Throughput can be improved over long distance links by changing the ACK/CTS timeouts and Slottime, see ACK Timeouts and the effects on distance links for more details.
RouterOS
The ACK Timeout option is found in the Winbox GUI under the “Advanced” tab for the interface settings – make sure “Advanced Mode” is clicked on. RouterOS has a feature which dynamically calculates the ACK Timeout, this is the default option and seems to work well for most installations. For manual setting of the ACK Timeout just type in the number in microseconds and click Apply.
Atheros cards using athctrl
athctrl is a small utility for Linux that changes the ACK Timeout, CTS Timeout and Slottime based on the maximum distance you specify, in meters. It’s usage is:
athctrl [-h] [-i device] [-d distance]
The maximum acktimeout /ctstimeout possible on atheros based cards is currently 409 µs, working out to a maximum distance of about 53-57 km.
Examples
athctrl -d 1000
Set maximum distance of 1000m for wifi0.
athctrl -i wifi1 -d 5000
Set maximum distance of 5000m for wifi1.
OpenWrt + Madwifi
The physical layer parameters are stored in /proc/sys/dev/wifiX
To view a parameter, such as acktimeout on wifi0
OpenWrt:~# sysctl dev.wifi0.acktimeout
dev.wifi0.acktimeout = 22
Example
I want to set up my 802.11a card, wifi0, for an 8 km link. The default slottime is 9 and the default acktimeout is 22 as discovered from the above command.
I must add 1 to slottime and 2 to acktimeout for every 300 meters over the initial 300. This means I need to add an extra (8000 – 300) / 300 = 25.7 to slottime. I will round up to 26 to be safe. So my slottime will be 9 + 26 = 35, and my acktimeout and ctstimeout will both be 22 + 26 * 2 = 74.
This can be manually done as follows
OpenWrt:~# sysctl -w dev.wifi0.acktimeout=74
dev.wifi0.acktimeout = 74
OpenWrt:~# sysctl -w dev.wifi0.ctstimeout=74
dev.wifi0.ctstimeout = 74
OpenWrt:~# sysctl -w dev.wifi0.slottime=35
dev.wifi0.slottime = 35
To make this permanent on bootup, one would add the following lines to /etc/sysctl.conf
dev.wifi0.ctstimeout=74
dev.wifi0.acktimeout=74
dev.wifi0.slottime=35
BSD
sysctl can be used in BSD based operating systems to access and modify the parameters, the format is similar.
Example
For example on interface wl1:
~# sysctl -w dev.wl.1.acktimeout=74
dev.wl.1.acktimeout: 22 -> 74
~# sysctl -w dev.wl.1.ctstimeout=74
dev.wl.1.ctstimeout: 22 -> 74
~# sysctl -w dev.wl.1.slottime=35
dev.wl.1.slottime: 9 -> 35
To make it permanent on bootup, one would add the following line to /etc/sysctl.conf
dev.wl.1.acktimeout=74
dev.wl.1.ctstimeout=74
dev.wl.1.slottime=35
OpenWrt Whiterussian + Broadcom
The dctrl utility by nbd or an nvram setting in Whiterussian can be used to specify the maximum distance between stations. dctrl usage is:
dctrl [max distance in meters]
In Whiterussian RC5 or higher, the nvram setting [code]wl0_distance[/code] can control the maximum distance, specified in meters. A small utility called sdist, attached to this page, can be used to verify that the correct registers have been changed. Remember to run /sbin/wifi to reload wifi settings after changing nvram.
Example
Checking initial values using sdist, then changing wl0_distance to 20kms, reloading wifi settings. Checking that the registers in the wifi card actually changed, then committing the nvram so the wl0_distance setting persists on reboot.
OpenWrt:~# ./sdist
shm: 0x9
reg 684: 0x207
OpenWrt:~# nvram set wl0_distance=20000
OpenWrt:~# /sbin/wifi
OpenWrt:~# ./sdist
shm: 0x8f
reg 684: 0x28d
OpenWrt:~# nvram commit
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HostAP + Prism ?
Note about Atheros: If using 802.11g it would be a good idea to lock the station into 11g, to prevent slottime resetting if a 11b client tries to connect. This is accessed in Linux/Madwifi through iwpriv <interface> mode 11g and in BSD through ifconfig <interface> mode 11g