This is the third installment of my “Tales from the Towers” series of articles about deploying large scale Wi-Fi networks in unlicensed bands. The theme: share and share alike. Although we were going to discuss putting access points (APs) on the poles in this week’s article, an incident occurred last week that I think is worth discussing before we go any further. Always keep in mind that unlicensed bandwidth is a shared commodity. It’s also a good idea to have a good relationship with your competitors or anyone in the area that is using unlicensed bandwidth. Sometimes that’s a little hard to do, but it’s definitely in your best interest to try.
The Glendale, AZ project
Traffic lights and cameras are a perfect application for 4.9GHz or 5.8GHz radios where fiber isn’t installed. For example, we connected several traffic lights in Glendale, AZ a month before the city hosted the 2007 SuperBowl. The entire network consisted of 5.8GHz SkyPilot equipment with Pelco cameras. The system is used to monitor the intersections and also as the SCADA system for the traffic lights. This allows the city to manually adjust the traffic lights before and after events to optimize traffic flow. The city just recently closed on a bid that ADOT managed for the next phase. For some reason that I still can’t explain, they removed the compatibility clause with the existing system and simply went for low bid. Due to the cost of the SkyPilot equipment, it was obvious that low bid would not be SkyPilot or a compatible system but something else. This means the city will now be supporting multiple mesh networks along with the extra long term costs. He who controls the funding, writes the rules but apparently doesn’t have to support it.
That led to another project to connect 13 traffic lights with 50 high-definition cameras at resolutions up to 1600×1200 pixels. We settled on Axis cameras for this project. The ideal cameras, fixed and PTZ, would be 1080p, 24 frames per second, and H.264. The project came close with most of these specs, but there were some compromises. We either got PTZ and H.264 at 720p, or we got 1600×1200 at reduced frame rates with MPEG-4. Future intersections may get 1080p cameras. This was sufficient to meeting the system requirements for the application which was traffic light management, license plate recognition, video analytics, and future facial recognition software.
Either way, we needed a lot of bandwidth at the City Hall to make this work. There were no other antennas on City Hall. A quick site survey showed additional equipment in the area with fairly low signal levels. The AP chosen was Ubiquiti 802.11N Rockets with 90 degree, 20dBi dual-polarity sector antennas. The system has 4 APs and sector antennas on City Hall for 360 degree coverage. Each light depending on distance to City Hall gets either a Nanostation 5M (integrated antenna) or Rocket 5M (external flat panel sector antenna). We are also adding 2.4GHz 802.11N to all the intersections for future use. It only adds $200 to the cost of each intersection and has an 800 feet range to a laptop.
The first AP was turned on about 6 months ago and covered one 3-way intersection with 3 cameras and a second client station for data backhaul to one of the city parks. The park had maintenance staff and a camera for the skate area. The second AP was scheduled to be turned on this week to cover the next 90 degrees off City Hall. That was when we discovered that between the time we turned on the first base AP and then went back to turn on the second base AP a couple of months later, the IT department had a local WISP put a Motorola Canopy radio on the roof without notifying the traffic department. Even more fun, it was in the 5.8GHz band.
Our traffic system was designed to use all 100MHz of the 5.8GHz band and was designed with signal levels for each link between -50 and -65dBm. Since the general rule is at least 10dB headroom on the link path and the minimum signal for APs with an MCS15 link (130-144Mbps modulation rate), 2×2 MIMO is -75dBm, you try to design for your worst signal at the receiver to be -65dBm. At those signal levels with directional antennas on the client side at 2 miles or less, it’s fairly hard to interfere with this system.
As a courtesy to the vendor, I thought that giving them a call and asking if they could change the Canopy to 5.1 to 5.4GHz before I turned on any more APs would be the professional thing to do. Unfortunately, when you run into inexperienced WISP technical support staff who think that whatever product they use is significantly better than whatever product you use, there is a problem. The conversation started with me asking if there was any chance that they had other frequency options. The technician asked me what equipment I was using. I replied, then he stated, “Our equipment will stomp all over your equipment.” I quickly established that this wasn’t the person I needed to talk to in order to resolve the problem.
