{"id":273,"date":"2018-08-07T11:16:26","date_gmt":"2018-08-07T09:16:26","guid":{"rendered":"http:\/\/netgab.net\/web\/?p=273"},"modified":"2018-08-07T11:17:19","modified_gmt":"2018-08-07T09:17:19","slug":"understanding-ap-power-levels","status":"publish","type":"post","link":"https:\/\/netgab.net\/web\/2018\/08\/07\/understanding-ap-power-levels\/","title":{"rendered":"Understanding AP power levels"},"content":{"rendered":"

WLAN power levels are easy, right? In Europe you “know”, that the maximum WLAN transmit power is 100 mW (20 dBm). However, this is true for the 2,4 GHz band and the limit describes the EIRP (equivalent isotropically radiated power<\/span>).<\/p>\n

This means, if there is a regulatory limit of 100 mW (20 dBm), the maximum power you are allowed to configure on the AP, depends on the antenna you are using, because the antenna gain<\/em> must be included in the calculation as well:<\/p>\n

TX(AP_max)<\/sub> = RegulatoryLimit – AntennaGain<\/p>\n

For example: If an antenna with 3 dBi is used, the max. TX power on the AP side (for the 2,4 GHz band) is 17 dBm<\/strong> (TX(AP_max)<\/sub> = 20 dBm – 3dB).<\/p>\n

Easy, right?<\/strong><\/p>\n

However, it becomes more complicated for the 5 GHz band, because it depends on the band (channel) you are using and on the 802.11h (DFC and TPC) capabilities. I illustrated the regulatory limit for Europe \/ ETSI in the following figure:<\/p>\n

\"regulatory<\/a><\/p>\n

Today, the WLAN equipment is typically DFS and TPC capable, to use may use the channels 36 – 140 in Europe. The channels 120, 124 and 128 are grayed out, because Cisco left them out, because there is critical radar (aviation, weather) equipment on these channels.<\/p>\n

A little bit more complex, but still easy, right?<\/strong><\/p>\n

However (sigh), this is still not the complete picture. The regulatory limits must be met, if multiple TX antennas send at the same time (MIMO).<\/p>\n

This means in theory (caution: result is in mW – input in dB)<\/p>\n

\"TXperAntenna\"<\/a><\/p>\n

For example: The AP transmits on channel 36 (Europe) using two spatial streams without beamforming (802.11n MCS index 8 – 15):
\n\"\"<\/a><\/p>\n

Using the same example from above, the result is 33mW (~15dBm) for three TX antennas and 25mW (~14dBm) using four antennas.<\/p>\n

 <\/p>\n

So in fact, for SISO client the max TX power per stream is higher compared to a MIMO client using multiple streams and beamforming.<\/p>\n

Caution: If a client is using three streams and additionally ClientLink (beamforming) is used, four transmit antennas are used in total!<\/strong><\/p>\n

Easy, somehow … whatever \ud83d\ude42
\n<\/strong><\/p>\n

So what is the take home message until now? The TX power per antenna is different and depends on:<\/p>\n