Abstract:
An objective, service-level specific voice call listening quality test scheme for a Voice Over IP (VOIP) network is presented. Test probes are deployed along the border of the VOIP network. Each test probe is capable of placing calls over the VOIP network to the other test probes at different levels of service and measuring call quality using an objective measurement algorithm such as PAMS or PSQM. The measurement results are collected on an ongoing basis to obtain information on the VOIP network&#39;s voice call quality. The information is compared to thresholds to measure performance against Service Level Agreement guarantees.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention relates to voice call quality testing.  
           [0002]    Packet-based networks, in particular, Voice Over IP (VOIP) networks, are rapidly emerging as a viable alternative to traditional telephony (that is, circuit switched networks). VOIP is viewed as an attractive option for voice transport in that it allows live voice conversations to be integrated with existing IP data and image applications. To be a truly competitive alternative, VOIP must emulate the performance of traditional telephony and do so using a protocol that was optimized for data traffic. The characteristics of data traffic are quite different from those of voice traffic, however.  
           [0003]    Unlike data traffic, voice traffic is extremely intolerant of delay and delay variation (or “jitter”), as well as packet loss. Much work has been done in the area of packet delivery to provide end-to-end Quality of Service (QoS). Service level agreements (SLAs) for VOIP, like those for conventional data IP networks, therefore tend to be based on conventional data network metrics, that is, guaranteed service levels are expressed solely in terms of packet level performance, e.g., packet loss, delay, jitter.  
           [0004]    Another important aspect of voice communications quality that is not reflected in the data network metrics, however, relates to the sound of a voice call from the perspective of the listener. Standardized techniques exist for measuring this aspect of voice quality. Typically, to support voice communications, VOIP networks encode the audio and format the encoded audio into packets for transport using an IP protocol. Consequently, the results of these voice quality tests are greatly affected by choice of speech coding techniques.  
           [0005]    One approach utilizes a standardized ranking system called the Mean Opinion Score (MOS). The MOS system uses a five-point scale: excellent-5; good-4; fair-3; poor-2; and bad-1. A level of quality indicated by a score of 4 is considered to be comparable to “toll quality”. A number of people listen to a selection of voice samples or participate in conversations, with the speech being coded by using the technique to be evaluated. They rank each of the samples or conversations according to the five-point scale and a mean score is calculated to give the MOS. Although the applicable ITU standard, ITU-T P.800, makes a number of recommendations regarding the selection of participants, the test environment, explanations to listeners, analysis of results, etc., because this type of voice quality testing is subjective, results tend to vary from test to test.  
           [0006]    Algorithms for performing a more objective voice quality test have been developed as well. These objective techniques for testing voice transmissions in packet-based networks include an ITU standard based algorithm known as Perceptual Speech Quality Measurement (PSQM) and Perceptual Analysis Measurement System (PAMS), which was developed by British Telecom. Both of these perceptual test algorithms evaluate whether a particular voice transmission is distorted from the perspective of a human listener.  
           [0007]    PSQM takes a “clean” voice sample and compares it to a potentially distorted version, that is, a transmitted version, using a complex weighting that takes into account what is perceptually important to a human listener, for example, the physiology of the human ear and cognitive factors related to what human listeners are likely to notice. PSQM provides a relative score (on a scale of 1 to 15, with 1 corresponding to the highest score and 15 corresponding to the lowest score) that indicates how different the distorted signal is with respect to the reference from the perspective of the human listener.  
           [0008]    PAMS is based on a perceptual model similar to that of PSQM and shares with PSQM the purpose of providing a repeatable, objective means for measuring voice quality, but uses a different signal processing model than PSQM and produces a different type of score as well. The scoring provided by PAMS includes a “listening quality” score and a “listening effort” score, both of which correlate to the MOS scores and are on the same scale of 1 to 5.  
         SUMMARY OF THE INVENTION  
         [0009]    In one aspect of the invention, providing service for use in a Voice Over Internet Protocol (VOIP) network environment includes selecting a service level and measuring voice call listening quality according to the selected service level for voice calls transmitted across a VOIP network to produce voice call listening quality metric values.  
