Hack 38. Graph Your Wireless Performance

Comprehensively track the performance of your wireless links over time.

The monitoring tools mentioned so far give you a good instantaneous reading of received signal and noise levels for a given radio link. While useful for proving a link and installing your gear, remember that radio conditions change over time. Doing the occasional spot check doesn't really give you the full picture of what is going on.

For example, take a look at Figure 2-42. This displays radio data for a one-mile link, averaged over several days. You can see that in the middle of each day, the signal drops by as much as 6 dB, while the noise remains steady. Remember that these are really negative numbers, so in this graph, a smaller number is better for signal. The repeating pattern we see indicates the effect of thermal fade.

Figure 2-42. Thermal fade in the middle of the day

This particular link is a simple waveguide antenna mounted in the middle of a low, sloping roof. As the roof and the rest of the environment heats up, the perceived signal is apparently less. At night, when things cool down, the perceived signal increases. The effect of thermal fade in this installation was later mitigated (by about 2 or 3 dB) by relocating the antenna, placing it closer to the edge of the roof. With less hot roof in the path, the effect of the day's heat was reduced. Nothing can completely eliminate the effects of the sun on a long distance path, but without a historical graph, it would be difficult to account for the effect at all.

Figure 2-43 shows another interesting artifact, this time averaged over several weeks. This link is about a mile-and-a-half long, and still shows some of the effects of thermal fade. But look at the noise readings. They are all over the map, reading as high as -89 and jumping to well below -100. This most likely indicates the presence of other 2.4 GHz devices somewhere in the path between the two points. Since the noise remains steady for some time and then changes rapidly to another level, it is probably a device that uses channels rather than a frequency-hopping device. Given the wide variety of perceived noise, I would guess that the most likely culprit is a high-powered 2.4 GHz phone somewhere nearby. It is probably impossible to ever know for sure, but this data might warrant changing the radio to a different channel.

Figure 2-43. A link with a noisy environment

These graphs were generated using Linux and some freely available tools. A modest Linux server can monitor a large number of radio devices, and the requirements on each of the access points (APs) is quite small. I will assume that you are using a DIY AP running Linux [Hack #63], although similar techniques can be used for just about any kind of radio device.

You can monitor signal strength using any available TCP port above 1024 on the access points. We'll use port 10000 here, but you should choose your own port based on your setup. On each of the APs to be monitored, as well as on the machine doing the monitoring, add lines like these to /etc/services:

	mon0	10000/tcp	
	mon1	10001/tcp
	mon2	10002/tcp

Then, add a line like this to the /etc/inetd.conf on the monitored machines:

	mon0	stream	tcp		nowait	nobody	/usr/local/bin/iwconfig iwconfig eth1

Be sure to use the path to iwconfig for your system, and specify the device to be monitored at the end. You can add as many lines as you like, using a different port each time:

	mon1	stream	tcp		nowait	nobody /usr/local/bin/iwconfig iwconfig wlan0

If you need more ports, add more lines to your /etc/services. When all of your radio cards are set up, restart inetd. You can verify that the changes have taken effect by using telnet:

# telnet localhost mon0

		 Trying 127.0.0.1…
		 Connected to localhost.
		 Escape character is '^]'.
		 eth1		IEEE 802.11-DS ESSID:"NoCat" Nickname:"pebble" 
				Mode:Managed Frequency:2.457GHz Access Point: 00:02:2D:1C:BC:CF 
				Bit Rate:11Mb/s Tx-Power=15 dBm Sensitivity:1/3 
				Retry limit:4 RTS thr:off Fragment thr:off 
				Power Management:off 
				Link Quality:14/92 Signal level:-83 dBm Noise level:-96 dBm 
				Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:6176330 
				Tx excessive retries:7880 Invalid misc:0 Missed beacon:0

		 Connection closed by foreign host.
		 #

To collect the radio data, you need Netcat installed on the machine that does the collection. Netcat is a free and insanely useful utility available at http://www.securityfocus.com/tools/137. Think of it as a scriptable telnet program that will handle just about any kind of network data you want to throw at it. I use Netcat to connect to each of the machines, and scrape the results with a Perl script.

