IP telephony is delivering proven benefits to business users, so it seems logical to extend it to the wireless realm. But VoIP places new demands on a wireless network infrastructure, and it pays to know in advance what you're up against. In this article, we'll look at the requirements and deployment considerations for the two basic components of a wireless VoIP system: the wireless LAN infrastructure that carries the traffic, and the wireless handsets that connect users.
WLAN infrastructureTo preserve call quality, the WLAN infrastructure must ensure reliable packet delivery, and it must do this as users move throughout the Wi-Fi coverage area. Here are some key considerations:
Pervasive coverage – The WLAN must be built to provide coverage in any area where users might need to place or receive calls. As with cellular phones, users will expect coverage wherever they happen to be. Companies used to "hotspot" AP deployments will have to expand them to provide continuous coverage in stairwells, hallways, and other common spaces as well as in offices and conference rooms. Most WLAN systems can provide such coverage, but many require costly and time-consuming RF engineering for optimum AP placement. This engineering must be repeated whenever the space is reconfigured, or even when key pieces of furniture are moved. When deploying a WLAN for pervasive coverage, IT engineers should consider not just whether a particular system will provide such coverage, but how much hassle and cost will be required both initially, and as the covered space changes.
Seamless roaming – Wireless users must be handed off as quickly as possible as they roam from one AP coverage area to another. Most VoIP calls will disconnect if they encounter a handoff delay of more than one second, but even delays as short as 50 milliseconds will cause audible clicks or pops in the call. For optimum call quality, delays from the wireless infrastructure should be 5 milliseconds or less. Remember, VoIP packets are delayed at several points during the transmission; if the WLAN infrastructure delays packets beyond a few milliseconds, this can be all it takes to cause problems.
Quality of Service – Each wireless handset user must have guaranteed access to the wireless network to ensure consistent packet delivery, even when many other users are connected through the same AP. The WLAN infrastructure must be able to distinguish between users making voice calls and users checking e-mail or accessing web sites, and it must be able to prioritize the traffic and provide dedicated bandwidth for the call. This type of QoS requires WLAN APs and a WLAN controller that can recognize each user and reserve bi-directional, client-specific bandwidth depending on each user's QoS requirements at the time. Many WLAN systems can't deliver QoS to individual users, or can prioritize packets only from the AP up to the WLAN controller rather than over the wireless link from the AP to the client.
User density – Companies have little control over the number of mobile users on a given AP as people roam or gather throughout the coverage area, but many WLAN systems can't handle more than half a dozen voice users on a given AP. Some WLAN vendors recommend overlaying an 802.11a network on top of an 802.11b/g network to reduce user density, but this technique doubles WLAN infrastructure costs and requires the IT team to support two WLANs instead of one. IT engineers should look for WLAN infrastructure that supports the highest user density to minimize the need for overlay networks. However, nobody should sacrifice QoS for density, or vice versa—effective WLANs need both.
Mobile VoIP handsetsThere will be a lot of development activity in the mobile VoIP handset market over the coming year or two. There are three considerations to keep in mind as handset options multiply.
Battery life – Many users expect wireless VoIP handset batteries to last as long as those in cellular phones, but this kind of longevity may not be necessary. In work environments, many employees are at their desks much of the time, and the handset can sit in a charging cradle when not in use. In this respect, the wireless VoIP handset should be thought of more as a cordless phone than as a cellular phone.
QoS compatibility – Wireless VoIP handsets use built-in QoS mechanisms to ensure high call quality. Earlier models of these handsets use proprietary QoS methods that may not be compatible with the WLAN infrastructure. Users should work with their WLAN vendor to choose compatible handsets that are Wi-Fi compliant.
Dual-mode functionality – Some manufacturers are now delivering handsets that can work either on wireless VoIP or mobile cellular networks. While these handsets don't yet allow seamless roaming from one network to the other while a call is in progress, they can enable users to have just one handset for all of their calling needs. To take advantage of this feature, however, users will need to wait for cellular providers to permit access to their networks.
By understanding these issues, IT managers and their bosses can minimize costly mistakes and avoid frustration when deploying IP voice over wireless LANs.
Joel leads efforts in defining, driving, and evangelizing Meru's technology for voice over WLAN. Prior to his role at Meru Networks, he played instrumental roles in the founding of the INS EnterprisePRO (now Lucent Netcare) network management software product line, the founding of the NETGEAR consumer and wireless product lines, and introducing the first commercially deployed voice over DSL system at CopperCom Inc. He has successfully brought over 60 products and services to market worldwide, garnering many awards ranging from Comdex best-in-show to a nomination for a ComputerWorld Smithsonian award.
Joel has written about wireless VoIP for numerous publications including Internet Telephony, Telephony Online, Wireless Business Review, Converge Network Digest, Business Communications Review, and Wireless Systems Design. He combines over 12 years of experience in the data communications industry with his BSEE degree from the Massachusetts Institute of Technology (MIT) to provide a practical technological voice to the wireless networking industry.