           [0010]    In another aspect of the invention, a computer program product residing on a computer readable medium for providing service for use in a Voice Over Internet Protocol (VOIP) network environment comprises instructions for causing a computer to: associate service levels with phone numbers; and, in response to a test voice call directed to one of the phone numbers, cause the test voice call to be transferred over the VOIP network to a destination corresponding to such phone number at the associated service level and causing a voice call listening quality to be measured for the associated service level to produce a voice call listening quality metric value.  
           [0011]    Particular implementations of the invention may provide one or more of the following advantages. The invention allows a network operator or service provider to provide a customer with an SLA that is based on voice call quality metrics, in particular, voice call listening quality metrics (such as MOS rankings), which reflect voice call quality as perceived by a human listener, as well as packet-based metrics. The voice quality tests are objective, repeatable tests and can therefore be implemented in an automated, production environment to enforce SLAs. The invention also enables the voice call listening quality metrics to be obtained for different service levels according to coding scheme and/or protocol.  
           [0012]    Other features and advantages of the invention will be apparent from the following detailed description and from the claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a block diagram illustrating a Voice Over IP (VOIP) network voice call listening quality test topology.  
         [0014]    [0014]FIG. 2 is a block diagram of an exemplary VOIP-telephony network that employs the test topology of FIG. 1.  
         [0015]    [0015]FIG. 3 is a detailed illustration of an exemplary VOIP Point of Presence (POP).  
         [0016]    [0016]FIG. 4 is an illustration of a VOIP gateway configuration supporting voice call listening quality testing for a VOIP network.  
         [0017]    [0017]FIG. 5 is an alternative embodiment of the test topology of FIG. 1.  
         [0018]    [0018]FIG. 6 is a VOIP-telephony network that employs the test topology of FIG. 5. 
     
    
     DETAILED DESCRIPTION  
       [0019]    Referring to FIG. 1, an exemplary voice quality network test topology  10  includes a packet-based network shown as an IP network  12  that transports voice traffic. The network test topology  10  further includes voice quality test probes  14   a  and  14   b , which are coupled to gateways  16   a  and  16   b , respectively. The gateways  16   a  and  16   b  each are connected to the IP network  12  and provide translation services between protocols of the IP network and a conventional telephony network, such as a Public Switched Telephone Network (or “PSTN”). The test probe  14   a  is connected to the gateway  16   a  by a first telephony transmission line, shown as an ISDN line  18   a  (e.g., E 1  or T 1 ), and uses an ISDN Primary Rate Interface (PRI) service. The test probe  14   b  is connected to the gateway  16   b  over a second telephony transmission line  18   b , also shown as an ISDN line  20   b  supporting ISDN-PRI service. Other PSTN physical and signaling interfaces can be used. For example, the lines  20   a ,  20   b  may be ISDN-BRI or CAS T 1 /E 1  lines. Alternatively, the lines can be implemented as analog FXO wires. Because the IP network  12  transports voice traffic, it is also referred to as a Voice Over IP (VOIP) network and IP communication devices coupled to the VOIP network  12  either directly (such as the gateways  16   a ,  16   b ) or indirectly (through another IP communications device) are also referred to as VOIP communications devices.  
         [0020]    The test probes  14   a ,  14   b  store a sample or reference voice file  22   a ,  22   b , respectively, for test purposes. The reference voice file  22  is formatted as a typical audio file, e.g., RIFF WAVE “★.WAV” file (as shown), or some other audio format. The test probes  14   a ,  14   b  also store a software algorithm implementing a perceptual or voice call listening quality test model. In one embodiment, the software algorithm is the Perceptual Analysis Measurement System (PAMS) algorithm. Other objective, repeatable voice call listening quality test algorithms, e.g., Perceptual Speech Quality Measurement (PQMS), can also be used. Although one of the two test scores produced by PAMS is known as “listening quality”, the term “voice call listening quality” as used herein refers to the quality measured by any perceptual voice call test technique, such as PAMS (and therefore encompasses both the PAMS “listening quality” as well as the PAMS “listening” effort”) or PQMS. The test probes  14   a ,  14   b  can store any type of reference voice file, but the voice file stored on both of the test probes  14  as the reference voice file must be identical in order for the PAMS (or other similar) voice call listening quality testing to work correctly. Preferably, for optimum test results, the voice file should include voice samples representative of many different types of voice activity.  
