VoIP services providers are still experiencing significant problems with audio delay in their calls, according to a study by Keynote Systems. And while the call quality of VoIP and PacketCable have continued to improve, both services still lag behind PSTN (Public Switched Telephone Network).

The study used indicators such as service availability, call completion percentage and audio delay to test 12 different VoIP and PacketCable (IP telephony over cable) phone services for their reliability, responsiveness and clarity. The study then compared the VoIP and PacketCable indicators with those of PSTN services from AT&T and Verizon.

Unlike previous studies, the latest only looked at VoIP hard phone services, which rely on actual telephones, and not the soft phone services, which are generally performed through computers.

"Our key finding here is that there has been significant improvement for VoIP services, but there is still a difference in terms of quality of calls," says Rajeev Kutty, VoIP product manager at Keynote.

The study found that VoIP service providers are still experiencing significant problems with audio delay in their calls. While both PSTN and PacketCable both measured a mean average audio delay of below 150 milliseconds, VoIP providers measure a one-way audio delay of between 150 to 250 ms.

The best mean audio delay for a VoIP provider came in at 149 ms, while the worst came in at 279 ms. Even so, this was far better than the worst delay recorded in Keynote's September 2006 survey, which clocked in at 335 ms.

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VoIP's call completion percentage was also found lagging behind PSTN, as only two VoIP providers studied had call completion percentages over 99.5 percent. Six providers had completion percentages under 99 percent, with one provider scoring as low as 86.7 percent. The most common audio problems for VoIP hard phone providers were audio holdover, which occurred in 68.13 percent of calls; front clipping, which occurred in 42.37 percent of calls; other kinds of clipping, which occurred in 58.27 percent of calls; and hum, which occurred in 31.98 percent of calls.

While PacketCable had significantly lower percentages of audio problems in nearly all of its indicators, it did experience a higher frequency of holdover at 72.32 percent of calls. PacketCable also performed better than PSTN in the number of calls that experienced back clipping, hiss, hum, other clipping and static.

The study was conducted by placing over 125,000 total calls between New York and San Francisco over a period of a month. The latest study is Keynote’s fourth in comparing call quality for PSTN, VoIP and PacketCable.