VoWi-Fi in the real world

19 Sep 2006

It's a natural fit: VoIP (inexpensive phone calls over the Internet) coupled with Wi-Fi (wireless connection to the Internet). In the enterprise setting, VoIP over Wi-Fi, or VoFi, provided acceptable voice quality while dramatically reducing long-distance phone costs for the organization.

But now VoFi is moving into the consumer world where operating conditions are not as stable. VoFi will have to work on standard 802.11 systems, not special enterprise networks that have been modified to handle real-time voice requirements. It remains to be seen how VoFi will sound in this environment.

The underlying problem is that packet networks such as LANs, Wi-Fi and the Internet weren't designed for voice traffic. In packet networks, there is no promise that packets containing voice data will arrive in a timely manner. Or even at all. But for a phone conversation to sound natural, voice signals must travel without noticeable delay between the callers, something on the order of less than 150 milliseconds.

Fortunately, many of the voice-quality problems have already been solved in wired VoIP systems. That is, the problems relating to inconsistent delivery of voice packets. If problems do pop up, it's often because the Internet, itself, is experiencing unusual problems or a LAN is constipated with data traffic and won't give VoIP applications priority.

Wi-Fi systems found in the consumer world, however, provide more opportunities for problems. Packet delivery can be affected by walls, weak signals, electronic interference or people passing by who become RF shields. Although both wired and wireless systems will have to deal with the same packet-delivery problems in essentially the same way, wireless systems seems to have more frequent problems, and are more severe.

The main culprits are the underlying causes of latency and jitter. Though not many test tools are yet available to help in this testing area, more testing needs to focus on delay and jitter metrics, not just on forwarding rates within a VoFi network.

Here's why

Voice quality starts to drop when the jitter levels of the associated voice packets start to exceed 50 milliseconds. For higher levels of jitter, the voice packets can arrive out of order, causing words to be jumbled. (One voice packets equates to about 20 to 30 milliseconds of speech.) A typical 802.11b network could have jitter with a standard deviation of 50 to 100 milliseconds; while an enterprise LAN would have jitter on the order of 2 to 3 milliseconds.

Last year the results of at least two benchmarking tests of wireless VoIP systems were published. However, the tests were conducted in highly controlled environments - environments that perhaps are consistent with those found in an enterprise network. What needs to happen now is an extension of those benchmark tests where the nature of a standard 802.11 network environment (interference, multipath, or other artifacts) is introduced into the test environment to see its affect on voice quality. Then we can have a check on whether VoFi is ready to move out of the enterprise world into the consumer world of 802.11.

Chris Everett is a regular contributor to Wireless Asia and Telecom Asia on test & measurement issues ([email protected])

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