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With VoIP, Transmission Quality, Not Just Speed, Linked to Call Caliber

Posted on: Monday, 26 March 2007, 06:00 CDT

By Leslie Cauley

Internet telephony -- also known as Voice over Internet Protocol, or VoIP -- isn't your average online service.

This increasingly popular form of phone service is a "two-way," meaning the upstream and downstream paths are equally important. With VoIP, transmission quality, not just the raw speed of your Internet connection, is directly related to call quality, says Patrick White, director of the wireless network security center at the Stevens Institute of Technology in Hoboken, N.J.

Why? White says it's because of the natural, even innate, pace of human conversation: When one person finishes speaking, the other typically pauses a moment -- about 200 milliseconds, to be exact -- before talking.

Any delays in transmission -- the lag between the time when one person speaks and when those sounds actually arrive at the other end -- can quickly disrupt the flow of the conversation and distort it. Think of the old ship-to-shore phones that practically echoed when you talked, making it almost impossible to conduct a running conversation.

"That can lead to a lot of frustration for everybody," White says.

The International Telecommunications Union, a global standard-setting body, says the maximum delay for voice transmissions in one direction is 150 milliseconds. "That is the maximum that is really livable," White notes.

Stevens' researchers recently put Internet telephony through the paces, with the goal of finding out which Internet connections deliver high-quality VoIP.

What they found:

*Dial-up works fine. Dial-up, which has constant delays of about 100 milliseconds in each direction, is perfectly adequate for VoIP.

His conclusion: "It's not a great connection, but it is usable."

*DSL and cable modem services are ideal. White says both services "work splendidly" with VoIP. Both have delays of about 30 milliseconds in one direction, he says, "which is pretty close to nothing."

*Wireless Wi-Fi and WiMax also work. These wireless connections to high-speed Internet access -- often through a network router -- typically clock in at 10 to 50 megabits per second. Both handle VoIP beautifully, White says. "If you're in a Wi-Fi hot spot, (VoIP) will work." Ditto for WiMax.

*Cellular data networks are a dud. White says the two most popular data standards today -- 1xRTT and Edge -- are subpar performers for VoIP. The first standard is embraced by Verizon Wireless. Edge is used by AT&T and also will support the new Apple iPhone.

"They're both unusable" for VoIP, White says. "That surprised us."

Their lag time -- known as latency in the industry -- is the major problem, he says. When researchers tried to use 1xRTT for VoIP, they had transmission delays of up to 600 milliseconds in one direction, he says. That's four times the ITU standard.

"Edge wasn't much better," he says, with delays of 200 to 500 milliseconds in one direction, on average.

The latency problems disappeared, however, when the speeds were kicked up a notch to the "3G" level -- the coming standard for cellular networks. (1xRTT and Edge are both considered "2.5G" technologies, a precursor to full 3G.)

White's advice to cellular data users: "Wait for (3G) before trying to use VoIP. Don't waste your time with 1xRTT or Edge." (c) Copyright 2005 USA TODAY, a division of Gannett Co. Inc.

Source: USA TODAY

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