Every wireless product handles interference in different ways. Better filtering, multiple polarity, diversity, beam-forming, better firmware and many other techniques are utilized to improve the quality of a connection. Obviously some equipment works better than others in high-interference environments. However, 802.11n doesn’t always play very well with 802.11 b/g or 802.11a radios. In addition, the physics on our planet has not changed. Throw in a 2×2 MIMO signal with very a high gain antenna and a total EIRP of 42dBi minimum on both polarities, and there is going to be interference. This is especially true if the competitor’s base station is 1000 feet away and their system is based on a 30dBm EIRP signal level from the APs. Since the traffic system was designed with a very high level signal path with multi-polarity 2×2 MIMO, there wasn’t much concern that the local WISP was going to interfere with the traffic system. On the other hand, it was clear that the traffic system was going to cause some interference to the WISP operation. I was wondering if that was already the case.
The last thing I wanted to do was start a war between a city government and a local WISP. Since the WISP was already serving some of the registered voters, this was probably not a good idea. The ego in me wanted to turn the remaining two sector antennas directly at the WISP base station across the street, program the two radios with a channel width of 40MHz to cover most of the 5.8GHz band, turn on 802.11n mode only, start multicasting “Transformers 2” in UDP mode to my other laptop and phone, and crank the power to 48dBm EIRP. I felt there might be a lesson to teach on RF, signal-to-noise ratio, and how to be polite when other companies are trying to do the right thing. Instead, I called back and talked to upper level management to arrange a meeting to see what can be done. That’s when I found out as I had suspected, that the traffic system’s first AP possibly might already been causing some interference issues. Unfortunately that traffic system AP also serves the city park and has been running for several months. As a courtesy, the best I could do was to turn the power down until we plan out a more cooperative strategy with the WISP.
Now we can come back to our design. The AP combo we are considering will have an EIRP of up to 36dBm and will be 802.11 b/g/n compatible in 2.4GHz. That is far more powerful than most indoor systems. There are ways to limit the interference and everyone sharing those bands should consider this. It’s good policy to always engineer for the least amount of power that you need. Also, you should consider using the least amount of bandwidth. If you don’t need a 40MHz channel, don’t use it. You gain 3dB on the receiver side with no more power if you use a 20MHz channel. Drop to 10MHz and you pick up 3dB more, etc… The tighter the channel, the better chance you have of getting a signal through.
There are many of you that may not even want 802.11b/g compatibility. If the system is only used where you control both the APs and the client radios, then you have some options. In that case, using 802.11n only is the best way to go. This results in fewer APs and better performance. It’s also possible in this same scenario to get as many as 100 users on an AP or as many as 300-400 on a single pole. If all the equipment is the same manufacturer, consider a 5-10MHz channel which will still deliver 12-25Mbps per AP. Running 2×2 MIMO doubles the throughput to 25-50Mbps in a 5MHz channel. This is probably more than most of us need for a link. With a 5MHz channel, there are 6 channels to work with in 2.4GHz instead of just 3.
Cooperation necessary in the unlicensed bands
Whatever gets installed will probably interfere with other existing systems. We have to realize that nearby systems might be critical for hospitals, voice systems, video surveillance, office networks, etc. For example, a few years ago, we saw an outdoor AP in Scottsdale stadium take down a wireless voice system at a nearby hospital. Since we put our phone number in the SSID, they could contact us quickly. We adjusted our antennas and resolved the issue. Site surveys have to be done everywhere to understand the ramifications of the install in addition to understanding your own coverage zones.
Based on all this, if I saw a little Motorola Canopy radio on the roof and thought it was a good idea to possibly contact that vendor, what was the installer thinking when he saw all the Ubiquiti equipment along with a Motorola PTP600 radio? He obviously didn’t do any kind of site survey. The reality is that this is standard procedure for most companies.
Many companies that rent roof space for APs do it from some type of property management company. There is usually no bandwidth management on the roof. When someone finally overlaps frequencies, an avalanche of problems will start. For example, administrators may turn up power to overcome the interference if they have no other options which causes even more problems (this gets into noise and channel filtering). The responsible thing to do would be to notify all parties involved and see if there is a way to work together. Obviously this sentiment is not shared by everyone, or some field installers aren’t trained well enough to watch for this situation. Then again, they just may not have cared.
Unless I run into another tech support person who hasn’t been taught the concept of cooperation and I need to vent, we will get back on our project in next article. I’m still working on some FCC equipment certification issues with the design. Not that we can’t do it but I want maximum performance and the equipment I need isn’t quite yet available. I’m also waiting for new products to test that may enhance the system. Anybody can build a system with unlimited budget. The fun is creating a carrier class dependable system on a shoestring budget since, that’s what most cities have in their coffers these days. Any new products that have the potential to help preserve that coffer is going to get their attention. Next article, we get back to work.