July 2005

From the Lab: Information Technology

Continued from page 3

By Monya Baker (editor)

Smoothing Out Speech
Internet phones get clearer

Context: People trying to converse over wireless local-area networks (WLANs ) -- using them to connect to voice-over-Internet-protocol, or VoIP, systems -- are often confounded by the poor quality of the transmission. Those frustrations may soon clear up, thanks to researchers at the University of California, Santa Barbara, who report a method to improve the clarity of VoIP conversations.

Methods and Results: Information is sent over a WLAN network in units called packets. But wireless signals can deteriorate over distance, interfere with each other, or otherwise introduce errors into the packets. If that happens, the transmission standard IEEE 802.11 requires that the packets be re-sent. The subsequent delay garbles real-time communication. While zero error tolerance makes sense for e-mail, it might be too strict a standard for voice: Ian Chakeres and colleagues have shown that digitized voice data can suffer some errors without degrading call quality. The researchers used a computer simulation of a network to test reliability. They then combined various network layouts, hardware settings, and traffic scenarios with different levels of permitted packet error and charted the resulting voice drop-out. This gave them the combination that allowed for the highest packet error and the best conversational quality.

Why It Matters: Users tap into WLANs via handheld devices with radio connections. Because of its ease, low cost, and mobility, voice transmission over WLAN is becoming more common, and video transmission is following suit. But current technology can't yet deliver smooth, clear voice or video communications, a drawback that keeps consumers from adopting it. The researchers' method could help bring WLAN voice, video, and multimedia into the mainstream.

Source: Chakeres, I., et al. 2005. Allowing bit errors in speech over wireless LANs. Computer Communications (in press).