Active Monitoring of Voice over IP Services with Malden

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1 Active Monitoring of Voice over IP Services with Malden Introduction Active Monitoring describes the process of evaluating telecommunications system performance with intrusive tests. It differs from passive monitoring which attempts to deduce performance from traffic statistics. Active monitoring will provide a more realistic view of the users experience, particularly where diverse end-points such as mobile phones or PSTN connections must be considered. Active monitoring can evaluate VoIP, mobile and TDM networks as well as mixtures of the technologies. It can also be useful with applications of mixed networks such as teleconferencing. The relative benefits of Active Monitoring and Passive Monitoring techniques may be summarised as follows: Feature Active Monitoring Passive Monitoring Measures live traffic No Yes Returns network performance data in Yes No absence of live traffic (quiet periods) Determines MOS prediction based on speech signal analysis Yes No (MOS prediction is estimated from packet loss, jitter and knowledge of Takes into account impairments occurring anywhere in the transmission path Yes codec) No (takes into account the packet statistics at the monitoring point only) Determines speech delay Yes actual delay May estimate based on packet timing data Determines speech power level Yes No Determines packet statistics during measurement period Yes Yes This document describes the technologies involved in Active Monitoring, how they may be applied and the results that can be obtained. Methodology Active Monitoring tests collect data to predict how a user will perceive the performance of a telephone call. In the case of a call made between two terminals, for example two IP phones, it is possible to replace the people with instruments which speak and listen. This is done by attaching the instruments at the point where the handset or headset is normally connected. 1

2 Figure 1: Performance measurement using instruments Typically a test will generate a variety of speech test stimuli at defined speech levels and synchronously record the received signal. Measurements of speech level and delay are easily obtained. A recording of the received speech can be compared with a reference file by a speech quality metric to calculate a prediction of Mean Opinion Score for Listening Quality. This should yield a result similar to that which would otherwise be obtained through a formal subjective test process described in ITU-T P.800 ( 1 ). Examples of current metrics are Perceptual Evaluation of Speech Quality (PESQ) ( 2 ) and Perceptual Objective Listening Quality Assessment (POLQA) ( 3 ). Techniques for VoIP Networks Two classes of technique are available for testing VoIP transmission performance. The first, described above, involves attaching test equipment to VoIP phones in place of the handset or headset. This method captures the key factors influencing users perception of call performance except for the acoustic environment. Whilst this method is useful, it does require significant effort in deployment. For a more flexible call quality monitoring solution on an International or wide-area VoIP network it is possible to deploy reference VoIP software to make routine test calls. This second technique is applicable where the main purpose is the assessment of the performance over time of the network that is carrying VoIP traffic. In this case reference software VoIP terminals may be deployed and used to make test calls between selected locations. Such testing may provide a snapshot of performance which may be useful for indicative purposes but the real value of this technique is derived when a routine pattern of tests is performed continuously. Test calls are scheduled at regular intervals throughout the day between the test points. Whilst this method is based on a series of intrusive tests, the effective outcome is to provide a model of the network s performance; it seems reasonable to refer to this as Active Monitoring. 2

3 The performance data derived from this process may be used to provide: - a performance benchmark - indication of any variation in performance which might be related to time-of-day or traffic loading - warning of any long-term drift in performance which might otherwise go unnoticed - alerting via or SNMP if measured performance falls outside pre-determined limits Specific tests might be set up to evaluate: - the likely pattern of performance degradation in the event of particular failure modes - the performance/bandwidth trade-off for alternative codecs or Voice Activity Detection (VAD) settings, which might be under consideration Figure 2: Performance measurement using Scalable Virtual Nodes 3