         [0021]    In operation, the test probes  14  transmit and receive the reference voice files (test.WAV files  22   a ,  22   b ) over the speech path within the VOIP network. One test probe acts as a resource to transmit the file. A second test probe acts as a resource to receive the file transmitted by the first test probe and perform the PAMS algorithm. For example, the test probe  14   a , serving as a “call initiator”, dials a telephone number corresponding to the other test probe, the test probe  14   b  (acting as a “call responder”). The VOIP gateway  16   a  directs the call over the VOIP network  12  to the VOIP gateway  16   b , which sends the call to the test probe  14   b . The test probe  14   b  answers the call by playing back to the caller, that is, test probe  14   a , the stored reference voice file. The test probe  14   a  records the played voice file as it “listens” and analyzes the voice quality of the recorded voice file using the PAMS algorithm. The PAMS algorithm compares the recorded voice file to the reference voice file stored by the test probe  14   a  and determines a difference between the files. When the analysis is complete, the test probe  14   a  translates the difference into a PAMS score (actually, a two-part score having separate scores for listening quality and listening effort) for the voice quality of the call. It will be understood that the same process can be used in the reverse direction, that is, when the test probe  14   b  acts as the call initiator and the test probe  14   a  acts as the call responder. The test probes  14  may be configured to allow a test administrator to view the resulting scores graphically or in some other form.  
         [0022]    Preferably, the voice call listening quality test is performed for each level of service as determined by the type of codec (i.e., coder/decoder) that is used by the VOIP communications device that is performing the voice encoding and decoding operations. In the network shown in FIG. 1, the gateways  16  implement one or more coding schemes to support voice encoding/decoding.  
         [0023]    Types of codecs include, but need not be limited to, the following: waveform codecs, source codecs and hybrid codecs. With waveform codecs, an incoming voice signal is sampled, coded and the coded samples converted to quantized values, which are used to reconstruct the original voice signal. Waveform codecs produce high quality sound but consume a significant amount of bandwidth. Source codecs try to match an incoming voice signal to a mathematical model of speech generation. That model is used to reconstruct the original voice signal. The source codec operates at low bit rates but tends to produce poor quality sound. Hybrid codecs use some amount of waveform matching as well as knowledge of how the original sound was generated. They tend to provide fairly good quality at lower bit rates than waveform codecs.  
         [0024]    The G.711 Pulse-Code Modulation (PCM) coding technique is a waveform codec and is one of the most common coding techniques that are used. It is the codec of choice for circuit-switched telephone networks. Other PCM waveform codecs include G.726, which offers Adaptive Differential PCM (ADPCM) coded speech. Lower bandwidth, hybrid codecs include the G.723 and G.729 codecs.  
         [0025]    The choice of codec is a major factor in high voice quality and voice quality test scores will vary with codec selection. The goal of any network operator or service provider is to offer “toll quality” voice, where toll quality voice relates to a MOS of 4.0 or better. The task of selecting the best codec for a given network is a matter of balancing quality with bandwidth consumption. Thus, it is desirable to test different codecs in an environment that closely matches expected network conditions.  
         [0026]    Still referring to FIG. 1, the test topology  12  tests for three different coding techniques, G.711, G.723 and G.729, although other coding schemes can be used in addition to, or lieu of, any one or more of these techniques. That is, each of the gateways  16  is capable of encoding and decoding in accordance with these different coding techniques. Consequently, there is a different phone number for each test probe and service level combination.  
         [0027]    Optionally, in addition to measuring voice call quality on a per-codec basis, test probes  14  can test voice call quality on a per-protocol basis as well. That is, it is possible to further associate each test probe phone number and service level combination with each protocol supported by the VOIP communications devices  16 . For example, commercially available gateways support such signaling protocols as H.323 and Session Initiation Protocol (SIP), as well as Media Gateway Control Protocol (MGCP). Other existing protocols, such as media gateway control protocol (MEGACO/H.248), as well as other protocols, may be supported as well. The protocol-specific testing could measure, for example, voice call setup and tear-down times.  
