Wednesday, April 2, 2014

10 Wi-Fi Terms You've Probably Been Using Incorrectly

Sometimes we fall into bad habits. Unfortunately, the improper use of terminology is quite common in the Wi-Fi industry. This can cause a great deal of confusion when people discuss technical topics. Therefore, as a Wi-Fi industry, I think we should start referring to the following terms using more accurate terminology so we are all on the same page.

Here goes:

  1. Over-the-Air Rogue APs - if it's not on your wired network, it's NOT a "Rogue AP" so let's start calling them Neighboring APs so we all know what someone is talking about rather than having to inquire each and every time someone mentions a rogue for clarification. And let's reserve using the term Rogue APs for when unauthorized APs are on the internal wired network.
    Correct Term: Neighboring APs

  2. Co-Channel Interference (CCI) - APs and clients that are operating on the same channel don't cause interference with one another, they contend for the same airtime and backoff if another one is transmitting. This is distinctly different from interference where a transmission cannot be properly decoded because the receiver can't distinguish the valid signal from noise.
    Correct Term: Co-Channel Contention (CCC)

  3. Collision - okay, here is one that most of you may not have really thought deeply about. Collisions don't actually happen on wireless networks (not in the traditional wired network meaning of the term 'collision'). Instead, the receiver simply cannot properly decode a valid signal because it can't distinguish it from the surrounding noise with the precision required by the modulation used.
    Correct Term: Interference

  4. Coverage Area - most Wi-Fi professionals refer to an APs coverage area as the physical area in which they intend for clients to connect to the AP, usually with an associated signal strength (such as -67dBm). However, the RF signal actually keeps going and can cause co-channel contention (see what I did there!) over a much larger area (usually out to a signal strength of around -85dBm). So, to refer to the area in which we expect clients to connect to the AP based on an RF design let's start using a different term such as Association Area and leave the term Coverage Area to refer to the area where CCC occurs.
    Correct Term: Association Area

  5. AES versus TKIP - this one is easy to get wrong, even for Wi-Fi professionals! Many times we interchangeably use AES, TKIP, and WEP to refer to the encryption on the wireless network. However, in so doing we confuse encryption protocols with cipher suites. For accuracy we should always mention like for like. CCMP, TKIP and WEP are all encryption protocols that we configure for a wireless network. Each of those protocols use a cipher suite to accomplish the heavy lifting: CCMP uses AES, TKIP uses RC4, and WEP uses RC4. Thanks to George Stefanick for bringing this up.
    Correct Term(s): Reference protocols (CCMP, TKIP, WEP) or ciphers (AES, RC4) but don't use them interchangeably

  6. 802.1x - I see this all the time in written material to refer to the IEEE 802.1X Port Based Network Access Control. Unfortunately, it should be used with capital letter 'X' since it is a (standalone) standard, whereas lowercase letters refer to amendments to standards (see here). So, whenever you reference it use the correct capitalization (802.1X).
    Correct Term: 802.1X

  7. WAP - many people use this term to refer to an access point and it's just annoying. It's just AP people. Referring to it as wireless AP (WAP) is just redundant.
    Correct Term: AP

  8. Antenna Gain in Decibels (dB) - many people refer to antenna gain in dB, which is incorrect. Decibels (dB) alone is a relative measurement and requires a point of reference. Instead, you should refer to antenna gain referencing either an isotropic radiator (dBi) or less commonly referenced to a standard dipole antenna (dBd). This establishes the absolute reference point for the measurement which actually gives it meaning.
    Correct Term(s): dBi or dBd

  9. 802.11b 1 Mbps and 2 Mbps Data Rates - do you reference all of the lower data rates of 1, 2, 5.5, and 11 Mbps as 802.11b? If you do, you've been using this amendment name incorrectly. The original 802.11 standard (802.11-1997) defined the 1 Mbps and 2 Mbps data rates as part of the DSSS PHY, as is generally referred to as 802.11 Prime. Then in 1999, along came the 802.11b amendment which added the 5.5 Mbps and 11 Mbps data rates as part of the HR-DSSS PHY. So, to be correct, when talking about 1 Mbps and 2 Mbps data rates you should reference 802.11-Prime (not 802.11b).
    Correct Term: 802.11 Prime (or 802.11-1997)

  10. 5 GHz Signals Attenuate Faster than 2.4 GHz Signals - it's common for many Wi-Fi professionals and writers to state that 5 GHz signals attenuates faster than 2.4 GHz signals in order to describe the common symptom that 5 GHz has less effective coverage area. However, this too is incorrect in most circumstances. 5 GHz signals attenuate through free space at the same rate as 2.4 GHz signals according to the FSPL (free space path loss) formula; it is not directly dependent on the frequency of the signal. Instead, the construction of the receiving antenna is a fractional multiple of the frequency to which it is tuned. This makes a standard 1/4 wavelength antenna for 2.4 GHz longer than a 1/4 wavelength antenna for 5 GHz, which causes a difference in antenna aperture. To put it simply, a 2.4 GHz antenna has a larger aperture than a similar 5 GHz antenna and can "capture" more of the signal as it passes by the antenna element.
    Correct Term: 5 GHz Antennas Have Smaller Apertures

Do you have any other terms that are misleading or misused and you think should be corrected? Drop a comment below!