4 Figure 3: The 'Active Monitoring' concept Measurements The following performance indicators may be considered as important for the Active Monitoring of a VoIP network: Listening Quality, derived from PESQ and/or POLQA Speech path delay Mean Active Speech Level Packet Loss Jitter A prediction of mean opinion score (MOS) takes into account the majority of impairments which influence the perception of quality of service. However, since MOS represents Listening Quality, it is not influenced by the absolute delay of the speech signal across the network, only the changes in delay occurring during active speech. Consequently, the measurement of delay is an important adjunct to the MOS prediction. The measurement of packet loss and jitter may also be of interest and value, since one or both may be relevant to the investigation of speech quality impairments. The effects of these factors, in so far as they cause audible impairments, are taken account of in the Listening Quality assessment. A measurement of received speech power level (for which a method is described in ( 4 )) may also be useful as means of confirming that a consistent and appropriate loss plan has been set in the network. Results can be presented as numerical reports or the key performance indicators can be displayed graphically. 4

5 Figure 4: Customisable NOC display Selection of Speech Quality Metric The choice of metric reflects the coding techniques and bandwidths to be evaluated. What are the bandwidths? Although the human voice contains a range of frequencies and harmonics, the frequencies below 4kHz contain sufficient information for the speaker to be understood and identifiable. This fact has traditionally enabled relatively scarce telephony bandwidth to be used efficiently. More recently, as bandwidth becomes more plentiful, network operators, network equipment / terminal vendors and enterprises can exploit the benefits of enhanced quality by extending the range of voice frequencies transmitted. 50Hz-3.8kHz Narrowband 50Hz-7.5kHz Wideband 50Hz-14Hz Super-wideband 20Hz-24kHz Full-band NB WB SWB FB (future use) Speech Quality Metric Options PESQ, (P.862 Perceptual Evaluation of Speech Quality), was very successful with the NB technology of its day in predicting MOS with reasonable accuracy. The rapid advance in speech coding 5

6 techniques and noise reduction technology over the last decade has, however, posed certain challenges for PESQ. POLQA (P.863 Perceptual Objective Listening Quality Analysis), delivers superior performance, particularly in these areas: 1) POLQA enables the effectiveness of noise reduction and voice quality enhancement to be measured in a way that corresponds to subjective opinion, (MOS). 2) POLQA uses an iterative method to resolve relative time difference between the reference and degraded speech (time alignment), so it can distinguish between timing changes which introduce noticeable degradations in speech quality and those which are not noticeable. POLQA is the metric of choice for modern codecs including EVRC, AMR, isac and ilbc. 3) POLQA s Super-wideband (SWB) model resolves the potential for confusion in both subjective testing and PESQ namely that NB and WB scores are not comparable. The test criteria are more exacting for WB. As illustrated below, NB subjective test uses a handset whereas for WB subjective test, headphones are used. The handset has a narrower frequency response and the coupling to the ear is leaky. The headphones are presenting a higher quality signal to the ear. If NB and WB samples are presented in the same subjective test then the NB samples will score lower due to the frequency response in the absence of any other impairment. 4) The POLQA SWB model can be used to assess the performance of NB, WB and SWB devices and networks with common test criteria. POLQA has a NB model so scores may be compared with NB subjective and PESQ scores. POLQA is also capable of measuring the performance of SWB applications including Skype. Integration with IP telephone systems A practical VoIP implementation typically consists of a soft switch and proprietary IP telephone terminals, the latter possibly being a mixture of physical telephones and soft phones. The terminals are registered with the soft switch and together use a defined protocol to manage the telephone calls setting up, clearing down and codec negotiation being amongst the functions performed for 6