         [0028]    For example, to illustrate service level selection based on both codec and IP signaling protocol, each of the test probes  14  may be assigned four unique telephone numbers. The four unique phone numbers include: a first unique phone for a service level associated with the use of G.711 coding and H.323 signaling; a second unique phone for a service level associated with the use of G.723 coding and H.323 signaling; and third unique phone number for a service level associated with the use of G.711 coding and SIP signaling; and a fourth unique phone number for a service level associated with the use of G.723 coding and SIP signaling. To place a call to the test probe  14   b , the test probe  14   a  initiates a call to one of the four unique phone numbers for the test probe  14   b  with the gateway  16   a . Gateway  16   a  is configured with resources to perform both types of coding and signaling, but selects the appropriate coding for the call to the test probe  14   b  and call signaling to establish a connection with the gateway  16   b  based on the phone number. Typically, the gateways include a configuration table which stores the called phone numbers with associated service level information for look-up, as will be described in further detail below. Thus, the gateway  16   a  determines from the service level information associated with the called phone number (for test probe  14   b ) how the audio is to be encoded and how the data connection is to be established. When the connection between gateway  16   a  and  16   b  is established, the gateway  16   b  is able to detect the type of coding in use and allocate an appropriate coding resource to encode the audio when it is receives the voice file transmission from the test probe  14   b.    
         [0029]    Although not shown, it will be understood that the test probes  14  further include the necessary hardware and software required to support applicable network layer protocols. In addition, and specifically in support of voice call quality testing, each test probe  14  includes a call generator. In one embodiment, the call generator provides a complete H.323 implementation package that is capable of initiating and responding to calls. The package thus simulates an H.323 terminal generating calls with (or without) a VOIP gateway, as well as opening logical channels and transmitting RTP voice packets. Other VOIP protocols, such as SIP and MGCP (as discussed above), can be used instead of or in addition to H.323.  
         [0030]    As indicated above, the test probes  14  are configured to perform automated voice quality measurements on a voice transmission and produce a score based on those measurements. This testing may be performed in a laboratory environment to simulate conditions of an operating network, or as part of the actual network operation, as will be described with reference to FIG. 2.  
         [0031]    [0031]FIG. 2 depicts an exemplary telephony-VOIP network  30 . The exemplary network  30  illustrates how the test topology of FIG. 1 is adapted for use in a production environment. The network  30  deploys a number of test probes like the test probes  14  (FIG. 1), indicated by reference numerals  14   a  through  14   f , at different points along the border of the VOIP network  12 . Although not shown in the figure, each of the test probes in configured with a copy of a reference voice file, as described earlier with reference to the reference voice files  22  of FIG. 1. Each of test probes  14   a  through  14   d  is connected to a respective one of gateways  16   a  through  16   d . Also connected to the gateway  16   c  is a server  31 . Each of the gateways  16   a  through  16   d  is connected to the VOIP network  12  and a respective one of PSTNs  32   a  through  32   d . In addition to the gateways  16 , VOIP communications devices include a VOIP server  34  and a VOIP telephone  36 . The test probe  14   e  is connected to the VOIP server  34  and the test probe  14   f  is connected to the VOIP telephone  36 . Each of the test probes  14  is controlled to generate test calls to others of the test probes  14  over the VOIP network  12 , perform PAMS testing on the voice files played back in response to the test calls, as well as play a reference voice file when acting as a recipient of a test call, much in the same manner as was described for the two test probes shown in FIG. 1.  
         [0032]    The test probes  14  attach to VOIP communications devices, such as devices  16 ,  34  and  36 , through digital or analog circuits. In addition, or alternatively, test probes can be deployed at other locations for different types of voice quality (and possibly packet-based) test coverage. For example, active test probes can connect to PSTNs through telephony interfaces for end-to-end voice quality testing, like test probes (TP 2 )  40   a ,  40   b , which are connected to PSTNS  32   c  and  32   d , respectively, or can connect directly to the VOIP network  12  through an IP interface and appear to the VOIP network  12  as another gateway, or some other VOIP communications device. The latter configuration, an example of which is illustrated by test probe TP 1   38 , only tests the VOIP network, not the gateway, however. A “passive” test probe may be connected between the gateways  16  (or other VOIP communications devices) and the VOIP network  12  to produce information for all of the voice calls it sees, in particular, packet-based and voice quality information for all of the voice calls and PAMS data for probe-generated voice calls. Test probe (TP 3 )  42 , which is coupled between the gateway  16   b  and the VOIP network  12 , is an example of a passive test probe.  
