Cellular Wi-Fi Voice

Voice-over-Wi-Fi (VoWi-Fi) promises to bring the benefits of Voice-over-IP (VoIP) to the mobile broadband user. When combined with metro-scale Wi-Fi, these benefits will extend to large outdoor coverage areas. Tropos Networks is committed to supporting standards-based VoWi-Fi in our products as the standards and market demand develops.

VoIP, the provision of high quality voice service over existing data communications infrastructure, is already being used extensively in lieu of traditional PBXs as a way of cutting telecom costs and adding features such a conferencing and white-boarding. The emergence of inexpensive Wi-Fi hot spots, hot zones and even city-wide Wi-Fi promises to extend these cost savings and quality improvements to the mobile user via VoWi-Fi.

Several vendors already offer VoWi-Fi handsets targeted for use within the bounds of an enterprise. These vendors include Spectralink, Cisco, Nortel and Alcatel. Form factors beyond the traditional handset are also being offers, with companies like Vocera delivering voice-activated VoIP badges.

VoWi-Fi using the 802.11b standard works reasonably well for small call volumes and under light load, but is Wi-Fi technology ready to support high-quality large-scale voice service? Two challenges must be addressed when attempting to use a data-optimized wireless network for voice—contention for resources and roaming handoff speeds. Both of these issues are currently being addressed by the IEEE in a series of updates to the 802.11 Wi-Fi standards. Wi-Fi has the potential to become a vastly improved mobile voice transport medium when these standards are implemented.

Traffic Contention. As data and voice streams are simultaneously transferred over the same communications medium, the streams experience contention for resources, placing limits on the number of simultaneous streams. The solution is to effectively isolate the voice and data streams by defining Quality of Service (QoS) parameters for each type of data, thereby maximizing the capacity of the medium. At the IP network layer, distributed mechanisms such as IP DiffServ and priority queuing have long been established to manage contention and enable traffic to flow without disruption through intermediate routers. As part of the draft 802.11e standard, distributed QoS mechanisms are being introduced to Wi-Fi to permit these streams to be either prioritized (WME) or even scheduled (WSM). The new standards will allow traffic over the air to be managed to ensure that all applications, including voice, are afforded the QoS they require. In parallel to these standard developments, Tropos is incorporating application-based QoS into our Tropos Sphere Network Operating System to ensure effective end-to-end voice handling through our cellular mesh system.

Roaming. Roaming is the ability to move seamlessly from cell to cell within a coverage area. As a wireless handset roams, the time taken to reconnect is called the handoff latency. In general, users can tolerate handoff latencies of at most 100-200 ms, with longer disruptions becoming distinctly noticeable. Handoff latencies for today's data-optimized Wi-Fi equipment can be one or two seconds in some situations, and these timings can be further exacerbated when supplementary standards such as 802.11i (security) and 802.11e (QoS) are implemented. The newly formed IEEE 802.11r group was set up specifically to fix this problem and is in the process of defining seamless sub-100 ms handoffs. These features will be fully incorporated into the Tropos products as they solidify.

Although in its infancy, VoIP over Wi-Fi has the potential to emerge as an important complement to traditional cellular voice communications, offering higher quality voice service at a lower cost. The combination of VoIP and Wi-Fi is entirely based on international open standards and operates in the same frequency band internationally. VoWi-Fi will be the first voice technology usable everywhere across the globe, promising tremendous scale economies and low user prices. Upcoming new handsets from manufacturers such as Motorola and Nokia will integrate Wi-Fi and cellular voice, enabling seamless handoffs between the traditional cellular network and Wi-Fi hot zones. The introduction of these devices is being driven by the rapid growth in public Wi-Fi hotspots, hot-zones and city-wide Wi-Fi, by the potential to reduce traditional cellular toll and roaming charges, and by the emergence of new phone-based broadband data applications.

With the IEEE leading the solution of these technical challenges, the growth in VoWi-Fi will be spurred by availability of this Wi-Fi access. The emergence of cellular Wi-Fi voice promises to create a seamless experience for the end-user that will improve traditional cellular voice services, and at a much lower cost.

For more information, see Wireless LANs Find Their Voice (COMPUTERWORLD) and VoWi-Fi Calls to Users (Network World).