Radio frequency requirements for Wireless telephony.

I will show you Radio frequency requirements for Wireless telephony and how to achieve them. Wireless IP Telephony requires careful RF planning.



RF planning includes:

1. Site survey - Inspection of wireless coverage level and identification of interference.

2. Proper distribution of APs and wireless antennas

Site survey will help you to determine number of access points and their proper configuration. You should also need to consider type of antennas that will provide you best coverage. Interference is also factor which you will also need to consider when dealing with Radio frequency requirements.

Different environment have different RF characteristics. So you always need to adopt equipment and their configuration based on that specific environment.There are various Site Survey tools on the market which you can use for Site Survey.

For better roaming characteristic you should use non-overlapping channels on your access points. Use manual RF channel configurations on access points instead of automatic channel option on your access points. Data for your manual channel configurations you should get from site survey. If you choose 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.



Check following points in channel selection:

1. Use minimum 5 channel difference - for example use channels 1, 6 and 11, or 2, 7 and 12.

2. Use the same non-overlapping channels through your all site. With this you will get smaller roaming times.

3. Always use diversity antennas for indoor and outdoor environment. With this you will reduce interference.

4. Deploy minimum 2 APs on non-overlapping channels with received signal indicator (RSSI) greater then 35, and only one AP per overlapping channels set with RSSI greater then 35. RSSI that is less then 35 can cause interference. RSSI of 35 is same as receiver threshold of -67 dBm.

5. Have minimum of 11 Mbps for your entire site. This includes both data and voice clients.

6. Maintain AP coverage overlap on 15% to 20%.

7. Keep packet error rate (PER) on maximum 1%.

8. Keep minimum signal to noise ration SNR of 25 dB.

9. Use same power on AP and phones. If you have different transmit powers set on your access points, set phone on highest transmit power of the APs.

10. Optimal settled and configured APs can handle 7 G.711 or 8 G.729 simultaneous phone calls. If you need more then this per one AP, plan to use load-balancing APs. You will need to include these extra APs also in site survey phase.

So as you can see, there are lots of radio frequency requirements you need to keep in mind, when dealing with wireless IP telephony.



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