WLW 019 – WLANs = Counter Intuitive

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WLANs = Counter Intuitive

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This week we’ll be talking about some of the counter intuitive issues concerning Wireless LANs

Many things in life are simple, easy, and just make sense. Take simple arithmetic. Two plus Two will equal Four. Easy to understand, a simple calculation. In our world of computer networks, there are other simple ideas. For example, look at the ‘Link Light’ on an Ethernet NIC. If the link light is off, we know where to look to solve the problem. (Physical Layer) But if the Link Light is on, we know to look up the stack for the issue causing a networking problem.

Our minds like to think about simple things. Things that can be easily understood. But all things in our lives aren’t always as easy or as understandable as we’d like.

On the surface, we think one thing, then as we come across empirical evidence, we soon learn our initial, might I say “intuative” reactions were wrong. Check out the following graphic of ‘additive’ colors. Without actually seeing the results, your first reaction might not be that White is the result of mixing all those other colors together.

Other times, in other situations, our perceptions can tend to distort reality. We have to look hard, and study well to make sure we really understand what is going on. The 802.11 protocols can be quite complex, and vendors have been given lots of latitude in their implementations, so a strong background in the fundamentals will help see through the vendor-hype. In the following graphic, the line drawing on the left suggests a box that is coming toward the viewer, or perhaps the box has it’s opening on the top. What do you see? (the box on the right is there just to mess with your mind)

This blog in going to focus on some of the things that are Counterintuitive with respect to Wireless LANs. And there are many aspects Wireless LAN Professionals need to be prepared for. The initial first intuitive reactions will lead you to failure. So pay attention, and lets talk through some of these counter intuitive issues.

Thanks to all those who helped contribute some of these issues via e-mail, twitter, and conversations. This is by no means an exhaustive list, just something to get you started.

By the way, it’s not just Wireless LANs that have counter intuitive issues…

On to our lists of counter intuitive issues with Wireless LANs -

  • Add more overhead, decrease throughput
  • More RSSI the better
  • I can see 16 APs from here, I’ve got great signal
  • VLANs on Wireless LANs separate collision domains
  • The Noise function in Wireless NICs will show us ambient RF issues
  • We need a different SSID for each purpose in our network, our system supports up to 16
  • If  you point your antenna right at the signal source it will work better
  • We designed our Wireless LAN for Voice, Video, Data, Bar-Code Scanners, and Location Tracking
  • 802.11n actually typically decreases collisions and retries to get higher throughput
  • We just installed 802.11n, we’ll get 300Mbs throughput
  • Multipath is good
  • Multipath is bad
  • We use Windows Zero Config, it gives us everything we need
  • The latest wireless drivers is all you need to fix the problem
  • The wireless network is responsible for clients deciding to roam
  • PoE is just PoE – they are all the same
  • They wouldn’t have let us configure AP to Channel 2 if it wasn’t alright
  • APs are just wireless switches
  • We’ve got great (-65dBm) RSSI everywhere, Voice over IP will run fine on our WLAN
  • Those little bar measurements actually reflect reality
  • Bigger is better with respect to antennas
  • I have four bars, I have good signal, right?
  • Its better to have our APs using all channels than ‘sharing’ only 1, 6, and 11
  • The SSID is unsecure so I can use it right?
  • AP Power settings from 1 through 10, from LOW to HI, right?
  • -90dBm is stronger than -40dBm —  90 is bigger than 40!
  • Mb or MB what’s the difference… They mean the same thing
  • APs route packets on the network
  • I turned on QoS so our voice will work on our WLAN
  • All our APs are on one channel
  • All our APs are on 1, 6, and 11 only
  • We like to use channels 1, 4, 8, and 11 to get more throughput
  • We’re in Europe, so we use 1, 7, and 13 to stay away from all the 1, 6, and 11 people
  • By using Wireless Range Extenders we’ll share our throughput with more people
  • But we have to buy all our equipment from the same vendor
  • We can force our neighbors to go to different channels and turn down their power
  • More power, more throughput
  • We had a problem in this one location, so we added APs to fix it
  • Getting RF coverage is hard to do

If you’ve said any of the above statements, or still believe any of them, I recommend going back to your studies. Setup small lab configurations and try it. I’m a firm believer in empirical evidence. Try these yourself until you can prove to your self the validity or non-validity of the statements.

I’m sure many of  you reading this, or listening to the podcast can think of other such silly statements you’ve heard, or even said yourself about Wireless LANs – please share them in the comments section below.

