Voice quality tests are key for VoIP

Not long ago, the use of the Internet Protocol to carry voice traffic (VoIP) was hotly pursued by Internet service providers hoping to capture some of the telephony market. With the dot-com crash, a lot of the media attention to VoIP vanished. But VoIP itself did not vanish. Telephony vendors are starting to quietly employ the technology; they're just not selling it by that name. Instead, they are selling voice channels, and customers may not even know the service is VoIP based.

Because of the increasing market for VoIP technology, the need for test is on the rise. Test needs can be broken into two major categories: end-user service quality and network performance. Network-performance measurements are the traditional tests for packet loss, jitter, and delay long used by the telecommunications industry. But end-user service-quality measurements, which include speech quality, echo cancelation, and voice delay, take on new dimensions when IP is the data carrier instead of time-division multiple access (TDMA).

In traditional TDMA systems, the relationship between network performance and end-user service quality was well understood. With VoIP, the relationship has become more complex. Network measurements alone are not enough, especially for testing voice quality. Voice quality over VoIP requires its own test.

In simplest terms, test engineers measure the voice quality of their VoIP systems by using a digital .WAV file to inject a known voice signal into an IP channel. They then capture and evaluate the signal as it exits the channel. The evaluation is not a simple task, however. In data traffic, a bit-for-bit comparison tells the quality story. For voice, direct data comparisons are meaningless. The evaluation must factor in psychoacoustics, or the way the human brain processes sound.

Fortunately, there is a standard method for making this evaluation: perceptual evaluation of speech quality (PESQ), defined by ITUT Recommendation P.862 (Ref. 1). First deployed less than five years ago, PESQ is now the de facto standard worldwide for evaluating telephony signals. For VoIP systems, testers are available that automate the entire process.

The trouble is, these are active tests. They are well suited to evaluating a design in a laboratory setting but are not convenient for testing a deployed system. Agilent Technologies (Palo Alto, CA; http://www.agilent.com) has recently addressed the need for a more passive test by incorporating speech quality prediction in its J6844A network analyzer. The system compares incoming and outgoing signals and calculates a predicted mean opinion score (MOS) for the telephony link (Figure 1).

Active or passive, the voice-quality tests are an essential element of providing a high-quality VoIP system. They are especially valuable in light of the trend of providers toward using VoIP for voice services not explicitly sold as VoIP. In these situations, users have the reasonable expectation that they can use their voice channels as they always have, including connecting fax or modem units to the channel. To meet that expectation, VoIP network testing has to evaluate how well the IP network carries the fax and modem connection. It also has to evaluate how the fax and modem signals affect voice quality.