Saturday, 18 July 2015

802.11 EIRP limitations in Australia


I have recently had the opportunity to design and implement an outdoor wireless mesh network at one of Australia's major airports. Although relatively straight forward to implement, we had challenges in deciding what channels to use on each Root Access Point (RAP). A spectrum analysis of the required locations (highly secure) wasn't in scope so we relied on Cisco's Clean-Air reports to tell us the least used channels in each location once the APs were installed.

Without going in to details it turns out that the 5GHz band at airports (or at least this one) is quite utilised and we couldn't just randomly assign the UNII-3 channels available to us. In order to provide a fast, yet stable network we had to choose a channel which had the highest consistent Signal-to-Noise Ratio (SNR) for each RAP. Some channels had a higher 'peak' SNR but would occasionally drop to an unusable level.

This was the first time I had to REALLY think about neighbouring WiFi networks and the potential performance impact I could impose on those networks (as well as my clients) by not planning carefully. It was also the first time I had to care what the transmit power laws for the 2.4GHz / 5GHz bands in Australia were. After all, I would hate to be responsible for a plane to drop out of the sky!

Technically a Wireless LAN Controller will limit an AP's Effective Isotropic Radiated Power (EIRP) to the limits of the regulatory domain that the AP is based in. However a WLC trusts that you have provided it with antenna gain values that match the ACTUAL gain of the antennas you are using. Entering a gain value that is lower than the real value could cause the WLC to use a higher transmit power on the AP's radio, potentially violating the laws of the domain.

Getting to the point of this post...

This got me thinking about what the EIRP laws for Wireless LANs are in Australia. Although the ACMA (Australia's version of the FCC or ETSI) and the ComLaw websites do provide the information, the sites aren't exactly intuitive or display the information in an easy to view manner, so I have created a chart that does exactly that (see below). There is still talk of the ACMA following the FCC and adding various other laws around the use of the 5GHz spectrum, but as far as I can tell these are not actually in place as of yet, so I have left these out of the chart.

Feel free to use to graphic as you please. All I ask is that the (c) be retained. I will update the chart as changes are made to the bands or the the laws are modified. I would love some feedback if you feel something needs to be corrected :)

-Brett

1 comment:

  1. Hi Brett
    Thank you for sharing your experience. I believe that your design must be in compliance with the ACMA regulations since the site is an airport and you know RADAR systems in the airport work at 5GHz too. So you must exactly do what ACMA has defined (maximum 30 dBm with TPC and 27 dBm without TPC if you want to use UNI2e). I think 30 dBm is relatively good power for 802.11 (WLAN short range)unless you want to make coverage for a big area with different obstacles. It also depends on the height of antennas you install, maybe you have to increase the number of APs to provide good coverage.
    You should do a Link Budget calculation (It is a common task in RF design) and try a coverage prediction software to see what will happen.
    Regards

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