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Chapter 4: You Should Never Hang APs With Your Spouse

The low-budget Muni-wireless system is ready to be deployed.

We have identified the perfect one square mile area to be covered.  The streetlights are 25’ tall and exactly 660 feet apart with full time electric power.  There are no trees and the houses are 20’ tall and made of wood.  Nobody in the area owns a microwave oven or any indoor WiFi routers.   This will be known as TriadLand.  It’s my make believe city. I get to name it. The goal for the first square mile is complete indoor coverage to a laptop.  Let’s find out if that is realistic.

Astute readers will notice that the firmware included with this radio doesn’t support mesh.  However, it does support WDS.  Basically, each AP will have to know the Mac address of the AP before and after it.  This had to be entered manually into each AP.  WDS then creates a layer 2 connection between each AP.   By using WDS instead of mesh, we lose 2 things.  The first is that if a radio fails, that path fails.  There is no recovery option other than to drive out and replace the AP.   However, it’s also possible to connect to the next AP wirelessly and manually bypass it.  The second is that the system does not provide any type of load balancing.  Not every mesh system does so that anyway.  We also have an upgrade path if there are clusters of high-bandwidth users that we will cover later.

The Ingress/Egress point has to be located so as to minimize the number of hops.  We will have a useable 50Mbps TCP/IP drop to about 10Mbps at 4 hops (I know that doesn’t calculate right but it’s processor limited, not bandwidth limited).   I will cover an upgrade later that delivers an additional hop and expands the first hop to 100Mbps or more. Keep in mind UDP is much higher.

Mounting this AP/antenna combination requires some ingenuity in terms of bracketing and power.  I am working in a simplified mount for it now.  The antenna is provided with a simple U-Bolt for up to a 2.0” vertical pole.  The radio simply screws into the bottom of it.  Power requires 12-24dc volts.  Interestingly enough, the power issue is nice for a solar application.  However, it requires a PoE that although less than $10, is not designed for outdoor mounting and is 110V.  If the radios will be mounted on horizontal poles, the system will require additional bracketing for this installation.  The power problem will require a small Nema enclosure to seal the AC/DC converter along with a Metropole power adapter or an electrician can mount a power outlet inside the pole.

A very simple formula for calculating the link path budget is (Power Output of the AP) + (Antenna Gain in dBi) + (Receiver Sensitivity at the speed we need).  The signal loss equation tells us what we are going to lose in signal level over distance or Signal Loss = (20 x Log10 (Frequency in MHz)) +( 20xLog10 (Distance in miles)) +36.6.  Since all of the connections are very short range, LOS or NLOS, and well below 10GHz, we can sort of ignore a lot of other variables that are used for longer range and higher frequency calculations.  So how far can we get 2 AP’s to talk to each other and keep a maximum modulation rate with a power level that matches the laptops?

Since the EIRP of the laptops is about 15dBm, we will start there.  The link path budget between APs is -

802.11N:   15dBm (radio output) +28dBi (antenna gain is a summary of both sides) + 74dBm (speed at MCS7, 65Mbps, for the Bullet M2 HP) = 117dB

The link path budget for legacy laptops is -

802.11b/g:  15dBm (radio output) +15dBi (antenna gain is a summary of both sides, assumes 1dBi on the laptop) + 74dBm (speed at 54Mbps for the Bullet M2 HP) = 104dB

The second equation needed is the free space loss.  I’m not going to go into the formulas here, but the result calculated at 660 feet was -83dB for the laptops. That means that we have an expected signal level of -56dBm.  From AP to AP, we should see a signal level of -49dBm.  So in TriadLand, we could install APs at 1320 feet with signal to spare at the full 802.11N or 802.11g bandwidth while delivering the same signal level on both sides.

The reality with an urban deployment is that the farther down the street the AP is located, the more houses the signal has to pass through.  We will assume that the AP is mounted on a street light.    Houses built of brick, stucco, or aluminum siding along with trees, bushes, and Uncle Bob’s motor home in the driveway, all add attenuation to the signal quality.  Therefore, for the sake of this design, we will assume that each house adds about 10dBm of attenuation to the signal.  The Bullet 2M, at the lowest connection speed in 802.11N mode, has a minimum sensitivity level of -96dBm.  A good rule of thumb for noise in 2.4GHz is about -85dBm.  That means we have -29dBm of attenuation room to make a connection or about 3 houses.  However, since we want a 10dB difference between signal and noise, we now are down to penetrating 2 houses at maximum speed and dropping off from there.

Attenuation is the biggest problem in an urban setting which means that LOS is more important than distance.  To guarantee connectivity in the worst locations with trees and brick houses, it will take APs about every 1/8th of a mile which means no client is further than 330 feet.  Cities such as Mountain View, St. Cloud, Scottsdale, and others learned this the hard way after setting up the system with fewer APs per square mile than really needed.  In addition, those APs used 7-9dBi.  We are adding up to 6-8 more dB gain on the antenna side which should add about another house of penetration.

