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…And If You Buy That Survey, I’ve Got Another Survey To Sell You

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I recently got into a little tiff on Twitter.  In part it was an argument about blogging and reaching a broad audience, but mostly it was about site surveys.  Site surveys are hot right now, but I find that surveyors often overlook an important aspect of WiFi: different devices act differently.

Conventional wisdom for WiFi site surveys is to get some site survey software, upload a floorplan and start a-surveyin’.  First predictive (letting the software estimate where coverage will go), then active (temporarily mounting access points in the locations chosen in the predictive survey and testing connectivity) and finally verification (walking the site after APs have been installed).

The problem with all three types of surveys (predictive, active and verification) is that they are done with site survey software.  Site survey software is great for selling APs or pacifying execs, but it usually requires using a specific adapter.  So every time you verify connectivity or see a certain received signal strength (RSSI), you’re not testing the network for users.  You’re testing the network for your survey adapter.  Usually, that means that your site survey software is giving you deceptive information.

I did a little test to illustrate the problem of different adapters acting differently.  I went out on my back patio and brought several WiFi devices to see RSSI.

First, I used AirMagnet WiFi Analyzer to view the RSSI of the Proxim 8494 (which is the same thing as the NIC-300 if you use Ekahau Site Survey).

The screenshot above shows an RSSI (circled in red) of -66 dBm, which is solid.  The graph above the RSSI shows that it fluctuated, but always stayed above -70 dBm.
I then viewed the RSSI of three devices.
My MacBook Air (802.11n – 2 streams) showed -71 dBm in the screen shot below, for the most part stayed BELOW -70 dBm with some moments above.
My iPhone 5 (not 5s or 5c), which has supports single stream 802.11n, was recorded at -72 dBm (seen below) and went as low as -79 dBm.  The iPhone did have moments above -70 dBm, but was consistently lower than the MacBook Air’s RSSI.
That makes things tough.  Most WiFi networks are going to support a heck of a lot more iPhones and MacBook Airs than USB adapters.  If I were to rely on that Proxim 8494/Ekahau NIC-300 adapter while surveying, I would be off by a few decibels.  In some cases a three to five dB difference may not matter, but in some cases it could create what I call “Dead Zones”, where the survey shows coverage but users get unacceptable WiFi access.
(There was some good news.  My Samsung Galaxy Tab 2 (below) showed a signal strength more or less on par with the RSSI of the USB adapter.  I took a screenshot at -67 dBm using the WiFi Analyzer app from farproc.)
So, reading RSSI in site survey software is a problem.  Great.  Now what do we do about it?
One direction I’ve been taking is to focus more on using actual devices and less on site survey software.  That means more focus on the “active” survey and less on the “verification”.  The verification survey is necessary if you have a manager or executive who likes looking at colors over a floorplan, but I’ve found myself avoiding that whenever possible.  I think it’s better to spend more time testing service to temporarily mounted APs using real devices and less time selling the traditional walk-around survey.

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ben at sniffwifi dot com

Twitter: @Ben_SniffWiFi


Written by sniffwifi

July 3, 2014 at 10:48 pm

Three Simple Ways to Boost Your WiFi

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Some days you wake up and say to yourself, “how can I be more like Buzzfeed?”  Buzzfeed is popular and beloved and has an office across the street from a great Mexican restaurant.  I have a few friends and wonderful parents, but I can barely cook a taco.  

What is it that I’m missing (besides venture money, a flock of ambitious MBAs and universal scorn from the intelligentsia)?  Lists!  That’s what I’m missing.  Hit-trawling, crowd-pleasing lists!

So here it is: the first in what hopefully will be a series of one.  A list of Three Simple Ways to Boost Your WiFi.  #LOL #cute #OMG #trashy

1) Add an 802.11n/ac USB adapter to old 802.11a/b/g laptops and desktops.

Some folks in the WLAN business like to use the term “5G” to refer to 802.11ac, but I refer to 802.11a/b/g as 1st generation WiFi and 802.11n/ac as 2nd generation WiFi.  The reason I do that is because big improvements to power consumption, receive sensitivity and channel bonding are available in almost all 802.11n and 802.11ac radios.

Smartphones, tablets and single-function terminals may be unable to support external WiFi adapters, but laptops and desktops have USB ports.  A small, low-cost, dual-band 2nd gen WiFi USB adapter like the Linksys AE6000 has worked well for me.  I’ve used it to give devices 5 GHz support (when the internal WiFi is 802.11b/g/n only) and to eliminate 802.11a/b/g associations that can slow down 2.4 GHz channels.

2) Use an external directional antenna on some APs.

Maybe you have an area where you can’t drop a Cat6 cable.  Maybe you want to make it so your client/stations don’t have to roam so often.  Maybe you want to divide up a high-density area.  Whether it’s one of those reasons or something else, external directional antennas can solve certain radio frequency problems.

Access points with internal antennas are simpler to install.  They also are often easier to use when planning and surveying, because experienced WiFi folks tend to be more familiar with them.  I’m not advocating that every AP for every installation use an external directional antenna, but if the coverage you need needs to spread out in one direction, then give external antennas a look.

3) Use a protocol analyzer.

Alert the press!  “WiFi Sniffing Blogger Advocates WiFi Sniffing”

Let’s tighten up this third rule, then.

3) Use a protocol analyzer instead of a spectrum analyzer or site survey utility.

Protocol analyzers tend to be the least popular troubleshooting tool.  They are more difficult to understand than spectrum analyzers, more expensive than discovery software and look less flashy than site survey utilities.  But they are better! (at least in my opinion)

Protocol analyzers are better than discovery software because they tell you more.  inSSIDer will allow you to see APs, channels and standards, but sniffers like WildPackets OmniPeek show client/stations and frames (packets) as well.  Discovery software can’t do that.

Spectrum analyzers like Metageek WiSpy/Chanalyzer are attractive because they show you all RF activity over a range of frequencies.  They are great tools.  But I believe that they are best as a backup to a protocol analyzer rather than a primary troubleshooting tool.  Protocol analyzers like Fluke AirMagnet WiFi Analyzer offer more detail about which specific WiFi devices are generating activity on the channel and how much harm that activity might be causing.

Site survey tools like Tamograph have a wider audience than protocol analyzers and it’s understandable why.  Reading a floorplan with color coded signal areas is a lot easier than reading a bunch of arcane statistics like data rates and Retry percentages.  But it’s also less specific.  Seeing that signal is strong/weak or noise is high/low or APs are sparse/saturated is nice, but it may fail to allow you to burrow down to the essential issue that is causing a WiFi problem.  Viewing device-by-device information in a protocol analyzer may be tedious at times, but it offers more salient information.

So there you have it.  Three things to try some time on your WiFi.  They might help you turn a frustrating deployment into something that runs smoothly.

If you like my blog, you can support it by shopping through my Amazon link or donating Bitcoins to 1N8m1o9phSkFXpa9VUrMVHx4LJWfratseU

Thank you.

Written by sniffwifi

February 11, 2014 at 1:01 am