Half Mile Point-to-Point WiFi Link

2009/01/02 - Jason Harper <ptpwificomments@truedesign.org>

The challenge: get a reasonably fast network link between two single-story homes. Both neighborhoods have HOAs and there are several two-story houses and trees in the path. There is definitely not a 100% clear line-of-sight, and probably not a 60% clear Fresnel zone. At least the terrain is flat. Google Earth calculates the path to be 0.52 miles, and provides handy maps to assist in aiming.

Map from above:

View from site A to site B:
      View from site B to site A:

One option was to use discrete antennas with short coax runs down to some wifi radios. However, first I wanted to try a simpler solution. There are many all-in-one devices available that incorporate radios and panel antennas into one weatherproof package. Some are quite pricey, while a few are fairly inexpensive. I figured I'd give one of the cheapest devices a shot before spending money on something more expensive. I found a generic device with a part number of CPE-2415-26 apparently made by www.aristar.com.tw. I had a hard time finding someone willing to ship small quantities to an end user, but finally stumbled across Geeks.com selling it on sale for $69.99 each. So, about $150 later, i had two of these:

Inside the plastic enclosure:

Panel antenna:

Radio itself. Looks like a PCB intended for a consumer broadband router (complete with port activity LEDs in place), modified slightly to accept power over one of the ethernet cables:

The radio board looks like it is probably a pretty standard Realtek reference implementation based around a RTL8186 SoC. It runs Linux, but I haven't been able to find an easy way to get direct access to the OS. I'd love to be able to ssh or telnet in. The web interface does allow you to view the system logs.

View example log of the system booting

The units are very light, so I just mounted them on PVC pipe. Here is site A:

And Site B:

Here is the view from the top of the unit at site B looking toward site A:

Conclusion: With the site A radio mounted, and the site B radio sitting on the pole about 6 feet off the ground, I was able to get a pretty solid 802.11b link, with usable bandwidth around 600-800kbps but a little bursty. Now that it is mounted up above the roofline, I am able to get an 802.11g link that hovers around 24-36mbps tx link speed with usable effective bandwidth in the 8-10mbps (megabits/second) range. It still has occasional minor pauses here and there in the data flow, but they generally only last a second or so. Packet loss (as measured by a ping flood) is near zero. Here's a ping -f that I left running for 15 minutes with no other traffic on the link:

PING ptp2.lan.truedesign.com (10.10.7.3) 56(84) bytes of data.
.................................................................................................................................................................................................................
--- ptp2.lan.truedesign.com ping statistics ---
342274 packets transmitted, 342065 received, 0% packet loss, time 901167ms
rtt min/avg/max/mdev = 2.044/2.814/717.455/9.701 ms, pipe 47, ipg/ewma 2.632/2.515 ms

Both ends are running in WDS-only mode with WPA2/AES encryption. I've experimented a little bit with various radio settings and so far seem to get the most throughput when using G-only mode, auto data rate, and setting the transmit level to the middle setting (20dbm). Since the effective throughput isn't quite as good with the power cranked up to max (24dbm), I'm guessing that these little radios might not be the best design? Or maybe I just don't know enough about RF stuff. In any case, I'm happy with the link speed I am getting!

I'm a little bit worried about how these will handle the 110+ degree (F) heat here in the summer. The main cpu chip doesn't have a heatsink on it, and the cases do get a bit warm at room temperature. I can't think of any good way to add better (any?) cooling to the units without risking water getting in. I'm also a little worried about moisture getting into the units and keeping the humidity high. I think I'll buy one or two more of these things just to have on hand as spares..

--Jason