CWNP Conference in San Diego, CA

At this year's CWNP Wi-Fi Trek Conference we will be having many great topics covered from some of the best Wi-Fi experts in the business.
I consider it an honor to be selected among this group to speak at the conference this year.
The Topic I will cover is "Alternative Power Sources".
Many access points have the option to be powered from various inputs and these can be leveraged for renewable energy sources such as solar power.
For Example: The Cisco 1552e access point operates on UPoE and requires a different power injector than many of the other Cisco APs. But... The 1552e also has a 12VDC input to where variations of power sources can be used.
One note to be aware of:
If your access point requires .625 amps of current from a 48VDC power source to achieve a required 30Watts of power,
Then...
It will require 1.25 amps of current from a 24VDC power source to achieve the same 30Watts.
One more step to 12VDC and you now require 2.5 amps to get the 30Watts you are looking for.
Here is a link to a helpful calculator for these numbers:
https://www.rapidtables.com/calc/electric/ohms-law-calculator.html
Electric power calculation
P = V ⋅ IP is the electric power in watt (W).
V is the voltage in volts (V).
I is the current in amps (A).
These are important factors when determining the voltage input for your wireless AP. The Power (Watts) is a given factor as you should know the required watts it takes to power the AP. If not, the technical papers on your AP will provide this information.
Once you have one of the values in the formula, i.e. P or (watts), then you can determine the rest based on the inputs made available to you. 
In the case above, we need 30W from a 12VDC power source:
P = 30W, V = 12VDC, I = ?
12 x n = 30   We solve for n
30 / 12 = 2.5
so our current needed is 2.5A
From the standpoint of a 12VDC battery, we look at "Amp Hours" to see the supplied current from the battery. In other words, a 100 Amp Hour Battery would supply 2.5 amps to our AP for approximately 40 hours (factors like battery type and chemistry, battery age, weather and temperature all factor into this number being accurate). 
Note: Deep Cycle batteries are typically rates at a 20 hour discharge rate. Lead Acid batteries are rated at a 10 hour discharge rate. What this means is the ability of the battery to sustain the load changes significantly past this time rating so if you want to run your AP 24/7 deep cycle batteries are highly recommended.Now let's look at the UPoE requirements: UPoE delivers up to 60 watts so our numbers change.
P = V x I
60W = 12VDC x n . We solve for n
60 / 12 = 5
so our current needed is 5A
The above mentioned 100 Amp Hour Battery would only supply current for approximately 20 hours.
This is important to know due to the discharge of the battery in a 24 hour time period VS the amount of time you have to recharge the battery.
If you are discharging the battery every 20 hours, then there is a 4 hour period that must be supported by the charging source itself while recharging the battery. This is where the math begins to really come into play.We will be discussing the details of the charging rate at the Wi-Fi Trek Conference.
See details of the conference at www.CWNP.com

Brett Hill CWNE #147

Sticky Clients

Nearby Office Complex with bad Wi-Fi


This past week, I was called on to help in a Wi-Fi nightmare at a nearby office complex here in Tennessee.

The issue was client devices having really poor throughput even though the building had a really good internet service.

After a few minutes onsite, I found there were no interferences anywhere in the building so I fired up my survey tools and performed a validation survey.

Contrary to my first thoughts, the survey was darn near perfect in every location and overlapping cells were textbook.

I decided that I would quickly replicate the issue before going forward. The layout is a 2 story building with a lot of open cubicle space and several walled offices.

I carried my laptop, iPad, and iPhone through the front entrance as if I were coming in for work. I went up the stairs and across the floor to the opposite end of the building and sat down at a vacant desk.

upon looking at my devices, I saw I was connected on every device but my iPad and iPhone had really slow connectivity and my laptop (which I just turned on) had great connectivity.

At this point, I began tracking the connectivity back to the actual access points each device was connected to and found that the iPad and iPhone was connected to the access point in the main lobby, while the laptop was connected to the  access point 20 feet from the desk I was sitting at.

This small amount of troubleshooting took me right to the IT Coordinator's office and I asked him to see his WLC.

He logged me into it and let me cruise through the configurations and I found that all the data rates were on, no rf profiles were applied to any of the SSID's and no limits were set to limit connections to each access point.

It was an office complex with just under 60 employees and 9 access points throughout the facility but when I checked connections 28 devices were connected to the lobby access point on 2.4 GHz at -77 dB or worse while 17 more devices were connected to access point in front of the elevator on the first floor at -76 or worse on 2.4 GHz. 13 more devices were connected to the lobby access point on the 5 GHz radio at -79dB or worse.

It became very evident at this point what the issue was.

I spent about an hour looking for devices that had to use 2.4 GHz  and found there were actually none that had to use it so I turned it off for testing.

I set rf profiles making 18MBs the lowest data rate available.

I set the maximum clients per AP to 15.

Once all of these settings were changed, I carried my devices out of the building, rebooted them and went into the building again.... but this time with everything on and operating.

I stopped in the lobby to be certain everything connected and they did, so I went on to the elevator and pressed the call button. My laptop switched to the elevator AP while the Apple devices stayed connected to the lobby AP until I stepped into the elevator.

Once I was in the elevator, the Apple devices switched to the break room AP which was a few feet down the hall from the elevator on the second floor.

When the elevator doors opened, my laptop switched to the 2nd floor office AP in the opposite direction of the break room.

I walked toward the cubicles and the Apple devices switched to the 2nd floor office AP with the laptop.

As I got to the far end of the building where I had sat before my laptop was on the AP 20 feet from the desk and the Apple devices were on an AP between the 2nd floor office AP and the one my laptop was on.