With the Netcat binary (nc) installed on your system, use the wrapper script in Example 2-2 to collect the data.

Example 2-2. radio.pl, a wrapper script that collects Netcat data

#!/usr/bin/perl -w
use strict;

my $ip = shift;
my $port = shift;
($ip && $port) || die "Usage: $0 ip port
";

open(NC, "nc $ip $port |") || die "Couldn't spawn nc: $!
";

while() {
 if(/Signal level:-?(d+).*Noise level:-?(d+)/) {
	print "$1 $2
";
	exit;
 } 
}

die "Warning: couldn't find signal and noise!
";

I put this script in /usr/local/sbin/ and invoke it with the IP address and port number on the command line. It simply returns two unsigned numbers representing the current signal and noise readings:

$ radio.pl 10.15.6.1 mon0

		 83 96
		 $

In case you haven't recognized it already from the graphs, the tool that actually stores the data and draws the pretty graphs is Tobi Oetiker's excellent Round Robin Database Tool (RRDtool); http://people.ee.ethz.ch/~oetiker/). Like many powerful tools, RRDtool can be imposing at first, sometimes to the point of frustration. Efficient data collection and accurate visual presentation isn't as simple as you might think. Of course, RRDtool will handle just about any data type you track, but getting it working can be tricky the first time.

Personally, I prefer to use a tool like Cacti (http://www.cacti.net) to manage my RRDtool graphs for me. It is fairly simple to install, has an easy to use web interface, and does the dirty work of managing your RRDtool databases. With Cacti installed, simply set up radio.pl (Example 2-2) as a data input script, and create a data source for each radio to be monitored. See the Cacti documentation for more details. Tutorials on how to set up and use Cacti are available at http://www.cacti.net/documentation.php. The Cacti pages also have an excellent web-based forum where you can get questions answered by developers and active users.

When properly configured, Cacti automatically generates Daily, Weekly, Monthly, and Yearly averages for all of your data sources.

If you prefer to roll your own RRDtool, use a command such as this to create your database:

	/usr/local/rrdtool/bin/rrdtool create 
	/home/rob/radio/radio.rrd 
	--step 300 
	DS:signal:GAUGE:600:0:150 
	DS:noise:GAUGE:600:0:150 
	RRA:AVERAGE:0.5:1:600 
	RRA:AVERAGE:0.5:6:700 
	RRA:AVERAGE:0.5:24:775 
	RRA:AVERAGE:0.5:288:797 
	RRA:MAX:0.5:1:600 
	RRA:MAX:0.5:6:700 
	RRA:MAX:0.5:24:775 
	RRA:MAX:0.5:288:797

Be sure that the rrdtool binary is in your PATH, and change the print "$1 $2 "; line in radio.pl to something like the following:

	`rrdtool update /home/rob/radio/radio.rrd --template signal:noise N:$1:$2`;

Add radio.pl to a cron job that runs every five minutes or so to regularly collect the data:

	*/5 * * * *				/usr/local/sbin/radio.pl 10.15.6.1 mon0

Finally, once you have collected some data, you can generate a graph like the one shown in Figure 2-43 with this command:

	/usr/local/rrdtool/bin/rrdtool graph - 
	 --imgformat=PNG 
	 --start="-604800" 
	 --title=" Radio" 
	 --rigid 
	 --base=1000 
	 --height=120 
	 --width=500 
	 --upper-limit=105 
	 --lower-limit=60 
	 --vertical-label="dBm" 
	 DEF:a="/home/rob/radio/ radio.rrd":signal:AVERAGE 
	 DEF:b="/home/rob/radio/ radio.rrd":noise:AVERAGE 
	 AREA:a#74C366:"Signal" 
	 LINE1:b#FF0000:"Noise" 
	 > graph.png

While the other tools mentioned in this chapter will give you a good instantaneous estimate of how well your link is doing, RRDtool, Netcat, and the script in Example 2-2 will give you impressive historical data that can be invaluable for troubleshooting your network. In addition to signal and noise, RRDtool can also track uptime, network traffic, associated clients, transmission errors, and any other data you can think of. The reward of having clear historical graphs is well worth learning to deal with the complexity of RRDtool.