         [0033]    All of the test probes store a copy of the same reference voice file and have the capability to generate PAMS scores for test call traffic. All but passive test probes can generate and answer test calls in the manner described above. Unlike the other probes, the passive probe TP 3   42  sees all voice calls, including test calls. It is able to identify a test call by a caller&#39;s IP address or the called phone number. Once a test call is detected, the passive probe can extract the audio from a test call and apply a PAMS test to it. Consequently, the passive probe provides a test result for a network location intermediate the call source and destination points. A combination of end-to-end scores, border-to-end (or border) scores, as well as a passive probe scores, therefore enables a test administrator to isolate a network trouble spot.  
         [0034]    In addition to PAMS testing, the end-to-end voice quality testing test probes  40  also include software to support other active test measurements for signaling and voice quality, including: post dialing delay; post gateway answer delay; background noise; audio level; insertion loss; round trip delay; echo and DTMF integrity.  
         [0035]    The VOIP communications device test probes, such as test probe  38 , can perform, in addition to the PAMS tests, the following signaling and voice quality tests: Q.931 setup time; RTP setup time; background noise; audio level; and insertion loss. The software can also simulate various packet-based impairments (e.g., jitter, total packet loss, packet loss burst, etc.) and assess their effects on the VOIP and the VOIP communications devices, e.g., the VOIP gateways  16 . The voice quality measurements by the VOIP communications device test probes can be end-to-edge or edge-to-edge.  
         [0036]    In addition to performing voice quality tests on PAMS-generated traffic, passive test probes, such as the test probe  42  can support, for all live traffic monitored on the VOIP network  12 , the following active test measurements: call statistics; setup time; jitter per RTP stream; and packet loss (and packet loss burst) per RTP stream.  
         [0037]    Thus, the deployment of the various types of probes throughout a network such as network  30  provides for a network-wide monitoring system. The different types of test probes, that is, the test probes  14 ,  38 ,  40  and  42 , and the manager  44  shown in FIG. 2 may be implemented using commercially available hardware and software, for example, using the various components of the Omni-Q Voice Quality Management System manufactured by and available from RADCOM.  
         [0038]    Still referring to FIG. 2, also connected to the VOIP network  12  is a management server (or manager)  44 . All test probes in the network  30  are configured and controlled by the manager  44 . The manager configures the test probe properties and test call generation schedules, as well as establishes alarms or thresholds to ensure delivery of service in accordance with Service Level Agreements (SLAs). It polls the probes on a periodic basis to gather test results, consolidates the test results for the entire network and stores the consolidated information in a database for analysis, reporting and historical trending.  
         [0039]    In one exemplary commercial setting, the VOIP network infrastructure of the network  30  is maintained by a VOIP network operator (or wholesale service provider) and made available to that VOIP network operator&#39;s customers, e.g., retail service providers who use the infrastructure and related services of the VOIP network operator to provide services to end-users. The network operator manages the VOIP network infrastructure and related services using the manager  44 . In a large-scale operation, the network operator supports a large number of VOIP Points of Presence (VOIP POPs), an example of which is indicated in dashed lines as VOIP POP  46 , in different geographic regions for coverage of a larger territory, e.g., national level coverage. Each VOIP POP provides a point of entry to and termination from the VOIP network backbone. The network operator supports outbound call completion to the PSTNs for calls placed over the VOIP network and inbound call delivery for calls initiated on the PSTN for delivery over the VOIP network. An example of such an inbound service may be delivery of calls from end-users to a customer&#39;s unified messaging server. To support such a service, the network operator provides a block of local Direct Inward Dial (DID) numbers to the customer so the customer has local access capability in a particular geographic market. A call from an end-user to one of the DID numbers results in a call passing through a gateway associated with that number to the customer&#39;s server (via a gateway hosting that customer). Examples of other inbound call services include, for example, conference call bridging, call delivery to call centers and call waiting servers. The network operator provides service level agreements based on voice call quality, which includes voice call listening quality metrics (i.e., test score values) as well as packet-based metrics, as described above.  
