Friday, October 15, 2010

Limit SSIDs & Data Rates to Maintain Network Performance

To maintain proper Wi-Fi network performance, limit the number of active SSIDs to reduce channel overhead used by network management frames, especially beacons. Most enterprise Wi-Fi solutions on the market allow administrators to configure multiple virtual SSIDs on a single access point, each creating a distinct Basic Service Set (BSS). Therefore, each BSS sends out beacon frames at the Target Beacon Transmission Time (TBTT), which typically defaults to every 100ms.

Enabling too many concurrent SSIDs results in more network overhead from beacons. This can cause performance degradation by utilizing a large amount of available airtime on the channel.

Tips for maintaining high network performance:
  1. Limit active SSIDs to 6 at maximum
    This is a general rule-of-thumb, and should be adjusted based on your environment and network design and performance requirements. Lower this value even further if you plan on deploying voice over Wi-Fi, perhaps down to 3 or 4 SSIDs max.

  2. Disable Low Data Rates
    Since beacons are sent at the lowest "basic rate" of the BSS, disabling lower data rates forces beacons to use higher data rates and reduces network overhead. Be sure to test changing data rates prior to implementation to ensure adequate coverage still exists and that no impact to clients in your environment results from this change. A good starting point is to disable the 1-2 Mbps 802.11b data rates!
Here is an example. Assuming 6 SSIDs per-AP, 100ms TBTT, and 3 co-channel APs in the same area (within signal range to induce CCA medium busy backoff amongst each other):

Note 2013-08-16: This assumes a 180 byte beacon frame, which is probably too low for modern WLANs which have added many features that result in more Information Elements. For example, I typically measure beacons at around 370-400 bytes today. 802.11u and Hotspot 2.0 capabilities will increase this further. So, these calculations on bandwidth utilization are probably about half of what they are today. Double these numbers :) I'll try to post another blog updating these numbers accordingly.

   Beacon Data Rate     Resulting Channel Bandwidth Utilization
   1 Mbps               25.92%
   2 Mbps               12.96%
   5.5 Mbps             4.71%
   11 Mbps              2.36%
   6 Mbps (802.11a/g)   4.32%
   12 Mbps (802.11a/g)  2.16%

Same network example, now with only 3 SSIDs per-AP:

   Beacon Data Rate     Resulting Channel Bandwidth Utilization
   1 Mbps               12.96%
   2 Mbps               6.48%
   5.5 Mbps             2.36%
   11 Mbps              1.18%
   6 Mbps (802.11a/g)   2.16%
   12 Mbps (802.11a/g)  1.08%

Limiting active SSIDs and disabling lower data rates can make a HUGE difference!

Additional Reading Materials / Tools:
Cisco's Beacon Bandwidth Estimator (Only available from Cisco Advanced Services)
Aruba Airheads Articles (3 in total)

Cheers,
-Andrew

9 comments:

  1. These are two great tips. Also, one technique I use to reduce the number of SSID's is by implementing Dynamic VLAN Assignment. See my previous post about it http://tinyurl.com/y9577wz

    Cheers,
    /steve

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  2. Hi Andrew

    This is a very interesting topic and I am wondering if what you are describing would also apply to Meru Virtual Port concept (every client getting his own bssid) ?

    Thx

    Alain

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  3. Hi Andrew,

    We actually recommend configuring the lower data rates as "supported" rather than disabling them, as like you said management packets are sent at the lowest mandatory/basic rate. Having them as supported helps our mobile voice clients when roaming. I cover this on my Blog as well. Good stuff!

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  4. Hi Johnny,
    The solution is dependent on your specific situation. Remember a few things though:

    1) Some clients are very sticky and will not roam properly if the low data rates are left supported.

    2) In high density installs, I would advise to actually disable the low data rates instead of leaving them supported, because it will help spectrum efficiency and reduce airtime utilization since higher data rates take less time to transmit the same amount of data.

    Specifically regarding our mobile voice clients, we have found the exact opposite. Disabling the lower data rates has helped our Cisco wireless phones roam better. They need a very good SNR as a best practice, like -67 dBm, so they shouldn't need to use the low data rates anyways.

    As with everything wireless, your mileage may vary!

    Cheers,
    Andrew

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    Replies
    1. I think another disadvantage to putting lower data rates to supported is that your clients might be connected at those lower data rates and not receive the beacons that are being transmitted at higher data rates.

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  5. Andrew, nice post. When you mentioned disabling lower data rates, does it include the 802.11n MCS 0 rate of 7Mbps rate as well?

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    Replies
    1. I've never disabled any 802.11n rates. Since AP beacons and broadcast traffic are sent out at the lowest "Basic" rate, and since 11n rates can't be a Basic rate, then it wouldn't help reduce network overhead. It would be purely to prevent clients from using those rates. I don't think that would have much effect on network performance.

      But feel free to test it and report back.

      Andrew

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  6. An old post but great info nonetheless, Thanks!

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  7. Hi Andrew, can you tell me what tools you used to determine the resulting channel bandwith utilization in this analysis? Thanks.

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