What are the radio frequency requirements for the wireless telephony? Check here ten useful tips for channel selection that help you to achieve the optimum WiFi network for the wireless IP telephony.
Radio Frequency Requirements
Radio Frequency Requirements and Planning
The process of the radio frequency planning includes the following activities:
- Wireless site survey – inspection of wireless coverage level and identification of the interference.
- Proper distribution of the access points and wireless antennas.
Wireless Site Survey and Interference
A wireless site survey helps you to determine the following information:
- A number of access points needed for the optimum coverage and enough data rates.
- Proper configuration of the access points.
One very important factor you need to consider for deployment of the WiFi network for the wireless telephony is an antenna. The type and positioning of the WiFi antenna have a great influence on the coverage in speed of wireless data transfers.
The other very important factor you must not forget in the deployment of WiFi networks is the interference.
Different environments have the different RF characteristics. You always need to adopt equipment and their configuration based on that specific environment. There are various applications on the market which you can use for wireless site surveys.
More about the site survey, the types, methods and the process, you can read in my post Wireless Site Survey – Proper Placement of the Access Points.
Better Roaming Characteristics
For better radio frequency requirements and roaming characteristic follow the rules below:
- Use the non-overlapping channels on your access points.
- Use manual RF channel configurations of access points instead of an automatic channel option.
The parameters you need for the manual channel configurations of the access points, you should get from site survey. If you choose an automatic channel distribution on your access points, AP will use different channel after reset. So your time spend on site survey was useless. Random channel selection also increases roaming time.
Tips for the Channel Selection
Use the following tips for the optimum channel selection:
- Use minimum five channel difference – for example, use channels 1, 6 and 11, or 2, 7 and 12.
- Use the same non-overlapping channels through all your sites. With this, you will get smaller roaming times.
- Always use diversity antennas for the indoor and outdoor environment. With this you will reduce interference.
- Deploy minimum 2 APs on non-overlapping channels with received signal indicator (RSSI) greater than 35, and only one AP per overlapping channels set with RSSI greater than 35. RSSI that is less than 35 can cause interference. RSSI of 35 is same as the receiver threshold of -67 dBm.
- The minimum speed of your whole WiFi network should be 11 Mbps. This includes both data and voice clients.
- Maintain AP coverage overlap on 15% to 20%.
- Keep the packet error rate (PER) on maximum 1%.
- Keep a minimum signal to noise ration SNR of 25 dB.
- Use the same power on AP and phones. If you have different transmit powers set on your access points, set the phone on highest transmit power of the APs.
- Optimal settled and configured APs can handle 7 G.711 or 8 G.729 simultaneous phone calls. If you need more than this per one AP, plan to use load-balancing APs. You will need to include these extra APs also in site survey phase.
A great document about the design and deployment of the WiFi networks for wireless telephony is Cisco 7920 Wireless IP Phone Design and Deployment Guide.
Radio Frequency Requirements – The Final Notes
So as you can see, there are lots of radio frequency requirements, you need to keep in mind, when dealing with wireless IP telephony.
More about the wireless IP telephony read in the article Wireless IP Telephony – Voice over WLAN (VoWLAN).