         [0040]    Referring to FIG. 3, a network  30 ′ having an exemplary VOIP POP 46′ that supports inbound service delivery and service level based voice call quality testing is shown in detail. In this example, the VOIP POP 46′ includes multiple gateways  16 - 1 ,  16 - 2 , . . .  16 - 8  to support different metropolitan areas, including San Francisco, Los Angeles and San Diego (as shown). The closest Internet access point is located in Los Angeles, so traffic from San Francisco and San Diego is directed to Los Angeles, where it is passed to an Internet backbone router  50 . Each gateway  16  in each metropolitan area has multiple connections  52  to support customer traffic and at least one gateway  16  in each area supports a single connection  54  for connecting to one of three test probes  14 - 1 ,  14 - 2  and  14 - 3 . The connections  52  in each area connect a respective PSTN to that area&#39;s gateways  16 . Thus, the two gateways in San Francisco, the gateways,  16 - 1  and  16 - 2 , are connected to a PSTN in San Francisco (SF PSTN),  32 - 1 , and the gateway  16 - 1  is also connected to the test probe  14 - 1 . The four gateways in Los Angeles, the gateways,  16 - 3 ,  16 - 4 ,  16 - 5  and  16 - 6 , are connected to a PSTN in LA (LA PSTN),  32 - 2 , and the gateway  16 - 6  is also connected to the test probe  14 - 2 . With respect to San Diego, the gateways  16 - 7  and  16 - 8  are connected to a PSTN in San Diego (SD PSTN),  32 - 2 , and the gateway  16 - 8  is also connected to the test probe  14 - 3 .  
         [0041]    In the illustrated example, and as discussed above, the VOIP network operator enables customers to deploy their services on a national level without having to make an investment in network infrastructure. Blocks of local DID numbers are made available to customers for use by end-users. In this example, it is assumed that one of the San Francisco gateways  16 - 1 ,  16 - 2  is configured to handle phone numbers having an area code “408” and a three-digit exchange of “123”, followed by a four-digit number in the range of 1000-3000. Two customers, customers A and B, wish to service users in the San Francisco area and thus require blocks of local DIDs to give to customers in that area. The identified gateway therefore serves as a central office and all the inbound traffic is aggregated to a single connection point.  
         [0042]    In general, and referring to FIG. 4, the gateways  16  maintain a configuration table  60  that includes, for each of a plurality of profiles  61 , a phone number  62 , an associated service level  64  and routing information  66 . At minimum, the service level  64  indicates the coding scheme to be used by the VOIP communications devices responsible for establishing the IP data path over which the voice transmission is to occur. The routing information  66  identifies by IP address a hosting gateway or server to which inbound traffic associated with the phone number  62  is to be directed.  
         [0043]    As an example, and thus intended for illustrative purposes only, the second, third and fourth table entries are populated with data to support configurations for inbound VOIP calls for the two customers, customer “A” and customer “B”, and SF test probe  14 - 1  via one of the gateways  16 - 1 ,  16 - 2  (FIG. 3). Numbers in the 1000-1999 range are allocated to customer A, who has requested G.711 service. The numbers in the range of 2000-2999 are allocated to customer B, who has requested G.723 service. The “3000” number is assigned to the test probe  14 - 1  for a specific service level, for example, G.711.  
         [0044]    Referring to FIGS. 3 and 4, when one of the San Francisco gateways, say,  16 - 1 , configured with the configuration data shown in the configuration  60 , detects an in-coming call from the PSTN  32 - 1 , it parses the configuration  60  to match the incoming phone number to any of the stored phone numbers. If the gateway  16 - 1  determines that the dialed number belongs to a particular customer such as customer A, it selects as a service level the service level requested by customer A, that is, the G.711 service level (specified by the service level field  64 ). It routes the call to the gateway identified by the routing information field  66  for hosting customer A, for example, the gateway  16   c  (shown coupled to the server  31 , representing customer A), using the G.711 service level. A call belonging to customer B is handled in much the same way, but according to the specific configuration information for customer B. That is, the call is routed to a hosting gateway or a server for customer B, shown in the figure as the server  34 , using the G.723 service. If the dialed number is 408-123-3000, the gateway  16 - 1  determines from the configuration  60  that the call is a test call to be placed to a test probe, for example, the test probe  14   d , using G.711 service. At this point, the process is as described above with reference to FIG. 1.  
