A: This is a great question. I'd recommend that you take these steps:
1. List the devices that will be used to make voice calls. These devices might include laptops, smart phones, or IP phones.
2. Look at the data sheets for these devices to determine their receiver sensitivity. The receiver sensitivity tells you how much signal strength a device needs to decode the received signal at a given data rate. This in turn tells you the minimum received signal strength that you'll need at the cell boundary so that these devices can connect to the access point (AP). For example, Cisco recommends a minimum data rate of 24Mbps for voice. Looking at the data sheet for a Cisco Unified Wireless IP Phone 7926G that's connected to an IEEE 802.11a/g AP, we can see that we'd need a receiver sensitivity of -82 dBm at 24Mbps.
3. Use a site-survey predictive-planning tool to predict how many APs you'll need if you deploy with this received signal strength. Several such tools are out there, including Fluke AirMagnet Planner, AWE Communications WinProp, iBwave RF-vu, and Motorola LANPlanner.
4. Add 15 to 20 percent to the predicted number of APs, to cater for unknown interfering sources.
5. Conduct a physical site survey and design your network with the desired received signal strength (which you determined in step 2) at the cell boundary. Design your coverage with 20-percent cell overlap between APs if you're operating in the 2.4GHz band. You can reduce this overlap to 15 percent in the 5GHz band, which contains less interference.
6. Enjoy your new voice over WLAN.
Of course, if you need help conducting your physical site survey, we'd be delighted to come and help!