Managing a Linux Wireless Access Point (Page 1 of 4 )
4.15 Turning Off Antenna Diversity
Problem
Your wireless interface supports using two antennas, but you’re using just one. You know that this means half of your broadcast and unicast packets are hitting a dead end, which can hurt performance. How do you send power only to one antenna?
Solution
Set Pyramid’s filesystem to read/write, then add the following lines to /etc/sysctl.conf:
dev.wifi0.diversity =0
dev.wifi0.rxantenna = 1
dev.wifi0.txantenna = 1
Then, load the new configuration:
pyramid:~# /sbin/sysctl -p
If the antenna is connected to the second port, just change 1 to 2 and reload sysctl.
Discussion
The Linux kernel sees the wireless interface as wifi0, which you can see by running dmesg | grep wifi. The MadWiFi driver creates a virtual interface named ath0.
Using two antennas might improve the quality of your wireless service, or it might not. Only one is used at a time, the one with the stronger signal.
Polarization diversity is when one antenna receives a stronger signal because it is lined up differently than the other one. Spatial diversity refers to distance between two antennas. A few inches might make a difference because of reflections, fading, physical barriers, and interference.
The radio hardware evaluates the signal strength at the beginning of the transmission and compares both antennas. Then, it selects the stronger antenna to receive the rest of the transmission. The only user-configurable options are to turn diversity on or off.
Multiple-input/multiple-output (MIMO) technology promises higher data rates and better performance by using both antennas at the same time. Different vendors mean different things when they say MIMO.
Some are referring to multiple data streams, while others use it to mean plain old channel bonding. The goal is the same: more bandwidth and reliability for delivering video, VoIP, and other high-demand applications.
There is considerable controversy and endless arguments over antenna placement, what kind of antennas to use, and how many. Pointless arguments can be fun; when that gets dull, whip out your 802.11 network analyzer and collect some useful data to help you figure it out.
See Also
Chapter 16, “802.11 Hardware,” in 802.11 Wireless Networks: The Definitive Guide, Second Edition, by Matthew Gast (O’Reilly)
Chapter 24, “802.11 Network Analysis,” in 802.11 Wireless Networks: The Definitive Guide, Second Edition
