Minding the VoIP...

Today’s Wi-Fi mostly sucks for voice over IP (coined “VoFi” by industry socialite Craig Mathias).

Originally developed for best-effort data delivery, Wi-Fi is a half-duplex, shared medium where users fight for access on a first-come, first-served basis. So as load increases, all users suffer equally by waiting longer to transmit. This is bad for VoIP.

Meanwhile, the 2.4 and 5 GHz bands used by 802.11 are shared by things like cordless phones, bluetooth headsets, satellite services, microwave ovens and neighboring Wi-Fi networks. These common interference sources make it a bitch for Wi-Fi devices to differentiate between legitimate transmissions and background noise. And it's not just noise.

Wi-Fi signals degrade by distance and objects in the way - even people's bodies (over 60 percent of which are comprised of water). Signals are weakened (attenuated) by walls, reflected by mirrors and bent (refracted) by almost everything else. This causes packet loss which drives retransmissions resulting in delays. This is bad for VoIP.

To really solve these VoIP problem, Wi-Fi systems must be able to constantly choose transmission paths between the AP and voice device that deliver the strongest signal, fewest delays and lowest packet error rates, while simultaneously attenuating any interference or noise.

Most of today's Wi-Fi systems just don’t do this (click on comparison chart on right). They can't dynamically cope with RF problems that crop up, other than changing the channel assignment or lowering the physical data rate. This only makes things worse because it will now take longer to send the same packets with a greater chance of interference and collisions. And despite its rock-star status, 802.11n does nothing to inherently benefit applications requiring consistent delivery of short frames (like voice).

Smarter Wi-Fi Adds New Value to VoFi
Smarter Wi-Fi technology has been developed that boosts RF signals but only in the direction of the client, steering these signals around interference. Using miniaturized antenna arrays and best path selection algorithms, these Smart Wi-Fi devices constantly find the best signal path for every packet. Smart Wi-Fi systems also employ dual-polarized (horizontal and vertical) antennas that help accommodate for the orientation of phones that constantly change. This is also immensely useful in increasing the reliability and efficiency of 802.11n networks by increasing the likelihood that spatial streams are traversing the best signal paths – allowing the network to operate as fast as possible and reducing interference.

Service Quality for All
While the IEEE created 802.11e (MAC enhancements for QoS), a subset of this standard, WMM (Wi-Fi Multimedia) is what most companies use. WMM defines four traffic queues (voice, video, data and background) that are shared among all users. The problem is if one VoFi device is lost (for whatever reason) the AP typically retransmits to that device before service other devices. The solution is to have a wireless LAN system that uses a per-client, per-traffic class QoS scheme so every client and traffic flow is fairly serviced.

Life Goes on for Batteries
By maximizing transmit signal strength and receiver sensitivity, VoIP handsets can send the same information in shorter times at lower power. The use of high-gain, directional smart antennas that focus RF energy toward each voice handset reduces retransmissions and extends battery life.  This makes for happier callers.

Roam, Forest, Roam
Finally there’s roaming. When clients move from AP to AP they must often re-authenticate. This can easily take hundreds of milliseconds which will cause crappy quality or complete disruption. Clients might also roam across subnets requiring the device to obtain a new IP address – thus breaking the call.

Well engineered WLAN systems incorporate central key management with the ability to cache user credentials on the controller. These smart WLAN controllers then “opportunistically” distribute these keys to APs around where the user is roaming. This eliminates the need to force the client to go back to the authentication server to re-authenticate.

And for roaming between subnets, smart WLAN systems use routable tunneling to get voice traffic from the handset back to the original subnet without requiring a new IP addressed to be assigned.

There you go.  Tips and tricks for crystal clear voice over IP over Wi-Fi.

Comments

[Devin Akin]

This whitepaper will explain how fast BSS transition (fast roaming) works.

[Ken]

Has anyone ever used your products with the T-Mobile UMA devices? I have 15 acres I need to cover.

T-Mobile is trying to use Linksys routers meant for @home to implement this. Results are not satisfactory.

Sughested solutions welcome.