antenna gain

During my work in TAC I have seen a lot of configs with improper usage of EIRP for APs with external antennas.

Basically for the APS with bult-in antennas, the system knows about the antenna gain and calculates properly the EIRP, not exceeding the regulations of the region.

When we use external antennas for indoor or outdoor environment, the system doesn’t know which antenna is installed. If that parameter is omitted, than the AP transmits with the power excluding the true value of the antenna gain. As a final result the EIRP sometimes exceeds the regulations.

WLC#show wireless regulatory device-type ap7522 pl
---------------------------------------------------------------------------------------------------
  #   Channel Set Power(mW) Power (dBm)    Placement          DFS       CAC(mins)        TPC     ----------------------------------------------------------------------------------------------------
 1    1-13        100       20          Indoor/Outdoor   NA             NA        NA               
 2    36-48       200       23          Indoor           Not Required           Not Required     
 3    52-64       200       23          Indoor           Required               Required         
 4    52-64       100       20          Indoor           Required               Not Required     
 5    100-116     1000      30          Indoor/Outdoor   Required               Required         
 6    100-116     500       27          Indoor/Outdoor   Required               Not Required     
 7    120-128     1000      30          Indoor/Outdoor   Required       10        Required         
 8    120-128     500       27          Indoor/Outdoor   Required       10        Not Required     
 9    132-140     1000      30          Indoor/Outdoor   Required               Required         
 10   132-140     500       27          Indoor/Outdoor   Required               Not Required     
----------------------------------------------------------------------------------------------------

Example:
The AP on channel 132 with power 30 transmitting with no knowledge of antenna gain. Antenna in use is 14dBi panel antenna. The EIRP will be around 44dBm (30+14-loss on cable) which far exceeds the ETSI regulations.
In 802.11n/ac’ APs there will be also MIMO gain based on the number of chains, therefore the initial power will be lower.

We will now focus how the issue might be resolved for each vendor.

Cisco (firmware version 8.0.152.0):

Wireless -> 802.11a/n/ac or 802.11b/g/n -> AP -> configure

At the bottom there is a configuration for the antenna gain. Strange thing is Cisco uses 0.5 dBi as an unit, therefore to use 3.5dBi value, 7 x 0.5 dBi shall be used.

As antenna gain is different for each band, 2 different entries should be configured, one for 802.11b/g/n radio and second one for 802.11a/n/ac radio.

1.PNG

 

Aruba (firmware version 8.2.1.0):

In Aruba’s world, an AP needs to be first provisioned with the basic networking settings and AP group.

An AP with an external antenna will not be provisioned unless the antenna gain values are entered.

As an example, taken from the user guide for version 8.3.x:

2.PNG

 

Extreme Wireless (firmware version 10.41.06.0013):

The configuration is in the AP settings:

3

An Extreme AP with external antennas’ SKU won’t start its radios and won’t broadcast any SSID if there are no antennas selected in the settings above.

What is interesting, Extreme provides a list of all available Extreme antennas for that specific SKU.

4

 

Extreme Wireless WING (firmware version 5.9.1.0):

The AP(s) might be configured as device override (single AP) or profile (the same AP SKU). The configuration needs to be done separately per each radio.

5

6

7

 

Summary:

Aruba and Extreme Wireless made systems, in which there is no possibility to have an AP with external antennas broadcasting SSIDs  without configured antenna gain.

This aspect of the configuration of the wireless system shouldn’t be skipped as it might cause problems related to exceeding regulations and unintentionally broadcasting SSIDs in a greater distance than expected.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s