Planning for Pitfalls
Not surprisingly, a key to a successful VoWLAN deployment is adequate wireless coverage so that users can connect from, at the very least, the most obvious places. As you've probably found already, positioning access points (APs) is part science and part trial and error. Keep in mind, though, that the areas needing coverage for laptops are much more obvious than those needed for phones.
Coverage for laptops is needed in gathering spots such as conference rooms while VoWLAN coverage needs to be more widespread. Laptop users rarely try to connect in stairwells, elevators, or bathrooms. But you can be sure as soon as people can walk away from their desks with their VoWLAN phones, they will be in exactly those places.
As a result, site surveys become more crucial than usual if you are planning to deploy VoWLAN. That's because contention and latency are bigger issues for voice than for data. Data traffic is fairly immune to short latency but voice traffic isn't. Also, while a single access point may support several laptop connections, there are thresholds for voice traffic that define the realistic number of simultaneous conversations.
Also, access point placement for VoWLAN has more restrictions than is the case for standard computers. VoWLAN doesn't take much bandwidth -- usually it is significantly below 64kb/s, which is a fraction of 802.11b's 11mb/s. However, each voice session needs an uncontended connection so latency doesn't become a factor. For that reason, most enterprise-class access point manufacturers specify no more than seven concurrent voice sessions per AP.