7 telephone calls. Whilst some manufacturers use proprietary standards for signalling, most soft switches and terminals support Session Initiation Protocol (SIP) for call management. When a test system is used in conjunction with an IP telephone network to make a continuous assessment of network performance, the test terminals ( reference VoIP soft phones ) may either use SIP in a peer-to-peer mode to establish calls, or be registered with the soft switch as if they were useable telephone extensions. Benefits of establishing the test terminals as registered extensions include: test terminals can call, or be called by, real extensions, to perform additional levels of testing the test calls can be viewed in the call data records (CDR) generated by the soft switch extension numbers assigned to the test terminals can be managed the same manner as real extensions Application to Teleconferencing The techniques described above for Active Monitoring are also relevant to networks which carry teleconferencing traffic. If the conferencing functionality is impaired then many people are inconvenienced. Conferencing products employ a range of audio codecs and support a range of audio bandwidths which may be classified as narrowband, wideband and super-wideband. They may also have gateways to the PSTN. The connections into the conference may be VoIP, PSTN, mobile phones or voice apps running on smartphones. The range of codecs, the variability of delay and impairment is much greater than that of the VoIP network considered above. It is important to evaluate the conferencing service both within and without a VoIP network. Test endpoints that can call in from PSTN or via mobile phones will confirm the continuing functionality of the gateways established to service conference participants outside the VoIP network. The impact of an NB conference participant upon the experience of the SWB conference participant will be assessed along with the effects of the impairments in the gateways and networks. Where the test system provides an implementation of the target codec then it will be beneficial to use that codec in the overlay test calls used to assess the network performance. In cases where the target codec is not provided in the test system, there is a choice: 1. Use a codec which offers the closest match in terms of audio bandwidth and bit rate, or 2. Employ instruments, as described above in the Introduction, attached directly to the audio ports of the teleconference terminals. By way of example, the tables below show the range of POLQA SWB scores for connections between two Skype Conference System participants on laptop computers, Skype on a 3G Mobile and a PSTN participant and the delay between the participants. The arrow indicates the direction of speech from speaker to listener and the measurements are made at the listener end. 7

8 Skype PC Skype PC Skype Mobile PSTN Skype PC Skype PC Skype Mobile PSTN Average MOS Prediction from P.863 POLQA SWB Note: The POLQA SWB scale has a maximum of 4.75, WB speech will not be higher than 4.5 and NB speech will not exceed 3.8 Skype PC Skype PC Skype Mobile PSTN Skype PC Skype PC Skype Mobile PSTN Average One Way Delay Estimate ms The scores, assessed on the POLQA Super-wideband scale, indicate that Skype works well when speaking to another Skype participant. The Skype PC to Skype Mobile is good. The Skype gateway to PSTN is satisfactory/poor but the Skype Mobile to PSTN gateway is slightly worse. The bandwidth of the speech files from the Skype PC is around 12kHz, Skype Mobile 5kHz and PSTN 3.5kHz. Delay to the Skype Mobile will make conversation difficult for all participants. The POLQA SWB scale provides a clear indication of the relative performance of participant media in a conference call even though the speakers and listeners have differing audio bandwidths. Conclusion Frequent, regular performance tests across the network and through gateways to other networks will establish the benchmarks for performance. Comparing current performance with the benchmark will indicate system degradation or even failure. VoIP network performance can be affected by a number of factors in addition to packet loss and jitter. Intrusive tests account for almost all the factors that affect user experience. Active Monitoring is already making a difference for many organisations, improving staff productivity, communication and collaboration. 8

9 References 1) ITU-T Rec. P.800: Methods for subjective determination of transmission quality 2) ITU-T Rec. P.862: Perceptual evaluation of speech quality (PESQ): An objective method for end- to-end speech quality assessment of narrow-band telephone networks and speech codecs 3) ITU-T Rec. P.862.1: Mapping function for transforming P.862 raw result scores to MOS-LQO 4) ITU-T Rec. P.862.2: Wideband extension to Recommendation P.862 for the assessment of wideband telephone networks and speech codecs 5) ITU-T Rec.P.863: Perceptual objective listening quality assessment 6) ITU-T Rec. P.56, Method B : Active speech level for other applications than those mentioned in method A 7) Malden MultiDSLA Enterprise Brochure 1 November 2012 Copyright Malden Electronics Ltd

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