is determined with reference to the background noise level in accordance with a certain rule. The code pattern is data of a combination of codes in the level range equal to or lower than the background noise level. A voice code chain is, e.g., data formed by bits of respective samples illustrating a same amplitude level in a frame formed by a plurality of samples. 
     The particular pattern to be detected by the pattern detection unit  903  is saved in the detection use pattern saving unit  904 . The pattern detection unit  903  reads the particular pattern from the detection use pattern saving unit  904 , and identifies whether or not the particular pattern exists in the level range equal to or lower than the background noise level in the non-voice frame while referring to the particular pattern (step S 1005 ). 
     Upon identifying the particular pattern as existing in the non-voice frame (S 1005  YES), the pattern detection unit  903  sends pattern replacement existence data indicating that the particular pattern exists in the non-voice frame to the detection identification unit. 
     Upon identifying the particular pattern as not existing in the non-voice frame (step S 1005  NO), the level estimation unit  905  calculates amplitude data (noise level) of inserted noise (step S 1006 ). The level estimation unit  905  identifies regularity of particular pattern detection positions of non-voice frames obtained in order, and estimates the amplitude data of the noise (noise level) inserted in the network system. In other words, the level estimation unit  905  identifies whether or not detection positions of the particular pattern in a series of the non-voice frames follow the given rule. Upon identifying the particular pattern detection positions as not following the given rule, the level estimation unit  905  estimates amplitude data of inserted noise (noise level) from the particular pattern detection position that does not follow the given rule. The pattern detection unit  903  sends pattern replacement absence data indicating that no particular pattern exists in a non-silent frame and the amplitude data of the inserted noise to the detection identification unit. 
     Further, if the voice analysis unit  901  identifies the frame that has been taken in as a voice frame (step S 1003  NO), the pattern test unit  900  does not test the particular pattern  319 . 
     As the pattern replacement unit  800  performs a replacement process of a particular pattern on a non-voice frame that flows in the network system and the pattern test unit  900  performs a detection process of the particular pattern, noise inserted in the network system can thereby be detected and further an amplitude level of the noise can be identified. 
     The network system of the embodiment can detect voice quality degradation caused by noise inserted in, the quality controlled area  115  in the network. Further, the network system of the embodiment can also detect voice quality degradation caused by a transcoding for converting a compression format of data in the quality controlled area  115  in the network. According to the test method of the embodiment, further, the pattern replacement unit  800  replaces a portion of the non-voice frame with the particular pattern. Thus, an amount of data that flows through the network system  100  does not increase. Hence, according to the test method of the embodiment, a traffic increase in the network caused by the test of voice quality degradation can be avoided. That is, the test method of the embodiment enables the voice quality degradation to be tested without causing ill effects such as a traffic increase in the network and resultant congestion. 
     Further, the test system is constituted by including the pattern replacement unit  101 , the pattern test units  102  and  103  and the detection identification unit  104 . The test system may be constituted by including the pattern replacement unit  800  instead of the pattern replacement unit  101 , and the pattern test unit  900  instead of the pattern test units  102  and  103 . Further, the digital signal corresponds to a signal into which an analog voice signal is encoded. Further, the particular signal corresponds to a code that constitutes the particular pattern. 
     INDUSTRIAL APPLICABILITY 
     The test method of the present invention is for testing voice quality degradation in a packet switching network. Thus, the test method of the present invention is quite useful for implementing detection of a factor of voice quality degradation inserted in the network without affecting voice communication even while the network is being operated. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and condition, nor does the organization of such examples in the specification relate to a showing of superiority and inferiority of the invention. Although the embodiment of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alternations could be made hereto without departing from the spirit and scope of the invention.