Thanks,
Andrew

23 comments:

  1. Nice writeup! Re: #10 - Would you agree that the 5GHz signal propagate through walls/obstaclesvegetation worse than 2.4 GHz? I think the confusion is there. Wish that we dealt with environments where FSPL ruled, but not on this earth. Comments?

    ReplyDelete
    Replies
    1. Hi Pete,
      Yes, higher frequency signals in general experience greater attenuation through some objects than shorter wavelengths. However, for most materials the loss is less than 1dB in difference between 2.4 GHz and 5 GHz. I'll point you to this research material for more information:

      http://www.ko4bb.com/Manuals/05)_GPS_Timing/E10589_Propagation_Losses_2_and_5GHz.pdf

      However, the antenna aperture plays a more constant role in users experiencing lower signal strength. It is present all the time and is a larger factor than obstacles in most cases (not all).

      Cheers,
      Andrew

      Delete
  2. Good one!!!

    What about #1 if there was an AP in the parking lot not on your wired network, broadcasting the same SSID as your corp / guest network and hijacking your client sessions? Would that not classify as rogue?

    ReplyDelete
    Replies
    1. Hi Sriram,
      That's considered a "Honeypot" AP.

      Andrew

      Delete
  3. #1 made me laugh. Noticed this when using Ubiquiti gear. Thanks for the article.

    ReplyDelete
  4. Nice one, but I won't agree on all points :)
    Rogues: The definition of a rogue differs between different vendors. Cisco at least defines a rogues, which is outside your own administrative domain (not in the same RF-group). Then there's a differentiation between the rogue states (friendly / malicious / unclassified). Long story short
    - Different definitions by different people / the 802.11 standard did not define the term rogue
    - I guess a rogue may be also an AP, which is not connected to your network. What about a honeypot using your SSID to catch your clients?

    Collisions vs. Interference:
    Even the 802.11 standard talks about collisions. The media access control mechanism has even the term in it (DCA).
    Here's a quote from the 802.11 standard
    "The CSMA/CA protocol is designed to reduce the collision probability between multiple STAs accessing a
    medium, at the point where collisions would most likely occur."
    So, if even the standard states that there are collisions within a wireless network, it can't be so wrong, right? :)
    But I totally agree, that collisions may result in a decoding problem on the receiver side, because the "good" modulated signal becomes noise.

    ReplyDelete
    Replies
    1. Wow ... Terrible typo in my post. It shouldn't be "DCA" in "Collisions vs. Interference" (sorry, I were configuring Cisco WLCs all day and all I see is DCA, TPC, RRM *argh*).
      Of course it should be "DCF - CSMA/CA". Sorry about that.

      Delete
    2. Hi Johannes,
      Regarding "collisions", even though the IEEE 802.11 standard makes use of the term it is not accurate. Professionals are apt to use terms that are familiar to convey a concept, and the term "collision" was likely carried over from the 802.3 standard.

      Being technically accurate, "collisions" do not happen with RF signals. I'll quote from a 2011 report by the National Research Council's Computer Science and Telecommunications Board:

      "It is commonplace to talk about radio signals interfering with one
      another, a usage that mirrors the common experience of broadcast radio
      signals on the same channel interfering with each another. Thus, the term
      “interference” might suggest that multiple radio signals cancel each other
      out, making their reception harder or impossible. However, this view is
      misleading because radio signals themselves do not, generally speaking,
      interfere with each other in the sense that information is destroyed.
      In fact, interference is a property of a receiver, reflecting the receiver’s
      inability to disambiguate the desired and undesired signals.

      Radio signals are electromagnetic waves whose behavior, as described
      by Maxwell’s equations, is linear. One consequence of this behavior is that
      radio signals do not, in general, cancel each other out. Each new communication
      signal is superposed on the entire field.13 Actual destruction
      of information requires energy input at the point of destruction, and this energy must be applied very precisely to cancel out the signal’s vector
      field in all six dimensions, which is a low-probability event, and applies
      only at a single point in space.