         [0045]    As already indicated, the manager  44  is operated by the IP services provider to control the test probes deployed throughout the VOIP network  12 . The manager  44  determines the frequency with which the test probes make test calls and schedules the test probes to generate and receive the test calls. It polls the test probes for test results (MOS scores) and is able to process the raw data for reporting, network repair/enhancements, and so forth. For example, the manager  44  can determine an average score from all of the test calls for each service level, that is, G.711, G.723 and G.729, during a given time period (e.g., on a monthly basis) and compare that average performance metric to a guarantee provided by an SLA between the VOIP network operator (service provider) and a service subscriber (such as the hypothetical customers A and B in the above-described example).  
         [0046]    Other embodiments of the voice quality testing topology and VOIP-telephony network of FIGS. 1 and 2, respectively, are contemplated. For example, and referring to FIG. 5, a voice quality test network topology  70  includes the VOIP network  12  and coupled to the VOIP network  12  are a test probe  72  and a VOIP communications device  74 . The test probe includes a reference voice file  76   a  and the VOIP communications device  74  has an embedded reference voice file  76   b . The test probe  72  can be functionally identical to the test probe  38  shown in FIG. 2. The test probe  72  and the VOIP communications device  76  each include a suitable interface to allow direct connection to the VOIP network  12  over an appropriate VOIP network connection  80 , e.g., an Ethernet connection. Thus, in contrast to the topology shown in FIG. 1, the topology  70  eliminates a second test by providing a voice file in a VOIP communications device having a codec to be evaluated. The VOIP communications device  76  can be any VOIP communications device that performs speech encoding/decoding in a particular VOIP network environment, for example, a gateway, a server, a telephone (like the gateway  16 , VOIP server  34  and VOIP telephone, respectively, from FIG. 2), or any other VOIP communications device.  
         [0047]    In addition to the voice file  76 , the VOIP communications device must also have a voice quality test support module  82  to enable the device to answer a test call by playing the voice file. As indicated in the figure, the module  82  can be implemented as an Interactive Voice Response (IVR) unit as is well known in the art. At present, many VOIP communications devices already include an IVR unit to support other functions, such as interactive call processing functions. In those instances, the voice call listening quality test scheme is able to exploit a device&#39;s inherent interactive voice response (IVR) capability. Alternatively, if the device does not include IVR functionality, then the device requires a script enabling that device to recognize a special phone number and understand that a call to that special phone number is to be answered by playing the embedded voice sample file.  
         [0048]    [0048]FIG. 6 illustrates an alternative VOIP-telephony network  90  generally configured in a similar configuration to that of the network  30  of FIG. 2, but modified so as to require only one test probe to produce PAMS scores for the voice call quality between the test probe and various connection points around the VOIP network  12  according to the test topology  70  shown in FIG. 5. That is, and with reference to FIGS. 2 and 6, the test probes  14  are eliminated by embedding voice files (VF)  92  in the VOIP communications devices to which the test probes  14  were coupled, that is, the gateways  16 , VOIP server  34  and VOIP phone  36 , shown with the embedded voice files in FIG. 6 as gateways  16 ′, VOIP server  34 ′ and VOIP phone  36 ′. The test probe  38  is responsible for generating calls to the gateways  16   a ′,  16   b ′,  16   c ′ and  16   d ′, as well as device  34 ′ and  36 ′ (on a scheduled basis under the control of the manager  44 ) and generating PAMS scores from the responses (that is, the playback of stored reference voice files) by the called devices  16 ′,  34 ′ and  36 ′. The functionality of the manager  44  is as earlier described with respect to FIG. 2.  
         [0049]    Additions, subtractions, and other modifications of the described embodiments of the invention will be apparent to those practiced in this field and are within the scope of the following claims. The test scheme and topology can be adapted to accommodate other different physical and signaling protocols. For example, the test probe  40  could be connected to an SS7 network, or the test probe  14  could connect to an SS7 or CAS gateway interface. Moreover, the packet-based network  12  need not be an IP network. The network  12  could be implemented as a Voice Over Frame Relay or Voice Over ATM network, and the interfaces and protocols supported by the test scheme could be modified accordingly.