Being more realistic, what are the options?  Let’s say that we assume 16 APs per square mile and all clients are 802.11N.  Also assume 14 houses per block and 2 blocks for 1/4 mile.    The light poles will be about 30’ from the front of the house.  The farthest house is 660’ away and will not have LOS except for the first couple of feet into the front of the house.  In the worst case, users may have to penetrate 8 or more houses at various angles, plus trees or vehicles may be in the way.  If the houses are brick, stucco, or have aluminum siding, the signal will definitely not go through it.  In this environment, we are installing 25-49 APs. The total cost for a 16 AP install is about $10k per square mile.  As mentioned before, $1000 upgrades the bandwidth to 60Mbps for the square mile and 10Mbps at 4 hops.  If you need more bandwidth than that, the cost jumps to $14K per square mile per 16 APs and carries a 100Mbps pipe through every AP.

Based on this analysis, you now have to make the decision as to the purpose of the network.  If it’s ubiquitous coverage, then you have no options.  It’s going to be a minimum of 25 APs up to 49 APs or even more in the worst environments. Even though the cost of each radio and antenna is $200, the cost of installation will drive this to about $14K-$42K per square mile with that many APs.  That sort of defeats the idea of a budget system.  If you were planning on this type of budget, then there are additional changes that should be done for many other reasons that we will cover in future articles.

If we just wanted outdoor coverage only to laptops on the main streets, 16 APs are probably more than sufficient.  If the goal is simply street level coverage for cameras on light poles and vehicles, 2-4 APs may cover that.  By eliminating the 100% coverage for indoor 802.11g laptops with their limited power and antennas, the system has more options and range goes up significantly.  There are so many variables in a metropolitan deployment due to the physics of microwave frequencies, that $3000 per square mile in Arizona might become $18,000 per square mile in California.

Now that we know the realities of RF, let’s stay with the original concept and change the expectations of the system to meet our budget.

  1. 100% street level coverage for cameras and public safety vehicles

  2. 100% street level coverage for laptops and portable WiFi devices (this does not mean back yard and between houses)

  3. 20-40% of the residential locations will have 100% coverage

  4. 20-40% of the residential locations will have varying coverage within the house

  5. 20-40% will require indoor equipment or a professional outdoor installation of a client device.

  6. The system will support legacy 802.11g equipment

  7. Minimum residential bandwidth from 1Mbps to 5Mbps

  8. 20 Mbps per square mile total available bandwidth (upgradeable to 60Mbps per square mile for an additional $1000)

  9. Expandable up to 300Mbps per square mile (for an additional $4000 per 16 APs)

Our second problem is user density.  Given that most APs are limited to 20-30 users per AP and each AP covers up to 80 homes, this isn’t an issue.  However, in some cities with apartments, density could reach into the 10,000 people per square mile range.  That could mean hundreds of people per AP.  If you find a place like that, don’t worry, we have plan C.

We now have to go back to the noise issue we discussed earlier.  Typically you want the signal to be at least 10dB better than your noise.  In Triadland, our noise figure is about -96dBm.  In the real world, it’s going to range from -65dBm (bad) to -85dBm (reasonably good).  Since our signal level at 1320 feet between APs with a 15dBm output is going to be -50dBm, we should be able to handle even higher noise areas.  Laptops should be connecting LOS at about -62dBm at 660 feet.

In some areas of the country, and this is the part I like, the scaled system could deliver 20Mbps – 50Mbps to a residential or business location.  Based on some numbers I received a few days ago, Internet bandwidth can be purchased for as little as $1 per 1Mbps per month at local data centers.  If a residential 10Mbps rate is sold for $30 and the over-subscription rate is 10-1, the potential revenue on a 1Gbps circuit is $30,000 month assuming you have a way to transport it to your WiFi area.  There are obviously a lot of capital costs here.  This will also take serious engineering and planning involved for this type of revenue stream, but it’s now possible.  4 years ago, nobody was making a profit at this.    IMHO, this now means that wireless can directly compete with cable and DSL services.

This design also solves another problem that WISPs are running into.  If you read the last article, it’s evident that WISPs using unlicensed frequencies are running out of bandwidth.  This design shortens up the distance from the client radio to the AP.  Instead of connecting at a few miles and increasing the risk of interference, all clients are within a few hundred feet of an AP; thus, minimizing the interference issue.  It also lowers the required altitude path.

Our basic network is defined. The budget is figured out. Our users in the first square mile of TriadLand have connectivity to the network.  Now, how do we get them connected to the Internet and keep track of them?  We will cover that in the next article.

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