      As a result, the superposition of any number of radio signals should
      be thought of not in terms of destroying information but rather in terms
      of the ambiguity it creates for a radio trying to receive any one specific
      signal. The difficulty of resolving the ambiguity relates to the energy emitted
      by other radios (with the implication that each radio sees multiple signals)
      and the unpredictability of the signals (which makes the individual
      signals harder to separate)."

      Here is a link if you would like to read the entire report (free registration required). The quote starts on page 54.
      http://www.nap.edu/catalog.php?record_id=13051

      Cheers,
      Andrew

      Delete
    3. Hi Johannes,
      Regarding rogue definitions, Cisco's terminology and implementation is especially bad! The purpose of identifying rogue APs is to alert the administrator to a situation that they must respond to, and the alert must contain actionable information. Lumping "unclassified" and "friendly" into the rogue discussion is unnecessary because these categories do not provide actionable information for an administrator.

      Since there is no formal definition of a rogue AP in the standard, the industry is left with varying interpretations. However, one thing is almost consistently the same - customers and WLAN admins only care about malicious rogue APs and any rogue AP list should not be cluttered with alerts for unclassified or friendly APs. I have found this interpretation to resonant highly with almost every single customer that I have worked with.

      Simply put, rogue APs were defined by the industry to give a name to malicious condition that administrators need to know about and take action on. Lumping any other non-malicious category of AP in with "rogue APs" is simply not what customers care about and adds no value for them. In fact, it only serves to cause confusion and security alerting systems that overwhelm administrators with data that is of no value and contains no actionable data. There is another term we use for that: "Alert Fatigue!"

      Cheers,
      Andrew

      Delete
  5. #10... "2.4 GHz antenna has a larger aperture than a similar 5 GHz antenna" ...
    Nowadays we have dual-band antennas in most APs... Dual-band - means it uses the same antenna for 2.4GHz and 5GHz band. So how 5GHz can be smaller? :)

    ReplyDelete
    Replies
    1. Hi Johny,
      It is my understanding that dual-band antennas still have multiple antenna elements inside one housing. I say multiple, because each frequency band may have more than one element for MIMO operation. However, some other antenna structures may be shared, such as a single antenna feed and antenna shorting pin.

      This is the way that I understand it at least.

      Check out this book:
      http://books.google.com/books?id=VbCCJhjO4OcC&pg=PA230&lpg=PA230&dq=dual-band+antenna+have+two+elements&source=bl&ots=ubeGk4AtIK&sig=cyXGmZWrint8G-MSNhkEUyg74Dw&hl=en&sa=X&ei=rOc-U77fAanr0gHopIDgCA&ved=0CIEBEOgBMAk#v=onepage&q=dual-band%20antenna%20have%20two%20elements&f=false

      Scroll down to page 233 and you can see a dual-band PIFA antenna with two "arms" on the antenna, one for GSM 900 MHz and GSM 1800 MHz (Figure 5.13 (a)).

      Cheers,
      Andrew

      Delete
    2. Here is another link that goes into dual-band multiple-element antenna structures for Wi-Fi applications:
      http://jpier.org/PIER/pier106/23.10052411.pdf

      Andrew

      Delete
  6. Andrew, #10 ... "Free-space path loss is proportional to the square of the distance between the transmitter and receiver, and also proportional to the square of the frequency of the radio signal." why you say: " it is not directly dependent on the frequency of the signal"?.

    http://en.wikipedia.org/wiki/Free-space_path_loss

    ReplyDelete
    Replies
    1. Hi Johny,
      Not that I take wikipedia as a true and accurate reference source, but you need to read further in your own link:

      "The FSPL expression above often leads to the erroneous belief that free space attenuates an electromagnetic wave according to its frequency. This is not the case, as there is no physical mechanism that could cause this. The expression for FSPL actually encapsulates two effects...

      The frequency dependency is somewhat more confusing. The question is often asked: Why should path loss, which is just a geometric inverse-square loss, be a function of frequency?. The answer is that path loss is defined on the use of an isotropic receiving antenna."

      Cheers,
      Andrew

      Delete
    2. Andrew, Looking into revision history of the FSPL Wiki article and related talk only re-emphasizes how important it is not to trust Wiki blindly. The sections you mention did not exist until a few weeks ago (Mar 18 2014)! Probably, most people citing it read it a long time ago.

      Am I right understanding that the common term 'FSPL' actually includes implicitly some antenna properties (i.e. aperture), and that it's derived from Friis equations?

      Delete
  7. Nice list on the correct Terminology Andrew :), regarding #10 I dont think FSPL or attenuation of the waves is directly related to Frequency of operation. Rather the wavelength of a 5Ghz Beacon from the AP is twice as smaller to the wavelength of the Beacon over a 2.45 Ghz so 5 Ghz waves will have difficulty while bending or while facing obstacles.
    This is the Problem which we see and Engineers assume that as an attenuation effect which is not true.