Thanks for listening.


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24 thoughts on “WLW 019 – WLANs = Counter Intuitive

    • You're not alone. Many people have thought because WLAN NICs show a Noise Variable, that it reflects actual ambient RF. But it's only showing what it calculates as Noise, based on the bits/chips coming into the NIC. But if there are no bits, then the WLAN NICs can't 'see' RF.

      Basically, it's showing you the effect of ambient raw RF on the Wi-Fi packets – NOT the raw RF itself.

      This is easy to prove with a demonstration using both Spectrum Analysis and Wi-Fi card side by side.

      • So i guess the NIC just says, OK, if the signal strength is below say -90dbm, i'm just going to consider it noise, because the packets i receive won't make sense anymore. So the NIC will just draw a cut-off line at -90 dbm and that'll show up as the “Noise” level .

        • Nope – not like that at all.

          There are some sophisticated algorithms to try and estimate what the actual noise floor is, based on how 'harmed' the chips and bits the WiFi NIC Receives. If there are no packets in the air, there can't be any 'Noise' variable. Thus when a jammer is turned on, it shuts down the APs and clients, nothing is talking, and so you see no noise at all. (yet the jammer is there causing the problem) weird, huh?

          • Hmm, i've another related question. Is the NIC's radio capable of doing spectrum analysis? I'm talking at hardware level. How different is the NIC's hardware compared to a… say WiSpy spectrum analyzer?

          • Normal Wi-Fi NICs can not do spectrum analysis. Not at all. Lately, however, a couple of chipset vendors have added a minor SpecAn feature to their Wi-Fi NICs, but this cannot work while the card is a Wi-Fi card. You have to stop the Wi-Fi features to access the SpecAn features. (AirMagnet uses this technique with their AirMedic product when tied to an Intel 5xxx series chipset) – and now Aruba is also offering it with some of their newer APs.

            For 'real' spectrum analysis, you need a chip specifically designed to 'see' raw ambient RF.

          • That answers my question. I was wondering if the driver was responsible for the chip behaving as a NIC or as a spectrum analyzer. That's another myth “Busted” :)

    • You're not alone. Many people have thought because WLAN NICs show a Noise Variable, that it reflects actual ambient RF. But it's only showing what it calculates as Noise, based on the bits/chips coming into the NIC. But if there are no bits, then the WLAN NICs can't 'see' RF.

      Basically, it's showing you the effect of ambient raw RF on the Wi-Fi packets – NOT the raw RF itself.

      This is easy to prove with a demonstration using both Spectrum Analysis and Wi-Fi card side by side.

      • So i guess the NIC just says, OK, if the signal strength is below say -90dbm, i'm just going to consider it noise, because the packets i receive won't make sense anymore. So the NIC will just draw a cut-off line at -90 dbm and that'll show up as the “Noise” level .

        • Nope – not like that at all.

          There are some sophisticated algorithms to try and estimate what the actual noise floor is, based on how 'harmed' the chips and bits the WiFi NIC Receives. If there are no packets in the air, there can't be any 'Noise' variable. Thus when a jammer is turned on, it shuts down the APs and clients, nothing is talking, and so you see no noise at all. (yet the jammer is there causing the problem) weird, huh?

          • Hmm, i've another related question. Is the NIC's radio capable of doing spectrum analysis? I'm talking at hardware level. How different is the NIC's hardware compared to a… say WiSpy spectrum analyzer?

          • Normal Wi-Fi NICs can not do spectrum analysis. Not at all. Lately, however, a couple of chipset vendors have added a minor SpecAn feature to their Wi-Fi NICs, but this cannot work while the card is a Wi-Fi card. You have to stop the Wi-Fi features to access the SpecAn features. (AirMagnet uses this technique with their AirMedic product when tied to an Intel 5xxx series chipset) – and now Aruba is also offering it with some of their newer APs.

            For 'real' spectrum analysis, you need a chip specifically designed to 'see' raw ambient RF.

          • That answers my question. I was wondering if the driver was responsible for the chip behaving as a NIC or as a spectrum analyzer. That's another myth “Busted” :)

  1. Keith – been to a couple of your three day classes – great stuff!. I can understand why many of the above statements are untrue, but some I agree with. Could you ever do a podcast that zips through some of these myths and explains them a bit?

  2. Keith – been to a couple of your three day classes – great stuff!. I can understand why many of the above statements are untrue, but some I agree with. Could you ever do a podcast that zips through some of these myths and explains them a bit?