Communications News
November 2007

The advent of dual-mode phones that can switch from the cellular network to the WLAN (also called fixed-mobile convergence) is expected to make voice-over-WLAN (VoWLAN) the norm in enterprise environments, as employees become more mobile, often communicating and accessing data from a single device. The success of VoWLAN, however, will depend on overcoming challenges that did not exist in the pre-voice Wi-Fi era.

Many of these challenges are related to the traditional cell-planned architecture of wireless systems, in which each access point transmits on one of the three non-overlapping channels available in the 802.11b/g standard, and in which access points are carefully positioned and "tuned" during the deployment process to mitigate the co-channel interference that can kill transmissions.

Enterprise-class systems are either optimized for coverage (wireless signal is available everywhere in the enterprise, but not with optimal bandwidth) or capacity (a maximum number of users can have optimal bandwidth, but not everywhere in the enterprise), but rarely for both at the same time. In the end, the systems do not address the real challenges of VoWLAN-that voice communications demand mobility and a reliable wireless connection, and voice drives convergence.

No matter what the topology, the interplay between a standard originally developed for asynchronous data transmission and voice technology that depends on regular, synchronous communication will always have some degree of complexity. Users should bear in mind a few facts about Wi-Fi that can be primary determinants of success or failure in a VoWLAN deployment.

Wireless does not always mean mobile. Voice communications require mobility, meaning continuous communications while traveling between point A and point B. This may seem like a trivial detail until realizing that the 802.11 Wi-Fi is, by definition, designed for portability, meaning communications between points A and B are not continuous. This, combined with traditional WLAN systems that organize access points in a cellular pattern, yields networks that are not optimized for mobility. The basic problem revolves around the "handoff," which is what happens as users move through cell-planned WLANs, losing the link from one AP and associating to another as they move. The delay introduced by handoff is usually not a problem for data transmissions but is often a deal-breaker for voice calls.

Mileage will vary. Another voice reality is its need for a constant, stable connection rate. In traditional WLANs, the negative impact of such things as rate adaptation, edge users, coverage holes and interference are not as strongly felt, since data communication, bursty by nature, can tolerate the widely varying data transmission rates that result from these effects. Voice performance, however, will suffer in such an environment, but the cell-based WLAN has no solution to this challenge.

Voice and data do not mix well. Convergence means adding voice to data-but these two traffic types clash. WLANs have been data-centric and, even with the advent of new standards, still have inefficient quality-of-service (QoS) mechanisms for prioritizing voice over data traffic. Effective QoS needs to be based on deterministic methods, otherwise the co-existence between converged voice and data will be uneasy at best and unworkable at worst.

An alternative approach to cell-based systems has been to allow all access points (APs) in the network to transmit and receive on the same channel, creating a "channel blanket" from the aggregate coverage of the APs.

The channel blanket eliminates the problems associated with supporting the mobile user, particularly the issues of handoff, reliable and stable connections, and QoS. The ease of deployment and maintenance of such systems, compared to more complex cell-based topologies, is an additional benefit of this architecture, since the RF cell planning of traditional systems is eliminated.

David Confalonieri is vice president, marketing, for Extricom, New York.

The original version of this article can be read online in the November 2007 edition of Communications News .