Why You Should Stop Disabling Low Wi-Fi Rates, Illustrated

The last Sniff Wi-Fi post; on why Wi-Fi professionals should stop disabling low data rates, was met some resistance.  Be it in the comments or on Twitter, several experienced Wi-Fi folks disagreed.

All arguments in favor of disabling low rates (the ones that were presented to me, at least) were refuted in the text of the Leave, Leave, Leave My Rates Alone blog post.  But text is a less accessible messaging method.  "A picture is worth a thousand words", as the old saying goes.

If pictures will get the message across better, then pictures are what I'll use.  What follows is an illustrated look at why disabling low data rates is a bad idea.

It's gauche to begin an illustrated work with text, but to understand the problem with disabling low Wi-Fi data rates one must first accept some facts about Wi-Fi devices (smartphones, laptops, etc.):

1. Wi-Fi devices -- not APs -- control associations and roaming.

2. Wi-Fi devices roam based on low received signal strength (RSSI) -- not missed Beacons, retries, low rates, interference, signal-to-noice ratio (SNR) or anything else.

3. Wi-Fi devices control which data rates are used when they transmit -- it has nothing to do with which rates the AP supports.

4. Successful de-modulation of Wi-Fi data is based on SNR -- not RSSI.

With that out of the way, here is an illustrated look at why low data rates should not be disabled:

LOW RATES CAN BE "HEARD" FROM FUTHER AWAY...

SO, IF YOU DISABLE LOW RATES, THEN FAR-AWAY DEVICES WON'T ASSOCIATE...

BUT THAT'S NOT HOW Wi-Fi WORKS...

IN REALITY, DEVICES OUTSIDE THE "BOUNDARY" STAY ASSOCIATED...

"WHY?... YOU ASK"...



BECAUSE FAR-AWAY DEVICES OCCASIONALLY "HEAR" A HIGH-RATE BEACON...

AND IF THE RSSI FROM THAT BEACON IS HIGH ENOUGH, THEN THE DEVICES STAYS...

NOW, THE WHOLE CHANNEL IS SCREWED...

I would love to be able to say, "don't worry, if you have another AP nearby then the far-away devices will quickly roam to the better AP".  But devices don't roam just because there's a better AP nearby.  Devices roam based on RSSI.  Watch this 2015 presentation from Cisco Wi-Fi expert Jerome Henry if you don't believe me:


So, please, stop disabling low data rates on enterprise Wi-Fi networks.  If you don't, then I'll have to assume that these illustrations weren't good enough.  I'll have spend my time learning how to use a slicker tool than Paintbrush instead of learning more about Wi-Fi, and that would be a major bummer.

***
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Comments

  1. This comment has been removed by the author.

    ReplyDelete
  2. Hello. I found you on Google. I want guidance from you regarding wifi business. Can you tell me your email address. Or mail me at vikrant01641@gmail.com
    Thanks

    ReplyDelete
  3. This comment has been removed by the author.

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  4. Hi there. Thanks for writing this topic - it's good to challenge the best practises time to time.

    In your blog you state that the client roaming decisions is always based only on RRSI. Could you explain this more? Countless documents say that the roaming decisions are based on the client driver code which can use packet errors and calculated noise together with RSSI.

    For example this is a direct citation from CWNA Study Fuide:

    “What actually causes the client station to roam is a set of proprietary rules specified by the manufacturer of the wireless radio, usually determined by the signal strength, noise level, and bit-error rate.“

    Thanks,

    ReplyDelete
  5. You replied to one of your readers:

    "Whoever told you that missed Beacons or Retries affects roaming either hasn't studied device behavior or was lying."

    For exampe a document which explains Marvell 88W8686 roaming behavior says:

    "This decision is made via roaming algorithms that employ metrics such as RSSI indication, beacon loss, and frame acknowledgment. When the algorithm decides that a roaming threshold is met by the metrics, it immediately triggers roaming."

    The document can be found from http://wi2wi.com/mgr/docs/Roaming_in_WLAN_rev0.2.pdf.

    How would you comment on this?

    Thanks,

    ReplyDelete
    Replies
    1. I am talking about what triggers roaming. Those documents refer to the decision of which AP to roam to.

      Delete
  6. One other thing. Your main idea of this blog post was:

    "Devices send data at whatever rate they want to send data at, regardless of what rates the AP says it supports."

    If your have set your SSID to support only lets say 18 Mbps rate, that's the rate what all the (a/g) stations on that SSID use for sending data. You can test this easilly by using a a/g client, an AP and packet capture.

    Some vendors allow to specify the supported MCS rates as well.

    The might be some misunderstanding but could you expain how the stations in SSID can use different rates than the SSID is supporting?

    Don't get me wrong I realy like your blog but if you ask me this doesn't make any sense.

    Many thanks,

    ReplyDelete
    Replies
    1. Stations just do. I don't know how else to explain it. A station or AP chooses whatever rate it wants to choose when sending data.

      Delete
  7. To summarize from a couple of your different blog posts, you do recommend turning off the 802.11B rates, but you recommend against turning off any of the slower 802.11G or N rates, and you outline why doing so could cause more harm than good.

    I use HP MSM access points, and when I look at my options for turning off 802.11B rates, the controller refuses to allow me to turn off all of them. If I try, it states "At least one of the rates for 802.11b must be enabled." So the closest I can come to turning off the 802.11B rates would be to turn off all of them except the 11Meg rate.

    I'm wondering your thoughts on this. Is turning off all but the 11Meg 802.11B rates accomplishing good by limiting access to B clients, or is it just an 802.11B only vesion of what you're cautioning against in this article while not successfully banishing 802.11B clients anyway?

    ReplyDelete
  8. Hey,

    I have a question that is off topic,

    Can we use wifi beaming to send data over long distances, by creating a network of routers.
    The reason I ask is ,can we replace wired last mile internet connectivity using this method,by fixing routers on inclined position like a lamp post.

    If you could reply to my mail it would be very helpful
    Dhanushalpha@gmail.com

    Many thanks,
    Dhanush

    ReplyDelete

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