None of them ever roamed to 2.4 GHz but all three devices roamed to a closer AP for better data rates as I traveled through the office.

Since all of the access points rebooted when the profiles were applied, I went around and checked with employees to see how their wireless was working now. Everyone said it was much better than before.

I told the IT Coordinator to check with them all in the morning when they came in and made the walk through the lobby to their desks.

GOOD NEWS!!! Every device roamed as it was supposed to and Wi-Fi was popular again!


Use your RF Tools, adjustments, and profiles.... They Work Great!!!


Brett Hill, CWNE #147

Alternative Power Sources for Wi-Fi Devices

One of the biggest issues facing a Wireless Engineer today is not the ability to get proper signal propagation into hard to reach areas, it is coming up with a power source to provide adequate power to the Wi-Fi equipment.

There are light pole taps and many other ways to provide power to your access points but I want to help you out here with designing your own reliable power source at a very affordable price.

Many companies offer solar solutions for powering 802.11 devices but they are usually very pricy (Up to $12,000) and most customers do not want to pay such a price to power one device.

If you understand the principle of power consumed over time, then you can calculate the power supply needed in order to sustain consistent, stable power to your device.

Lets start with time -

Amp Hours are better explained as how long a power source can provide power.

For example: If a battery is rated for 10 Amp Hours then you could power a device requiring

1 Amp for 10 hours, 2 Amps for 5 hours or 5 Amps for 2 hours.

So if your device requires 2 Amps to operate and you want it have a 48 hour power reserve, then you

need to multiple TIME by POWER USED

2 Amps x 48 Hours = 96 Amp Hours

Disclaimer: These values are always assumed calculations and climate and temperature conditions can effect the totals so you should always provide a buffer as a reserve.

If you need 96 Amp hours you should provide 120 Amp Hours to be on the safe side.

Now that we know our discharge rate (2 Amps per Hour), we know we need to provide a charging rate to keep our power source (battery bank) charged.

Discharging faster than we are charging will eventually result in a power failure.

To figure out a dependable figure to work with, we need some values and a formula to work with:

DC watts to amps calculation

The current I in amps (A) is equal to the power P in watts (W), divided by the voltage V in volts (V):

I_(A) = P_(W) / V_(V)

The current is Amps and since solar panels are measured in Watts, we need to convert based on the voltage we are using.

If you are using a 12VDC power source, we use 12 as our V.

If we are requiring 2 Amps, we use 2 as our I.

Since we are solving for P or watts, we multiply 12V x 2A and we get 24 Watts.

Now we know how many watts we are burning per hour so we can start looking for a way to provide battery recharging to prevent our batteries from a complete discharge.

The Sun is not always in an optimum position for a solar panel to produce power at 100% all the time so we MUST plan for this. 

First plan for a maximum of 50% utilization of your solar panels to be on the safe side of engineering. With the above calculations, a 50Watt solar panel would fit the bill, except...

- THE SUN INSN'T ALWAYS UP -

Your solar panels may get 6 or 7 hours per day of peak solar power but it could be as little as 3 or 4 

or cloudy or winter days. With this information, you should plan on providing enough power in a 4 hour period to recharge your battery bank.

If you discharge at a rate of 2 Amps per hour then you will discharge 48 Amps in one 24 hour day. you need to replace those 48 Amps during the daylight hours, preferably you will calculate your recharge rate based on the worst conditions, so we will use 4 hours.

In 4 hours time, we need 48 Amp Hours recharged so we need to replace 12 Amp Hours per Hour. This will quickly recharge the batteries from the power consumption of the night before.

12V x 12A = 144W

I_(A) = P_(W) / V_(V)

In 4 hours time, we need (at Minimum) a 144 Watt charging source provided to keep our system running.

Using these Factors, you now know your MUST HAVE recharge rate based on your device's discharge rate.

NOTE* use an overcharge circuit protector to keep from damaging batteries and other equipment.

I personally have used Harbor Freight's 100W solar panel kit ($145.00) and their solar charging protection circuits to do this very thing and it works great.

For less than $600.00, I have successfully built a dependable solar power source for an 802.11 

wireless access point.

Enjoy and I hope this helps!

Brett Hill, CWNE #147

Wi-Fi Humor

Just a humorous conversation to share about a dual band Wi-Fi network that consisted of outdoor mesh and indoor access points:

Me and a co-worker were troubleshooting an issue on the 2.4 GHz Wi-Fi when I just randomly asked him a question.


"What happens during the open authentication process?"

and he just stared at the table...

so I explained the frames and the information element exchanged during that process and what was  advertised in the probe response and access point beacons. 

The information element advertises the data rates available on a particular Wi-Fi network. Data rates are 

controlled by a type of modulation that provides the data rates. If there are no data

rates advertised, then there are no modulation types offered that are

compatible with your device in order for your device to associate to. 

If you can’t associate, then you probe another available option immediately.
(Such as the 5GHz network).

Then he just randomly says, “Well, once the phone associates to the 5GHz radio, it might try

to roam to the 2.4GHz radio during a phone call and drop the call”.

So…. I reply, “Your controller based network is equipped with Fast BSS

Transition, CKM, Neighbor Messages, and neighbors sharing their neighbor

messages. Your phones will know all of the great places to roam to and the

places they cannot roam to, way before they ever roam”.


His reply….. “I don’t think that’s right”.😖


Moral of the story- If you are going to troubleshoot any technical project, please know the technology you are 

working on.


Brett Hill - CWNE #147