    To sum it up

    * The longer the wavelength, the further it goes
    • The longer the wavelength, the better it travels/ bends through and around things
    • The shorter the wavelength, the more data it can transport

    your thoughts..?

    ReplyDelete
    Replies
    1. Hi Akram,
      You get the description accurate, then fall into the very misconception I attempted to clear up your first summary point.

      Quick points on this topic:
      1.) 5 GHz signals travel just as far as 2.4 GHz signals. (This is the point I was trying to clarify)
      2.) Users typically experience lower signal strength from 5 GHz than 2.4 GHz signals because the antenna aperture is smaller, meaning it collects less signal out of the air.
      3.) 5 GHz signals DO attenuate more than 2.4 GHz signals through some obstacles, and this also leads to the user experiencing a lower signal strength when not in direct line of sight.
      4.) Shorter wavelength signals such as 5 GHz when compared to 2.4 GHz can transport more data based on Shannon's Theorem. However, this is not being realized today with 802.11 Wi-Fi networks since the modulation rate is the same for OFDM whether in 2.4 GHz or 5 GHz (how often the signal is changed using QAM). Other factors are in play giving us higher data rates in 5 GHz which do not take advantage of the shorter wavelength, such as more aggressive encoding with 256-QAM in 802.11ac (but is available in 2.4 GHz with proprietary Broadcom Turbo-QAM), and wider channel widths which is just using more spectrum to carry data on more subcarriers, not using the shorter wavelength to modulate at a faster rate.

      Hope this helps clarify!

      Cheers,
      Andrew

      Delete
  8. You, Sir, have a doubful privilege of becoming the first Wi-Fi Grammar Nazi ☺

    Please explain #10: clearly FSPL formula has frequency in it, and materials DO have different attenuation/reflectivity in different frequencies. Did you mean to say that antenna aperture is the main contributing factor? What about dual-band antennae?

    (Since we're getting really strict now, I should remind the "antennae" thing!) ☺

    Sriram, that's Impersonation or "Evil Twin".

    Support Johannes on Collisions, but not on Rogues. Cisco, MS and Apple are well known of bending terminology towards their minute needs. Talking to admins, who learned networking using MS books, is quite an experience...

    ReplyDelete
    Replies
    1. Hi Arsen,
      Please see my comments in reply to others as I believe that I've covered all of your questions regarding FSPL, antenna aperture, and dual-band antennas.

      Also, both plural forms of antenna are accurate (antennas, antennae):
      http://www.merriam-webster.com/dictionary/antenna

      Andrew

      Delete
  9. Thanks for the article, it clears up things. I myself have been promoting some of these, this really helps.

    ReplyDelete
  10. Great article. On #9 I've personally been trying to refer to them using the PHY. DSSS, HR-DSSS, etc. Either works though.

    ReplyDelete
  11. With point #2, I agree co-channel contention more accurately describes what is going on. Do you think also ACI could be described this way?

    ReplyDelete
  12. Here are my pet peeves:
    - "5.0GHz". The band starts at 5.150GHz. Channel 36's lower edge is 5.170GHz, and it's centered at 5.180GHz. Wi-Fi does not operate at 5.0GHz.
    - "A-band vs. B-band (or G-band)". It's 5GHz vs. 2.4GHz. 802.11n works in both, so which band is "N-band", huh?
    - "Wireless router". Most consumer home gateways with Wi-Fi can either do NAPT gatewaying, which is much more than a router, or they can be simple bridging APs, but few can be a real layer-3 IP forwarding router separating routable subnets, and fewer can actually participate in any routing protocols. I prefer "Wi-Fi home gateway", or just "Wi-Fi AP".
    - "PAT". It's a Cisco-ism. The IETF standard term is NAPT. Also, don't write "NAT" when you really mean "NAPT".
    - "CCK rates", when used to refer to the "1, 2, 5.5, and 11" rate set. Not only are 1 and 2Mbps not part of 802.11b, they don't use CCK. Only 5.5 and 11 use CCK.
    - I strongly second your disgust for "WAP". Back when Apple started popularizing 802.11b to consumers in 1999, WAP still stood for "Wireless Application Protocol", which was the way the barely-data-capable mobile phones of the day could connect to a very early version of the mobile web, using what, as I recall, was a stripped down version of XHTML and HTTP. Aside from that name collision, using the term "Wireless Access Point" gets the role of the AP backwards. When 802.11 was designed, APs were thought of the places where these newfangled wireless clients could get access to the existing wired LAN. So they were really "Wire Access Points" in the original mindset, not the other way around.

    ReplyDelete